Authors: Todd M. Doley and Waldon R. Kerns*
*Graduate research assistant and professor, respectively; Department of Agricultural and Applied Economics, Virginia Tech
Publication Number 448-406, June 1996
Table of Contents
Introduction
Alternative Wastewater Treatment Systems for Individual Homeowners
Cluster Systems and Small Centralized Community Systems
Alternative Collection Systems
When the site or soil conditions are not conducive for the proper functioning of a conventional septic system, the result can be less than adequate performance or the outright failure of many of these systems. A malfunctioning septic system can pose a significant health risk, be detrimental to the environment, and be rather costly to repair. Therefore, it is very important that the property owner or owners use a system that is specifically designed to overcome these limiting factors. Alternative systems may be necessary when:
When faced with the difficult decision of choosing among several costly alternatives, it is often in the best interests of homeowners in a given area to join together and construct a system that will meet the needs of several houses instead of just one. Many systems that can be used by individual homeowners can be modified in design to serve several homes. Systems built to serve a small number of properties are referred to as cluster systems. Those options best suited for individual homeowners will be discussed first, followed by those that can serve both individual residences and several properties, and then those that can serve the needs of an entire small community.
To understand the advantages and disadvantages associated with a particular system, it is important to first understand the various stages involved in wastewater treatment.
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Most aerobic treatment units consist of an aeration chamber and one or more clarifying chambers. Typically, the first compartment is a clarifying or settling chamber where solids and liquids separate and some anaerobic digestion occurs. This chamber performs the same function as a septic tank, and if the unit is preceded by a septic tank, this first compartment may not be necessary. From the settling chamber, wastewater then passes into the aeration chamber where a pump supplying a constant flow of compressed air and a stirring mechanism are used to oxygenate the water, creating optimum conditions for aerobic organisms to decompose organic compounds. This allows more of the organic matter in the water to be digested, reducing the amount of pathogens and other pollutants. The third compartment is another clarifying chamber that allows for further removal of particulate matter before the effluent leaves the unit. All aeration treatment units are required to be equipped with an alarm system that detects both failure of the aeration pump and high water levels.
The main advantage of this alternative is the high quality of the effluent it produces. The soils of a drainfield are able to assimilate a greater volume of wastewater that has been treated in an aerobic treatment unit than from a conventional septic tank. Aerobic treatment units are often used when a drainfield is not available and direct discharge must occur. Under these circumstances the effluent would need to be disinfected before it could be discharged. The least expensive and most commonly used disinfection process is a chlorination and dechlorination process. The main disadvantages associated with aerobic treatment units are the need for an outside power source and the higher amount of maintenance required to ensure proper system operation. The system usually fails when homeowners try to assume complete responsibility for the maintenance of the aerobic treatment unit and the disinfection process. This is why many localities require (and the authors highly recommend) a maintenance contract with an outside contractor before an aerobic treatment unit is installed.
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There are several different designs for sand filters. Filters placed beneath the ground are referred to as buried filters, while those located above ground are known as open filters. Buried filters are often used when aesthetic considerations are important, as open filters can occasionally cause odors. However, buried filters are not easily accessible and they need to be designed to operate effectively without periodic maintenance. Because of this, buried filters are usually not suitable for treating large volumes of wastewater on a daily basis. Larger volumes of wastewater tend to clog the surface layer of sand, forming a thick crust or mat which inhibits the effectiveness of the filter. Buried filters are also not recommended where bedrock or seasonal water tables are located close to the surface.
Open filters must be readily accessible so they can be routinely raked by the owner to break up the crust, and after a period of time the filters sand should be entirely replaced to keep the system running properly. Open filters can be designed to recirculate a portion of the wastewater back to the septic tank so it can pass through the system again. The added benefit of a recirculating filter is that it is able to remove a greater amount of nitrogen from the wastewater than the other types of sand filters. Their primary drawback is that they require a pumping system to recirculate some of the effluent.
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The medium used in the cell is typically gravel, although sand or a gravel/sand mixture can be used as well if dictated by the design. Bulrushes, cattails, reeds, rushes, and sedges are common types of vegetation used in constructed wetlands. Native wetland vegetation can be found in all regions of the country and has the advantage of being well adapted to the climate as well as being better able to contend with local pests. Water irises are very attractive wetland plants that perform well in constructed wetlands. Most species of vegetation will require at least six hours a day of good light. If the site does not permit that much light to come through, more shade loving plants such as ferns can be considered.
