Cluster System ( For Community Usage )
A cluster system is a wastewater collection and treatment system that serves two or more dwellings. Individual septic tanks or aerobic units will pretreat wastewater from several homes before it is transported through a low cost, small pipe sewer to a drain field.

Small satellite treatment plants or soil absorption systems that have low-cost collection sewers are called cluster systems. Cluster systems treat wastewater from a group of dwellings and/or businesses and are most appropriate in moderately populated areas. These systems serve two or more dwellings (but not usually an entire community) and are located near the buildings they serve. The wastewater from each dwelling or business flows into its own interceptor (septic) tank to settle out and allow solids to break down. From the tank, the effluent is able to travel through a smaller diameter, therefore less expensive, collection pipes. These pipes are buried at a shallower depth than large pipe sewers and run relatively short distances to smaller, less maintenance-intensive treatment and disposal units. These units often use soil absorption fields or effluent recycling rather than discharging the treated wastewater into surface waters.

Decentralized System ( For Community Usage )
A decentralized system is an onsite or cluster wastewater system that is used to treat and dispose of relatively large volumes of wastewater, generally originating from an individual, or groups of dwellings and businesses that are located relatively close together. Onsite or cluster systems are commonly used in combination.

Why Decentralize?
When town leaders face having to upgrade wastewater treatment, the first choice is usually to build a centralized collection and treatment facility. However, centralized collection and treatment may not be the right answer for every community's wastewater disposal needs. Small and rural communities often cannot afford these expensive facilities, and their populations may be too spread out to make centralized treatment a realistic option. Additionally, some existing onsite systems may function effectively, so they don't need to be replaced. In circumstances like these, decentralized wastewater treatment is often the best solution for wastewater management. Decentralized treatment involves using a combination of treatment technology options, both traditional and innovative, where they are most appropriate in a community. Conventional onsite systems, alternative onsite systems, cluster systems for groups of homes and businesses, and some use of centralized treatment can all be included when considering decentralized community wastewater management. The decentralized system is then managed (with varying degrees of control) to ensure each component functions properly.

The decentralized approach to wastewater treatment is seen as beneficial for a number of reasons. This approach:

• saves money by deciding on a preventive strategy (such as assessing a community's needs and conditions) to manage wastewater before a crisis occurs, thereby avoiding unnecessary cost;
• allows homeowners to continue to use their properly functioning septic systems;
• enables better watershed maintenance by eliminating the large transfers of water from one watershed to another that happens with centralized treatment;
• may be the most cost-effective treatment strategy for rural communities with sparse populations;
• is appropriate for varying site conditions including ecologically sensitive areas-treatment methods can be tailored to suit different site conditions.
• federal bond money is available, through the Clean Water Act, for communities.

Decentralized Systems Offer Flexibility
A decentralized system employs a combination of onsite and/or cluster systems and is used to treat and dispose of wastewater from dwellings and businesses close to the source. Decentralized wastewater systems allow for flexibility in wastewater management, and different parts of the system may be combined into "treatment trains," or a series of processes to meet treatment goals, overcome site conditions, and to address environmental protection requirements.

Managed decentralized wastewater systems are viable, long-term alternatives to centralized wastewater treatment facilities, particularly in small and rural communities where they are often most cost-effective. These systems already serve a quarter of the population nationwide and half the population in some states. They should be considered in any evaluation of wastewater management options for small and mid-sized communities.

How does a community decide which management approach is right for its wastewater treatment? Community leaders first need to ask some questions and then create a management plan. What circumstances are causing a reevaluation of present wastewater treatment? Are local septic systems failing? Is residential development stifled because of a lack of adequate wastewater treatment facilities? An organized plan will help managers clearly define the problems,review the possibilities, and assess the costs associated with each potential solution. Many options now exist for wastewater treatment and disposal in rural areas and small communities. Each technology has advantages, as well as limitations, so a treatment technology must be selected specifically to meet local conditions and treatment objectives. Similarly, every community's own financial, physical, and regulatory factors must be evaluated to find the best technology for their circumstances. Onsite systems now include a number of alternatives that surpass conventional septic tank and drain field systems in their ability to treat wastewater.Alternative onsite processes, such as sand filters, peat filters, aerobic treatment units, pressure distribution systems, drip irrigation, and disinfection systems, can be employed in a wide range of soil and site conditions. Alternative systems require more monitoring and maintenance, making a strong case for these systems to be managed.

