Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region	Report Contents Report in Word Format
	Caribbean Environment Programme Technical Report #40 1998	All CEP Technical Reports

TABLE OF CONTENTS

Conventional Gravity sewers

DESCRIPTION

Conventional gravity sewers carry raw sewage from households, public facilities, and businesses. Pipes are 200 mm or more in diameter to prevent clogging. Conventional gravity sewers are installed at a slope so as to maintain a flow of 20 cm/s minimum velocity by gravity. When this is not possible, pump stations are used to pump the sewage. Conventional gravity sewers are expensive to build and can be difficult to maintain, but they are the most common collection systems being built today.

APPLICATIONS

Conventional gravity sewers are appropriate in large urban centres with a high population density or for more dispersed development. They have historically been the primary method of sewage collection and transport.

• Peak flow rate should be determined in designing a collection system. Inflow and groundwater infiltration (I&I) into the sewer pipes should be accounted for in existing systems. In new construction, I&I should be limited. Inflow connections should be allowed.

• Sewers conveying raw sewage should be at least 200 mm in diameter.

• Sewers should be designed so that sewage has a mean velocity not less than 60 cm/second in average flow conditions so that solids do not settle and build up in the pipes. Excessive velocities are not desirable.

• Manholes should be installed at the end of each line, at a change of grade or pipe size, and at least every 100 m.

PERFORMANCE EFFICIENCY

Conventional gravity sewers effectively convey the wastewater flows they are designed for. However, I&I entering the sewer lines through the manholes and pipe joints creates an additional volume of waste that must be treated. I&I can be controlled with modern designs.

DISADVANTAGES

The biggest disadvantage of conventional gravity sewers is their high capital cost. In areas with high water tables, extensive subsurface rock formations, or unstable soil conditions, conventional gravity sewers are even more expensive to build due to the excavation and dewatering costs. Also, because conventional gravity sewers carry solids, a minimum velocity or slope is needed to prevent excessive solids deposition. This means that excavations can end up being very deep in order to maintain necessary slopes, or that pump stations will be needed, which can be expensive to maintain.

RESIDUALS GENERATED

N/A

OPERATION & MAINTENANCE

Sewer pipes need to be periodically flushed out to prevent solids accumulation. If pump stations are used, normal mechanical maintenance is required. Special provisions should be made for any grit accumulation in wet wells.

WCR INSTALLATIONS

Conventional gravity sewers are used throughout the WCR.

REFERENCES

Herbert, J.C. et al. 1992; Inter-American Development Bank 1992; Kaijun, W. et al. 1995; U.S. Department of Commerce 1991; U.S. EPA February 1980; U.S. EPA October 1991.

Pressure Sewers

DESCRIPTION

In STEP systems, septic tank effluent flows to an intercepter tank, which is basically a septic tank. At a specified high water level, the effluent is pumped to its destination. In GP systems, a grinder pump grinds the solids before pumping the flow to a central line or its final destination. In both systems, the connection lines and pressure mains are made of inexpensive polyvinyl chloride (PVC) or similar platic piping.

APPLICATIONS

Pressure sewers are typically used in low density areas where the terrain does not permit gravity flow to a central location or treatment facility. They can also be used where soil conditions are rocky or unstable, or where the groundwater level is high. Construction costs are much lower for these small diameter sewers because the material costs less, excavations do not need to be as deep (to prevent pipes from damage), and PVC piping is flexible, making pipe-laying easier.

• Connection lines are typically made of PVC (or other plastic) piping and are typically 25 to 50 mm in diameter.

• Pressure mains are made of PVC (or other plastic) piping and are typically 75 mm in diameter or larger.

• A minimum design velocity is not important in STEP systems as in gravity or GP systems because few solids are transported.

• To avoid solids accumulation in GP systems, flow must attain a minimum velocity of 90-150 cm/second once a day for a period long enough to scour the system clean. This duration varies with pump capacity and overall system size.

PERFORMANCE EFFICIENCY

Pressure sewers experience much less inflow and infiltration than conventional sewers.

DISADVANTAGES

The main disadvantage of pressure sewers is the maintenance of mechanical equipment at each entry point to the system.

RESIDUALS GENERATED

N/A

OPERATION & MAINTENANCE

Sewage conveyance in pressure sewers relies on pump operation. Because there is a pump at each entry point, maintenance costs are significant, but less than a conventional gravity system with pump stations.

WCR INSTALLATIONS

KCM has no knowledge of installations in the WCR.

REFERENCES

Inter-American Development Bank 1992; U.S. Department of Commerce 1991; U.S. EPA October 1980; U.S. EPA October 1991; U.S. State Department 1994.

Vacuum Sewers

DESCRIPTION

Vacuum sewers use a central vacuum source to convey sewage from individual households to a central collection station. A valve separates the atmospheric pressure in the home service line from the vacuum in the collection mains. The valve periodically opens based on volume stored to allow wastewater and air to flow into the vacuum collection mains. The wastewater is propelled in the collection main from the differential pressure of a vacuum in front and atmospheric pressure in the back. Eventually the air pressure in the collection main equalises, and all flow ceases until the next valve from a service line is opened. Through this process, wastewater is conveyed to a central collection tank. From there, it can be conveyed by gravity or by a pump station through a force main to its final destination.

APPLICATIONS

Like pressure sewers, vacuum sewers are typically used in low population density areas where the terrain will not permit gravity flow to a central location or treatment facility. They can be used in mildly undulating terrain, but perform better with relatively flat topography because the vacuum systems are limited in the amount of lift they can generate. They can also be used where soils are rocky or unstable or where the groundwater level is high. Construction costs are much lower for these small diameter sewers because the material costs less, excavations do not need to be as deep (to protect the pipe from damage), and the PVC piping used is flexible, making pipe-laying easier.

• A vacuum of 0.5 to 0.8 atmospheres is maintained in the central collection mains.

• The lateral piping is typically made from PVC of 80 mm in diameter, while mains start at 100 mm.

PERFORMANCE EFFICIENCY

Vacuum sewers experience much less inflow and infiltration than conventional sewers because they are air tight.

DISADVANTAGES

Vacuum pumps can only generate a maximum lift of 10 metres of water. This limits the terrain in which vacuum pumps can be used. Also, there can be an odour problem from the venting of odourous off-gases. A minimum of about 70 dwellings is required to utilize this system effectively.

RESIDUALS GENERATED

N/A

OPERATION & MAINTENANCE

Vacuum sewer stations require dailiy maintenance and yearly inspection of the valves at all connection points. The vacuum and discharge pumps typically require major repair or replacement every 10 years.

WCR INSTALLATIONS

KCM has no knowledge of installations in the WCR.

REFERENCES

Inter-American Development Bank 1992; U.S. Department of Commerce 1991; U.S. EPA October 1980; U.S. EPA October 1991; U.S. State Department 1994.

Small-Diameter gravity Sewers

DESCRIPTION

Small-diameter gravity (SDG) sewers convey septic tank effluent by gravity to a centralised treatment location. Because the septic tanks remove most of the suspended solids in the wastewater, there is little clogging, so the piping can have a smaller diameter than for conventional sewers. PVC piping is typically used for SDG sewer installations.

APPLICATIONS

SDG sewers are typically used in low to medium population density areas where the terrain permits gravity flow to a central location or treatment facility. They require less slope than conventional gravity sewers and can be used where it would be difficult to provide adequate slope for conventional sewers. They also can be used where soil is rocky or unstable or the groundwater level is high. Construction costs are much lower than for conventional sewers because the material costs less, excavations do not need to be as deep (to protect the pipes from damage), and the PVC piping that is used is flexible, making pipe-laying easier.

• Typical pipe diameters for SDG sewers are 80 mm or more.

• The slope of the piping should be adequate to carry the daily peak hourly flows

• SDG sewers do not need to be designed to meet a minimum velocity.

• The depth of the piping should be the minimum necessary to prevent damage from anticipated loadings. If no heavy loadings are anticipated, a depth of 600 to 750 mm is typical.

• Cleanouts need not be placed at any regular interval short of that dictated by the sewer cleaning technique employed. A cleanout is a pipe that forms a tee with the collection main, providing access to the main. Cleanouts are used instead of manholes because SDG sewers are not designed to carry solids or grit, and manholes are a source of solids and grit to collection mains. Cleanouts also are much cheaper to construct and maintain than manholes.

PERFORMANCE EFFICIENCY

Small-diameter gravity sewers experience much less inflow and infiltration than conventional sewers.

DISADVANTAGES

The main disadvantage of SDG sewers is they are an emergent technology. Some previous applications have performed inadequately because of poor design and construction practices.

RESIDUALS GENERATED

N/A

OPERATION & MAINTENANCE

The main operation and maintenance needs of SDG sewer systems are removing septage from the septic tanks and occasionally checking collection main connections.

WCR INSTALLATIONS

KCM has no knowledge of installations in the WCR.

REFERENCES

Herbert, J.C. et al. 1992; Inter-American Development Bank 1992; U.S. Department of Commerce 1991; U.S. EPA 1980; U.S. EPA 1991.

Septic tank systems

DESCRIPTION

ˇ Septic tanks with drainfields

ˇ Septic tanks with mounds

ˇ Septic tanks with evapotranspiration beds

Septic tanks with evapotranspiration beds. Septic tanks can also be used with evapotranspiration (ET) beds. ET beds are a sand bed with an impermeable liner and wastewater distribution piping. Wastewater fills the pores in the sand and rises to the upper portion of the bed by hydraulic pressure and capillary action. In the upper portion of the bed the water evaporates in the soil through direct vaporisation and through the leaves of rooted vegetation grown on the surface of the bed. In evapotranspiration/absorption (ETA) systems the liner is omitted and water can also escape by seepage into the underlying soil. A further modification of the evapotranspiration system is to drain toilet drainage only to the ET bed and to discharge drainage from sinks and showers ("grey water") to soil absorption pits or surface discharge. A serious limitation of evapotranspiration systems is that they function only when evaporation exceeds precipitation during every month of the year.

APPLICATIONS

Septic tanks with drainage fields are used primarily in rural or suburban areas for single households or for small clusters of homes. Septic tanks with mound systems are used when soil conditions are not suitable for an underground drainage field, primarily in rural or suburban areas for single households or small clusters of homes. Mounds are appropriate when soil permeability is less than 25 mm/hour, the bedrock is shallow, or the water table is close to the ground surface. ET systems are applicable only in climates where evaporation exceeds precipitation for every month of the year.