The medium and vegetation present in the cells provide several functions for the system. The principal function of the plant roots is to transpire oxygen and thus aerate the water. Aerobic conditions allow for a larger variety of microorganisms to attach themselves to the surface of the roots and medium. These microbes are the primary source of treatment, feeding on the waste products in the water. The medium and plant roots help polish the water by trapping tiny particles. Another positive function of the plant roots is to take up some of the water, reducing the amount that will need to be discharged.
Wastewater from the house goes directly to a septic tank, where solids and liquids are allowed to separate. Bacteria which thrive under anaerobic conditions begin the process of breaking down waste products within the effluent. The water then passes to the first cell of the wetland system. Here the microorganisms attached to the substrate proceed to further break down the waste in the water. From here there are two options for the disposal of the water. In the case of a very small system, the water can flow to an unlined cell where the water evaporates, is taken up by the roots of the vegetation, or is allowed to percolate into the soil below. For systems with a larger volume of water flow, the addition of more cells or a drainfield may be necessary.
The designs of constructed wetlands can be readily adapted to accommodate different site and flow characteristics. These systems have proven to be effective at removing nitrogen from wastewater and can be used in areas where nutrient enrichment is an environmental or health problem. Constructed wetlands are typically inexpensive when compared to other alternatives, and they typically require a modest amount of maintenance. As with conventional septic systems, the septic tank should be pumped every three to five years to prevent the overflow of solids. If it is a subsurface system, then a small observation tube should be installed in each cell so the homeowner can periodically check the water level to ensure that it is not too low or too high. Finally, the owner will need to care for the wetland plants by removing dead plants and any weeds or saplings that have taken root.
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Irrigation systems can be classified into two basic types depending on how the wastewater is delivered to the soil. Spray irrigation systems use sprinklers to distribute the effluent over the surface of the ground. Drip irrigation systems distribute the effluent just below the surface of the ground. With spray irrigation, the effluent is distributed more evenly over the surface area which allows for a greater amount of evaporation to occur. Spraying the effluent also exploits the entire assimilative capacity of the soil profile, since it enters the soil at the surface. Because the effluent is distributed through the air, a potential exists for pathogens to be carried by the wind, causing a possible health hazard. Wastewater must first be treated by either an aerobic treatment unit or a septic tank-sand filter combination and then disinfected through a chlorination process before it can be sprayed. It is recommended that these fields be enclosed by a fence to ensure that children and animals will not wander into the area. The greater amount of treatment prior to spraying and the large field size can make this an expensive system to employ.
Drip irrigation systems utilize pressure compensated drip tubing to slowly and evenly dispense the wastewater just below the soil surface, but still within the root zone of the vegetation. After first being treated by a septic tank, the wastewater goes to a dosing chamber. The dosing chamber then periodically sends effluent through a series of disk filters before delivering it to the network of tubing. The system regularly back flushes effluent through the filters to prevent them from becoming clogged. The pressure compensated tubing is designed to distribute wastewater uniformly over the entire drainfield. Since the effluent is never airborne, drip systems do not allow for as much evapotranspiration to occur as with the spray irrigation. Yet, because the wastewater is never exposed to the air, aerobic treatment and disinfection are not always necessary with drip irrigation. This also enables the system to avoid the potential problem of odors. Both irrigation systems require an outside power source for the operation of the dosing chamber.
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The primary purpose of grey water systems is to separate household effluent. Wastewater from toilets or kitchen sinks with garbage disposals is referred to as black water and needs to be thoroughly treated before it can be discharged. Water from sources such as the laundry, tub, or bathroom sink is known as grey water and can be discharged with a minimum of treatment. The household effluent is partitioned through the installation of separate plumbing systems. Typically, grey water is delivered to a septic tank or sand filter where it is treated before being delivered to its own drainfield. Grey water can be routed to a holding tank where it is stored until needed for flushing the toilet. Black water is still required to undergo the necessary treatment stages before it can finally be discharged in a drainfield. The benefit of this type of system is that it allows for a greater amount of treatment time in the septic tank for the black water. Grey water systems can be readily incorporated into the plans of a house being built with little additional cost. However, altering the plumbing of an already existing house could be expensive.
