The useful life of soil absorption systems can be greatly prolonged by achieving uniform distribution of effluent throughout the soil absorption area. It was pointed out in the August issue of the "On the Wastewater Front" that uniform distribution is not achieved with conventional gravity flow of effluent from the septic tank to the soil absorption system.

One method of achieving uniformity is to use small diameter pipes perforated with small orifices, from 1/8" to 1/4" in diameter, and to pressurize this small pipe with a pump or a syphon. This article deals only with the use of pumps. Pressure distribution systems are designed to operate under low pressure of

1 to 3 psi. For individual homes, the pumps required are fractional horsepower, usually ½ horsepower or less. The pump is chosen by the designer so that it develops just enough flow to feed the desired number of orifices at the desired pressure.

When the system is operating, the entire pipe network fills with effluent before much flows out of any of the holes. As soon as the pipes are filled the system comes up to the operating pressure and all orifices flow at about the same rate for a set period of time or for a set volume of flow. Research has shown that the best treatment of the effluent in the soil is achieved when the dose volume delivered to the soil is as low as possible. Older systems were designed with the recommendation that the pump operate no more than 4 times-a-day to deliver flow to the soil absorption system. However, the goal of a pressure distribution system is to provide uniform, unsaturated flow in the soil and this is best done with many small doses rather than a few larger ones. The size of the system and the number of orifices will determine the required flow rate. Designers attempt to keep the volume of water delivered per dose to less than about 30 or 40 gallons per dose.

Dose volume can be controlled either by floats that turn the pump on when the water level in a pump chamber reaches a certain height and off at a certain height or by operating the pump on a timer so that the pump operates for a preset period of time. Timer operation is preferred because it not only controls the dose volume but it prevents frequent doses to the drain field when there is a large volume of wastewater discharged from the source over a short period of time.

The goal of pressure dosing is to create unsaturated flow in the soil absorption system. This means that the wastewater moves over the surface of soil particles or soil peds so that the effluent is continuously in contact with the soil air. Soil pores are never filled with water. This rapidly converts an anaerobic effluent from the septic tank into an aerobic condition and provides for aerobic microorganisms to treat the effluent as it moves through the soil profile.

Keeping the soil absorption system aerobic will reduce the development of a clogging mat or "biomat" on the bottom of the soil absorption system and thus prolong the life of the soil system.

The Michigan Department of Environmental Quality requires that all larger systems (greater than 2,000 gpd) be dosed and they highly recommend pressure-dosing anytime a system is dosed. Pressure dosing adds cost and enhances the need for maintenance but will greatly extend the life of an onsite wastewater treatment system.

Ted Loudon