Deconstructing Onsite Wastewater Treatment
Examining some of the technical options and management opportunities
Having a properly functioning wastewater treatment facility is vital to protect public health and environmental quality in developing areas where humans congregate.
Historically, there have been two options available to meet the wastewater management needs of communities and individuals: the community collection and treatment system, commonly called a publicly owned treatment works, or the individual onsite wastewater treatment system, commonly called a septic system.
Throughout the country, onsite wastewater management systems commonly are used in rural and urban fringe areas. Presently, many state laws allow a variety of onsite wastewater management options and alternatives. Prior to determining which of the options to utilize on any parcel of land, the local environmental health specialist accomplishes a comprehensive site and soil analysis to determine the treatment potential of the property. This analysis of the land is essential to assure that the system selected will protect public health, environmental quality, the homeowner investment in the property, local tax base and the community’s investment.
Site and soil investigation
The site evaluation examines the area available on site for wastewater management, the slope and topography of the site and the landscape position occupied by the property. This assessment is essential to assure that the property is sufficiently large to host the wastewater system and to ensure that when installed, the onsite wastewater system is buffered adequately from wells, surface waters and the adjoining property.
The soil evaluation is required to determine the soil properties deemed critical for a properly functioning soil absorption system. The properties evaluated include depth to limiting layers or horizons (such as rock or shallow groundwater) in the soil, soil texture and structure, mineralogy and consistence, the estimated permeability of soil on any receiver site and whether the native soil is adequate to provide the necessary treatment of wastewater applied.
Each of these factors is critical in the design process. The soil depth is critical because state laws and rules generally require a minimum separation distance between the zone of septic waste application and any restriction such as rock or seasonal saturation. In North Carolina, the laws and rules allow a separation distance of 12 in. to rock or seasonal saturation for heavy textured soils is allowed and this distance is increased to 18 in. for coarse sand. Each state has specific laws and rules addressing separation distance and these local rules must be consulted. In several states including North Carolina, wastewater which has been treated to secondary levels can be applied where the separation distance may be as little as 6 in.
The selection of the wastewater management option or alternative is dependent on maintaining the appropriate separation distance between the zone of waste application and any restriction that will reduce treatment capacity of a site. Each state and many local jurisdictions have similar requirements, however many states and local jurisdictions have requirements that are significantly more stringent. Local rules must be consulted prior to design and specification for any onsite wastewater treatment system.
Wastewater treatment options
Maintenance of these separation distances is important. Where soil is deep, a conventional or traditional gravity dosed soil absorption wastewater treatment system is often adequate. These traditional systems are typically placed in a 30–36 in. wide by 30–36 in. deep trench. The trench is typically filled with approximately 12–18 in. of gravel, expanded polystyrene, or a chamber type system, all of which serve to support a trench type system and allow gravity to facilitate the distribution of wastewater to the soil. Soil material is used to fill and close the trench. These traditional systems require a soil at least 42 in. deep to maintain adequate soil cover over a system and adequate separation distances to a restriction. In some jurisdictions around the country, the soil depth required to install a traditional, gravity dosed wastewater soil absorption system is as much as 6 ft.
Where the depth of the soil is restricted, one of the pressure-dosed options may be designated. The low-pressure pipe (LPP) system was developed in North Carolina in the late 1970s and has been utilized extensively where the soil depth measures between 24–30 in. A typical LPP system consists of a narrow (12–18 in. wide) shallow (12–18 in. deep) trench with approximately 6–8 in. of gravel fill over which the pressure distribution pipe is placed. Once the pipe is placed in the trench, soil material is used to cover the trench.
In some instances, soil depth may be a serious limitation. Here soil material can be imported onto a site to construct an elevated at-grade or mound wastewater treatment system. The at-grade or mound system utilizes the pressure distribution network. At-grade systems may be utilized where the separation distance between the natural soil surface and a restrictive layer is as little as 18 in. Mound systems can be placed in areas where the separation distance to shallow groundwater is as little as 12 in. Mound systems are not well suited for areas with very slowly permeable soils, and site geometry strongly influences potential for treatment. At-grade and mound systems are expensive because of the need to develop a pressure distribution system and the need to import a large volume of suitable fill. Ideally, these systems are placed with a long on-contour dimension and a minimum cross contour dimension.
