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

Chapter 6.
Methodology for selecting
appropriate technology

 

The current chapter presents methodologies for identifying appropriate technologies for sewage pollution control. The methodologies have been developed with the target audience in mind—government and funding agency planners, local officials, and engineers in the Caribbean region who must develop or evaluate plans for sewage pollution control for a given pollution source. Each methodology has been developed using a "decision tree"—a structured series of questions leading the reader to an appropriate technology or group of possible technologies that can abate or solve the problem at hand. The technologies identified in the decision trees are described in detail in Appendix D. Methodologies have been prepared for four broad areas of pollution control:

· Collection Systems

· Domestic Wastewater Treatment

· Industrial Wastewater Treatment

· Solids Treatment and Disposal

 

Collection Systems

Until recently, an engineer designing a sewage collection system had few options from which to choose. The oldest sewage collection system, and most common system to this day, is the system that in this report is called "conventional gravity sewers." These are gravity driven pipelines or channels that carry raw sewage away from homes and businesses. The conduits are constructed with a constantly downward slope so that gravity drives the flow. The main advantage of conventional gravity sewers is that design criteria are well established. However, conventional gravity sewers have many disadvantages compared to alternative systems. They are expensive to build, especially when the water table is high or soils are rocky, and can be susceptible to infiltration and inflow (I&I) of groundwater and suspended solids into the waste stream. Wastewater treatment facilities must be sized to handle the wastewater flow plus the I&I. Other, newer collection system technologies include small-diameter gravity sewers, pressure sewers, and vacuum sewers. These newer systems address some of the disadvantages of conventional gravity sewers.

Figure 6-1 is a decision tree for selecting an appropriate sewage collection system. The main factors which must be considered in choosing a system for sewage collection are population density, surface topography, and subsurface conditions. Collection systems considered in the decision tree and presented in Appendix C include :

· Conventional Gravity Sewers

· Small-diameter Pressure Sewers

· Vacuum Sewers

· Small-diameter Gravity Sewers

 

Figure 6-1. DECISION TREE FOR APPROPRIATE SEAGE COLLECTION
(Click on image for full figure.)

Decision-Tree Criteria

Below are the most important criteria for selecting appropriate technologies for sewage collection. The relevance of each criterion in the decision process and its implementation in the decision tree is discussed. The main factors in choosing a domestic wastewater conveyance technology are water availability, the prevailing slope of the terrain, hydrogeological considerations, and social considerations.

 

Water Availability

The first question in the decision tree is whether piped water is supplied to homes and businesses to be served. If little or no piped water is available, the volume of wastes generated will be minimal, and excreta and other household wastes can be disposed of in household systems, such as pit latrines or other non-water carriage toilets. Septic tanks should not be considered in such cases because they will operate the same as latrines or composting toilets, due to lack of fluid, but cost much more to install. Typically, not enough wastewater volume is generated to use a septic tank when residents do not have piped water supplies.

 

Surface Topography

If the surface topography allows sewers to be laid at a downward slope from homes and businesses to a sewage treatment facility, then gravity systems can be used. Gravity systems should always be preferred over pumping. Large pumping (lift) stations dramatically increase operation and maintenance costs, and may increase capital costs as well.

 

Subsurface Conditions

Unstable soils, rocky soils, and high groundwater levels make conventional gravity sewers more expensive to build and maintain. In these conditions, small-diameter or vacuum sewer systems may be cost-effective. Small diameter gravity sewer lines, made from PVC pipes, can bend to accommodate unstable soils, virtually eliminate I&I, and can be constructed around rock outcroppings relatively easily. Because small diameter gravity and septic tank effluent pressure (STEP) sewers do not carry a significant amount of suspended solids (they generally carry septic tank effluent), they can be installed at a lesser downward slope than conventional sewers (conventional sewers carry raw sewage, and must maintain a minimum flow velocity to prevent excessive deposit of solids in the sewer). This saves in construction costs since excavations for small diameter sewers are not as costly as for conventional sewers. Vacuum sewers can be used most effectively under conditions of flat terrain and high water table. Under these conditions vacuum sewer lines can be placed in shallow trenches to minimise construction cost. They are sealed systems from the house vacuum valve to the central vacuum station, so infiltration and inflows are eliminated. I&I can still enter the system through the house lateral line, however, since it is a conventional gravity pipe.

