Enhanced primary treatment in floating filters

Experiments have been carried out on the use of a floating filter for primary treatment of municipal wastewater. The floating filter used is based up-flow filtration through a randomly packed filter-bed consisting of plastic biofilm carriers that are normally used in the Kaldnes moving bed biofilm process. It is demonstrated that because of the high porosity of the filter bed, a low head-loss is exhibited, resulting in long filter runs and high sludge accumulation capacities. The SS-removal efficiency experienced was in the range of 75-85 % at a filtration rate in the range of 5-15 m/h, making this concept a very interesting one for primary treatment. The key design parameter was found to be the sludge loading rate (kgSS/m2h). At a typical filter depth of 1 m, a filter run time of 5 hrs was experienced at a surface loading of 2 kg SS/m2h while doubling the filter run time required halving of the sl udge surface loading. Pre-coagulation with cationic polymer, possibly in combination with a low metal-coagulant dose, did not significantly improve performance.