Up-gradient drainage may be beneficial to facilitate the proper functioning of these systems.
Drip and spray irrigation systems can be used to treat wastewater from single family homes, commercial developments or communities. These sophisticated effluent treatment systems can de used where site limitations such as slowly permeable soil and shallow groundwater combine to render a site undevelopable with any other technology. Drip and spray irrigation equipment is used to place liquid either on the soil surface or just below the soil surface. These systems have been utilized where soil depth is as little as 12 in. and there is 12 in. of suitable soil material over a shallow watertable or other restrictive horizon. Drip systems can be installed in direct earth contact and no fill is required. Both drip and spray systems are expensive because of the pretreatment requirements and the electronic controls required to disperse effluent into the soil.
In areas where there are serious site or soil limitations, where the environment is particularly sensitive, or where there are sources of drinking water that may be impacted by onsite wastewater systems, some form of advanced treatment may be required before liquid is placed into the soil for final treatment and dispersal. In other instances, there may be no option available to repair an improperly operating onsite wastewater system than a mechanical treatment device. In either of these examples, aerobic treatment units or media filters may be employed to provide extensive pretreatment of the wastewater before it is placed in the receiver environment. In order for these systems to function properly for the life of the property, continuous, high level operation, maintenance and management is essential.
All onsite wastewater treatment and dispersal systems including the traditional onsite wastewater treatment systems will require some routine and recurring inspection, operation and maintenance and management. In order for a county to issue a development or improvement permit which specifies one of these mechanically intensive options, a public or private, certified management entity must be available. This can be accomplished either as contract or service agreement with a private management entity or through an agreement with a county management entity. Both public and private management entities are operating in North Carolina.
Recently, the EPA developed a comprehensive set of management guidelines which, although voluntary at this time, encourage local units of government to become much more involved in the management of onsite and decentralized wastewater management systems. These systems are a permanent part of the wastewater management infrastructure and they must be managed accordingly. The EPA has proposed five levels of management for onsite and community wastewater treatment systems. Communities are strongly encouraged to examine management needs associated with onsite wastewater programs. These range from management programs that encourage data collection and records of individual system location to renewable/revocable performance based operating permits.
The management levels range from simple programs that increase levels of awareness of system management needs to more complex management efforts where all onsite wastewater treatment systems in a community are owned and operated by a responsible, independent, certified third party service provider. The management levels distinguish between system ownership and system management with individual system ownership remaining with the property owner for the first four management levels and ownership reverting to the service agency at the highest management level.
Each of the management levels contain a series of management programs or elements that must be accomplished as a part of a comprehensive management effort. These management functions are common throughout all of the management levels. The program activities associated with these management functions or elements become more intense as program level increases. Determination of the appropriate management level remains the responsibility of the local community, but the EPA is developing a guide intended to provide communities with sufficient detail to assess management requirements based on the level of risk in a community and the level of risk associated with selection of a specific technology. In areas with significant environmental or health risk where sophisticated onsite treatment systems are employed, high levels of management are required.
Management programs exist throughout the country today. Implementation of the EPA recognized basic management programs would require little additional effort on the part of many local health agencies. Implementation of a performance-based program will require local agencies to implement tools to:
- Assess environmental risk (as accomplished through a site and soil analysis);
- Technology risk with the more sophisticated system describes as inherently more risky than a simple basic system; and
- The risk that the management entity including the management personnel and the organization itself
Management programs incorporating these elements of risk exist today. Management models have been implemented to assure the long term and sustainable management of individual onsite wastewater systems, cluster systems and community based decentralized systems. These management programs are effective in protecting public health and environmental quality, homeowner investment, and community investment and image.