 

Social Considerations

Although not specifically mentioned in the decision tree, social considerations play an important part in selecting an appropriate sewage collection system for a community. The conventional gravity sewer system has been widely used with a variety of community types because it is the simplest system which requires no routine operational attention. It has been used in both high and low-income urban communities and for clusters of rural homes. Alternative systems, which may be of lower cost for initial construction, are either more complex or require more maintenance than a central gravity collection system. Small-diameter pressure sewers, for example, require a grinder pump in each house. This proliferation of powered equipment requiring routine maintenance is a significant disadvantage of this type of system in many communities. The experience with this system in developed world is that this type of system is very difficult to keep operating properly even in a fully developed economy. Vacuum sewers are less complex, but still require a valve to be maintained at each house and, generally, more vacuum/pump stations than would be required for a comparable gravity collection system. Small-diameter gravity sewers are used with septic tanks at each house which must be desludged at regular intervals.

Planners of collection systems should ask the question "Will the community accept maintenance of equipment in the house or permit access of utility personnel onto private property for such maintenance?" If the answer to this question is "no," then a conventional gravity collection system is indicated. Water carriage sewage collection facilitates the modern convenience of indoor toilet facilities provided in each community household. This convenience may not be required or even desired, however, in a given community where a community latrine would be a more easily accepted public waste collection strategy. Likely public acceptance of collection system strategies should be assessed through well advertised public meetings in the community, distribution of explanatory material, and community opinion surveys.

Domestic wastewater treatment

Choosing technologies for domestic waste disposal is a complex process involving many factors. Figure 6-2 is a decision tree for selecting an appropriate treatment technology for domestic wastewater. The tree is intended to help the reader arrive at an appropriate technology for a given community (here defined as a cost-effective technology that provides adequate treatment and that the local community has the finances and skilled labour force to operate and maintain.) Selecting the most appropriate technology for a given community requires an analysis of cultural factors, a site evaluation, and a cost analysis. The decision tree is intended as an aid in identifying appropriate technology. For a final selection, however, it must be supplemented with a detailed analysis for each community based on local factors and needs.

Decision-Tree Criteria

Below are the most important criteria for selecting appropriate technologies. The relevance of each criterion in the decision process and its implementation in the decision tree is discussed. The main factors in choosing a domestic wastewater treatment technology are water availability, presence of a collection system, housing or population density, availability of skilled management and operating personnel, land availability, availability and cost of power, receiving water requirements, hydrogeologic conditions and climate, and availability of opportunities for effluent reuse.

Figure 6-2. DECISION TREE FOR APPROPRIATE DOMESTIC SEWAGE TREATMENT
(Click on image for full figure.)

 

Water Availability

The first question in the decision tree is whether piped water is supplied to homes and businesses to be served. If little or no piped water is available, the volume of wastes generated will be minimal, and excreta and other household wastes can be disposed of in household systems, such as pit latrines or other non-water carriage toilets. Septic tanks should not be considered in such cases because they will operate the same as latrines or composting toilets, due to lack of fluid, but cost much more to install. Typically, not enough wastewater volume is generated to use a septic tank when residents do not have piped water supplies.

 

Collection System

If no waste collection system exists, a home or small community has few options for waste treatment and disposal. A community with a collection system has many more options. For use in this decision tree, the definition of collection system includes septic tanks as well as community sewers.

 

Housing or Population Density

For dispersed rural homes, central sewage collection facilities are not economical due to the high cost of piping wastewater to the central treatment facility. The housing density at which central systems become more economical compared to on-site systems varies widely. It depends upon the prevailing soil type, land cost, evaporation/precipitation balance, ground water hydrology, and local costs for construction materials. No density can be specified that will serve to make a hard and fast selection of the desirability of on-site versus central treatment systems for all community types.

 

Availability of Skilled Labour and Management

The complexity of a treatment technology that a community can expect to operate and maintain successfully is determined by the local availability of skilled labour. This is an important consideration; many activated-sludge package treatment plants in the U.S. and the Caribbean do not function properly because they are not operated or maintained correctly. In many small rural communities, where there are no skilled workers to operate an activated sludge process properly, a simpler process such as a lagoon or a wetland should be used. As a rule, low-maintenance technologies should be preferred over high-maintenance technologies, even if some treatment efficiency is sacrificed. This rule is reflected in the decision tree—all of the technologies applicable to communities without skilled labour must be easy to operate and maintain. Availability of a management infrastructure to process and collect user charges and manage expenses in another prerequisite for effective operation of more complicated sewage treatment processes. To some extent, all treatment systems must be part of an effective management infrastructure, but land-intensive, low power treatment systems are more forgiving of operations and management breakdown and should be the preferred technology where management systems are developing.

 

Land Availability

Where land is abundant and low cost natural treatment systems are usually appropriate, since they require little maintenance, are easy to operate and provide adequate treatment. Where land is scarce and expensive, mechanised, energy-intensive treatment processes, which require less land, may be more cost-effective than natural systems.

