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

Technology Options

Conventional onsite septic systems in Maine include a septic tank and a stone bed disposal field (or an equivalent soil absorption system). In some cases, conventional systems also include optional pre-treatment components such as grease traps (particularly for restaurant kitchen wastewater), proprietary pre-treatment technologies to deal with specific wastewater components or reduce the size of the disposal field in poor soil conditions, and secondary (post-septic tank) advanced treatment systems prior to discharging into a disposal field.

The Treatment System and Disposal Field

The traditional septic system for a single-family residence consists of a watertight 1,000-gallon septic tank, a distribution box (in most cases) and a soil absorption system or disposal field, also known as a leach field. Effluent filters at the septic tank outlet are installed in almost all modern-construction septic systems. These filters reduce the amount of solids that leave the septic tank, reducing the potential for premature clogging of the disposal field and possible hydraulic failure, also known as “ponding.” Maine’s Subsurface Wastewater Disposal Rules specify numerous proprietary and standard components that can be used to construct reliable onsite septic systems. For a list of all currently allowed wastewater treatment products in Maine, click here (Excel spreadsheet).

System design engineer Joan Brooks, PhD, a.k.a. "The Peat Lady" describes a Peat Filtering System for Wastewater Treatment, an alternative to the traditional septic system:

(4 min. 57 sec. Video)

In Part II of this interview, Brooks provides specifications of a Peat Filtering System, and the availability of a pre-fabricated Peat-Bed-in-a-box:

(6 min. 36 sec. Video)

See the conclusion of this three-part interview with Brooks.

Gravity flow systems

Gravity flow systems are designed so that the wastewater flows downward from the septic tank to a distribution box (or to serially connected chambers) for further distribution into the field. Although gravity flow systems are usually the simplest and least expensive option, pressurized distribution (using pumps) can yield significant benefits, including the ability to either distribute effluent evenly across the entire field, or make allowances for dosing and resting of portions of the field.

The first step when installing a new onsite septic system is to evaluate the property, to determine the best locations to place the disposal field. A Maine-licensed site evaluator will determine the wastewater design flows (for example, a 3-bedroom residence would have a design flow of 270 gallons per day) and determine the predominating wastewater components that must be treated, based on the proposed land use. The location, size and depth of each disposal field is determined by the land use, soils, and site conditions found on the property. Positioning on the landscape, slope, and setbacks are all considered in selecting candidate areas for soils evaluations. The site evaluator will then conduct soils testing on the most promising sites with either a hand auger or a backhoe. He or she will record the characteristics of the soil profile over the depths required to properly place a septic system in the topographic conditions of the site.

The soil conditions are then compared to the design classes described in Maine’s wastewater rules. The rules specify minimum subsurface conditions (depth to bedrock, seasonal high groundwater table, and impervious soil layers) for each design class. New onsite disposal systems are not allowed where the depth to seasonal high groundwater table or the restrictive layer is between zero and seven inches. Some towns in Maine have additional local rules for septic system placement. For example, the Town of Auburn requires a 36-inch vertical separation distance between the bottom of the disposal system and any limiting soil factor within the Lake Auburn watershed.

Pre-treatment systems

Pre-treatment systems can be added to conventional system designs to improve performance. These systems may be required for a “first time” (new) system with a variance request to install a system in marginal soil conditions, where the depth to seasonal high groundwater table or the impervious layer is between 7 and 15 inches. They also can be used in sensitive environmental areas to improve treatment performance, as pre-treatment systems can achieve water quality standards equivalent to those established for secondary and tertiary treatment processes in wastewater treatment plants. Pre-treatment technologies are often used to reduce specific nutrients in wastewater effluent that will be discharged in or near surface water, such as nitrogen and phosphorus. Pre-treatment systems for disinfection are now available for hospitals and similar land uses where the wastewater stream may be heavily loaded with human pathogens.

