Decentralized Wastewater Systems

Decentralized wastewater service refers to the use of small and/or large-scale onsite or clustered systems which may be managed either by individual property owners, a public utility, or some other designated entity.  Decentralized systems may consist of combinations of individual onsite systems and clustered commercial and/or residential systems.  Clustering facilities onto the same final treatment and final effluent dispersal system can in many cases result in significant cost savings due to favorable economies of scale.  It also can result in more efficient and lower cost operation and maintenance needs over time.

Roughly one-fourth of the overall U.S. population relies on decentralized, or “onsite” systems for their wastewater service, according to the U.S. Census Bureau. In some U.S. states and territories, that percentage is much higher. Maintaining good quality and cost-effective wastewater service is essential for both centralized and decentralized systems. However, lack of familiarity and attention to important details often contribute to key considerations being ignored for onsite systems. Most families and business owners relying on decentralized systems have one time or another likely experienced the costs and frustrations of unreliable or failing systems.

In areas where current centralized wastewater service is not available, or where costs of extending service may be high due to adverse slopes, shallow rock or groundwater, or where there are long distances to connect to existing centralized collection and treatment systems, clustered or individual onsite systems may well be the most cost-effective options.  In cases where it may be most cost effective to connect to a centralized system for final treatment and disposition of the effluent, it may be most cost effective to use an “alternative” collection system to accomplish this.  One such option is an effluent collection system, which can be used either for clustered systems, or to connect to a centralized wastewater system.  The effluent may be collected in either small diameter gravity lines, or a pressurized line, or combinations of pressure and gravity connections.  An example of a project which uses a combination of gravity and pressure connections can be found under project examples on this website for the Camp Coca-Cola Youth Camp.

In an effluent collection system, homes and/or businesses are served by septic tanks to provide removal of most solids and oils/grease, and effluent from the septic tanks is collected in small diameter gravity or pressure lines in the street.  Effluent is transported either to a common location for final treatment and soil/land dispersal, or conveyed to a centralized wastewater collection and treatment system.  Substantial cost savings can result from eliminating the solids handling component from collection systems and lift stations, as compared with conventional raw wastewater gravity collection and pump stations.  Both initial capital cost savings can often be achieved as well as operation, maintenance and repair costs over time.  CES has designed numerous effluent lift stations that have functioned very reliably and without problems, and which seldom require any type of maintenance or trouble responses.  This does not tend to be true for most raw wastewater lift stations.

CES has worked with several developers on the planning of some large scale effluent collection systems with as many as 500 homes.  In some cases, plans have called for clustering homes onto a number of separate treatment systems located throughout the development, and in other cases the developer has planned to connect the effluent collection system to an existing centralized collection and treatment system.

Effluent collection systems are typically used for clustered decentralized wastewater systems.  Many of the same treatment and final effluent dispersal technologies used for individual onsite systems are also used for clustered systems.  CES assists clients with the evaluation of which options may be best-suited for their particular circumstances.

A good additional source of general descriptive information from the U.S. EPA about decentralized systems can be found at the following website:

The houses located on the hill in the above photo are an example of an area that could be very cost-effectively served by a clustered decentralized wastewater systems.  The hills in this area are very rocky, and lead down to areas where there can be high groundwater close to the lake.  The homes are located sufficiently close together to be able to cluster them onto a single effluent collection system, and locate a final treatment and soil dispersal somewhere near the homes.  In this way, there is less concern about having to either locate suitable soil on each site for final disposition of wastewater effluent, or treating to a higher level due to adverse site conditions with the accompanying higher maintenance costs for each home.  Economies of scale for a clustered system in this type of setting make it cost-effective to provide the higher level of treatment needed in these challenging site conditions, while limiting most of the routine operation and maintenance needs to a single system serving all of the homes.  At each residence, inspections every one to three years and periodic septic tank pumping would be needed.

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