Strategies for Rural Development in Areas with Limited Public Infrastructure: Alternative Septic Systems

Clustered Collection, Treatment & Disposal

Clustered systems (also known as community or shared systems) are septic systems that serve more than one property. They may require an individual septic tank and pumping equipment onsite, but the effluent is generally piped offsite for combined treatment and disposal.  Individual septic systems for properties containing multi-family housing or small businesses, with design flows of less than 2,000 gallons per day, can be installed using the same design criteria as a system for a single-family residence. The advantages of clustered systems include cost, flexibility in land use, centralized maintenance, and increased environmental protection when the system is thoughtfully located and designed to minimize ecological impacts.

Small multi-user disposal systems

Small multi-user disposal systems with design flows of less than 2000 gallons per day may be designed by site evaluators using the criteria outlined in the Maine wastewater rules, as long as the soil conditions and topography allow for placement of a suitably-sized common disposal field on one of the lots.  However, if three or more sites share a common disposal field, all system components  on all lots must be owned and managed by a single, independent, legally established entity, such as a homeowners’ association, sanitary district, or town government.   This rule is intended to ensure appropriate long-term maintenance and monitoring for the system, so that one property owner’s failure to perform routine septic tank pumping (for example) will not place the entire shared disposal field in jeopardy.   

Large clustered systems

Large clustered systems with design flows exceeding 2000 gallons per day must be designed and stamped by a Maine-licensed Professional Engineer.  Engineered systems are also required (regardless of flow rate) if the wastewater effluent will contain Biological Oxygen Demand (BOD) and Total Suspended Solids (TSS) in combined amounts greater than 2000 milligrams per liter, since these waste components typically require pre-treatment and a more robust disposal disposal field design than would normally be indicated by the onsite soils and topographic conditions.  Engineered systems may also require higher levels of treatment, additional soils evaluations, site setbacks, hydrogeological analyses, the formal identification of responsible management and ownership entities, and defined operation and maintenance duties including any reporting and monitoring requirements.

The State of Maine’s Minimum Lot Size Law requires a minimum size of 20,000 square feet for residential lots with onsite septic systems. This minimum lot size will need to be increased by an adjustment factor for land uses characterized by heavy organic loading in their wastewater, such as restaurants.  A small restaurant with only 10 seats may need almost twice as large a lot as a single-family residence.

Variances

Variances are often granted for new systems on smaller lots, particularly if the soils and site conditions are favorable (or if pre-treatment systems are used to reduce the size of the required disposal field).   The latest modifications to the Minimum Lot Size Law clarified and standardized the request, review, and approval process for property owners seeking a variance, and authorized local plumbing inspectors to approve lot size variances if specific criteria could be met.  However, the law specifically precludes the use of lots smaller than 20,000 square feet for clustered systems with design flows in excess of 2000 gallons per day, and for individual septic systems of any design flow that are intended to treat wastewater with a high organic content.

A 20,000 square foot lot will generally accommodate both an onsite water supply and a septic system with all of the required minimum setbacks.  Some lots may require slightly more acreage to accommodate onsite water and wastewater facilities, depending on site conditions such as slope, bedrock outcrops, availability of suitable soils, and the type of water supply and septic system that are being used.  A drilled well has the smallest minimum setback distance if an onsite water supply is needed.   Connection to a public water supply will reduce the lot size requirement, since the drinking water source does not require onsite protection.

The smallest land area allowed for an individual onsite septic system would be for a traditional septic tank and disposal field on sandy, well-drained soils (approximately 600 square feet ) plus all required minimum offsets.  As soil permeability decreases, the disposal field size increases to as much as 1200 square feet,  plus fill extension areas in some cases, with sufficient additional land surrounding the field to accommodate all minimum offsets.  However, pre-treatment systems can reduce the size of a disposal field in low-permeability soils by up to 50 percent, and can potentially eliminate the need for fill extensions.

Clustered offsite septic systems 

Clustered offsite septic systems can greatly reduce lot size requirements, in many cases to just slightly larger than a lot connected to a municipal sewer system.  Land use planning for an offsite clustered system should incorporate easy access to all lots for construction and maintenance of individual tanks and lines, adequate space to install septic tanks (and pre-treatment systems, if used) on each individual lot with all required minimum offsets, and sufficient acreage in suitable site conditions to place both a primary disposal field and a reserve area.  

To estimate the amount of land that may be required to construct these systems under different soil conditions, consider the hypothetical  case of two clustered systems, one with a design wastewater flow of 4,000 gallons per day (up to 14 single-family residences) and the other with a design flow of 10,000 gallons per day (up to 37 residences).   The smaller system will require 0.6 to 1 acre for the common disposal field, while the larger system will occupy 1.2 to 2 acres, all of which can be attractively integrated into the project as open space.  It is clear that clustered systems can help developers achieve compact, village-style developments.

 

Related Work Plan Components

Workgroup Contacts

In Aroostook County: Jay Kamm, Ken Murchison, Joella Theriault

In Washington County: Judy East