DESIGN CRITERIA

• Septic tanks must have sufficient liquid volume for a 24-hour fluid retention time at maximum sludge depth and scum accumulation. For a single home, a tank volume of 2 to 3 times the daily flow is adequate.

• Shallower tanks generally provide better performance than deep tanks.

• Tanks with multiple compartments remove BOD and suspended solids better than single-compartment tanks.

• Septic tanks with drainage fields require a minimum groundwater percolation rate of 25 mm/hour.

• Seasonal high groundwater level should be at least 600 mm below the bottom of the drainage field.

• The area required for the drainage field is based on flow rate and soil percolation rate, as shown in the following table:

• Mound systems are effective where soil permeability is between 15 and 25 mm/hour.

• The mound height in the centre should be between 900 and 1500 mm, and the side slopes should be no steeper than 3:1 horizontal-to-vertical.

• The sand fill depth for mound systems is 300 to 600 mm beneath the distribution piping, depending on the groundwater level.

• Effluent should be applied to the mound at a rate of 4 to 50 L/m2/day.

• The frequency of discharge to the mound should be once every 1 to 4 days.

• For non-discharging systems, the hydraulic loading rate should be determined by an analysis of the monthly net evaporation (pan evaporation minus precipitation) experienced during the wettest year of a 10-year period. Under these conditions loading rates of 1.2 to 3.3 L/m²/day have been found acceptable for arid regions.

• Where occasional discharge is acceptable, loading rates may be less restrictive than for non-discharging systems, for example, based on the minimum net ET in a normal year.

• Distribution piping networks should be constructed of 100 mm diameter perforated plastic or clay pipes in drain rock and surrounded by filter fabric.

• Sand bed depth should be 600 to 900 mm covered with 0 to 100 mm of topsoil.

• Clean and uniform sand in the size of D50 = 0.1 mm (50% by weight smaller than or equal to 0.1 mm) is desirable.

• Synthetic liners should have a thickness of at least 10 mil. It is preferable to use a double thickness of liner to permit staggering of seams, if seams are not avoidable.

• Synthetic liners should be cushioned on both sides with layers of sand at least 50 mm thick to prevent puncturing during construction.

 

PERFORMANCE EFFICIENCY

The performance of a septic tank with absorption system is a function of design, construction techniques, type of soil (permeability and composition), and loading. In properly designed systems, the soil removes BOD, suspended solids, bacteria, viruses, phosphates, and heavy metals from the effluent. However, nitrates and chlorides easily pass through coarser soils. A septic tank alone will remove 30 to 50 percent of BOD, 40 to 60 percent of suspended solids, about 15 percent of phosphorus, and 70 to 80 percent of oils and grease. The performance efficiency of a mound system is similar to that of a septic tank with drainage field. ET systems have no discharge.

DISADVANTAGES

ET systems require much lower loading rates than either drainage fields or mounds and are applicable only in arid climates.

RESIDUALS GENERATED

The residual associated with a septic tank system is sludge build-up in the septic tank of about 0.04 m3 per person per year.

OPERATION & MAINTENANCE

Sludge must be removed from the septic tank every two to three years. Mound systems have associated costs for pump energy consumption and maintenance.

WCR INSTALLATIONS

Septic tanks with drainage fields are widely used throughout the Caribbean islands. KCM has no specific knowledge of mound systems in use in the region. ET systems have been used successfully in Jamaica.

REFERENCES

HOLDING TANK

DESCRIPTION

A holding tank receives and stores wastewater from homes or commercial establishments until it is pumped out and hauled to a wastewater treatment facility. The tank must be watertight and airtight and have an alarm to indicate high fluid levels. It should have capacity for at least two days of use after the alarm engages.

APPLICATION

Holding tanks are used primarily in areas where septic tanks with drainage fields or mounds are not feasible. They also are used in environmentally sensitive areas, where nutrients must be prevented from entering the groundwater.

• The most important criterion for a holding tank is that its volume not exceed the capacity of the pump truck that will service it.

• The alarm should set off when the tank has capacity remaining for about two days of use.

• Water conservation devices should be used to minimise how often the tank must be pumped.

• A typical family of four in the U.S. with piped water supply will need a 4-m3 tank pumped about once a week.

PERFORMANCE EFFICIENCY

Some anaerobic digestion occurs in the tank, like in a septic tank. Otherwise, the system is highly reliable if designed and built properly and if proper servicing techniques are maintained.

DISADVANTAGES

Pumping can be very expensive if the tank is far from a wastewater treatment facility. The pumping service must be reliable and a suitable treatment facility is also needed.

RESIDUALS GENERATED

The only residual associated with a holding tank is the wastewater hauled to a treatment facility.

OPERATION & MAINTENANCE

Frequent pumping and travel costs are associated with the pumping truck as well as the costs of discharge and treatment.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

EPA, September 1992; U.S. Department of Commerce, 1991.

HOUSEHOLD SYSTEMS

DESCRIPTION

• Pit latrines are holes in the ground where small amounts of excreta and wastewater are stored and liquids leach slowly into the ground.

• Incinerating toilets are small units that incinerate excreta and other wastes. The waste collects in a chamber and is incinerated periodically with fossil fuel or electricity.

• Composting toilets are designed to aerobically convert the organic matter from wastes into a safe humus that can be applied to soils. The waste is mixed and heated to evaporate excess liquids and to stimulate the biological activity needed for composting. Composting can take place in a chamber included with the toilet or in a larger, separate unit, and generally requires external mixing and aeration energy.

• Oil recirculating toilets use a petroleum fluid to flush wastes into a collection chamber. The solids are separated from the petroleum fluid and stored for subsequent disposal.

APPLICATION

Household systems are appropriate in areas with little or no piped water supply and waste collection system.

• Pit latrine volume should accommodate a solids accumulation of 0.05 to 0.06 m3 per year per person.

• Typical pits are 0.3 to 1.1 m2 in area and 2400 to 3000 mm deep.

• It is usually cheaper to build two smaller latrines than one very large; this approach minimises the need for wall support and maximises distance from groundwater.

• Adequate holes should be provided for ventilation of odour and solar heating.

• Criteria and fuel requirements vary with manufacturer.

• The criteria for sizing the composting chamber, aeration, mixing, and bulking agent addition vary with each manufacturer.

• Criteria vary with manufacturer; required holding tank volume can be up to 1.4 m3.

PERFORMANCE EFFICIENCY

Pit latrines provide excellent treatment if designed and loaded properly. The degree to which the effluent is treated before reaching groundwater depends on the soil characteristics, i.e. depth to groundwater, soil permeability, and soil composition. The benefit of incinerating toilets, composting toilets, and oil recirculation toilets is that their pollutant load is removed from the grey wastes, thus making their treatment easier and less costly.

DISADVANTAGES

Pit latrines can only handle small flows of wastes. They are not suitable in environmentally sensitive areas. They need to be properly designed for adequate treatment. Odour and pestilence or vector problems can develop. Incinerating toilets have a capacity of about three uses per hour. Frequent maintenance is required for both fuel- and electric-powered designs. Electric-powered toilets have high energy costs. Composting toilets with separate composting units serve households of only up to five people. Smaller, non-separated units can serve households of only about two people. These toilets require knowledge and care for proper usage. Oil-recirculating toilets require filtration equipment to separate solids from the petroleum-flushing fluid. Solids disposal is difficult because the solids are very oily, and no successful domestic applications are known. All of these systems may be aesthetically displeasing.

RESIDUALS GENERATED

Pit latrines generate 0.05 to 0.06 m3 of sludge per person per year. Incinerating toilets generate a harmless ash which must be disposed. Composting toilets can generate a soil conditioner provided the sludge is stabilised properly. Oil recirculating toilets generate an oily-solids residual that is difficult to dispose of properly.

OPERATION & MAINTENANCE

Pit latrines require decommissioning or sludge pumping every few years. Incinerating toilets require a high level of maintenance in the form of cleaning and have significant energy costs. Composting toilets require the periodic addition of mulch, grass, or some other vegetation for bulking agents. Mixing will be required to obtain aerobic conditions. Oil-recirculating toilets require cleaning or replacing exhausted filtration media, disinfection, and replacing lost oil.

WCR INSTALLATIONS

Pit latrines are widely used in rural areas in the WCR. The other disposal facilities have not gained acceptance in the region.

REFERENCES

EPA, October 1980; U.S. Department of Commerce, 1991; World Bank, 1982.

LAGOONS (STABILISATION PONDS)

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DESCRIPTION

After treatment, effluent can be disposed in one of three ways. Continuous discharge is the simplest and most common method of effluent discharge. Controlled release is discharge of effluent only when its water quality is good or during high flows in the receiving water (if discharge enters a stream or river). The third option is to dispose of effluent by evaporation and percolation into the soil rather than discharging to a receiving water. This can be done only when the combined rate of evaporation and percolation equals or exceeds the wastewater influent flow.

APPLICATIONS

Lagoons are a versatile wastewater treatment process. They can be used for domestic and industrial sewage. Aerobic, facultative, and anaerobic lagoons may be used as the first step in a treatment process, without pre-treatment, but the influent should be screened to remove floating materials. Facultative or aerobic lagoons also can be used as a final process to polish the effluent before final discharge. Maturation ponds are usually designed to allow sufficient detention time and contact with sunlight for pathogen removal or die-off. Anaerobic lagoons are especially useful for industrial wastes with high BOD loads. Anaerobic lagoons usually need to be followed by an aerobic or facultative lagoon since effluent will need further treatment.

DESIGN CRITERIA

PERFORMANCE EFFICIENCY

Anaerobic lagoons remove about 40 to 60 percent of influent BOD. The other types of lagoons can reliably achieve an effluent BOD concentration of 30 mg/L, and even better if designed well. Suspended solids (SS) concentrations are typically higher than 30 mg/L. Some lagoons can achieve final SS concentrations of 20 to 30 mg/L, however most can only achieve effluent SS concentrations between 30 and 90 mg/L. Effluent faecal coliform concentration varies greatly. Detention time, exposure to sunlight, pH, and lagoon geometry all affect coliform removal. If maturation ponds are used as a polishing step, faecal coliform counts as low as 200 to 400/mL can be reliably achieved without chlorination. Some nitrogen removal is achieved through uptake in algae, and through nitrification (ammonia conversion to nitrates) and denitrification (nitrate uptake in carbonaceous BOD removal.)

