Trends in the Industry

Dr Bob Rubin
Dept Of Bio & Ag Engineering
N C State Univ
Box 7625
Raleigh NC 27695

Joyce Hudson, P.E.
USEPA - OWM

Throughout the country onsite wastewater management systems are commonly used in rural and urban fringe areas. Presently many state laws (see North Carolina Laws and Rules for Onsite Sewage Disposal) 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.

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 for Onsite Wastewater Treatment Systems 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 inches to rock or seasonal saturation for heavy textured soils and this distance is increased to 18 inches in 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 inches.

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. These are summarized based on North Carolina Laws and Rules in the table below. 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.

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 1970's and has been utilized extensively where the soil depth measures between 24 inches and 30 inches. A typical LPP system consists of a narrow (12 inch to 18 inch wide) shallow (12 inch to 18 inch deep) trench with approximately 6 inches to 8 inches 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 inches. Mound systems can be placed in areas where the separation distance to shallow groundwater is as little as 12 inches. 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 inches and there is12 inches of suitable soil material over a shallow water table 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. The USEPA recognizes these management requirements and is developing a comprehensive guide for management of onsite wastewater treatment systems.

Soil based onsite wastewater treatment systems will function most effectively when they are placed with long on contour dimensions and short cross contour dimensions. This design factor is called system geometry and it does influence system performance. Liquid applied to the soil must move through the soil medium in accordance with Darcy's Law. Optimum solutions to the Darcy's Law requirements are achieved with long contour systems and narrow cross contour systems. In some instances, the installation of up-gradient drainage will facilitate the long term performance of a land based onsite wastewater treatment system. Drainage installed to facilitate system performance must drain to the environment. The need for drainage must be included as an element of the site and soil evaluation and the interpretation of critical site and soil information.

All wastewater treatment systems including the traditional, gravity based onsite wastewater treatment systems and the most sophisticated on the Municipal AWT facilities will require 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 USEPA 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 USEPA has proposed 5 management levels for onsite and community wastewater treatment systems. Local units of government, private entities, and communities are strongly encouraged to examine management needs associated with onsite wastewater programs.

Please address any questions to Dr. David Lindbo.

This page created by
Roland O. Coburn, Research Technician I on 1/8/03.