Small Community Wastewater Treatment and Disposal Options

Author: The Water Quality Program Committee, Virginia Tech*

*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."

Publication Number 448-404, July 1996


Table of Contents

Introduction
Lagoons
Sequencing Batch Reactors
Oxidation Ditches
Trickling Filter
Conclusion

Introduction

Pollution from poorly functioning septic systems is a major cause of water quality problems for communities across Virginia. If a community recognizes the need for improved wastewater management, there are several options they can consider. Traditionally, the answer has been to rely on large projects for the entire jurisdiction, yet this is not always the most cost-effective solution. Within a county or city, there is likely to be a wide variety of wastewater treatment needs and the use of smaller community systems is a way to accommodate these differences. The number of properties that will be served by a system will be a crucial factor in its design. Small, centralized community systems can serve between 50 and several thousand households. Cluster systems is the term used for those which serve communities of less than 50 properties. These systems are often small enough that they can make use of systems designed for individual households. The systems mentioned in VCE Publication 448-403, Alternative On-site Wastewater Treatment and Disposal Options , can be used as part of a cluster system, depending on site requirements. Some of those systems, such as irrigation and constructed wetlands, can be adapted for larger communities. Several systems that can be designed for both cluster and small community systems are listed below.

Return to Table of Contents

Lagoons

Lagoons are essentially artificial ponds, called cells, that are built into or above the ground. The number and size of these cells depend on the needs of the community. Systems that use several smaller cells have proved to be more effective than one large cell for treating wastewater. Cells are lined with clay or other impervious materials to prevent groundwater contamination. There are four basic types of systems: anaerobic, facultative, aerated lagoons, and stabilization ponds. They vary according to the amount of dissolved oxygen typically present in the water. Their purpose is to retain wastewater for an extended period of time. This acts to stabilize wastewater as heavier particles sink to the bottom and lighter ones rise to the surface. The retention period gives microorganisms time to feed on the nutrients and trace elements in the water.

Anaerobic lagoons have steep sides and can be as deep as 20 feet. They develop a thick crust on the surface which inhibits oxidization, but 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 cause 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 less odor, but may have problems functioning during 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.

It is recommended that lagoons be preceded by a holding tank so solids can settle out. Effluent from lagoons can be discharged to a drainfield, if water quality standards allow, or if necessary they can be followed by sand filters and disinfection before being directly discharged. These systems tend not to need a great deal of operation or repairs, so maintenance costs can be minimized. Yet construction of the lagoons can be costly and they do require more land than most other alternative systems.

Return to Table of Contents

Sequencing Batch Reactors

Sequencing batch reactors use an aeration process to treat wastewater. A specific volume of wastewater, called a batch, is first screened to remove larger particles within the water. It then may undergo primary treatment in a septic tank before delivery to the reactor. The reactor is a tank into which air is pumped to ensure that a sufficient supply of oxygen is present for aerobic biochemical processes to occur. The addition of oxygen allows microorganisms to consume dissolved organic matter in the wastewater that are not removed by a screening or settling process. After a specified period of aeration, the wastewater in the reactor is allowed to settle. The sludge that settles on the bottom now primarily consists of the microorganisms that have fed on the organics in the wastewater. Sequencing batch reactors utilize an activated sludge treatment process. After the treated effluent is discharged, all but a small portion of the sludge, which is rich in microorganisms, is removed from the reactor. This helps quickly reestablish a population of microorganisms within the next batch of wastewater delivered to the reactor, reducing the amount of time necessary for treating each batch. Usually more than one reactor is needed so that while one batch of wastewater is being treated, additional flow can be directed elsewhere. The number of reactors ultimately depends on the expected volume of wastewater flow and the amount of time allowed for treatment of each batch in the reactor. A longer retention period produces less sludge and cleaner effluent.

The main advantages of sequencing batch reactors is that they produce effluent low in organic compounds and thus can be used to 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 are required. Other advantages are that it can be located on a small area of land, and it is relatively easy to expand this system by adding additional reactors. However, the operation of this system is more complex than others. The system does tend to be more costly to construct and operate than most others, yet it usually has fewer maintenance problems over its lifetime.

Return to Table of Contents

Oxidation Ditches

An oxidation ditch is a large holding tank with a shape similar to that of a race track. The tank is shallow, averaging about 3 feet deep, and is constructed with an impermeable lining. This gives the wastewater plenty of exposure to the open air for the diffusion of oxygen, helping prevent anaerobic conditions from occurring. 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 resemble a large circular brush. The aerators slowly rotate to introduce 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 will allow for a greater amount of organic matter within the sewage to be broken down by the aerobic bacteria. After treatment the effluent 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 is removed from the bottom of the settling tank and a portion is returned to the ditch to facilitate microbial activity in the next batch of sewage.

Oxidation ditches can be built to accommodate the needs of several thousand people. They are suitable for communities with limited access to land. Initial construction costs are relatively expensive, yet the system's energy demands are moderate. The system requires a moderate amount of skill to operate and maintain, and it works well under most weather conditions.

Return to Table of Contents

Trickling Filter

Trickling filters are circular tanks containing a media of either rock or plastic. Trickling filters using rocks are shallow with wide diameters, and made with reinforced concrete built into the ground to support the weight of the rocks. Systems designed with plastic media do not need as much support and so can be built above ground. Because of this, they are referred to as tower filters. Tower filters are typically 20 to 30 feet high and have a smaller diameter than the filters with rock media. Most systems use a rotating distributor to spray effluent onto the surface layer of the media, but some use a fixed distributor. Rotating distributors spread the effluent more evenly than fixed distributors, but they require more energy to operate. The distribution of the effluent can either be intermittent or continuous depending on the system. When continuous, a portion of the wastewater is recirculated back to the distribution system.

Sewage first goes to a settling tank where much of the solid matter settles out of the wastewater. The wastewater is then pumped to the distributor, which sprays it onto the surface of the media. The media act as a substrate to which microorganisms attach themselves. 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. The microbe population eventually grows to form a layer of slime over the media. Portions of this slime 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 are good at removing nitrogen and organic matter from the wastewater. This makes them 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, and energy requirements will vary depending on the system. These systems are not well suited for very cold climates and can cause odor problems.

Return to Table of Contents

Conclusion

The disposal of the wastewater is an important consideration when choosing system designs. If land conditions or availability prohibit using it for effluent disposal, direct discharge to an open waterway will be necessary. Communities that will need to directly discharge treated effluent should consider those systems that produce cleaner effluent, thus meeting water quality standards set by Virginia's Department of Environmental Quality.

Return to Table of Contents

Visit Virginia Cooperative Extension