DISADVANTAGES

The primary disadvantage of lagoon systems is their large land requirement. Relatively high levels of effluent suspended solids compared to well-operated conventional mechanised treatment plants are another disadvantage. If land is abundant and the receiving water is not sensitive to discharge of moderate levels of suspended solids, lagoons or ponds are appropriate treatment options for most communities. If a high level of removal is required, polishing processes are needed. Algae is often the main contributor to suspended solids in the effluent. If low levels of suspended solids are needed, algae can be filtered or removed by other processes such as dissolved air flotation. One potential solution to the problem of excess algae production in lagoons is to use several maturation ponds in series, each with a detention time too short to allow the growth of algae. Discharge to wetland systems for polishing is another potential solution. In pond systems where algae control is a problem effluent should be withdrawn from well below the surface, since most algae float. Flies can be a nuisance in some tropical climates. Talapia, a hardy fish species, can help control this problem, as well as strategic placement of lagoons in breezy, open areas, and vegetation maintenance to eliminate insect habitats.

RESIDUALS GENERATED

It has been reported that sludge is generated in aerobic or facultative lagoons at a rate of about 0.04 cubic metres per person per year. Many lagoons do not experience a significant build-up of sludge, however, even after decades of loading. Others, like the Beetham Lagoons in Port of Spain, Trinidad, fill up rapidly. Designs must take into consideration sludge removal requirements based on rational calculations of sludge build-up under design conditions of loading. Small barge-mounted dredge pumps can be used effectively to remove sludge from lagoons, if sludge build-up is modest.

OPERATION & MAINTENANCE

Lagoons may require sludge removal every few years and regular vegetation maintenance. Regular maintenance of mechanical components, such as recirculation pumps, mixers, or aeration equipment, is also required for some lagoon designs.

WCR INSTALLATIONS

Lagoons are commonly used throughout the Caribbean region wherever space is available. The Los Guayos plant in Valencia, Venezuala is an lagoon system with pimary anaerobic cells, facultative cells, and effluent recirculation, designed to serve an ultimate population of 1.5 million. The Rodney Bay wastewater treatment plant in St. Lucia is an AIPS which has performed effectively. The Beetham Lagoons in Port of Spain, Trinidad were designed in the late 1950s as anaerobic and facultative lagoons to serve 150,000 persons.

REFERENCES

Archer, A.B., 1990; Archer, J.P., 1983; Curtis, T.P., 1992; Ellis, K.V., 1991; Evans, B., 1993; Ghrabi, A., 1993; Kruzic, A., 1994; Lansdell, M., 1996; Lansdell, M., 1987; Lansdell, M., 1991; Mayo, A.W., 1996; Mendes, B.S., 1995; Millette, W.M., 1992; Mills, S.W., 1992; Oragui, J.H., 1995; Phelps, H.O., 1973; Picot, B., 1992; Rich, L.G., 1996; Sweeney, V., 1996; U.S. EPA, 1983; U.S. EPA, 1992.

CONSTRUCTED Wetlands

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DESCRIPTION

Constructed wetlands are an excellent treatment process for removing BOD and suspended solids, as well as other particulates, from domestic and industrial sewage. Two types of wetlands are commonly used in wastewater treatment: free-water surface and subsurface flow. In a free-water surface (FWS) wetland the wastewater flows through a shallow bed or channel and is in contact with emergent vegetation and the atmosphere. The wastewater is treated by the anaerobic microbial community associated with the plant stems and root mounds, as well as by aerobic communities in the open water zones. In subsurface flow (SF) wetlands, a foot or more of gravel or coarse sand is used to support the root zone of emergent vegetation. The wastewater is treated primarily by the microbial community in the root zone and the rocks below. Subsurface flow wetlands usually have a clay barrier or membrane liner between the flow being treated and the groundwater to prevent contamination. The effluent can be collected or, more commonly, discharged to a river or ocean. Wetlands require a large land area but they can be easily managed and operated by unskilled labour. FWS systems are best suited following lagoons, while SF systems should follow septic tanks or other treatment systems.

APPLICATIONS

Wetlands can treat anything from septic tank effluent to effluent from secondary treatment. They can be used as buffer zones to treat urban stormwater runoff and because they are excellent solids removal systems, they are capable of removing metals from the waste stream. Wetlands provide excellent removal of BOD and suspended solids as long as they are not overloaded (hydraulically or in pollutant load). Both wetlands also remove faecal coliforms and other pathogens. Constructed wetlands are most appropriate for medium- or low-density communities where sewage is collected, and where adequate land is available for construction. They are easiest to build on flat terrain, but can be built successfully in a tiered form on hillsides. They are both excellent denitrifiers and can provide good nitrogen removel when following nitrification systems.

DESIGN CRITERIA

• Wetlands should be sized with an area of 5 to 10 m2 per person served, assuming 100 to 200 L per day per person of wastewater generated. The requirement may be lower if the wetland is used as tertiary, polishing step in the treatment process.

• Free water surface wetlands for domestic wastewater should be sized for a hydraulic loading of 8 to 40 L/ m2/day.

• The wetland should be sized for a BOD loading of 1 to 20 kilograms per hectare per day, or about 10 metres square / person.

• Appropriate hydraulic detention time ranges from 7 to 40 days. When high strength or higher quality effluent is needed, it is better to use a series of wetlands, each with a detention time of 20 days.

• Subsurface flow wetlands for domestic wastewater should be sized for a hydraulic loading of 20 to 400 L/m2/day, or about 5 metres square / person.

PERFORMANCE EFFICIENCY

Wetlands can achieve very high BOD if influent BOD is in particulate or large colloidal states, but 80 to 90 percent removal—for BOD and suspended solids—is more typical. Nitrogen removal depends on the influent nitrogen form and detention time; some submerged flow systems have achieved over 90 percent removal, but more typical systems remove about 30 percent. One-to two-log removals of faecal coliforms have been observed, yet faecal coliform removal is not as reliable in wetlands as in stabilisation ponds.No phosphorus removal is expected after initial startup unless vegetation is harvested (up to 15% removal).

DISADVANTAGES

FWS wetland systems need a large area to operate properly. They are proven and reliable if the organic and hydraulic loading is not too high. When the soluble organic loading rate increases, the BOD and suspended solids removal becomes less reliable. Removal of faecal coliforms also is unreliable, due in part to the use of constructed wetlands by birds and animals; certainly direct reuse without disinfection or filtration is risky. For many receiving waters, wetland effluent requires disinfection and reaeration, as the process is inherently anaerobic. Flies and mosquitos can be a nuisance in FWS wetland areas. This can be partially controlled by planting Talapia, a hardy breed of fish, into open areas of the wetland.

RESIDUALS GENERATED

The BOD and nutrients removed from the waste stream fuel growth of emergent vegetation and biomatter attached to vegetation roots and filtration media (if a subsurface flow system is used). Typical vegetation growth is 56 to 80 kg/hectare/day. Normally, there is no harvesting of SF vegetation. Properly designed and maintained FWS systems require regular harvesting.

OPERATION & MAINTENANCE

The primary maintenance activity is harvesting new FWS vegetation growth. If toxic metals are present in the waste streams, the roots and leaves of the vegetation should be properly disposed of and not ingested by humans or animals. Inlet, outlet, pumping, and other mechanical maintenance may be necessary. Overall, the operational and maintenance requirements are low for wetland processes.

WCR INSTALLATIONS

Wetlands are usually used as a polishing or tertiary final process in the treatment chain in the Caribbean. They are most effective if used in this manner. They are usually overlooked as a secondary process because of land requirements. Wetland treatment is not extensively used in the Caribbean, but it is a promising technology because of the warm, moist, Caribbean climate.

REFERENCES

Boutin, C., 1993; Choate, K.D., 1990; Green, M.B., 1995; Kreissl, J.F.; Kruzic, A., 1994; Mitchell, D.S., 1995; Netter, R., 1993; Perfler, R., 1993; Polprasert, C., 1996; Sweeney, V., 1996; U.S. EPA, 1980; U.S. EPA, 1980; U.S. EPA 1988; U.S. EPA 1992; Urbanc-Bercic.

Land Treatment

DESCRIPTION

In the slow rate process, primary or secondary effluent is applied to a vegetated surface and is treated as it flows through the vegetative root zone and the soil. Underdrains may be provided if the effluent is to be reused or disposed of elsewhere. In rapid infiltration, primary or secondary effluent is applied to moderately or highly permeable soils. Treatment is achieved as the wastewater percolates through the soil. Underdrains are not usually provided, and the treated wastewater can serve to recharge the groundwater. Overland flow is the uniform application of primary or secondary effluent at the top of grass-covered slopes. The wastewater flows over the vegetated surface and is treated before it collects in runoff ditches below. This process is most suited to impermeable soils but can work with soils of low or medium permeability as well.

APPLICATIONS

Land treatment processes can use wastewater that has received primary or secondary treatment. The higher the level of pre-treatment the wastewater has received, the less land is required. The slow rate process is most suitable for soils of low to medium permeability. It is a good way to recycle water and nutrients and grow a useful product or crops. Rapid infiltration is appropriate in soils with high permeability and deep groundwater levels. Overland flow is appropriate in impermeable soils on terrain which has a steady, uniform slope; it is very expensive if earthen construction or excavation is needed to create the right slope.

DESIGN CRITERIA

PERFORMANCE EFFICIENCY

DISADVANTAGES

Land treatment processes are limited by climate, the slope of the land, and soil conditions. Wastewater application may have to be reduced or even stopped during rainy periods. This would require adequate wastewater storage space during wet periods. Other disadvantages are that land requirements are very high and potential odour and vector problems can occur if inadequate pretreatment is employed.

RESIDUALS GENERATED

The residual associated with land treatment is vegetation growth and the solids generated from pretreatment processes.

OPERATION & MAINTENANCE

Overland flow and slow rate infiltration vegetation growth must be harvested regularly, while rapid infiltration vegetation is harvested periodically. Growth rate depends on the type of vegetation used and the volume and strength of wastewater. If there are no metals or other toxics in the wastewater, harvested vegetation can be fed to cattle and other farm animals. Pumps and distribution pipes need to be serviced and cleaned regularly.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

Braungart, M., 1997; Kruzic, A., 1994; Goldstein, N. 1981; U.S. E.P.A., 1980; U.S. E.P.A. 1992; U.S. E.P.A. 1984.

