PHOSPHORUS REMOVAL AND DENITRIFICATION IN DEEP-BED TWO-MEDIA FILTERS / Fosforavskiljning och denitrifikation i djupa tvamediafilter i Henriksdals reningsverk i Stockholm
Simultaneous suspended solids separation, denitrification, and phosphorus precipitation in a down-flow filter were studied as a final process step at Henriksdal treatment plant. The research was performed in a stainless steel tube with a diameter of 0.4 metre filled with sand of different grain sizes and layer heights over a layer with expanded clay. The total bed height was 1.5 metre which corresponds to a full-scale filter. The hydraulic load
was mainly 10 m/h and phosphate precipitation was accomplished with either FeS04 or FeCl^. Sodium acetate acted as a carbon source in the denitrification tests. Sludge flight and primary settled wasrewater by-passing the biological treatment were simulated in some tests. No significant difference was obtained with FeS04 compared to FeCl^ as precipitation agents for the concentration of total phosphorus or phosphate in the filtrate. The phosphate concentration never exceeded 0.05 mg P04-P/1 when iron was added. Primary settled wasrewater consists partly of liquid fat with organic bounded phosphorus. This fluid was not removed by the filter. Primary settled wastewater acts as an efficient carbon source for the denitrification in the filter. The filter could effecttively separate suspended solids with concentrations as high as 60—70 mg SS/1 but the time before clogging then decreased to around 10 h at a hydraulic load of 10 m/h. With a suspended solids concentration of 2-8 mg SS/1, the rime before clogging became about 100 h at 10 m/h. Different operational conditions and chemical additions gave clogging times of 4—35 h.