Use of Layered Geotextiles to Provide a Substrate for Biomass Development in Treatment of Septic Tank Effluent Prior to Ground Infiltration

The use of layered geotextile filters for biological treatment of septic tank effluent prior to ground infiltration was investigated. The goal was to provide secondary treatment and prevent soil clogging by fostering biomass growth in the internal porosity of commercially available geotextiles. The study used primary effluent from a water pollution control facility that serves a combined sewer area. To identify sustainable operating conditions, the hydraulic loading rate (HLR) and other parameters were varied in tests with columns packed with multiple geotextile and granular layers. At continuous high hydraulic loading rate application, over 90% of total suspended solids and biochemical oxygen demand was removed, but the three layer filters eventually clogged. When a 365 L/m^2 day (9.0 gal/ft^2 day) HLR was applied in dose and drain cycles to two filter layers, not only was there little loss in permeability, but ammonia (NH3) and nitrate (NO3-) concentrations in the effluent were reduced to below 5 and 10 mg/L, respectively. Scanning electron microscope images showed that the biomass morphology was not a continuous biofilm as was expected, but a discontinuous floc trapped within the geotextile pore structure. This provided intimate contact between substrate, oxygen, and biomass to produce the desired effluent quality and limited loss in filter permeability.