Filtration

DESCRIPTION

The other type of filters are those that do not have backwashing mechanisms and are loaded at far lower rates than backwash filters. When the top layer of these slow sand filters begin to clog, they are simply scraped off and replaced. Buried sand filters are constructed below grade; the upstream ends of the underdrains extend above grade to help ventilate or aerate the wastewater. Open (or intermittent) sand filters are constructed at grade, with an exposed surface, which allows easy access for inspection and cleaning. Recirculating gravel filters are open filters that recycle 300 to 500 percent of the influent flow. The treated effluent is continuously mixed with the pre-treated influent and applied to the filter. All of these filters nitrify well (convert ammonia into nitrates). Only recirculating filters can denitrify (convert nitrates to nitrogen gas). Nitrification increases the nitrate level in the effluent, which may be an issue if it is to be discharged near a drinking water source. The remainder of this fact sheet only describes the recirculating, open, and buried sand filters.

APPLICATIONS

Sand filters are a reliable and proven method for treating wastewaters from septic tank effluents to secondary treatment effluents. They are most suitable for rural communities, small clusters of homes, individual residences, and businesses, where land is available. They are easy to operate and maintain by local labor, which makes them suitable for rural areas where skilled labour may not be readily available.

• Wastewater requires a minimum of primary treatment (e.g., sedimentation or a septic tank) before application to sand filters. The filter medium will clog quickly if the wastewater is not pre-treated adequately.

• The medium should be 600 to 900 mm deep.

• Smaller filter media provide better contaminant removal but require more frequent cleaning.

• Hydraulic loading and medium size should meet the criteria in the following table.

PERFORMANCE EFFICIENCY

DISADVANTAGES

Passing wastewater through filters requires about 1 metre of hydraulic head. This may necessitate pumping for effluent disposal if the topography of the land is not suitable. Recirculating filters will require pumps in all circumstances. Other disadvantages are that open filters may produce undesirable odours, and that suitable filter media may not be available locally. If filter media are not available locally, other granular materials such as peat derivatives may be suitable.

RESIDUALS GENERATED

A small amount of biological matter is produced in the top region of the filter medium which needs to be raked and removed for disposal.

OPERATION & MAINTENANCE

Operation and maintenance requirements are low for non backwashing sand filtration systems. Periodic cleaning (every 6 to 12 months) of the top layer of the filtration medium is required to prevent clogging. Regular maintenance of pumps and wastewater distribution equipment also is required.

WCR INSTALLATIONS

These systems are being studied and applied in parts of Florida, U.S.A.

REFERENCES

Bennani, A.C., 1996; Boutin, C., 1993; Check, G.G., 1994; Evans, B., 1993; Rich, L.G., 1996; U.S. EPA, 1980; U.S. EPA, 1984; U.S. EPA, 1980; U.S. EPA, 1992; Yang, P.Y., 1994.

PRELIMINARY TREATMENT

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DESCRIPTION

Influent wastewater usually flows through screens that remove floatable material and rags. The separation between bars can vary from 5 mm 50 mm. Where downstream treatment equipment problems are to be avoided, the bar spacing should not exceed 12 mm. Grit removal, when provided, removes inert solids and sands that would damage pumps and other mechanical equipment in downstream processes. There are many different types of grit removal processes, but most include a small chamber through which wastewater flows, large enough to detain the flow so that heavy, inert solids settle to the bottom.

APPLICATIONS

All treatment processes, with the exception of septic tanks and household systems, require some sort of preliminary or screening process to remove large and floatable objects. For mechanically intensive wastewater treatment systems, screening and grit removal are strongly recommended. Grit removal is not necessary in most natural systems, but should be considered in highly mechanised wastewater treatment systems to prolong equipment life. The presence of a significant amount of grit in wastewater quickly wears down pumps and other mechanical equipment.

• The bar spacing for screens may be from 5 mm to 50 mm, depending on the type of treatment processes downstream. The wider the spacing, the less material retained.

• Typical screenings volumes are 0.037 to 0.22 m3 per 1,000 m3 of flow.

• The approach channel to the bar screen should be sized so that the approach velocity is at least 30 to 60 cm per second for average flow conditions.

• A conventional aerated grit chamber is sized to provide 2 to 5 minutes of wastewater detention time. Other types of grit removal tanks have different criteria. Vortex grit chambers are designed for overflow rates of approximately 66 m/hr at maximum daily flow.

• The volume of grit generated varies with the type of sewage collection system used and its degree of inflow. Grit chambers typically generate from 0.0024 to 0.18 m3 per 1,000 m3 of flow.

• Circular designs are used for vortex units; aerated grit chambers are rectangular. Headlosses across the units vary from negligible to 0.6 m.

PERFORMANCE EFFICIENCY

Screens reliably remove all items larger than the bar openings. Most grit chamber designs remove about 95 percent of inert particles larger than 0.21 mm. Some modern designs can remove inert particles even smaller than 0.21 mm.

DISADVANTAGES

Screening and grit removal increase capital and operation and maintenance costs. In most cases though, grit removal is less expensive than the additional maintenance cost for downstream systems that would be incurred if grit and screenings removal is not provided.

RESIDUALS GENERATED

Screenings and grit are collected in these processes. After being washed, drained, and compacted, the residuals are usually disposed in landfills. Typical volumes of residuals are described above in the section on design criteria.

OPERATION & MAINTENANCE

Basic operational requirements for preliminary treatment are residuals removal, washing, and compaction (dewatering). Screenings and grit can be removed mechanically or manually. Grit can be removed manually by shovelling, but this requires a redundant grit chamber so that each chamber can be isolated and drained for shovelling. Usually, grit is removed from the tank bottom with mechanical buckets, inclined screw conveyors, or grit pumps. Grit pumps must be very durable because they pump very abrasive material. For aerated grit chambers, blower operation and maintenance add further costs.

WCR INSTALLATIONS

Screens are used for all types of treatment facilities in the WCR. Grit chambers are used in some larger, conventional treatment facilities. The treatment plant in San Fernando in Trinidad has a grit chamber, as do the Dos Cerritos and Mariposa plants in Venezuela.

REFERENCES

Millette, E.M. 1992; Sweeney, V. 1996; U.S. EPA 1992; Water Environment Federation & American Society of Civil Engineers 1992.

PRIMARY TREATMENT

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DESCRIPTION

Dissolved air flotation (DAF) is another type of primary treatment process commonly used for industrial wastewater. A DAF process removes oil and grease in less space than by primary sedimentation. Wastewater and air are pressurised to 3 to 5 atmospheres and released in a tank open to the atmosphere. This releases small bubbles from the solution, which float to the top. The bubbles become enmeshed in the light solids and oils and bring them to the surface. A skimmer then collects solids on the water surface, and the clarified liquid continues to downstream processes. Other types of oil-water separating processes are also widely used in the petroleum industry.

APPLICATIONS

Sedimentation tanks are used as a primary treatment process for most large, conventional domestic sewage treatment facilities and some industrial applications. DAF is used mostly for industrial sewage that contains oil, grease, and other easily floatable solids. Oil refineries, meat packing factories, and dairy processing plants commonly use DAF for primary treatment.

• A surface overflow rate (flow/tank surface area) of 0.8 to 1.5 m/hr for the average design flow is an accepted value in the U.S.

• Sedimentation tanks should be 2 to 5 metres deep.

• Both rectangular and circular tanks are widely used.

• A hydraulic detention time of 20 to 30 minutes is adequate for solids separation.

• Other important design criteria are pressure, recycle ratio, and influent solids concentration and characteristics.

PERFORMANCE EFFICIENCY

A conventional sedimentation tank removes 25 to 40 percent of influent BOD, 40 to 70 percent of total suspended solids, and about 50 percent of the bacterial load. DAF devices can produce an effluent with as little oil as 1 to 20 mg/L.

DISADVANTAGES

DAF treatment processes have more complex operation and energy requirements than plain sedimentation tanks. DAF processes are usually chosen when sedimentation tanks do not provide adequate removal of light solids and oils. For primary sedimentation tanks, the sludge (which is high in organics) should be withdrawn rapidly before denitrification processes generate gaseous nitrogen, which can resuspend some of the solids.

RESIDUALS GENERATED

Solids, scums, and oils are the main residuals collected in primary treatment. The volume generated depends on the volume of wastewater flow, the composition of the wastewater, and the effectiveness of the treatment. For a medium-strength wastewater, the amount of sludge generated in a primary sedimentation tank is about 0.10 to 0.17 kg/m3 of wastewater.

OPERATION & MAINTENANCE

Although primary treatment mechanical processes are relatively simple, routine maintenance is necessary. For conventional sedimentation tanks, the majority of the maintenance is upkeep of pumps, sludge scrapers, scum collectors, and motors. DAF processes require a more intensive maintenance plan for the pressurised pumps, pressure relief valves, and collector systems.

WCR INSTALLATIONS

Sedimentation tanks are used at most conventional, mechanised treatment systems. DAF systems are used mostly in oil refinery and petrochemical waste facilities.

REFERENCES

Bryant, J.S. 1991; Eckenfelder, W.W. 1989; Engelder, C.L. 1993; Millette, E.M. 1992; Rhee, C.H. 1988; Sweeney, V. 1996; Water and Environment Federation & American Society of Civil Engineers 1992.

SECONDARY TREATMENT

DESCRIPTION

• In the first step, the treatment bacteria are brought into contact with the soluble and suspended organic material in the wastewater. This is accomplished by directing the wastewater to a well-mixed tank containing the treatment organisms (a "suspended growth" system) or passing it over a fixed surface on which the bacteria grow (a "fixed film" system).

• In suspended-growth systems, aerobic bacteria need sufficient oxygen to metabolise the organic material in the wastewater. This is provided by a mechanical aerator, a diffuser, or some other process. Aerators introduce air, or oxygen, into the wastewater.

• The bacteria that metabolise the organic material in the wastewater must subsequently be separated from the wastewater flow. Except for sequencing batch reactors (SBRs), all secondary processes discussed here have a separate secondary sedimentation tank to settle this flocculated cell mass in the same way that primary sedimentation tanks settle suspended organic material. The effluent continues to the discharge or to downstream processes.

• In suspended-growth activated sludge systems, sludge is returned from the sedimentation tank to the aeration tank, which maintains a viable concentration of bacteria to metabolise the incoming organic material. This is called return activated sludge, or RAS. Sludge that is removed and not returned is called wasted activated sludge, or WAS. Sludge return is not necessary for fixed film processes or the SBR process.