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These alternative systems that serve more than one residence can collectively be called community systems. Community systems that serve fewer than 50 properties are referred to as cluster systems, while those that serve from 50 to as many as several thousand are known as small centralized community systems.
There are several important factors to consider when communities are evaluating the use of alternative wastewater treatment or collection systems. First, the technical feasibility and the reliability of the possible alternatives need to be evaluated. Secondly, the costs of installation, management, and maintenance need to be considered as well. Finally, these decisions can be difficult to make, and so the community should be sure to enlist the help of state and local officials. Planning assistance is available for each community from its local sanitarian, but it can also come from regional planners, the local soil and water conservation department, or the local Cooperative Extension agency. Communities may decide to hire a consulting engineer to provide them with professional advice on many of these decisions.
Choosing the proper design for the system is the important first step. If the determination has been made to use a community system rather than going with individual on-site treatment, special attention needs to be given to the land where it will be located. Site evaluations, soil structure analysis, topography, and average precipitation measurements are all needed. The likely peak and average volume of wastewater the system will need to accommodate is also an important consideration. This information will help determine if the wastewater can be discharged into a mass drainfield, or if it will have to undergo advanced treatment and disinfection before being directly discharged to a waterway.
Land values are an important cost consideration when deciding on systems. Certain systems, such as stabilization ponds or mass drainfields, require a large amount of land per person served. If the price of land is relatively high, it may be less expensive to build a system that might cost more to construct, but requires the purchase of much less land. Therefore, two communities with similar site conditions may be better served by different systems. As an example, a rural community where land prices are low may be best served by a system that requires a great deal of land, but is inexpensive to construct and operate. While a new suburb being planned in an area with high real estate values might be better served by a system that is more expensive to build and operate, but requires only a small parcel of land.
The financial costs of a new system need to be considered early on in the process. Each community needs to investigate possible funding sources, sources of planning assistance, and methods for financing the long-term maintenance of the system. Outside funding typically entails applying for either state or federal assistance to plan and install the system. This financial assistance usually comes in the form of grants or low interest loans. Other important considerations are:
There are several approaches that the locality can take toward the long-term operation and management of the system. For instance, cluster systems can be owned and managed by a homeowners association, owned by the homeowners but managed by the locality, or owned and managed completely by the locality. Many Virginia localities have had problems with homeowners assuming responsibility for a system without being adequately prepared to handle the necessary operation and maintenance of the system. This has led to system failures and in some cases pollution problems. It is therefore recommended that arrangements be made with the local government to assist in the management of the system. This could simply be assuring that fees are collected and contracts with private operating firms are binding and renewed on a regular basis. This partnership between the homeowners and local government should be worked out before the system is installed. This is to ensure that each party's responsibilities are well defined before major commitments of time and money are made.
Small centralized community systems should be owned and operated by the locality in which they are used, since it is usually necessary to acquire easements and pass local ordinances before a system of this size can be installed. However, the active participation of the public in selecting the institutional arrangements is crucial to long term public cooperation. The management of a community system will involve routine inspections, operation, maintenance, and monitoring. Inspections will need to occur both during the installation of the system and on a regular basis afterwards. This involvement is best achieved by providing the public with several means by which they can learn about and have access to the decisionmaking process.
Information on the wastewater treatment needs of the community and what the likely options are is a good way to begin education efforts. Efforts to inform the public about waste reduction, water conservation, and homeowner maintenance are important as well. Public meetings should be held at each stage of the process. This will allow the public to ask questions and make comments during the various planning stages. Opportunities for direct public participation in making certain decisions could be provided by holding referendums and by forming a citizens advisory committee. It is particularly important to involve the public in deciding how the construction and long-term operation and maintenance will be paid for.
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Anaerobic lagoons are the deepest systems (up to 20 feet), have steep side walls, and require long periods to stabilize wastewater. They develop a thick crust on the surface which inhibits oxidization and serves to trap in heat. This makes anaerobic lagoons more suited to colder climates. They typically use less land but require a longer retention period than the others. They can produce odors, especially when being cleaned.
