Conventional wastewater treatment systems for individual homes typically consist of a septic tank followed by a soil absorption system. The latter is usually either a series of trenches containing gravel and 4" pipes for distribution or a bed that is a wide excavation containing a 4" pipe network for distribution. The flow of wastewater through the septic tank and to the soil absorption system is by gravity so there is no pump or power requirement.

Since the flow is by gravity, the flow rate from the septic tank is very low (usually less than 1 gal/min). With 4" pipe in the distribution network and large holes approximately 30E off the bottom center of the pipe, the distribution of effluent within the soil absorption system is non-uniform. Most of the flow comes out of a few holes, either near where the flow enters the system or near an end-cap if distribution lines have slope. This produces local over-loading of the soil absorption system which results in the development of a clogging mat where the wastewater enters the soil. Septic tank effluent varies in strength. The strength of the wastewater and the flow quantity affect the development of this clogging mat.

Septic effluent is very low in dissolved oxygen content because the septic tank is an anaerobic environment. Therefore, the clogging mat that develops is the result of anaerobic processes including filtering out of suspended solids and growth of anaerobic bacteria which produce slimes that clog soil pores. As long as this clogging mat is influenced by septic effluent it will continue to grow and spread throughout the soil absorption system. This process can eventually completely clog a soil absorption system. This is commonly known as system failure.

Alternative systems are designed to reduce the development of the clogging mat by providing more uniform distribution of an effluent with less suspended solids and some dissolved oxygen. By requiring the soil to filter less suspended solids at each location the solids can be more completely biodegrated. By having dissolved oxygen in the effluent that is applied to the soil, there is more activity of aerobic organisms which digest organic solids faster and do not excrete slimes to form a clogging layer.

Small diameter pipe with small holes and pressure distribution utilizing a pump or a syphon that discharges over 30 gallons of effluent at one time can result in quite uniform distribution of effluent throughout the entire soil absorption system. By doing this, the individual point loading of septic tank effluent can be kept to a level such that the solids filtered by the soil are decomposed by aerobic organisms in the soil. By keeping the soil environment aerobic the development of an anaerobic slime is greatly reduced and the system can function for many more years before a clogging mat is formed. If the loading rate is low enough, because few people live in the home and/or the effluent is of very low organic strength, a clogging mat may never form. Pressure distribution is the simplest form of alternative system to protect and extend the life of a soil absorption system.

A number of other alternatives exist and have been discussed in previous newsletters. Alternatives currently being used in Michigan are mounds, constructed wetlands, aerobic treatment units, and sand filters.

Ted Loudon

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