 

Receiving Water Requirements

Water quality requirements for the effluent receiving water (e.g., a lake, a stream, groundwater, an estuary, or open ocean) or effluent reuse significantly affect treatment requirements. Two criteria affect water quality requirements for the receiving water and, consequently, how much effluent can be discharged to the receiving water:

• Volume of receiving water—Large bodies of water have more assimilative, or diluting, capacity than smaller bodies of water.

• The intended use of the receiving water—Drinking water, shellfish harvesting, primary contact recreation, and irrigation all have different water quality requirements.

 

Appropriate treatment technologies for rural communities will provide adequate contaminant removal for most receiving waters or reuse needs. Consideration of the effluent receiving water is much more important for urban communities due to the volume of waste they generate. Selection of an appropriate treatment technology for urban communities requires knowledge of the degree of treatment required for the receiving water. If the effluent is discharged through a submarine outfall to an open ocean, primary treatment may be sufficient. If the effluent is discharged into an estuary, bay, lake or stream, eutrophication is a concern, and nutrient removal must be considered. If there is uncertainty about how much waste can be discharged into a receiving water, a mixing zone analysis should be conducted.

 

Hydrogeologic Conditions and Climate

Because treatment processes for low- and medium-density communities rely on natural systems more than those for high-density communities, some are more affected by hydrogeologic conditions of the treatment site than large systems.

For subsurface treatment or disposal processes, the following hydrogeologic conditions must be known:

• Soil permeability—Soil permeability sometimes with depth and location. If the soil is not permeable enough to accommodate the effluent flow rate, effluent will flow to the ground surface. This is known as ponding.

• The seasonal high water table—Adequate treatment of effluent requires sufficient travel time in the unsaturated zone above the water table to prevent groundwater contamination and allow oxidation.

 

In an arid climate, evaporation ponds can be considered for effluent disposal. For this to work, average annual evaporation must be greater than average annual precipitation, which is not common in the Caribbean basin.

 

Social Considerations

Residents’ knowledge, attitude, opinions, and prejudices about waste disposal can determine whether a treatment technology will work in a particular culture. For example, some cultures have an aversion to any contact with human wastes, so a composting toilet would be inappropriate for their communities. Local consultants and government officials should account for cultural issues when choosing a treatment technology.

 

Effluent Volume

The volume of effluent to be discharged determines appropriate effluent disposal methods. Low and medium effluent volumes can often be discharged below the ground if local soil conditions are suitable. If the effluent is high in contaminants, and the local drinking water source is groundwater, a different option should be considered. For higher volumes, marine outfall disposals may be more suitable because of the large diluting capacity of the open ocean. Planners must ensure that water quality standards for the receiving water are met.

 

Opportunities for Reuse

In many locations in the Caribbean properly treated effluent and sludge from wastewater treatment plants can be reused for beneficial purposes. Reuse has the double benefit of removing a discharge of nutrients and other contaminants from receiving waters while reducing pressure on water supply systems by providing an alternate water source. Wastewater can be used for many purposes including street washing, cooling water, and other industrial uses, irrigation of feed or fodder crops, landscaping irrigation, use in separate toilet water flushing systems, or in indirect or direct potable reuse.

The scope of this report does not provide for detailed development of reuse requirements and controls. Wastewater reuse for irrigation requires careful design of the overall water management program including, often, provisions for wastewater storage when irrigation demands are low. Wastewater loading may be limited by several factors including nutrients, hydraulic needs, or heavy metal or total dissolved salt content in the wastewater. In many cases wastewater application rates are determined by hydraulic requirements. Often sludge application rates are controlled by crop uptake rates for sludge nitrogen or by heavy metal content in the sludge.

Depending on the use, effective disinfection is a key requirement for reuse systems. Regulations for reuse in many states in the United States require effluent filtration and nearly complete removal of pathogen indicators prior to unrestricted use of wastewater effluent for irrigation. Indirect wastewater reuse for potable purposes is practised in many locations where wastewater effluents enter groundwater, either through direct infiltration or through exfiltration from lakes and streams, which becomes a subsequent source of water supply. In these cases, removal of nitrates is often required to limit build-up of nitrate concentrations in the ground water.

Assumptions Used to Develop the Decision Tree

The following assumptions were used in developing the decision tree for domestic sewage treatment processes:

• A reasonable attempt should be made to reduce the amount of wastewater generated. The less wastewater generated, the less costly the treatment.

• For low-income, rural communities, nutrient removal and advanced treatment may not economically or socially feasible. Many low-technology processes, like wetlands or lagoons, can be effective in removal of nitrogen without need for sophisticated operations control. These processes are not so effective in removal of phosphorus, however.

