Evaluation of Wastewater Filtration

Tertiary filtration of secondary wastewater is frequently used to improve wastewater treatment plant effluent quality. Four experimental filter columns were operated at the Preston, Idaho, Wastewater Treatment Plant to evaluate the effectiveness of granular media, gravity filtration. The Preston plant is a trickling filter secondary treatment plant and services a population of approximately 3600 people. Four filter medium configurations were studied. Multi-media, dual-media, and single-media beds were constructed with the following media configurations: (1) coal-sand-garnet; (2) coal-sand; (3) sand-garnet; and (4) all sand. The filters were operated at two hydraulic loading rates. Effluents from the primary clarifier, trickling filter, and secondary clarifier were filtered to compare the difference in filter operation and performance when filtering different effluents. Wastewater quality parameters used to monitor filter performance were biochemical oxygen demand (BOD5) and suspended solids. The quality of filter influend affected the quality of the filter effluent. Typical total BOD5 and suspended soilds removal efficiencies were 30 percent and 75 percent, respectively. Soluble BOD5 was not significantly removed by granular filtration. The four filter beds were equally effective in removing suspended solids and biochemical oxygen demand. The coal layered filters operated for 22 hours maximum. The longest filter run time for the sand filters was 9 hours. Filtration of the Preston treatment facility effluent did not consistently produce and effluent that would satisfy the 10 mg/l BOD5 effluent discharge requirement. A survey conducted to review approbal criteria and design standards for wastewater filters employed by state regulatory agencies indicated the following. Most state standards allowed the installation of either gravity flow filters or pressure filters. The majority of state agencies base the allowable hydraulic loading rate on the type and configuration of media employed. The majority of the design standards for wastewater filters permitted the following media types: (1) sand; (2) anthracite; (3) sand and anthracite; (4) sand, anthracite, and garnet or ilmenite. All wastewater filter design standards called for backwast appurtenances complete with air scour or mechanical scour.