This bulletin presents an overview of home (onsite*) sewage treatment involving both conventional and alternative systems—not detailed analyses of the various systems. Your county health department has detailed bulletins on each system.

The goal in system design is to provide a long-term solution for onsite sewage treatment and disposal. In many soils, a properly designed, constructed, and maintained onsite sewage system will last for years. Proven guidelines, however, must be followed in the design and installation of a sewage treatment system. A sewage treatment system rarely fails: it is either improperly designed, constructed, maintained, or is overloaded.

Use of the septic tank and standard soil drainfield for onsite disposal of home sewage wastes is both efficient and economical if the soil and site possess the right characteristics. However, many soils and sites are unsuitable or have moderate restrictions for onsite sewage disposal. Septic systems in such soils do not adequately treat wastes. Systems can seldom be made to operate properly in unsuitable soils. Fortunately, there are alternative systems that will overcome some of the limitations of unsuitable soils and unsuitable sites. Unpleasant odors, pollution of surface and/or groundwater, and danger to public health are costly consequences of malfunctioning systems.

Goals in sewage treatment are to:

1. Remove solids,

2. Break down organic compounds,

3. Kill disease-causing microorganisms, and

4. Remove harmful chemical substances and disagreeable colors and odors.

Sewage from individual homes is a complex mixture of all manner of things that go down drains and toilets. The composition varies from day to day, from house to house, and from hour to hour. Average domestic sewage consists of about 99.9% water (by weight) and 0.02 to 0.03% suspended solids and other soluble organic and inorganic substances. Sewage also contains bacteria, viruses, and other microorganisms from the digestive tract, respiratory tract, and skin which make their way to toilets and drains.

In a single family house, the laundry and kitchen each account for about 10% of the wastewater volume, while the bathtub, shower, and handwash basins account for about 40%, and the toilets account for the remaining 40%. The organic chemical content of domestic sewage comes primarily from human wastes, soaps, and food wastes (especially in homes where a garbage grinder is used).

Aerated tanks can achieve an effluent of better quality than septic tanks when functioning well, but certainly not a fully degraded effluent. As with a septic tank, further treatment must be achieved by subsurface discharge in soils. Unfortunately, aerated tanks are frequently erratic and discharge excess solids which can clog the drainfield soil.

The effluents from septic tanks and aerated tanks require additional breaking down before they can be regarded as fully treated and safe for other uses. Furthermore, a residual sludge builds up in all of the processes, and this sludge must be removed from the system periodically. Disposal of sludge often poses difficult environmental problems. Fortunately, septic tank pumping companies are available to provide this disposal service.

Soils vary so much from place to place that it is impossible to give specific guidelines on the use of soils as absorption fields that will fit all sites. Information on soils and their suitability for onsite disposal at specific sites can be obtained from soil scientists with the Soil Conservation Service and Cooperative Extension, and from published soil surveys available from these agencies. County environmental health specialists should also be consulted, because they ultimately will approve or disapprove the permit for the sewage treatment facility.

In general, how well a septic tank sewage disposal system performs depends upon the rate at which effluent moves into the soil (infiltration) and through the soil (permeability). Soil and other site properties that affect performance are: permeability, texture, structure, depth, groundwater level, underlying materials, and slope. Distance from streams and lakes is also an important characteristic. Failure to properly evaluate any one of the site properties can lead to system malfunction, pollution of surface and/or groundwater, and potential risk to human health.

Inspections by county environmental health specialists have shown that drainfields often fail to work properly because (1) septic tank effluent has clogged the soil pores and reduced the absorption capacity, or (2) the soil permeability is very slow. Slow soil permeability occurs because the soils are either poorly drained or have a perched watertable, or because the soil became compacted during drainfield construction.

Poorly drained soils are saturated with water during wet weather and contain no space for septic tank effluent. Drainfields in such soils may function well in dry weather and fail to function in wet weather. If a soil has very slow permeability, the effluent may rise to the surface in dry weather, and in wet weather the situation is even worse.

Site conditions frequently rule out the use of the conventional drainfield. The following alternative systems may overcome site limitations and be acceptable to the county health department.

Fill Systems
Where shallow soils are underlain by hardpan, creviced or channeled rock, or there is a high groundwater table, aboveground fills may be used. The design of a successful fill requires a firm understanding of the principles of movement of liquids through soils because a fill must be custom-designed to the particular soil conditions and topography. Fills are manmade filter fields and are constructed from soil, sand, and gravel which are trucked to the disposal site. Effluent is pumped up to trenches or to an absorption bed located in the interior of the fill. If the terrain slopes properly, it is possible to build the fill downslope of the sewage treatment tank and thereby avoid the need for a pump.

