Department Process Engineering

(Complementary investigations to the national MicroPoll project)n zum nationalen

Ozonation or powdered activated carbon (PAC) addition after biological treatment are promising technologies for complementing nutrient removal plants to strongly reduce micropollutants and ecotoxicity in the receiving water.

Due to the stringent P removal conditions a lot of WWTP in Switzerland are already equipped with a flocculation filtration step. WWTP Kloten/Opfikon near the city of Zuerich operates a sand filtration to reach effluent suspended solids concentration smaller than 4 mg/l. Flocculants addition to the flocculation tank in front of the sand filter is not in use due to a very efficient secondary clarifier. The aim of the project is to evaluate the significance of PAC addition directly to the filtration step using the flocculation tank as contact tank for PAC addition. The filter is normally back flushed every 24 hours giving a few hours contact time for the accumulated PAC in the filter.To evaluate general operation parameters for PAC and flocculant addition, the flow regime and the hydraulic retention time in the contact tank as well as the back flush interval for the planned full scale application at WWTP Kloten/Opfikon, a pilot sand filtration system was built in the experimental hall of Eawag in summer 2008. The lay out as well as the dimensions of the pilot plant comply with the full scale facilities. The full scale trials at the WWTP were conducted in autumn 2008.

First results in 2008 for the retention of PAC in the filter showed inconsistent behavior between the pilot and full scale application. In addition micropollutants measurements by BfG showed a heterogeneous picture with elimination rates up to 80% in maximum. To optimize the process and to enhance the elimination rates additionally trials are planned at Eawag and at WWTP Kloten/Opfikon for 2009. The investigation are closely linked to the national Bafu-project MicroPoll.