• For many communities in the WCR, land-intensive, low-cost, and low-maintenance technologies (natural systems) are appropriate. Hydrogeologic conditions affect the selection of an appropriate treatment technology. Most of the technologies provide excellent treatment, but some fail to remove nutrients. If the effluent is discharged into an estuary, bay, lake, or stream and eutrophication is a concern, nutrient removal processes should be considered.

• For urbanised areas with effective management control and access to skilled labour, conventional, energy-intensive technologies may be appropriate because land is too expensive for natural systems. The most appropriate technology for a given problem depends in this case on receiving water requirements.

• For discharge to non-sensitive areas such as to open marine water through a long outfall pipe, primary or lagoon treatment may meet discharge receiving water requirements and no further treatment is necessary. If discharge is to a river or estuary which is sensitive to dissolved oxygen depletion, then secondary treatment, as a minimum, is called for. If effluent is discharged to an environment that is sensitive to nutrients, such as a coral reef, estuary or lake, then nutrient removal may be needed to avoid destruction of the coral reef community or eutrophication of the lake.

• Conventional, mechanical treatment technologies do not necessarily provide better treatment efficiency than natural treatment systems such as lagoons, wetlands, or sand filters. Where natural systems would be effective and space is available, they are always recommended over mechanical systems because they are easy to operate and virtually maintenance-free.

 

The decision tree is intended to be used as a guide for selection of appropriate technologies for domestic wastewater treatment for communities in the Wider Caribbean Region. Unusual needs or circumstances, however, may make it appropriate to use technologies for a given community which would not be indicated by the decision tree. Planners need to use their own good judgement when special circumstances arise to identify and select the most appropriate technologies for a given community.

The questions listed in the decision tree, such as "Is inexpensive land available?" or "Is high power use cost-prohibitive?" were intended to be relative. Different options need to be compared to establish the right technology for a given community. For a given community a land-based alternative such as lagoons or wetlands could be initially compared to a conventional alternative, either secondary treatment or primary treatment and outfall discharge, depending on the receiving water requirements. Whether power use is "cost-prohibitive" or not depends not just on the local cost of power, but also on the relative cost of other alternatives. Only after the local costs and impacts of different alternatives have been compared can the relative questions in the decision tree be finally answered. In this way a series of alternatives can be screened to isolate the single alternative that is best for the community.

Industrial wastewater treatment

Domestic dry-weather sewage flows to municipal treatment facilities are fairly uniform in daily volume, pollutant type, and pollutant concentration. BOD and TSS concentrations range from 150 to 400 mg/L, and there are seldom excessive concentrations of toxic chemicals. For this reason, municipal treatment facilities are designed to handle domestic wastewater that falls within a narrow range of pollutant composition. The range for different types of industrial wastewater is much broader.

Industrial wastewater is the liquid waste generated by industries such as oil refineries, metal processing plants, leather tanneries, medical facilities, bottling factories, distilleries, and sugar processing plants. Industrial wastewater has a very wide range of volume, pollutant type, and pollutant concentration. The pollutants can be extremely complex, and often include more harmful chemicals and toxics than found in domestic sewage. The wide range of pollutant composition in industrial wastewater, along with the number of available processes and combinations of processes, precludes a brief, simple description of all the treatment processes used for its treatment. Even similar industries produce wastewater of highly varying composition, depending on the production processes used.

The methodology presented here focuses on removal of pollutants considered to be priority contaminants in the WCR; the scope of this study did not allow consideration of all important pollutants and processes for removing them. The absence of discussion about an industrial pollutant in this report is not intended to indicate that removal of that pollutant can be overlooked in selecting treatment technologies. The following steps should be taken before beginning the process of identifying appropriate technologies for an industrial waste stream:

• An extensive survey must be completed of waste stream characteristics. Because the pollutant composition of wastewater from every factory or industry is unique, it is crucial to identify the wastewater content precisely.

• Provisions should be made for spill containment.

• Every effort should be made to minimise the amount of waste produced. This involves experimentation, alteration, and fine-tuning of the production process. It is often less expensive to reduce waste than to treat it. Treated wastewater should be reused within the plant whenever it is cost-effective. Many factories and oil refineries can reuse treated wastewater as cooling water or for housekeeping, but this usually requires a very high quality effluent.

• It must be determined where the treated wastewater will be disposed and the degree of treatment needed to preclude adverse impacts to human health and the environment. If it will be disposed in the surrounding environment, the wastewater must be treated to a high degree of purity. This is often uneconomical. It is required, of course, where there is no municipal facility to accommodate the wastewater. Where discharge is to a municipal facility, pre-treatment is necessary because municipal treatment facilities are designed to handle waste within a narrow range of pollutant composition. Since industrial wastewater rarely falls into this range, its discharge without treatment could impair municipal treatment processes. Therefore, the goal of industrial sewage treatment processes is not always to produce a high quality effluent, but to make the wastewater suitable for municipal treatment.

