Small Community Wastewater Collection Systems

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 448-405, July 1996


Table of Contents

Introduction
Small Diameter Gravity Sewers
Pressure Sewers
Vacuum Sewers
Conclusion

Introduction

An important consideration that is often neglected when small community wastewater systems are being designed is the type of sewer system that will be used to transport the wastewater to the treatment system. The collection system generally comprises 70 to 90% of the total construction costs for a new wastewater treatment system. Conventional gravity sewers were designed to serve high density urban/suburban areas. When used as part of a system that serves small or rural communities, conventional sewers often add significant and unnecessary costs. The U.S. Environmental Protection Agency recommends that small communities consider the use of an alternative wastewater collection system, or a combination of conventional and alternative systems.

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. They 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. 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

Small diameter gravity sewers use gravity to transport sewage, much like conventional sewers do. However, small diameter gravity sewers are always preceded by a septic tank. The settling that first occurs in the septic tank eliminates much of the solid matter from the wastewater. This enables the collection pipes to have a smaller diameter and a more gradual incline. The pipes used are made of light weight plastic and can be buried at a relatively shallow depth. Manholes are not required for small diameter gravity systems; instead, clean out ports are used to service collector pipes. When it is necessary for the flow to be directed upwards, effluent pumps can be utilized to move the wastewater to higher elevations. High water alarms are normally installed in the septic tanks to alert property owners of any potential problems with their part of the system.

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 to 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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Pressure Sewers

Instead of relying on gravity, pressure sewers utilize the force supplied by pumps, which deliver the wastewater to the system from each property. Since pressure sewers do not rely on gravity, the systems network of piping can be laid in very shallow trenches that follow the contour of the land.

There are two kinds of pressure sewer systems, based upon the type of pump used to provide the pressure. Systems that use a septic tank effluent pump combination are referred to as STEP pressure sewers. Like the small diameter gravity system, STEP pressure sewers utilize septic tanks to settle out the solids; this allows for the use of piping that is extremely narrow in diameter. The effluent pump delivers the wastewater to the sewer pipes and provides the necessary pressure to move it through the system.

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. 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 Sewers

Wastewater from one or more homes flows by gravity to a holding tank known as the valve pit. When the wastewater level reaches a certain level, sensors within the holding tank open a vacuum valve that allows the contents of the tank to be sucked into the network of collection piping. There are no manholes with a vacuum system; instead, access can be obtained at each valve pit. The vacuum or draw within the system is created at a vacuum station. Vacuum stations are small buildings that house a large storage tank and a system of vacuum pumps.

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 requires 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.

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Conclusion

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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