Fill systems are normally trapezoidal in cross-section with the wider dimension at the base. The absorption trenches or bed are no wider than the top of the trapezoid, and the tapered sides serve to divert precipitation from the fill and to help contain effluent within the fill area. The top of the fill is arched slightly upward to aid in runoff diversion. The top and sides of the fill are normally planted in grass to increase evapotranspiration from the fill and to reduce erosion of the fill by wind and rain.

Effluent entering the fill receives treatment in the fill soil just as it does in native soil. Some of the moisture may be removed by evapotranspiration, and the remaining liquids are expected to infiltrate the native soil beneath the fill without surfacing. Infiltration into the native soil is enhanced by rough plowing to mix surface vegetation with native soil and by the trapezoidal geometry which provides a much greater ground contact area than the bottom area of the trenches alone.

Evapotranspiration Systems
An evapotranspiration system receives effluent from the septic tank and disposes of it by a combination of evaporation from the soil surface and transpiration from plants. There are two types of evapotranspiration systems—total evapotranspiration and open evapotranspiration. The total evapotranspiration system is a large bed located beneath the soil surface that is lined with an impermeable membrane to prevent leakage. The open evapotranspiration system is unlined and thus allows some of the effluent to be disposed of by deep percolation to groundwater.

Evapotranspiration systems are limited to areas where evapotranspiration exceeds precipitation plus the volume of the septic tank effluent. Generally, evaporation must be twice the precipitation for an evapotranspiration system to be successful. In Washington, the total evapotranspiration system requires an unreasonably large surface area which severely limits its applicability. In dry regions with great evapotranspiration the open evapotranspiration system may have limited applicability.

An evapotranspiration system may be considered for use where there is a high water table or where soils are impermeable or excessively permeable: that is, where standard drainflelds are unacceptable.

Incineration Toilets
Incineration toilets reduce non-water-carried human sanitary wastes to ash and evaporate the liquid. Wastes are deposited directly into the combustion chamber for incineration. Incineration is fueled by gas, fuel oil, or electricity. Incineration toilets are not intended to handle the total wastewater generated in a home. They are usually designed for fecal and urinary wastes plus combustible sanitary supplies. All other wastewater must be collected, treated, and disposed of in an acceptable manner.

The units have their greatest applicability where sufficient water is not available, such as in a mountain cabin, or to replace privies and chemical toilets. Incineration toilets can, however, be used anywhere a conventional toilet can be used, provided a means of handling the remaining wastewater is provided.

Composting Toilets
A composting toilet is designed to store and compost by bacterial digestion the human urine and feces which are not water-carried and small amounts of kitchen wastes. The other wastes which are watercarried must be collected, treated, and disposed of in an acceptable manner. The units have their greatest applicability where a water supply is not available, such as in a mountain cabin, and to replace privies and chemical toilets. The compostlng toilet, however, can be used anywhere a conventional toilet can be used, provided a means of handling the remaining wastewater is provided.

Graywater Systems
Human sanitary wastes are termed "blackwater" whereas "graywater" is all other wastewater, such as sink and laundry wastes. The recommended guidelines where nondischarging blackwater treatments are used (incineration toilets, composting toilets) are to reduce the graywater septic tank volume 50% and to reduce the size of the graywater disposal system 40%. The graywater system can be any system that is acceptable for the final treatment and disposal of septic tank effluent.

Sand Filters
The sand filter is a biological and physical wastewater treatment unit consisting of an underdrained bed of sand to which pretreated effluent is applied intermittently. Treated effluent from the sand filter is disposed of in an approved drainfield. The subsurface sand filter is located beneath the soil surface and is covered with a cap of topsoil. Pretreated wastewater is distributed in gravel trenches within the sand bed.

The recirculating sand filter has an exposed sand bed and a means of recirculating the collected effluent through the bed before final disposal. The sand filter can be used on irregular topography and its location is not dependent upon most site conditions. Sand filters provide a high-quality effluent and are useful where such quality is necessary, such as on sites overlying shallow aquifers. The sand filter requires less imported soil than a fill system and may require less land area. However, it does require more maintenance than a fill system.

1. "The Care and Feeding of Septic Tanks," EB 0707, is available from your Cooperative Extension office or county health department.

2. "A Septic Tank System for Your Home" is available from your county health department.

3. A series of technical guidelines for a wide variety of systems, including those discussed herein, is available from your county health department.

4. County soil surveys are available through local offices of the Soil Conservation Service and Cooperative Extension.