• Identification of the appropriate treatment processes, using the decision tree described below, should take place after characterising the wastewater composition and determining the level of treatment needed.

• When an appropriate treatment process has been identified, pilot, or small-scale, tests should be run to find out how effective the process is on the waste to be treated. It is crucial to continue monitoring the effluent to find out the effectiveness of the treatment process. After fine-tuning the process, the selected treatment technology should be applied to the entire waste flow.

 

Decision Tree Criteria

Figure 6-3 provides a simplified decision tree for selecting an appropriate treatment technology for industrial wastewater. Using the tree generates a list of technologies that can be used as the best available technology. The decision tree for industrial wastewater treatment identifies processes that remove specific pollutants that typical industries in the WCR produce. It requires knowledge of the pollutants present in the waste stream. Selecting an appropriate technology from the decision-tree list requires an in-depth analysis of the wastewater constituents in the waste stream and the degree of treatment needed before discharge into a municipal sewer. Fact sheets in the appendix to this report describe the technologies and list references for more detailed analysis. The main pollutants that industrial pre-treatment processes must remove before discharging to public sewers are oils, metals, volatile and refractory organic materials, dissolved and suspended solids, and concentrated BOD loads. Only the most common unit processes for treatment of industrial wastes common in the WCR are included in the decision tree. See the section "Other Processes" below for consideration of some of the unit processes that have been omitted.

 

Oils and Greases

Of all WCR industries, oil refineries discharge the greatest BOD load to marine waters. Other industries, such as slaughterhouses and food processing factories, also produce large quantities of oil and grease. Not only do oils generate a high BOD demand on receiving waters, they also are toxic to aquatic life, clog screens and filters, and reduce activated sludge efficiency in downstream municipal treatment processes. Oil-water separation devices are very effective for oily waters, but are not effective for emulsified oils. Emulsified oils and particularly greases can accumulate in sewers and conveyance lines, causing a severe reduction in flow capacity.

 

Metals

The primary sources of metals are metal-processing and plating plants, hospital or medical facilities, oil refineries, tanneries, pesticide producers, and the paint industry. Most metals are highly toxic to aquatic life and humans, so they should be removed prior to biological treatment. Metals can accumulate in aquatic life, so even if effluent discharges contains metal concentrations below toxic levels, concentrations in aquatic animals, particularly shellfish can accumulate to dangerous levels. Some strains of microbiology are able to continue functioning when metals are present in significant concentrations, but they always function more efficiently if the wastewater is free from metals. Coagulation/precipitation and demineralisation processes remove metals from waste streams.

 

Volatile Compounds

Volatile organic compounds and other volatile chemicals will eventually be removed by natural processes. However, some of these compounds are odorous or hazardous, and should be removed into a controlled environment rather than into the open atmosphere. Air stripping and aerated biological processes remove volatile compounds.

Figure 6-3. DECISION TREE FOR APPROPRIATE INDUSTRIAL SEWAGE TREATMENT
(Click on image for full figure.)

 

High Soluble BOD Loads

Municipal wastewater facilities are designed to remove biochemical oxygen demands in the 150 to 400 mg/L range. If BOD concentrations are not significantly higher than this, then industries don’t need to remove BOD before discharging to a municipal sewer. However, many industries, particularly food processing and bottling industries, distilleries, chemical manufacturing plants, slaughterhouses, and meat packing plants produce high-strength wastewater with BOD concentrations up to 50,000 mg/L. If such a high-strength wastewater entered a municipal treatment process, it would overload the biological processes, may not be treated adequately, and could be discharged as an effluent of very poor quality. Anaerobic and aerobic biological processes remove high soluble BOD loads.

 

Suspended Solids

Most factories and industries produce waste streams high in suspended solids concentrations. High suspended solids concentrations have an adverse effect on the environment and make other wastewater treatment processes less efficient. Sedimentation processes remove large amounts of suspended solids, and filtration processes are effective as polishing processes.

 

Refractory Organics

Refractory organics are not biodegradable, so they are difficult to remove through biological treatment. Phenols are the primary refractory organic in industrial wastewater. Very high concentrations of phenols are found in wastewater from food processing plants, oil refineries, metal processing and plating factories, and many other industries found in the WCR. Refractory organics are extremely toxic to aquatic life and will inhibit biological treatment of the degradable pollutants. High concentrations of refractory organics are typically treated with solvent extraction processes while activated carbon adsorption or chemical oxidation is commonly used to remove refractory organics at more moderate concentrations.

