Septic systems use drain fields to treat the septic tank effluent for the removal of impurities and contaminants. The field is made up of trenches typically containing washed "drainrock" or gravel. In some cases perforated pipes are used within the drainrock, but pipes are more typically used for gravel-less systems. The types of gravel-less drain fields used are large-diameter single-pipe systems, smaller diameter multiple-pipe systems, Enviro-septic® pipe systems, and plastic chamber systems.
According to Tool Base Services' Gravel-less Pipe Leach Field Web page, a single-pipe system uses eight-inch to 12-inch diameter, corrugated, perforated pipe wrapped in a filtering fabric. The perforations are located high enough on the pipe wall so solids can settle without plugging the holes, and the corrugations allow the effluent to circulate under the fabric and discharge to the soil from all parts of the pipe's circumference. The fabric-coated pipe is placed in a trench and the trench can be filled with native soil.
A multiple-pipe system uses smaller pipes, 4 to 4½ inches in diameter, and the pipes are not wrapped in fabric. The system consists of three to thirteen pipes in an array with the primary effluent pipe at the top-centre of the array. The idea behind the array is that bacteria grows on the inside and outside surfaces of all the pipes, and the effluent then flows downward into and out of all the surface area of the pipes allowing the bacteria to digest the impurities. This system may not be permissible under some local regulations.
The trademarked Enviro-septic® system uses a corrugated, perforated pipe surrounded by a plastic fibre mat and wrapped with fabric. The pipe is etched with ridges to increase the surface area for bacterial growth and to provide cooling to the waste. Each perforation in the pipe has a small tab to prevent undigested grease and solids from leaving the pipe and clogging the fibre mat. In addition to providing a large bacterial treatment area, the fibre mat acts as a filter to protect the outer fabric. The outer fabric also provides bacterial treatment area and prevents soil from passing into the fibres and pipe.
Plastic Chamber System
Plastic chambers are not pipes in the strictest sense of the word, but they do fill the same role in a septic system. They are louvered, arched, plastic chambers or vaults, open on the bottom. They receive the effluent and allow it to move through the louvres into the soil around them. The effluent is typically piped into the chambers with four-inch PVC (polyvinyl chloride) pipe. The chambers themselves are six feet long, one foot high and twenty to forty inches wide. According to Economics, an American environmental remediation company, health jurisdictions are recognising the value of this technology, and its use is becoming common across the U.S.
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