• Activated sludge

• Oxidation ditch

• Trickling filter

• Sequencing batch reactor (SBR)

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In the SBR process, all steps of the treatment process take place in a single complete-mix tank, to which influent is directed intermittently. The treatment process consists of discrete, timed processes: fill, mix/aerate, settle, withdraw effluent, and withdraw sludge. Some SBR manufacturers combine these processes and develop proprietary timing cycles, but all SBRs use a combination of the above five elements. Historically, SBRs were only used for small treatment facilities. In recent years, there has been a resurgence of interest in the SBR process because it entirely eliminates the need for secondary sedimentation and RAS pumps.

APPLICATIONS

These secondary treatment processes are usually most appropriate for large, high population density communities because of their high cost and the high level of skill required for operation and maintenance. Although these processes produce good-quality effluent for large flows if operated and maintained properly, they produce very poor effluent quality if operated improperly. Oxidation ditches have the highest land requirements of the processes described in this fact sheet, and SBRs have the lowest. Both are appropriate for medium-sized communities due to their high reliability. Trickling filters have a high capital cost, but low operational costs compared to an activated sludge plant because no aeration is needed.

• The mixed liquor suspended solids concentration (MLSS) ranges from 1,500 to 3,000 mg/L.

• The hydraulic detention time is from 6 to 24 hours.

• The solids residence time is from 3 to 20 days.

• The hydraulic detention time is 24 hours or more.

• The solids residence time ranges from 10 to 30 days.

• Flow channels are from 2 to 4 m deep.

• Channel velocities should be from 24 to 36 cm/second.

• The hydraulic loading rate has a very wide range. The most commonly used trickling filters use a loading rate per filter surface area of 1 to 9.2 m/day.

• The organic loading rate is 175 to 1,000 kg BOD/day/1,000 m3.

• Unless the filter medium used is lightweight plastic, the filter depth is 1 to 3 m. For plastic media, depth can be as high as 12 metres.

• The hydraulic detention time ranges from 24 to 40 hours for most applications.

• The solids retention time ranges from 5 to 40 days.

PERFORMANCE EFFICIENCY

DISADVANTAGES

Secondary processes generally require a high degree of skilled labour for operation and maintenance. They are mechanically intensive, and produce poor effluent quality if key equipment is not working properly. These processes also generate a higher volume of sludge than natural processes used for wastewater treatment. Sludge treatment and disposal is a significant cost associated with secondary treatment processes. Flies can be a serious nuisance with trickling filters, as they live and breed within the filter medium.

RESIDUALS GENERATED

Secondary treatment can generate 0.10 to 0.15 kg of sludge per day per cubic metre of wastewater. Trickling filters generate a comparable quantity of sludge. The sludge generated is generally high in volatile solids and it can become septic quickly, producing offensive odours if not treated or disposed immediately.

OPERATION & MAINTENANCE

Operation and maintenance requirements are extremely high for secondary treatment processes. Except for the SBR process, all require flow and/or sludge recycling. While the capital or energy cost may not be excessive, pump maintenance is crucial for proper operation. Except for the trickling filter, all processes require aeration. Aeration is usually provided with a blower. The energy needed to run a blower or aerator makes it the single most costly operational element in a wastewater treatment process. A standby generator must be provided for pump and blower operation in case of electricity supply failure; if outages are longer than a few hours, then standby power for aerator equipment is prudent. Another operational consideration is the amount of sludge to be generated. As the sludge volume increases, it is more cost effective to perform sludge treatment before final disposal. This introduces further equipment and operation and maintenance costs.

WCR INSTALLATIONS

Extended aeration activated sludge is used at the Dos Cerritos, Venezuela plant. Modified sequencing batch reactors (MSBRs) are used in Juangriego, Venezuela. Trickling filters are in use in Arima, Trinidad and San Fernando, Trinidad. Small package activated sludge plants are used throughout the WCR.

REFERENCES

Millette, E.M. 1992; Sweeney, V. 1996; U.S. Department of Commerce, 1991; U.S. EPA 1980; Water Environment Federation & American Society of civil Engineer 1992.

NUTRIENT REMOVAL

DESCRIPTION

ˇ The A2/O process for biological phosphorus and nitrogen removal

ˇ The MLE process for biological nitrogen removal

ˇ Chemical precipitation for phosphorus removal

The A2/O Process. Many treatment systems remove BOD, suspended solids, and nutrients through microbiological activity. A typical biological nutrient removal (BNR) process is the A2/O (anaerobic, anoxic, and oxic) process. An oxic, or aerated, zone has "free oxygen" (O2) available for microbiological respiration; an anoxic zone has nitrate; and an anaerobic zone has neither.

The A2/O process generally uses the same mechanical equipment as the conventional activated sludge process, with additional reactor zones provided before the secondary sedimentation tank instead of just one. These zones may be separate tanks or separated areas of a single tank. Raw sewage or effluent from primary treatment flows first to the anaerobic zone, then to the anoxic zone, and finally to the oxic zone before discharge to a secondary sedimentation tank, where the cells settle out.

Most other biological nutrient removal processes are variations of these processes. Other biological processes that can remove nitrogen are upflow granular filters and some sand filters. Many of the biological nutrient removal processes are patented, which increases the cost of construction. Some nutrient removal is effected in processes such as wetlands and oxidation ponds. For discussion of nutrient removal features of these low-technology processes see the references cited for the fact sheets for these processes.

APPLICATIONS

Most receiving water standards in the WCR do not specify allowable nitrogen or phosphorus concentrations. Consequently, nutrient removal is rarely practised in the region. However, most of the coastal waters in the WCR are nutrient poor. This means that any amount of nutrients discharged into enclosed water bodies such as estuaries or bays may cause eutrophication problems. Many nutrient removal processes are expensive and complex and suitable only for dense population centres. However, they should be considered whenever wastewater effluent is discharged to receiving water other than open ocean. High ammonia-nitrogen concentrations are toxic to fish and animals, and high nitrate concentrations in drinking water are toxic to humans and can quickly kill infants.

DESIGN CRITERIA

Key design criteria for the MLE and A2/O processes are summarised in the following table. Additional design criteria include such factors as dissolved oxygen concentrations and temperature. Theoretical precipitant doses for phosphorus removal are indicated in the next table. In actual practice dose rates required for complete removal of soluble phosphorus are 50 to 100% more than the theoretical requirement.

PERFORMANCE EFFICIENCY

Typical effluent concentrations from the A2/O process range from 0.2 to 5 mg/L of total phosphorus and 5 to 10 mg/L of total nitrogen. Average concentrations are about 1 mg/L for total phosphorus and 8 mg/L for total nitrogen. Variations on this process can achieve higher removals. Comparable effluent nitrogen concentrations can be achieved with the MLE process. Upflow and fluidised bed filters (also known as denitrification filters) can remove 80 to 95 percent of influent nutrients. Recirculating sand filters can remove 40-75% of the influent nitrogen. Conventional activated sludge treatment processes produce effluent with 10 to 15 mg/L of total nitrogen and 2 to 6 mg/L of total phosphorus depending on the influent concentrations. Chemical precipitation can remove soluble phosphorus to low concentrations (less than 0.1 mg/L.) For complete removal of phosphorus, inorganic phosphorus included in effluent suspended solids must be removed, typically by filtration.

DISADVANTAGES

Nutrient removal processes are more complex and expensive than secondary treatment. The extra tanks and recycle lines add a high capital cost and increase the operation and maintenance cost. Also, it is crucial that the solids produced in the process be treated or disposed of correctly. Through solubilisation, aerobic and anaerobic solids digestion processes can produce liquid side streams very high in nitrogen and phosphorus. If these side streams are returned to the main plant flow, the effluent quality will degrade. Another disadvantage is the variability of phosphorus removal in biological systems. Chemical removal of phosphorus requires a continuing expense for chemical precipitant and additional costs for disposal of the resulting sludge.

RESIDUALS GENERATED

The volume of sludge generated in biological nitrogen and phosphorus removal processes is the same as or less than that for conventional activated sludge plants. Chemical precipitation can increase sludge loads substantially.

OPERATION & MAINTENANCE

Operations and maintenance costs increase when nutrient removal is included in treatment. Capital costs include the construction of additional tanks, pipes, and recirculation pumps. Ongoing costs include maintenance of the aeration systems, pipes, and pumps. Processes are complex and require skilled labour for efficient operation. Chemical costs for chemical precipitation of phosphorus can substantially increase plant operational expenses.

WCR INSTALLATIONS

The Mariposa treatment plant in Venezuela has been designed for partial BNR.

REFERENCES

DISINFECTION

DESCRIPTION

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Pond disinfection is the natural process of pathogen removal in successive stabilisation ponds. Visible light and ultraviolet radiation from the sun, sedimentation, and natural die-off are the mechanisms for pathogen removal in ponds.

APPLICATIONS

Chlorination is appropriate for most wastewater, and is the most popular disinfection process in the world. Ultraviolet radiation performs well, but performs less well with effluents high in turbidity or suspended solids. Sand filtration prior to UV radiation is common. Ozonation disinfects more powerfully than chlorine, and with no harmful by-products. It is usually used to disinfect highly treated secondary or filtered effluent. Ozone must be generated on-site, which can be costly and requires a reliable power supply. Pond disinfection is a simple-technology, maintenance-free process that requires a large land area.

• Chlorination, UV radiation, and ozonation all require a specified contact time between the wastewater and the disinfectant. To ensure an adequate contact time between the wastewater and the disinfectant, the disinfection chambers should be designed to minimise hydraulic short-circuiting (fast, direct flow between the chamber’s inlet and outlet).

• For a contact time of 1 hour, the typical chlorine dosage is 10 to 25 mg/L for septic tank effluent, 2 to 5 mg/L for secondary treatment effluent, and 2 to 10 mg/L for rapid sand filter effluent.

• An alternate dosage guideline is to produce a chlorine residual of 0.5 mg/L in the wastewater after 15 minutes of contact time.

• Violent initial mixing should be provided.

• Ultraviolet radiation supplies a great amount of energy, thus contact times between the wastewater and UV lamp are typically very short. A contact time of 1 minute or less is common. This disinfection process is preferred over chlorine and ozone where dechlorination is required before discharge.

• The hydraulic detention times in an ozone contactor chamber range from 30 seconds to 15 minutes depending on the type of contactor used.

• The EPA recommended ozone dosage is 5 to 15 mg/L for disinfection of wastewater effluent.