 

Dissolved Solids

Effluent with high dissolved solids concentrations is not only harmful for freshwater aquatic life, it creates a scaly build-up and other corrosion problems as it travels through pipes and conduits. This is a problem if the effluent is discharged to public sewers or reused within the plant. If reuse water at a plant is consistently high in dissolved solids, the scaly build-up in the plant reuse piping will quickly cause complications. Demineralisation processes remove dissolved solids.

Other Processes

As previously mentioned, several processes used to treat industrial wastewater are not addressed in the decision tree or the fact sheets provided with this report. Some of these include the following:

• Equalisation is a very important process for most industrial wastewater treatment plants. An equalisation basin serves as a holding tank that controls fluctuations in wastewater flows to ensure good performance of processes downstream. The basin receives the wastewater, which varies in composition and volume, and discharges a steady flow of uniform composition. Mechanical mixing is usually provided. The main purposes of equalisation for industrial treatment processes are as follows:

– To dampen surges in the flow volume

– To control pH

– To provide a continuous feed of wastewater to biological systems even when no wastewater is being generated

– To prevent a "slug" of toxic material from upsetting downstream biological processes.

• Neutralisation, or pH control, occurs naturally to some extent in equalisation basins. If the waste stream is not neutralised, lime, caustic, or acid can be added to lower or elevate the pH. Most biological treatment processes operate optimally when the wastewater is within the range of 6 to 9 pH units. The purpose of pH control is to ensure that the wastewater is within this range.

• Supplemental nutrients may be necessary with certain industrial wastewater. Because some industries produce wastes with extremely high BOD loads, and relatively low concentrations of nutrients (nitrogen and phosphorus), nutrients may need to be added to ensure proper operation of biological processes. Biological processes will be impaired if nutrients are deficient.

• Chemical oxidation is a process used to break down pollutants, such as pesticides, that are ordinarily difficult to biodegrade. Common chemical oxidants are chlorine, ozone, hydrogen peroxide, and potassium permanganate.

 

Assumptions Used to Develop the Decision Tree

The following assumptions were used in developing the decision tree for industrial sewage treatment processes:

• Most appropriate treatment technologies require a medium to high level of operator skill. It is assumed that personnel qualified to operate industrial treatment facilities are available.

• Some of these processes are expensive, but cost is not explicitly addressed in the decision tree.

• The order in which the decision tree questions appear is the order in which the treatment chain usually progresses. However, there are exceptions. An example is that refractory organics can be removed in biological activated sludge processes by adding powdered activated carbon. They also can be removed with granular activated carbon filtration units, which are used later in the treatment process so that suspended solids do not clog the filtration media. Other examples are given in the facts sheets.

• There is some overlap in the role of each of the removal mechanisms. Coagulation processes remove not only toxic metals, but also suspended solids. Biological treatment removes not only soluble BOD, but also some volatile organic material. The user should be aware of this overlap.

• With the exception of lagoon systems, most industrial sewage treatment processes can not use natural systems as many domestic sewage treatment processes do. Most industrial sewage treatment processes are energy-intensive, mechanised processes. Therefore, industrial sewage treatment processes are more immune to environmental conditions than domestic sewage treatment processes.

 

Solids treatment and disposal

All technologies for removing pollutants from sewage and industrial wastewater generate residual materials in the form of waste solids, or sludge. In developed countries in northern climates, sludge treatment typically requires as much capital and operating and maintenance cost as treatment processes for liquid flows. In developing regions in equatorial climates, sludge management typically consists of sludge lagoons and drying beds with disposal of residuals to the land, which is generally less expensive to build and operate than liquid treatment technologies. If the liquid treatment technology is lagoon treatment, sludge treatment facilities normally are not required, since sludge is left to stabilise on the bottom of the lagoon. Periodic removal by dredging is the only sludge disposal practice required. For more mechanised liquid treatment technologies such as activated sludge and fixed film processes, however, significant quantities of residual sludge are generated that must be treated and disposed.

This discussion addresses only the basic sludge treatment technologies of thickening, stabilisation, and dewatering. For industrial sludge and for special needs in treatment works for high density population centres, high temperature processes such as incineration, heat drying, and high temperature wet air oxidation may be appropriate, but these technologies are not discussed in this report.