Facultative lagoons have an aerobic top layer and an anaerobic bottom layer. They tend to be large and shallow (3-8 feet) to allow for maximum diffusion of oxygen, which occurs at the surface, and for the maximum amount of algae growth to take place. The algae helps the treatment process by using nutrients in the wastewater. Facultative lagoons cause fewer odor problems, but may have problems functioning during prolonged cold periods when ice forms on the surface.
Aerated lagoons and stabilization ponds are both aerobic systems. Stabilization ponds are the shallowest of all the systems, usually just 2 feet deep. They rely on surface diffusion of oxygen and algae growth to oxygenate the wastewater. Stabilization ponds require a large area of land, typically about 1 acre for every two hundred people, and are usually located in areas where the climate permits year round algae growth. Aerated lagoons create aerobic conditions through mechanical means. Mechanical aeration allows these lagoons to use 60% to 90% less land area than stabilization ponds. Limitations of surface space or cold winter temperatures are two reasons for mechanical aeration. (Mechanical aeration can save on space and thus initial construction costs, but it will be more costly to operate and maintain.) In aerobic lagoons, algae can be harvested and used as a component of animal food or as a soil conditioner, and fish can be grown as well.
Lagoons can be preceded by a septic tank, but primary treatment is generally not required. The septic tank can be followed by a sand filter or be discharged directly to a drainfield. If the lagoon is large enough, evapotranspiration and ground seepage underneath the lagoon can take care of disposal; however, this has the potential for possible groundwater contamination, so it is not usually recommended and the cells are usually lined with clay or an impervious material. These systems tend not to require a great deal of operation and maintenance, so maintenance costs can be minimized. Construction of the lagoons can be costly and they do require more land than most other alternative systems.
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Sequencing batch reactors utilize an activated sludge treatment process. After the treated effluent is discharged, the sludge is then removed from the bottom of the reactor. Yet, a small portion of the sludge, which is rich in microorganisms, is left in the reactor. This helps to quickly establish a population of microorganisms within the next batch of wastewater delivered to the reactor. This helps reduce the amount of time necessary for treating each batch. Usually, more than one reactor is needed to ensure that while one batch of wastewater is being treated, additional wastewater can be directed elsewhere. This ultimately depends on the expected volume of wastewater flow and the amount of time allowed for treatment of each batch in the reactor. The longer the retention time, the less sludge and cleaner effluent.
The main advantage of sequencing batch reactors is that they can produce an effluent very low in organic compounds and thus can meet strict effluent standards. The system can be effectively used as part of a larger system when the removal of the nutrients, nitrogen and phosphorus, is required. Another advantage of this system is that it can be located on a small area of land. It is relatively easy to expand this system by adding additional reactors. However, the operation of this system is more complex than others. All sequencing batch reactor systems require a trained system operator, and systems with two or more reactors typically require a computer to control distribution of the batches and retention time in each tank. Another drawback of this type of system is the need for frequent and regular disposal of the sludge. Finally, the system tends to be more costly to construct and operate than most other systems, yet it tends to have fewer maintenance problems over the lifetime of the system.
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An oxidation ditch is essentially a large holding tank with a shape similar to that of a race track. The tank is lined with an impermeable lining and is built on the surface of the ground. This allows the wastewater to have plenty of exposure to the open air for the diffusion of oxygen. The liquid level in the ditches tends to be shallow, averaging about 3 feet; this helps prevent anaerobic conditions from occurring at the bottom of the ditch. One or more mechanical surface aerators are attached to the side of the ditch. Although surface aerators can vary depending on the design of the system, most tend to resemble a large circular brush. The aerators slowly rotate to facilitate the introduction of oxygen to the wastewater without causing too much turbidity.
Raw sewage is delivered to the ditch where it is slowly mixed by the aerators. Longer retention time within the ditch will allow for a greater amount of organic matter to be broken down by the aerobic bacteria. After treatment, the sewage is then pumped to a settling tank where the sludge and the water are allowed to separate. From here most of the wastewater goes on to other treatment processes. The sludge that has accumulated on the bottom of the settling tank is then removed and a portion of it is returned to the ditch to facilitate microbial activity in the next batch of sewage to be treated.
