Hydraulics of peat filters treating septic tank effluent

Peat is an alternative filter medium for the treatment of various waste str eams including septic tank effluent. The water holding capacity and adsorpt ion capacity of peat make it a favorable filter medium over sand or gravel which are commonly used as the filter medium for the drainage field of sept ic systems. This paper presents the results of a field study to evaluate th e hydraulics of a peat filter used to treat the septic tank effluent from a public school west of Ottawa, Canada. An experimental box was placed withi n the filter during its construction to provide access to the vertical prof ile of the peat layer. The filter is periodically pulsed with septic tank e ffluent, which is distributed over the top of the filter and flows vertical ly through the peat. The filter was instrumented with tensiometers and tran sducers to monitor the pore water pressures in response to a pulse of septi c tank effluent. An in-depth study of the hydraulics of the system was comp leted. The soil moisture retention curve and the hydraulic conductivity as a function of density were determined in laboratory experiments. A one-dime nsional unsaturated flow model was used to predict the pressure response du e to a pulse. A comparison of the field and model results illustrates the i mpact of the density variations, and the corresponding hydraulic conductivi ty variations, on the model predictions. The compaction of the peat is an i mportant design consideration for the filter since it directly impacts the flow characteristics and the hydraulic retention time within the filter.