Loadings

The first step in planning for sludge treatment and disposal is to identify the quantity of sludge produced by the liquid process. The following formula is useful for predicting sludge quantities for a number of activated sludge secondary treatment processes:

TSS_p = TSS_in + (Y * SBOD_r - k_d * INV_vss)/VSS_r - E_T

where

TSS_p = Total sludge production, kg per day (kg/d)

TSS_in = Total suspended solids influent to the secondary treatment process, kg/d

Y = Yield coefficient (0.5-0.8), kg volatile sludge produced per kg soluble BOD removed

SBOD_r = Soluble BOD removed in the liquid treatment process, kg/d

k_d = Decay coefficient, 1/day = 0.03 - 0.08

INV_vss = Inventory of volatile solids in the liquid treatment process, kg

VSS_r = Ratio of volatile to total solids in the liquid treatment inventory

E_T = Effluent suspended solids, kg/d

 

For systems that operate with a very long sludge age, so that volatile solids influent to the liquid treatment process have an opportunity to break down, the following formula may be more appropriate:

TSS_p = (Y * TBOD_r - k_d * INV_vss)/VSS_r - E_T

where

Y = Yield coefficient (0.5-0.8), kg volatile sludge produced per kg total BOD removed

TBOD_r = Total BOD removed in the liquid treatment process, kg/d

k_d = Decay coefficient, 1/day = 0.03 to 0.08

 

For fixed growth processes, the following formula is suggested (U.S. EPA, 1979):

TSS_p = P_x + TSS_in - E_T

where

P_x = Y * BOD_r - k_d *Am

Am = Media surface area in the reactor, square meters

For primary and other physical or chemical treatment processes, solids mass balances must be performed and chemical reactions considered to predict the appropriate quantity of sludge that will be produced under full-scale operation.

Decision Tree Criteria

Figure 6-4 presents a simple decision tree for selection of basic solids treatment and disposal technologies.

 

Thickening

Sludge wasted from the liquid treatment process may be very dilute. Since sludge stabilisation treatment reactors can be very expensive and are frequently designed on the basis of hydraulic residence time, it is advantageous to reduce the water content of sludge sent to solids treatment. A waste sludge from the aeration tank of an activated sludge process, for example, will typically have a concentration of 2,000 to 3,000 mg/L or 0.2 to 0.3 percent dry solids by weight. Thickening processes can increase the solids content of such sludge to 6 to 8 percent, an increase of over 30-fold. This decreases the size of subsequent treatment reactors by a corresponding amount.

 

Stabilisation

If sludge is to be beneficially reused as a soil amendment or otherwise come in contact with the community, it is imperative that putrescible materials in the sludge be decomposed to prevent odours at the application site and attraction of rodents and other "vectors" that can spread contaminants to the human population. In the United States, the Environmental Protection Agency has completed an exhaustive process of regulatory review leading to the promulgation of sludge disposal regulations that include requirements for reducing "vector attraction" in sludge that will be applied to the land. Typical stabilisation processes are anaerobic or aerobic digestion, composting, and sludge lagoon storage.

 

Dewatering

Disposal or reuse of sludge may be more economical or efficient with further reduction in water content following treatment. Processes similar to those used for thickening sludge may also be used to dewater them further prior to final disposal or reuse.

 

Cold Digestion/Drying Lagoons

A sludge management technique that is especially cost-effective for WCR applications in hot climates with a prolonged dry season are cold digestion/drying (CDD) lagoons. CDD lagoons fulfil all of the functions of sludge thickening, stabilisation, dewatering, and storage in a series of earthen basins. Waste activated sludge can be pumped to CDD lagoons in relatively dilute form and converted to a dried product of 25-30 percent solids concentration after a fill period of one year and a drying period of an additional year. Where land area is available CDD lagoons are a highly appropriate technology for the WCR. Although not shown in the decision tree in Figure 6-4, a fact sheet for CDD lagoons is provided in Appendix C.

 

Land Application

Wastewater treatment sludge may have agronomic value. It can provide nutrients—especially nitrogen and phosphorus—and organic material that contribute to soil tilth by building the humic resources of the soil. Sludge disposal by land application is therefore a widespread and sound method of disposal which may provide for beneficial reuse of sludge nutrient and organic value. Land application may be by tank truck, by spraying through large bore sprinklers, by injection, ridge and furrow application, or by spreading of dewatered material. Consideration of detailed land application methodologies and limiting loading rates is beyond the scope of the current report. In general, sludge appreciation to agricultural land is limited by sludge nitrogen uptake by the agricultural or silivicultural crop. Heavy metal content, however, may also limit long-term loading rates. The EPA sludge disposal regulations provide good background data and a methodology for determining limiting sludge loading rates.

 

Landfill

Sludge that contains heavy metals or other toxic materials that prevent its use as a soil amendment must be disposed of in a landfill. Sludge landfilling can be achieved in various ways—sludge only trench fill, sludge only area fill, and codisposal with refuse. See EPA 1979 for detailed criteria.

