Primary Effluent Filtration: Results from a Field Comparison of Three Filters

In primary effluent filtration (PEF), primary effluent is treated to reduce TSS, BOD, or other contaminants without resorting to secondary treatment in trickling filters or activated sludge basins. If PEF can meet the effluent quality requirements applicable to a wastewater stream, substantial cost reductions can be realized by using PEF instead of secondary treatment. In other instances, PEF can reduce the secondary treatment load if it is used between the primary and secondary treatment processes. Previously, no directly comparable data have been available for the various technologies that could be used for PEF. The Orange County Sanitation District conducted a project to test three filters simultaneously, representing three different approaches to PEF, to produce direct comparison data. The test involved two types of sand filters (intermittent and continuous backwash types) and a filter using a synthetic filter medium that operates at higher hydraulic loads than sand filters. The tests explored the ranges of hydraulic load and operating settings appropriate for each filter. The filters' effects on TSS and BOD levels were of particular interest, since these are of most concern for NPDES discharge purposes, but various other characteristics also were measured to better differentiate the performance of the filters. Measurements of particle size distributions in filter influents and effluents showed that the effect of filtration on the particle distribution depended on the filter being studied, the particle size being evaluated, and the operating conditions of the filter. Operating cost comparisons based on the test results were done for the three filters, as were cost comparisons of PEF to other treatment technologies.