6. A family of six in a five bedroom house would like to treat their wastewater

Question

6. A family of six in a five bedroom house would like to treat their wastewater in an onsite septic system, constructed wetland, and tile drainage (leach field) system. They will be generating a maximum of 1.2 m3/d of septic tank effluent. The septic tank effluent characteristics are 140 mg/L BOD, 50 mg/L TSS and 40 mg/L nitrogen (as ammonia nitrogen). They would like their constructed wetland to reduce their BOD to 15 mg/L and their nitrogen to 10 mg/L before the water enters the tile drain/leach field.    No treatment objectives are required for TSS or pathogens in the wetland. The available gravel medium has a K of 9,144 m/d, with a porosity of 0.42. Decaying plant matter provides a background BOD concentration (C*) of 7 mg/L. Assume a wastewater temperature in the wetland of 12

Explanation / Answer

Answer

One proposed solution to the problem of wastewater treatment in small communities is the use of a large-scale septic tank-leachfield system (Otis, 1978; Diodata, 1980; Rubin and Carlile, 1981). In this system each home or business has an individual septic tank connected to a small diameter gravity or low-pressure sewer system that conveys the tank effluent to a large-scale soil absorption bed (commonly called a large-scale leachfield and typically handl i ng 13.22 m3 (3,500 gallons or more per day). The treatment of the wastewater begun in the septic tank is then completed in the unsaturated soil beneath the leachfield. The type of system descri bed above has severa1 advantages for an existi ng sma 11 communi ty. The cost of constructi on is genera11 y lower than the conventional system because small-diameter plastic PVC pipe can be used for collection purposes since most of the solids have been settled out in the septic tank (Otis, 1978). The cost of operation and maintenance is lower since trained operators are not required to operate and maintain the plant 24 hours a day (Otis, 1978). Operation and maintenance of the septic tanks (pumping out solids) is also more uniform because pumping is no longer the responsibility of the individual homeowner (Englehardt, 1983). Another advantage of the above system is that in areas where only a limited area of soil is suitable for a leachfield, the suitable area can be used for a leachfield while homes can be built on areas without suitable soils (Rubin and Carlile, 1981). Also, this system replenishes groundwater, but with added nitrates, whereas the conventional system usually discharges the treated effluent into a surface stream (Laak, 1980)

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