Long Term Performance of a Septic Tank - Rock Filter System

The overall performance of rock plant filters in Louisiana has been spotty at best. Previous investigations have found that most Louisiana facilities maintained little or no operational data on which to base rational judgments. This paper summarizes the performance of a septic tank – rock filter system at an interstate rest area on I-49 approximately 40 miles south of Alexandria, Louisiana. At present, treatment occurs in two cells, each 150' long by 30' wide, connected in series. The Louisiana Department of Transportation and Development (LADOTD) is using this facility to investigate whether subsurface flow technology might be a more economical and reliable means of treating waste from rest areas as opposed to existing mechanical systems in situations where flows are small and skilled operators are not available. Discharge limits for this facility are 45 mg/l of BOD and TSS. There is no nitrogen limitation at this time.The following data have been collected every other week since March of 1997 at five points throughout the system: biochemical oxygen demand (BOD), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), ammonia (NH3), nitrate (NO3), pH, conductivity, dissolved oxygen (D.O.), temperature and fecal coliforms. Chemical oxygen demand (COD), total phosphorous (TP), volatile suspended solids (VSS) and carbonaceous oxygen demand (CBOD) tests are conducted periodically as are tests for selected metals and hydrocarbons.The facility is not maintenance free but requires essentially no proactive operation except for periodically adding chlorine tablets for disinfection. Maintenance requirements are minimal and can be performed by rest area personnel. The waste strength at this facility is higher than reported for other rest areas. Mean BOD and TSS concentrations of the waste entering the treatment facility are 521 and 682 mg/liter respectively. The mean BOD and TSS concentrations of the waste entering Cell 1 are 177 and 73 mg/liter respectively. The BOD loading to Cell 1 is substantially higher than the North American average.Mean effluent BOD and TSS levels from Cell 1 are 37 and 25 mg/liter. Corresponding values from Cell 2 are 20 and 20 mg/liter respectively. The last permit violation occurred in August 1998.