Systems Must be Managed
Management is the key to keeping decentralized treatment systems functioning properly and can encompass planning, design, installation, operation, maintenance, and monitoring onsite and cluster systems. Regular inspection and maintenance form the basis of any management program. Using one management strategy over another may depend upon local environmental sensitivities, the complexity of treatment technology and equipment, and the local regulatory agency's authority and resources. More than one management model might be effective under particular circumstances, but any model should give the regulatory agency enough authority to make sure failing systems are repaired or replaced.

The National Onsite Wastewater Recycling Association (NOWRA) suggests these seven elements be included in any management model:

• system performance requirements that protect human health and the environment;
• system management to fulfill specific and measurable performance requirements;
• compliance monitoring and enforcement to ensure adequate system performance;
• guidelines for all aspects of design, construction, and operation;
• education for all service providers, regulators, planners, and owners;
• certification/licensing for service providers and regulators; and
• program reviews to resolve shortcomings and to correct problems.

The U.S. Environmental Protection Agency (EPA) is in the process of developing a number of management models for decentralized wastewater systems. The EPA's models start from a hands-off level of involvement where a regulatory agency inventories systems, but property owners are responsible for their own systems' maintenance. Four additional models have been proposed with increasing levels of oversight. The far end of the scale suggests a public or private utility own and manage all aspects of the decentralized system, including both onsite and cluster systems. This model is most appropriate where a complex network of advanced onsite systems and cluster systems is in place, and where the environment may be especially sensitive.

Secondary Treatment Options
In decentralized systems sand filters may be utilized. The primary purpose of improving the quality of the effluent from a septic tank system is to provide a cleaner effluent. Due to new technologies alternate filtration medias can be used improving effluent quality. The alternate media work with the same principals as the original sand filters.

Alternative media filters provide an option beyond a conventional septic tank and drainfield, which consists of several trenches with gravel beds and perforated plastic pipes. These filters may allow a higher soil loading rate, use less space, and use material that is easy to obtain. Alternative media filters are suitable for lots with sizing constraints or where water tables or bedrock limit the depth of the drainfield. Some states may offer a sizing reduction allowance for alternate media filters because of their high loading.

Alternative Medias:

Peat: Peat is a permeable, absorbent medium used for onsite wastewater treatment. Because peat is a natural material, significant variations in composition have been noted. Several manufacturers enclose the peat in fiberglass housings.

Foam: The foam cube filter is similar in performance to an intermittent sand filter, but has been tested at 10 times the loading rate. The filter is housed in a 6 ft x 6 ft container, with four feet of foam media. Wastewater is sprayed on top of the media and withdrawn from the base of the unit. Alternatively, filter cubes installed in pre-assembled cylinders can be placed in a tank.

Textile: This media consists of textile chips know as " coupons". The medium is placed in a filter housing similar to a sand filter, with wastewater applied by spraying it at the top of the filter.

Operation and Maintenance:
Alternate media filters require more initial operational control an maintnenace. Primary Operation and maintenance tasks include filter surface maintenance, dosing equipment servicing, and influent and effluent monitoring. With continued use, filter surfaces become clogged with organic biomass and solids. Once operating, infiltration rates may fall below the hydraulic loading rate and permanent ponding of the filter surface may occur. If this happens, the filter should be taken off-line for rest or media removal and replacment. Buried filters are designed to operate without maintenace for their design life. Filters exposed to sunlight may develop algae mats controlled by surface shading. For community systems, disinfection is required prior to discharge, but disinfectant quantity requirements are low due to the high quality of the effluent.

Other Topics
Aeration Systems - Do's & Don'ts | Engineering and Design for Difficult Sites | Health Departments - Friends or Foe? | Potential Home Owners Reference | Potential Property Owners Reference | Why Systems Fail | Sewage Protection Program

Barrow Company specializes in the engineering and design of all types of systems, including Decentralized and Cluster. If you would like additional information on these or any other system, please use the Contact Us section on this website, e-mail us at info@barrowcompany.com ,or contact us by telephone or fax. Office hours are 7:30 a.m. to 4:30 p.m. Monday thru Friday.

Source of information in part: National Small Flows Clearinghouse, West Virginia University (800) 624-8301