• This is the most expensive disinfection choice.

• Pond disinfection should be used as a polishing process, after most of the BOD has been removed.

• Disinfection ponds should be shallow to maintain aerobic conditions. Most disinfection ponds are 300 to 1000 mm deep.

• Several small ponds in series provide better coliform and pathogen removal than a large pond with the same total area.

• Algae will be generated where detention times exceed 2-3 days.

• The efficiency of this process relies heavily on the presence of sunny conditions.

PERFORMANCE EFFICIENCY

The contact time and recommended dosages provided produce a final effluent with a maximum of 200 faecal coliforms/100 mL.

DISADVANTAGES

Chlorination produces many undesirable organic compounds that are toxic to humans and aquatic life. Sometimes dechlorination is necessary to lower the residual chlorine concentration in the effluent. Chlorine gas is a hazardous element, and safety features must be employed where it will be stored. Ozonation is a very expensive disinfection process that currently is not in wide use for wastewater disinfection, so limited design data and experience are available on the process. Ozonation, and to a lesser extent, ultraviolet radiation should only be used for high-quality effluent. Otherwise, slime and scaling accumulate on the lamps, greatly decreasing the radiation transmittance and thus the disinfection power, or excessive ozone demands result. Slime accumulation and mineral scaling may necessitate frequent cleanings of UV lamps. Pond disinfection requires a great deal of space.

RESIDUALS GENERATED

Chlorination is the only disinfection process discussed here that can produce harmful organic by-products. For this reason, it is desirable to remove as much of the organic material as possible in previous treatment processes before adding chlorine.

OPERATION & MAINTENANCE

Disinfection processes require effluent monitoring to verify pathogen removal. Chlorination processes require a feeder mechanism to introduce the liquid, gas, or tablet form of the chlorine. Typical maintenance includes replacing chemicals, adjusting feed rates, and maintaining the mechanical components. Most chlorine systems are designed for minimum maintenance. Ultraviolet radiation requires little maintenance other than regular cleaning and replacement of the lamps. Ozone generating and feeder equipment uses a large amount of electricity and is complicated. The EPA estimates that 8 to 10 kW-hours are used for each pound of ozone generated.

WCR INSTALLATIONS

Most large treatment facilities and some smaller aerated package plants in the WCR use chlorine to disinfect the effluent. Ultraviolet radiation has found some uses, but is not widely practised. Pond disinfection has been successfully used in Venezuela.

REFERENCES

Effluent Disposal

DESCRIPTION

There is some overlap in what is considered land surface and subsurface disposal; for this fact sheet, land surface disposal refers to an evaporation pond. Effluent flows into the pond, and most of it evaporates. Subsurface disposal is the application of effluent to the land surface, a subsurface absorption bed, or any other mechanism that eventually leads the effluent to the groundwater. Most subsurface systems are soil absorption systems. Surface water disposal in the WCR is generally effluent discharge to estuaries, bays, and the open ocean through a simple outfall pipe. Outfall pipes can be as short as several metres and as long as several kilometres.

APPLICATIONS

Land surface disposal is most appropriate in dry or arid climates. An evaporation pond may work in the most arid parts of the WCR, but most areas of the region receive too much rainfall for evaporation ponds to be effective. Subsurface disposal systems are commonly used for on-site treatment systems, especially septic tanks. They also can be used with high-density treatment systems, provided the soil is permeable enough and there is no significant risk of groundwater contamination. Because soil treatment systems are very effective in removing BOD, suspended solids, and pathogens, primary treatment is the only treatment required prior to subsurface disposal. A secondary function of subsurface disposal (provided there is adequate distance between the discharge point and the water table) is groundwater recharge. Surface water disposal is the most common method of wastewater disposal in urban, high-density areas. This is particularly true for most of the large coastal urban centres in the WCR.

• Evaporation plus percolation must be greater than or equal to the influent wastewater flow plus precipitation.

• The volume of wastewater effluent that can be discharged into a subsurface area depends on the soil permeability and the depth of the water table.

• Some design criteria are given in Fact Sheet #1.

• Marine outfalls dilute wastewater effluent with seawater as it flows out of the diffusers. The dilution level depends on such factors as the receiving water current velocity, the velocity and volume of discharge, the depth of the receiving water, and density differences between the effluent and receiving water. The U.S. EPA has produced computer programs to calculate this dilution; these programs are available to the general public.

• The level of treatment needed prior to surface water disposal depends on the receiving water requirements:

– In open ocean situations with a properly designed outfall, wastewater may be disposed of with only preliminary or primary treatment because dilution will lower the pathogen concentration below World Health Organisation (WHO) standards.

– In sensitive areas such as estuaries or coral reefs, the diluting capacity of the ocean must lower pollutant concentrations enough to prevent harm to the sensitive area; this may require advanced treatment or nutrient removal.

– The outfall must be very long (1 to 5 km), and preferably in deep water so that strong currents dilute and move the wastes farther offshore. Ocean currents must be analysed in great detail to ensure that the wastes are not drawn back to land or other sensitive areas. If a short outfall is used, treatment with disinfection prior to disposal is adequate to maintain pathogen concentrations below WHO standards.

PERFORMANCE EFFICIENCY

N/A

DISADVANTAGES

N/A

RESIDUALS GENERATED

N/A

OPERATION & MAINTENANCE

Operation and maintenance requirements for effluent disposal systems depend on the quality of the effluent and the type of discharge. The only maintenance required for all effluent disposal systems is ensuring that the discharge orifice is not clogged with debris and performing any mechanical maintenance of pumps. The better the effluent quality, the fewer problems will develop with clogging in the distribution system. If the discharge can be achieved with gravity flow, very little operation or maintenance is required.

WCR INSTALLATIONS

In the Caribbean, the majority of wastewater effluent is disposed of through rivr or ocean outfalls. Unfortunately, in most cases there is little or no wastewater treatment before disposal. Subsurface disposal is practised throughout the WCR wherever septic tanks are used. In Barbados, effluent from septic tanks is discharged into 6 metre deep wells excavated into the thick coral limestone rock formation overlaying the ground water aquifers. The coral rock layer varies from 200-300 feet thick and acts as a natural filter for the purification of effluents. This is not allowed in Zone (I) water protection areas, however, where potable water is abstracted from the aquifer. Zone (I) areas are sized to allow an average travel time of 300 days through the rock to the aquifer source. Jamaica also practices subsurface effluent disposal. In Venezuela, most wastewater effluent is disposed to rivers with a short reach to the Caribbean Sea.

REFERENCES

Archer, A.B. 1990; Bartone, C.R. et al. 1984; Compton, A.W. 1973; Faruqui, N. 1993; Ruiz, C.S. et al. 1995; UNEP 1994; U.S. EPA February 1980; U.S. EPA October 1980; U.S. EPA 1992.

Oil-Water Separation

DESCRIPTION

Sometimes, the skimmed product is placed in a secondary reservoir, where further separation occurs, with the oil passing over a weir and the skimmed water being removed from below. This allows almost complete separation.

APPLICATIONS

Gravity separators with skimmers provide inexpensive and effective oil-water separation for any type of oily wastewater, such as wastewater produced by oil refineries, petrochemical plants, food processing plants, slaughterhouses, and many other industries.

• The tank should provide enough detention time to allow the oil and water to separate.

• Turbulence should be minimised because it encourages the oil to emulsify (break into small droplets), which decreases skimming efficiency

PERFORMANCE EFFICIENCY

DISADVANTAGES

Very low oil concentrations are difficult to achieve using only gravity separators with skimmers. Other processes such as sand filters and reverse osmosis membranes are needed to achieve very high oil removals. Usually, a gravity separator with skimmers will not produce effluent clean enough to be re-used as cooling water. However, in most cases, it will bring oil concentrations low enough so that the effluent can safely be discharged to a public sewer.

RESIDUALS GENERATED

The volume of collected oil will depend on the process flow, and the percentage of oil removed. Often the oil can be re-used or recycled.

OPERATION & MAINTENANCE

There are no maintenance requirements other than regular lubrication and cleaning of the mechanical parts.

WCR INSTALLATIONS

KCM has no knowledge of installations in the WCR.

REFERENCES

Benedek, A. 1992; Beychok, M.R. 1967; Borup, M.B. et al. 1987; Bryant, J.S. et al. 1991; Chigusa, K. et al. 1996; Copeland, E.C. et al. 1991; Engelder, C.L. et al. 1993; Galil, N. 1990; Hobson, T. 1996; Jones, H.R. 1973; Mitchell, D.B. et al. 1994; Park, T.J. et al. 1996; Rhee, C.H. 1988; Viraraghavan, T. et al. 1994; Wong, J.M. 1995.

Coagulation/Precipitation

DESCRIPTION

Precipitation is the addition of a lime or caustic to a waste stream so that metals removal can be enhanced. The idea is to add enough lime or caustic so that the pH of the wastewater solution is at the metal’s minimum solubility, thus encouraging the metal to precipitate (form as a solid) as a hydroxide or other complex. As precipitates, metals are removed by settling or by filtration.

APPLICATIONS

Coagulation has many applications for wastewater treatment, particularly for industrial wastewater. Coagulation removes very fine suspended matter, including colloids, metallic ions, iron, phosphates, suspended organic material, and fine oil droplets. It is also used for pH control. Paperboard industries, oil refineries, and rubber, paint, and textile and some food processing factories use coagulation as a wastewater treatment process. Precipitation is used to remove metals from waste streams.

DESIGN CRITERIA

• Sulphide precipitation to remove arsenic

• Sulphate precipitation to remove barium

• Alum precipitation to remove mercury.

PERFORMANCE EFFICIENCY

DISADVANTAGES

Although most coagulants are inexpensive, the cost can be high for an ongoing supply of them, particularly in some parts of the WCR. Another disadvantage is the volume of sludge generated, which includes the solids removed from the waste stream as well as the coagulants that are added. If any metals or toxics are coagulated or precipitated, then the sludge must be disposed of carefully and cannot be reused.

RESIDUALS GENERATED

A high volume of sludge is generated. The amount depends on the amount of coagulant added, the amount of precipitate formed, and the amount of solids removed.

OPERATION & MAINTENANCE

Operation and maintenance for coagulation and precipitation processes are several times that required for ordinary sedimentation tanks, plus the additional cost of the additives.

WCR INSTALLATIONS

KCM has no knowledge of installations in the WCR.