Pre-treatment systems are usually placed between the septic tank and the disposal field. Commonly used technologies include sand filters (intermittent and recirculating) and proprietary filters that use various biological processes and media. Other available proprietary devices include aerobic treatment units, trickling filters, fixed-film activated sludge units, peat biofilters, sequencing batch reactors, microwave systems to destroy pathogens, and “grey” (i.e., from all plumbing sources other than toilets, quaintly referenced in the rules as “water closets”) wastewater treatment and recovery systems. There are many pre-treatment technologies that are fully tested and approved for installation in Maine (Excel). Devices not currently approved in Maine may be proposed for conditional approval and operational testing, according to the procedures set forth in Maine’s wastewater rules. Many of the proprietary systems on the approved list include as-needed technical assistance and remote monitoring technologies as part of the purchase agreement, to ensure appropriate long-term maintenance by trained operators.

Disposal fields

Disposal fields can be constructed from many different materials, although most still use the traditional combination of stone beds, perforated pipe, and filter fabric or hay covered with topsoil. Many alternative materials are provided as components of proprietary systems, and some require specialized construction methods. Peat disposal beds are approved under certain conditions. Subsurface drip disposal is gaining in popularity in Maine, since it allows property owners to irrigate lawns and plantings while dispersing their treated wastewater. Pressurized disposal (using pumps) is required for conventional systems where the disposal field is higher than the elevation of the building, and it is an essential component in many proprietary pre-treatment systems. Pressurized disposal helps to ensure a more even distribution of the effluent throughout the disposal field (or pre-treatment process), as compared with gravity flow systems.

Variance requests

Variance requests are sometimes granted by state regulators to permit the installation of first-time systems on challenging sites. The system designer must explain the reason for the variance request, show that granting the variance will not lead to contamination of nearby water sources or other sensitive environmental features, and identify any additional measures (such as pre-treatment) that will be used to achieve equivalent or better effluent treatment standards if the variance is granted. A variance will not be granted by the State of Maine if it would contradict a more stringent local ordinance or shoreland zoning requirement. Soil conditions are an essential component in the variance approval criteria, and a minimum point value assessment system is established in Maine’s wastewater rules to quantify the review and approval process.

Holding tanks

Holding tanks, where wastewater is stored, pumped, and transported offsite for treatment and disposal, are still allowed for residences and businesses under certain limited conditions. They are most suitable for land uses where flows are very low or intermittent (such as small-scale seasonal facilities). They are sometimes considered as the last best alternative to replace a failed conventional septic system, if there are no other suitable locations on the site to place a new disposal field. If a holding tank is installed, there must be appropriate monitoring and management systems in place to ensure that the tank gets pumped before it is filled or annually, whichever comes first. Holding tanks are no longer allowed in Maine for first-time systems, unless the town has enacted a holding tank ordinance. Even with a local ordinance in place, holding tanks will never be an optimum solution for new development in growth areas. Cesspools and drywells are no longer allowed for either first-time or replacement septic system installations in Maine, although a few grandfathered systems are still in use in rural parts of the state.

Conditional approvals

Conditional approvals for new or experimental wastewater technologies require submission of an acceptable backup design that can be implemented if the conditionally approved system fails. Proposals are reviewed and conditionally approved on a case-by-case basis by state regulators, following the procedures outlined in Maine's wastewater rules. When granted, conditional approvals require additional measures to permit long-term analysis and evaluation of the new technology, including system performance criteria, remote monitoring if applicable, and periodic inspections of the conditionally approved elements to ensure that they are working as intended.

Shoreland zones

Shoreland zones require additional setbacks and special requirements for first-time disposal systems. Systems on sites with marginal or unsuitable soils are not allowed in these areas, and lined disposal fields are required in very permeable sands and gravels to prevent phosphorus from entering the nearby water body. Excessive phosphorus discharges can create the potential for widespread algae blooms across inland lakes and ponds during summer months.