Drainfield failure and biology

As the effluent is discharged into the soil absorption system, bacterial growth develops beneath the distribution lines where they meet the gravel or soil. This layer is known as the clogging mat, clogging zone, biocrust, and bio-mat. This biomat (biological mat) is a black, jelly-like layer that forms along the bottom and side-walls of the drainfield trench. This clogging zone reduces infiltration of the wastewater into the soils. The biomat is composed of anaerobic microorganisms (and their by-products) that anchor themselves to soil and rock particles. Their food is the organic matter in the septic tank effluent. Less than one centimeter to several centimeters thick, the biomat acts as the actual site for effluent treatment. The biomat forms first along the trench bottom near the perforations where the effluent is discharged, and then up along trench walls. It is less permeable than fresh soil, so incoming effluent will move across the bio-mat and trickle along the trench bottom to an area where there is little or no biomat growth.  Biomats tend to restrict the flow of effluent through the drainfield, but are crucial because they filter out viruses and pathogens. As the biomat develops, the soil infiltration rate decreases. Once the hydraulic loading rate exceeds the soil infiltration rate, ponding starts. At some point wastewater will either back up into the home or break out on to the soil surface. Total biomat formation cannot, and should not, be prevented, but septic tank filters, proper organic loading, and proper maintenance of the septic tank can slow the rate at which it forms. Septic tank filters prevent excess suspended solids from flowing into the drainfield and can be retrofitted to existing systems.

In 1987, a Multi-Flo aerobic treatment unit was installed at a site where the effluent entered a “failing” soil absorption area. “Failing” meant that effluent was ponding in the trenches of the soil absorption system and/or on the ground surface. Installation of this unit was completed in July 1987, and effluent began to leave the unit about a week later. After one year, all of the effluent was still discharging to the previously failed soil absorption area.¹⁵ Two other Multi-Flo units were installed in 1990. All three systems were monitored by the University of Wisconsin. Based on preliminary results, the State of Wisconsin allowed owners to install pretreatment units for the purpose of renovating failed soil absorption areas.¹⁶  In 1994, a follow-up survey of 17 installations was conducted. Based on this survey, University researchers concluded that aerobically pretreated effluent successfully renovates failing soil absorption systems.¹⁷

15 Converse, J. Correspondence of August 12, 1988, to Jim Baker, relating to the installation of a Multi-Flo unit serving a residence at the Poultry Farm at the University of Wisconsin Experimental Farm in Arlington, WI
16 Burks, B. “Aerobic Wastewater Treatment Units.” Memorandum to Private Sewage Staff and County Code Administrators, 1991. Wisconsin Department of Industry, Labor and Human Relations, 201 West Washington Avenue, Madison, WI
17 Converse, J. and Tyler E.J. “Renovating Failing Septic Tank-Soil absorption Systems Using Aerated Pretreated Effluent.” Madison: University of Wisconsin, Small Scale Waste Management Project, Publication 8.24, 1994, pp 1-8.
18 Converse, J., Converse, M., and Tyler, E.J. “Aerobically Treated Effluent for Renovating Failing Soil Absorption Units.” Madison: University of Wisconsin, Small Scale Waste Management Project, Publication 10.20, 1997, p 7. Data was averaged from Table 2.