REFERENCES

Eckenfelder, W.W. 1989; Water Environment Federation & American Society of Civil Engineers 1992.

Air Stripping

DESCRIPTION

Air stripping processes remove volatile organic or chemical materials. The volatile constituents come into contact with air that is bubbled through the wastewater flow. They then diffuse into a gaseous state and are removed from the wastewater as the air bubbles out. This happens naturally in aerated biological processes and is engineered to occur at a faster rate in packed tower air strippers. Air that has passed through the process flow (or exhaust air) is passed through a gas scrubber if the constituent concentration is too high to allow direct emission to the atmosphere. Otherwise, it is vented to the atmosphere.

APPLICATIONS

Air stripping’s primary use is to remove volatile organic compounds (VOCs) such as those generated by petrochemical industries. It can also be used for ammonia removal.

DESIGN CRITERIA

• The removal rate increases as the air flow increases.

• The removal rate increases as the air and water temperature increases.

• The removal rate increases as the air-water interface area increases.

• Compounds with a higher "Henry’s constant" (a constant describing a gas’s solubility in water) are removed more quickly than those with a low Henry’s constant.

PERFORMANCE EFFICIENCY

The performance efficiency depends on the constituent solubility, the packing tower dimensions, and the temperature.

DISADVANTAGES

If the constituent concentrations in the exhaust gas are high, or if the exhaust gas is odorous or hazardous, it should be sent to a gas scrubber. This increases the cost of the operation considerably. Another disadvantage is that additional pumps or blowers may be required to operate an air stripper.

RESIDUALS GENERATED

Air stripping generates a gas containing VOCs. The volume of the exhaust gas is the amount of gas that travels through the stripping columns. The concentration depends on operating conditions.

OPERATION & MAINTENANCE

Operation and maintenance requirements for air stripping are standard maintenance of the pumps that send air and water flow through the packing columns and any additional maintenance associated with a gas scrubber, if one is used. The only maintenance required for the actual column is an occasional cleaning of the filter medium.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

Eckenfelder, W.W. 1989; U.S. EPA 1980; Water Environment Federation & American Society of Civil Engineers 1992.

Biological Treatment of industrial waste

DESCRIPTION

• The anaerobic filter can be operated in an upflow or downflow mode, where upflow or downflow describes the direction of process flow through the filter. The anaerobic organisms grow on the filter medium and degrade the organic material in the wastewater as it flows through. The physical filtration helps eliminate or minimise the need for solids removal downstream.

• The fluidised bed reactor is a filter operated in an upflow mode. The filter medium is sand, and the flow velocity through the filter must be high enough to expand the space between the sand particles, filling the entire reactor.

• Upflow anaerobic sludge blanket (UASB) reactors have gained much popularity in the last decade, particularly in Latin America. Wastewater flows into the bottom of the reactor then upward through a blanket of biologically formed granules, which provide treatment as the wastewater flows through. The UASB process requires a relatively low hydraulic detention time compared to the other anaerobic processes.

APPLICATIONS

• Industrial wastewaters can have COD values as high as 100,000 mg/L. Aerobic treatment processes would require a very large aeration capacity to treat this level. (Anaerobic processes are not aerated.)

• Anaerobic processes produce one-fourth to one-third as much sludge as aerobic processes.

• Anaerobic processes generate a significant amount of methane gas. In medium to large reactors, it is economically feasible to capture and reuse the methane to generate energy.

Aerobic Processes

Design criteria for aerobic processes can be found in Fact Sheet #5—Lagoons and Stabilisation Ponds, and Fact Sheet #11—Secondary Treatment.

Anaerobic Processes

PERFORMANCE EFFICIENCY

The performance efficiency of anaerobic processes ranges from 40 to 90 percent. Typical efficiencies are in the 60 to 80 percent range.

DISADVANTAGES

Anaerobic processes do not achieve high quality effluent unless an aerobic treatment process follows as a polishing step. Anaerobic systems also require large land areas and have long start-up times; it is 2 to 3 months before an anaerobic process operates efficiently. This is a problem for seasonal industries, such as some food processing plants and dairy farms.

RESIDUALS GENERATED

Both aerobic and anaerobic systems produce sludge. The volume generated depends on the wastewater composition and the degree of treatment. A good rule of thumb for sludge production is that aerobic processes produce about 0.6 to 1.2 kg of sludge per kg of BOD removed; anaerobic processes produce about one-fourth to one-third as much. Anaerobic processes also produce about 5.6 cubic feet of methane per pound of COD removed.

OPERATION & MAINTENANCE

The operation and maintenance requirements for anaerobic processes are very similar to those for secondary treatment processes. Routine maintenance for piping and pumps is necessary. A key difference is that anaerobic processes are not aerated, which is the primary expense for aerated treatment processes. The level of operator skill necessary to operate most anaerobic processes is not as high as for a typical activated sludge plant. However, it is still a skilled position.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

Alaerts, S. et al. 1993; Boopathy, R. et al. 1991; Borzacconi, L. et al. 1995; Capobianco, D.J. et al. 1990; Carter, J.L. et al. 1992; Chigusa, K. et al. 1996; Copeland, E.C. et al. 1991; Eckenfelder, W.W. 1989; Filho, B.C. et al. 1996; Galil, N. et al. 1990; Gavala, H.N. et al. 1996; Martinez, J. et al. 1995; Park, T.J. et al. 1996; Polprasert, C. et al. 1996; Sendic, M. 1995; Tyagi, R.D. et al. 1993; Viraraghavan, T. et al. 1994; Yue-Gen Y. et al. 1996; Zhang, R. et al. 1996.

Suspended Solids Removal

DESCRIPTION

The primary suspended solids removal processes are coagulation, sedimentation, and physical filtration. These processes are applicable for suspended solids removal from any wastewater. Information on the suspended solids removal processes are included in Fact Sheet #8—Sand Filtration, Fact Sheet #10—Primary Treatment, and Fact Sheet #20—Coagulation/Precipitation.

Activated Carbon Adsorption

DESCRIPTION

The other method is to add powdered activated carbon (PACT) to an activated sludge treatment process. The PACT adsorbs pollutants, then settles out from the flow in a secondary clarifier.

APPLICATIONS

Activated carbon processes are an excellent way to remove non-biodegradable organic materials, colour, taste, odour, and refractory organic material from waste streams. Activated carbon processes are sometimes, though infrequently, used in domestic wastewater treatment. Activated carbon is commonly used to treat wastes from food processing industries, textile factories, petrochemical industries, oil refineries, and metal processing or plating industries. For GAC processes, most of the suspended solids and biodegradable organic material should have been previously removed so that the carbon’s adsorption capacity is not wasted on constituents that can be removed by other processes.

DESIGN CRITERIA

• There is a wide range of activated carbon quality. Each type of activated carbon has a different adsorption capacity.

• The chemicals to be adsorbed, or the adsorbate, each have different affinities for the activated carbon. This needs to be determined through pilot testing.

PERFORMANCE EFFICIENCY

Carbon adsorption processes can achieve removals up to 99 percent; typical removal efficiencies are from 90 to 95 percent.

DISADVANTAGES

When the activated carbon reaches its adsorption capacity, it must be regenerated or replaced. This is the most expensive aspect of activated carbon adsorption processes. GAC columns are economical if they are used continuously. However, if they are only used a few months out of the year, it makes sense to use PACT processes because there is no capital for setting up a PACT process if an activated sludge process is in place. PACT processes are not as economical if they are used continuously because of the excess sludge build-up. Also, because the spent carbon is mixed into the sludge, regenerating the carbon is a more difficult.

RESIDUALS GENERATED

GAC columns generate activated carbon with an exhausted adsorption capacity. PACT processes generate exhausted activated carbon also. However, in PACT processes, the exhausted carbon is mixed with the biological solids from the activated sludge process.

OPERATION & MAINTENANCE

In addition to routine mechanical maintenance of pumps, piping, and activated sludge processes (for PACT), adsorption systems require fresh carbon regularly. If little carbon is exhausted, it may be economical to replace the exhausted carbon with fresh carbon; if a significant amount is exhausted, regenerating it on-site is more economical. Regeneration for exhausted activated carbon from columns is usually accomplished in hearth furnaces at temperatures of 650 to 1,000ēC. Regenerating exhausted carbon from PACT processes is a more involved process known as wet-air oxidation. It requires temperatures near 450ēC at a pressure of 40 atmospheres.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

American Water Works Association 1990; Eckenfelder, W.W. 1989; Weber, W.J. Jr., 1972.

Demineralisation

DESCRIPTION

Membrane separation processes act like a filter. Semi-permeable membranes allow water or solvents to pass through, while keeping ions, metals, or other molecules too large to pass through the membrane pores on the upstream side. A pressure differential is generated between the upstream and downstream end of the membrane, which forces the waste stream through the membrane. The concentrated solution collecting on the upstream side of the membrane is disposed of and can be as high as 100,000 mg/L. The most common membrane material is cellulose acetate. A common membrane process is called reverse osmosis (RO).

APPLICATIONS

Membrane separation processes can be used as a final step in treating waste streams with undesirable ions, colloids, and oily emulsions. To minimise clogging the membrane, or fouling, pre-treatment processes should remove suspended matter, bacteria, and any precipitable ions. This will also prolong the membrane life.

• The minimum bed depth should be 600 to 750 mm.

• The treatment flow rate can be 16 to 40 m3/hour per cubic metre of resin.

• The regenerant flow rate is typically 8 to 16 m3/hour per cubic metre of resin.

• Rinse water volumes are 4 to 14 m3 per cubic metre of resin.

Membrane separation

PERFORMANCE EFFICIENCY

DISADVANTAGES

Membrane separation processes provide very good removal, but operation costs are very high. Pressure differences across membranes are nearly 40 times atmospheric pressure. Also, membranes have a history of problems with fouling. Membranes should be used only for waste streams of already very high quality.

RESIDUALS GENERATED

Membrane separation processes generate very concentrated brine streams with concentrations up to 100,000 mg/L of dissolved solids.

OPERATION & MAINTENANCE

Ion exchange processes require that the operators have a good understanding of the process. Membrane separation processes require frequent cleaning and backwashing. Also, operational costs are very high for membrane processes. Maintaining a pressure difference across the membrane of 40 atmospheres is expensive.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

American Water Works Association 1990; Eckenfelder, W.W. 1989; Weber, W.J. Jr., 1972.