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Sewage must first go to a settling tank where much of the solid matter is allowed to settle out of the wastewater. From there the wastewater is pumped to the distributor, which sprays it onto the surface of the media. The rock and plastic media act as a substrate to which microorganisms attach. Empty space between the media allows for the presence of air, creating an aerobic environment for the microorganisms. Plastic media have significantly more empty space, thus allowing for a greater oxygen transfer. As the wastewater passes over the media, these microbes feed upon organic material within the wastewater. The population of microbes eventually grows to form a layer of slime over the media. Portions of this slime layer are sloughed off each time wastewater passes through the filter. After it has been collected in an underdrain beneath the filter, the wastewater is then sent to a second settling tank where slime debris is allowed to settle out.
Trickling filters do a good job of removing nitrogen as well as organic matter from the wastewater. Because of this, they are beneficial for communities with strict nutrient discharge standards. Trickling filters can be expensive to build and systems that use a rock media are usually more expensive than those that use plastic. Moderate skill requirements are needed for maintenance and operation. Energy requirements will vary depending on the system. These systems are not well suited for very cold climates and can cause odor problems.
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Conventional sewer systems depend on gravity to deliver the sewage from each property to the treatment plant. Therefore, the system's collection pipes must continuously slope downwards. Solids are not separated from the wastewater before it enters the network of collection pipes. To ensure that the pipes do not become clogged with solid material, the downward slope of the pipes must be at a steep gradient that is uniform throughout the system. The pipes must also be laid in straight alignments between manholes to ensure that when a stoppage does occur it can be readily accessed. For conventional sewer systems that serve a large area, there will likely be elevation differences within the network of collection pipes that will require a lift station to transport the sewage to the higher elevation. These requirements can make conventional sewer systems very expensive to install.
The principal advantage that alternative collection systems have over conventional systems is the lower cost of installing the network of collection pipes. The network of piping for an alternative collection system can be laid in much shallower and narrower trenches. The pipes also do not need to be laid in a straight line or with a uniform gradient. This means they can be laid in such a manner as to easily avoid obstacles. Nevertheless, there can be disadvantages in using an alternative collection system. Some systems require the separation of solids before the liquid can enter the network of collection pipes, while others need the aid of a mechanical device to propel the sewage through the system. When the population density is high for an area, and the required length between service connections is short, the additional requirements of alternative systems can make them more costly than a conventional system. However, where the use of alternatives is appropriate, the EPA estimates that communities can reduce overall collection costs by 25% to 90%. The consideration of alternative collection systems is appropriate when:
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Small diameter gravity sewers are well suited for communities where the houses are far apart, or where most houses are served by an existing septic tank. Areas with a high housing density or with extremely hilly terrain are not as conducive for the use of this type of system. Operation and maintenance costs for small diameter gravity sewer systems are compatible to that of conventional gravity systems. Depending on the size of the system, one or two persons can be employed on a part time basis to handle operation and maintenance, although at least one person should be on call at all times. The only additional maintenance requirement is the periodic pumpout of the septic tanks, which is usually done every three to five years by a contractor hired by the community.
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The other type of pressure sewer uses a grinder pump. Wastewater from each property goes to a tank containing a pump with grinder blades that shred the solids into tiny particles. Both solids and liquids are then pumped into the sewer system. Because the effluent contains a mixture of solids as well as liquids, the diameter of the pipes must be slightly larger. However, grinder pumps eliminate the need to periodically pump the septic tanks for all the properties connected to the system. Both the STEP and grinder systems are installed with high water alarms. Pressure sewers are well suited for hilly areas. Because of the addition of the pumps, pressure sewers tend to require more operation and maintenance than small diameter gravity sewers. Operators can usually be hired on a part time basis, as long as someone is on call at all times. Operators will need training on both the plumbing and electrical aspects of the system.
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Vacuum sewer systems are limited to an extent by elevation changes of the land. Rolling terrain with small elevation changes can be accommodated, yet steep terrain would require the addition of lift stations like those used for conventional sewer systems. It is generally recommended that there be at least 75 properties per pump station, for the use of a vacuum sewer system to be cost effective. This minimum property requirement tends to make vacuum sewers most conducive for small communities with a relatively high density of properties per acre. The maintenance and operation of this system require a full time system operator with the necessary training. This can make the operation and maintenance costs of vacuum sewers exceed those of other systems.
