A cluster septic system is defined as a wastewater treatment system designed to serve two or more sewage-generating dwellings or facilities with multiple owners. They can be privately or publicly owned and managed. Cluster systems are commonly utilized to solve an existing problem in a community or in a new development to preserve open space, wildlife habit and to simplify management.  

With cluster systems, sewage collection and treatment for a group of homes and businesses occurs at a single facility with community-level collection, treatment and dispersal, as opposed to each dwelling where treatment and dispersal is on each lot. Many options are available to treat wastewater from a cluster of homes and businesses through the design of a septic system. The options are scalable to collect, pretreat and disperse the effluent back into the watershed.

The advantages of cluster systems include:

• One location for most management functions for service providers and tracking of performance through regulatory programs.
• More consistent flow rates can improve performance particularly with advanced technology.
• The ability to solve challenges associated with small lots, areas of problematic soil conditions or sensitive environments.
• Can be scaled up in new development as community grows.
• Can be designed to be connected to larger treatment system in the future.
• Typically, more cost effective than regionalization or building of a new wastewater treatment plant.

Cluster solutions are not for every project as there are challenges as well:

• Typically requires an engineer to design the system.
• Land must be acquired or set aside for the initial and replacement system.
• Permitting is often done at the state level and as flows increase the design standards often increase as well adding additional requirements. 
• As system size increases, operation and maintenance costs can increase as well — particularly with advanced treatment. 
• Forming of management structure may take additional time and resources.

Solving Problems

There are many situations where an existing property does not have the room to locate a new septic system. For many of these parcels, the only solution on their property is a holding tank, which is undesirable to many property owners due to associated maintenance activities and costs. A cluster septic system can be utilized if nearby land can be found to handle wastewater generated by multiple homes. 

An example of this is near my home in Afton, Minnesota. This community has 77 residential homes and 25 businesses with a combined peak flow of over 50,000 gpd. Afton is a river town where flooding of septic systems has commonly occurred since the community was settled. A community treatment site was located and a treatment system was installed. It included community septic tanks, flow equalization, secondary treatment with nitrogen reduction (recirculating gravel filter followed by an anoxic denitrification unit) and tertiary treatment including disinfection and a polishing ATU followed by final soil treatment. The solution was not simple but was required due to the scale and to assure protection of ground and surface waters.

New Development

Typical zoning practices establish minimum lot sizes, setbacks and widths that developers must follow when they design subdivisions. This leads to developments that maximize the number of lots based on the total acreage of a parcel. For instance, if the code requires a minimum lot size of 2.5 acres and the developer has a 40-acre parcel, the site will be developed with 16 residential units unless there are major site limitations.

Cluster development protects open space by establishing the number of units allowed for a parcel completely independent of any minimum lot size. The open space can then be used by residents, to preserve agricultural land, or to protect wildlife habitat. Clustered septic systems for new developments can be located in the best location to maximize treatment and therefore reduce environmental impact.

Variations in Clusters

There are many variations in the design and installation of cluster systems. One of the biggest considerations with cluster systems is how the wastewater will be collected and where primary treatment will occur.   

1. Gravity – If topography allows wastewater may be able to reach the treatment site by gravity but often lift stations will be needed. Traditional gravity sewer is prone to infiltration due to the need for manholes and lift stations and often can get expensive due to the depth of installation. Gravity collection can be used to transport raw wastewater or wastewater that has undergone primary treatment with a septic tank at each structure generating sewage. The diameter of gravity collection tends to be larger than pressure but is dependent upon the flow from the community. 

2. Pressure – With pressure sewer collection, individual dwellings and businesses discharge into a small diameter collection pipe under pressure which discharges into a common septic tank(s) or a stilling tank at the treatment site. If each structure has a septic tank this can be done with an effluent pump but if it is done with raw sewage, a grinder or ejector pump is needed. Pressure sewers are less likely to have infiltration and tend to be installed shallower than gravity collection.

Once at the treatment site, many options are available to treat wastewater from a cluster of homes. Most technology utilized on individual properties can be scaled up but must be done so carefully to assure the system is not either over- or underdesigned.  

Management

Community owned and/or managed wastewater treatment systems are infrastructure, as are roads, power lines and other components of a community. An organized community structure should be responsible for operating, monitoring, maintaining and paying for these infrastructures. Typically, communities requesting public funding must have a legal entity in place that can levy funds and own or hold permanent access easement rights before they are eligible for funding. 

In most rural areas, the manager for an individual system is the homeowner. For cluster systems and some individual systems in designated areas, a responsible management entity with legal authority and administrative capabilities is needed to provide the necessary services and be accountable. These RME form the “community structure” for the systems. Homeowner associations are frequently created when parcels of land are subdivided or parceled out and can potentially serve as an RME but may lack long-term accounting and management abilities and skills and can have difficulty with fee collection.