sal system should last longer than your house and cause fewer headaches. Process As mentioned in the earlier chapter on disposal systems, unsatu- rated soil can provide excellent treatment for sewage effluent. Pro- vided that ground water is not too near the bottom of the trench, that the soil is moderately permeable, and that the rate of the sew- age discharge into the soil is not too great, the concentration of particles (including bacteria and viruses) should be reduced by many times, and the effluent should be reasonably safe as it com- bines with the ground water. It should be realized, however, that some chemical contaminants are not removed by filtration, and thus dilution with uncontaminated ground water is the only way to reduce the danger of these contaminants. More information on chemical contamination is presented in Lot Evaluation, Chapter 10. After a newly installed trench system has been in operation for several months, a layer of bacteria and other microorganisms, along with filtered particles from the septic tank and various insoluble chemicals, collect at the trench bottom to form what is known as a ONSITE WASTEWATER DISPOSAL Figure 5.1 An absorption trench. slime layer or biological mat. This mat is the first and most effectiv L barrier in preventing ground water contamination, providing filtra- tion and breakdown of solid material, and transformation of certain chemicals. In soils with rapid percolation rates, this mat filters out a majority of the particulate material in the effluent and slows the infiltration rate of the effluent, allowing unsaturated flow to occur below the trench. This slowing of the infiltration rate must be considered when sizing a drain field in loose soils. This leads us to a discussion of the old debate of shallow versus deep trenches. Initially, almost all of the waste effluent will soak through the bottom of the trench (Figure 5.1). As time goes on, clogging of the soil pores by particulate matter and biological mate- rial increases and may reach the point where the