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

· Septic tanks with drainfields

· Septic tanks with mounds

· 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

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

· The MLE process for biological nitrogen removal

· 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