 

Septage Handling and Disposal

With a large percentage of the populace in the WCR served by septic tank systems, the need exists for consideration of septage handling and disposal. A common practice at present is for septage to be dumped at landfills and sewage treatment plants. The Bahamas has a septage treatment facility which has worked effectively. The Arima Sewage Treatment Works in Trinidad has a septage handling tank that is aerated and equipped with pumping units to transfer septage to the anaerobic digesters. It was beyond the scope of the current effort to evaluate technologies for septage handling in detail. A US EPA handbook, Septage Treatment and Disposal (EPA 1984) gives design data for septage characterisation, receiving station design, land disposal of septage, cotreatment of septage and sewage, and independent treatment of septage. It also provides fact sheets for receiving stations, land disposal, lagoons, composting, lime stabilisation, and odour control.

Costs

A crucial element in the process of selecting an appropriate technology for wastewater treatment is to identify realistic costs for alternatives. Cost estimating is local by its nature. With over 29 countries included in the WCR speaking at least four major languages and with widely varying levels of economic development, it has not been possible to provide comprehensive cost data for wastewater treatment technologies that would be applicable throughout the region.

Figure 6-4. DECISION TREE FOR APPROPRIATE SOLIDS TREATMENT AND DISPOSAL.
(Click on image for full figure.)

The literature review prepared as a part of this report did not uncover any comprehensive cost guides that would be helpful to local planners in the WCR. The U. S. EPA in the 1970s prepared a series of cost curves that were used widely in wastewater technology fact sheets that have been referenced in this report. An example would be the Innovative and Alternative Technology Assessment Manual (US EPA February 1980.) This manual contains fact sheets for approximately 100 different wastewater treatment technologies. Most of these fact sheets contain cost curves for construction and operating and maintenance costs. These costs were based on the surveys conducted by EPA in the mid 1970s. Today these data are of limited value, since comparable studies have not been completed to update the costs to current conditions. Furthermore, these cost data were gathered in the United States and would not be applicable to different countries where costs for labour and imported equipment vary greatly from the conditions found in the United States. By necessity, therefore, cost comparisons of technologies for wastewater treatment in the WCR must be prepared locally, by planners and engineers with an understanding of the local economy and construction industry.

Achievable Treatment efficiencies

This report has not considered receiving water quality needs based on chemical, oceanographic, or ecological requirements of the marine waters of the Caribbean region. The report has rather considered wastewater treatment technologies and their potential to remove contaminants. To the extent that effluent standards are based on the capabilities of available technology, however, this report can serve as supporting documentation for the standards setting process for the WCR. Table 6-1 provides a guide to treatment efficiencies that can be expected for the domestic treatment processes described in the fact sheets in Appendix C. To achieve these efficiencies, the treatment processes must be designed and operated properly, and must not be hydraulically or organically overloaded.

 

 

TABLE 6-1. ACHIEVABLE EFFICIENCIES FOR DOMESTIC WASTEWATER TREATMENT PROCESSES
	Effluent Concentration or Process Removal Efficiency
	BOD	TSS	Ammonia	Phosphorus	Faecal Coliform
Septic tank	100-150 mg/L	40-70 mg/L	40-60 mg/L	6-7 mg/L	1-2 log removal
Septic tank + soil	0-10 mg/L	0-10 mg/L	0-40 mg/L	0-2 mg/L	6-7 log removal
Holding tank	N/A	N/A	N/A	N/A	N/A
Household Systems	N/A	N/A	N/A	N/A	N/A
Lagoons	20-30 mg/L	30-80 mg/L	20-30 mg/L	5-7 mg/L	3-5 log removal
Wetlands	5-30 mg/L	5-20 mg/L	5-15 mg/L	0-10 mg/L	1-3 log removal
Land treatment	2-15 mg/L	0-20 mg/L	0-5 mg/L	0-6 mg/L	4-6 log removal
Sand filtration	2-25 mg/L	0-10 mg/L	0-10 mg/L	0-2 mg/L	3-4 log removal
Preliminary treatment	0% removal	0-10% removal	0% removal	0% removal	0 log removal
Primary treatment	25-40% removal	40-70% removal	0-10% removal	0-10% removal	0-1 log removal
Secondary treatment	5-40 mg/L	5-40 mg/L	1-10 mg/L	5-10 mg/L	1-2 log removal
Nutrient removal	5-30 mg/L	5-30 mg/L	0.1 -5 mg/L	0.1-1 mg/L	0-1 log removal
Disinfection	0% removal	0% removal	0% removal	0% removal	5-6 log removal

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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.