Some communities may find that their wastewater collection needs cannot be adequately met by any one particular system. It is often the case that a combination of various systems, including conventional systems, will be needed to overcome all the site limitations for the lowest cost. An important fact to remember when considering alternative systems is that they tend to require a greater amount of participation by the homeowner. Therefore, the need for community involvement in the choice of systems is important. Questions concerning homeowner maintenance requirements, or the possibility of hiring a contractor to routinely service the system for the community, need to be discussed before any final decisions are made.
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Step #1 The local office of the Virginia Department of Health (VDH) will answer many of the homeowner's initial questions and will provide the necessary applications to begin the permit process. The type of permit needed will depend on whether the effluent is directly discharged or disposed of in a drainfield. It will also depend on whether the expected average daily load of effluent to the system will be greater or less than 1,000 gallons per day. The Virginia Department of Environmental Quality (VDEQ) becomes involved in the permitting process when there is the possibility of the discharge affecting the quality of local water resources.
Step #2 A site evaluation will need to be done by health officials to determine if the ground is suitable for use as a drainfield. If it is suitable for a drainfield, the homeowner could have a conventional septic system installed. If the site is only marginally suitable for a drainfield, the homeowner will have to work with the health department to determine what alternative system or combination of systems will be best suited for the site. Several things will need to be taken into consideration at this point:
Step #3 If the site evaluation determines that the area is unsuitable for use as a drainfield, there is still the possibility that a direct discharge system could be used. Again, the homeowner should work with the health department to determine what alternative system would be best for the circumstances. Things to consider are:
List of Important Steps for Small Communities
Step #1 When a community is deciding on a new wastewater treatment system, it is important to start the process by gathering all the information necessary for an informed decision. First, clarify the reasons for constructing a new system and the likely future needs of the community. Second, determine how much the community can afford to spend on a new system, and the available sources of financial assistance. Finally, be aware of the various regulations and permit requirements that the community must comply with. Four suggested offices to contact are:
| Average Daily Load less than 1,000 gallons pre day. | Average Daily Load greater than 1,000 gallons per day. | |
| Drainfield | A general permit from the VDH. | A general permit from the VDH. Reviewed by the VDH Office of Water Programs. VDEQ may be asked for comments as well. |
| Direct Discharge | A general permit from the VDH. Reviewed by the VDEQ | A Virginia Pollution Discharge Elimination System (VPDES) permit from the VDEQ. Construction permit from VDH. |
Step #2 Communities will need to hire the services of a qualified consulting engineer to help them with the various stages of planning and implementing the project. The choice of an engineer is important because a considerable amount of money and resources will be committed on his/her recommendations.
Step #3 Early on in the process, well before an actual system is decided upon, the community must determine what type of management institution needs to be formed and what type of payment system will be instituted. Different systems will have different long-term maintenance requirements and costs, and the public should be consulted to see what arrangements are most acceptable to them.
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"Alternative Wastewater Collection Systems," United States Environmental Protection Agency, Washington D.C., October 1991.
"Information on the Use of Alternative Wastewater Treatment Systems," United States General Accounting Office, Washington D.C., December 1994.
"It's Your Choice, A Guidebook for Local Officials on Small Community Wastewater Management Options," United States Environmental Protection Agency, Washington D.C., September 1987.
"Wastewater Engineering Design for Unsewered Areas," Laak, Rein, Lancaster, Pennsylvania, 1986.
"Onsite Wastewater Disposal," National Environmental Health Association, Perkins, Richard J., Chelsea, Michigan, 1989.
"Low-Maintenance, Mechanically Simple Wastewater Treatment Systems," Rich, Linvil G., New York, 1980.
The authors are grateful for the assistance provided by Ray Reneau, professor, CSES, and Eldridge Collins, professor, BSE, Virginia Tech, in the preparation of this publication.
Funding for this project was provided, in part, by Extension Service, USDA, under grant number 91-EWQI-1-9034, "Residential Watershed Management," and by the Virginia Department of Conservation and Recreation, under grant number 94-0612-10, "Residential Water Quality Management."
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