CHEMICAL OXIDATION

DESCRIPTION

Chemical oxidation is a process to transform reduced inorganic and organic contaminants that are resistant to conventional biological treatment into non-hazardous or less toxic substances that are more stable, less mobile, or inert. Chemical oxidation can convert inorganic compounds to a stable oxidation state that permits precipitation or discharge to a municipal sewer system or receiving water with substantially reduced impact. Chemical oxidation of organic compounds converts organic compounds into carbon dioxide, water, and oxides of nitrogen, or to simpler organic products that are amenable to conventional biological treatment

APPLICATIONS

Chemical oxidation has been used to oxidise organic constituents including: halogenated volatiles (TCE, DCE, PCE, TCA, MeCL), halogenated semi-volatiles, non-halogenated volatiles (alcohols, ketones, aldehydes, acetates, hydrazine, nitrated esters), non-halogenated semi-volatiles (phenol, quaternary amines), PCBs, pesticides, dioxins/furans, and organic cyanides. Chemical oxidation also is effective for inorganics (volatile metals, non-volatile metals, inorganic cyanides and sulphides). Chemical oxidation has been used to destroy metal complexes to allow chemical precipitation of toxic metals. Alkaline chlorination is frequently the most appropriate technology for cyanide destruction. Chemical oxidation technology has been used to treat industrial wastewater generated by the petrochemical industry, chemical formulators, paint and ink formulation industry, textile dying and finishing, metal plating and finishing, and the agricultural chemicals industry.

DESIGN CRITERIA

The half life of ozone is 20-30 minutes at 20ēC, therefore it must be produced on-site.

PERFORMANCE EFFICIENCY

Performance and efficiency depend on the contaminant involved, the specific oxidation system used and the presence of interfering or competing substances.

DISADVANTAGES

On-line process monitoring systems are often necessary to monitor pH, flow rate, temperature, contaminant of concern, and residual oxidant concentration.

RESIDUALS GENERATED

Metal oxides may be formed as a by-product of the oxidation reaction. Sedimentation or filtration may be required prior to reuse or disposal of the water. Chemical oxidation employing ferric or ferrous catalysts can generate significant quantities of sludge depending on the quantity of catalyst used. Other residuals formed can include partially oxidised products if the oxidation is incomplete, which may require supplemental treatment (biological, activated carbon adsorption, etc.).

OPERATION & MAINTENANCE

Incomplete oxidation may be caused by an insufficient quantity of the oxidation chemicals, inhibition of oxidation reactions by a pH that is too low or too high, the strength of the oxidising chemicals, the presence of interfering compounds that consume chemicals, or inadequate mixing or contact time between the oxidant and the target contaminant.

WCR INSTALLATIONS

KCM has no knowledge of specific installations in the WCR.

REFERENCES

Patterson 1985; EPA 1991b.

Sludge Thickening

DESCRIPTION

• Gravity thickening

• Lagoon thickening

• Gravity belt thickening

• Centrifuge thickening.

The dissolved air flotation (DAF) process has been used in the past for sludge thickening, but today it has been almost entirely replaced by GBT and centrifuge thickening for applications where a compact thickening process is required.

APPLICATIONS

Lagoon thickening is appropriate for many applications in low to medium population density communities in the Caribbean region because of its simplicity and economy. Gravity thickening uses less land area than lagoon thickening, but requires more operator attention and equipment maintenance. GBT and centrifuge thickening are appropriate for high population density communities and industrial use.

DESIGN CRITERIA

PERFORMANCE EFFICIENCY

DISADVANTAGES

Lagoon thickening requires a larger land area than gravity thickening or mechanical thickening processes such as GBT and centrifuge thickening. Gravity thickening has higher maintenance and operating requirements than lagoon thickening. GBT thickening requires higher operator attention and regular maintenance by qualified technicians. Centrifuge thickening has high power requirements. Maintenance work for restoration of scroll and bowl coatings or tiles can require highly skilled maintenance workers and expensive shipment from outside the country for replacement materials.

RESIDUALS GENERATED

All thickening processes produce effluent flows that must be returned to the plant or otherwise disposed of.

OPERATION & MAINTENANCE

Regular operation and maintenance of lagoon thickeners includes management of sludge pumping and periodic dike maintenance. Gravity thickening, GBT, and centrifuge operation require close operator attention for control of loading rate. These equipment-intensive thickening processes will require regular equipment maintenance and may require periodic import of maintenance parts from outside the Caribbean region.

WCR INSTALLATIONS

All the installations visited by the KCM team in the Caribbean region used either no thickening or lagoon thickening of sludges.

REFERENCES

U.S. EPA, 1979.

Sludge Stabilisation

DESCRIPTION

• Aerobic Digestion

• Air Drying

• Anaerobic Digestion

• Composting

• Lime Stabilisation

Lime stabilisation is the addition of alkaline compounds to raise the pH of the sludge mixture. Holding the sludge mixture at a high pH for an extended period of time will remove pathogens.

APPLICATIONS

For high density areas, digestion and lime stabilisation are appropriate because of their relatively low land requirements compared to the two other processes. They also require a high degree of operator attention and equipment. Composting is not very intensive, but piping and compost handling equipment are needed. Air drying is the simplest stabilisation process. It only requires land space, a sunny climate without extended periods of rainy weather, and equipment to apply and remove the sludge from the drying beds.

DESIGN CRITERIA

Lime stabilisation requires that sufficient lime is added to the sludge to raise the pH of the mixture to 12 after two hours of contact.

PERFORMANCE EFFICIENCY

The above design criteria are rules of thumb for achieving the sewage sludge criteria in the U.S. EPA’s Sludge Disposal Regulations. The goal of the regulations is to achieve a minimum of 38 percent of volatile solids reduction.

DISADVANTAGES

The disadvantages to digestion processes are that the equipment, operation and maintenance costs can be very high. Also, trained operators are needed for proper operation. Composting and air drying can be low-tech processes but they require large land areas and large amounts of organic materials such as wood chips or waste plant material as a bulking agent. Air drying is easiest to operate, however, it may not be suited to rainy areas in the Caribbean.

RESIDUALS GENERATED

All stabilisation processes produce a sludge that can be disposed of by land application. Anaerobic digestion produces a useful by product, methane gas, which can be used as a fuel source.

OPERATION & MAINTENANCE

Regular operation of digesters includes management of sludge pumping, mixing, and controls. Equipment intensive processes will require regular equipment maintenance and may require periodic import of maintenance parts from outside the Caribbean region.

WCR INSTALLATIONS

The Arima and San Fernando plants in Trinidad have anaerobic digesters. The small package plant in Charleyville, Trinidad has air drying beds. All the facilities in Venezuela included in the site visit for this study use sludge lagoons for stabilisation and drying.

REFERENCES

U.S. EPA, 1979.

Sludge Dewatering

DESCRIPTION

• Belt filter press dewatering

• Centrifuge dewatering

• Screw press dewatering

• Plate and frame dewatering

Plate and frame presses are an old, high maintenance, and high cost dewatering processes. They achieve high sludge cake solids concentrations at the expense of high chemical and power costs.

APPLICATIONS

Belt filter press, centrifuge, and screw pump dewatering are appropriate for high population density communities and industrial use.

DESIGN CRITERIA

PERFORMANCE EFFICIENCY

DISADVANTAGES

Belt filter presses are very sensitive to incoming feed sludge characteristics. They also require operator attention and regular maintenance by qualified technicians. Centrifuge dewatering has high power requirements. Maintenance work for restoration of scroll and bowl wear-resistant coatings or tiles can require highly skilled maintenance workers and/or expensive shipment from outside of the country for replacement materials. Screw presses are a new technology, so design criteria are not well established.

RESIDUALS GENERATED

All dewatering processes produce effluent flows that must be returned to the plant or otherwise disposed of.

OPERATION & MAINTENANCE

Belt filter press, centrifuge, and screw pump operation requires close operator attention for control of loading rate. Equipment for these intensive dewatering processes requires regular maintenance and may require periodic import of maintenance parts from outside the Caribbean region.

WCR INSTALLATIONS

None of the installations visited by the KCM team in the Caribbean region used dewatering processes.

REFERENCES

U.S. EPA, 1979.

  COLD DIGESTION / DRYING LAGOONS

Click on image to view full photograph (700K)

DESCRIPTION

Digestion and stabilisation takes place in the lagoon at ambient temperatures. Two lagoons are needed. One lagoon is used for fill while the other is used for maturation. At the end of the one-year filling period the fill lagoon is isolated and allowed to dry for a period up to one year and sludge fill is directed to the alternate basin. Rooted aquatic plants such as scirpus grow on the surface during the maturation period and assist in sludge drying by evapotranspiration. When these plants change colour to brown from green due to desiccation, the sludge may be removed.

APPLICATIONS

Cold digestion / drying lagoons may be used in tropical climates when conditions of rainfall and evaporation permit. Evaporation should exceed rainfall by at least 500 mm for best results. Sludge from conventional activated sludge plants, extended aeration plants may be conveniently processes in CDD lagoons. Primary sludges should not be applied where odours could not be tolerated.

DESIGN CRITERIA

• Depth of sludge and water should not exceed 0.7 m

• Area should be 1 square meter per 5 to 20 persons served, depending on climatic conditions.

• Two or more lagoons should be built

• Side slopes should be lined with concrete.

• Access should be provided for sludge removal equipment in the form of an earthen ramp into the interior of the lagoon.

 

PERFORMANCE EFFICIENCY

Solids concentrations in the dried sludge may be as great as 25-30% for cake 300 mm deep.

DISADVANTAGES

A larger land area is required than for mechanical thickening, digestion, and dewatering. Limited to use in hot climates with a prolonged dry season.

RESIDUALS GENERATED

Excess supernatant water needs to be pumped back to the plant inlet. Dried sludge requires disposal or beneficial use.

OPERATION & MAINTENANCE

CDD lagoons require little operation or maintenance during filling. Sludge is lifted by means of wheeled mini-loaders or agricultural tractors with large wheels depending on the characteristics of the lagoon floor (normally unlined.)

WCR INSTALLATIONS

CDD lagoons have been in use at the Juangriego, Dos Cerritos, and Cruz del Postel plants on Margarita Island in Venezuela since 1989.

REFERENCES

Lansdell 1996.

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Chapter 1. | Chapter 2. | Chapter 3. | Chapter 4. | Chapter 5. | Chapter 6. | Chapter 7. | References | Appendix A.  | Appendix B. | Appendix C. | Appendix D. | Appendix E.