Optimization of flocculation for advanced phosphorus removal via microsieve filtration

In the future, advanced phosphorus removal will be necessary in many WWTP in order to meet the demands of the European water framework directive. The project OXERAM deals with the comparison of different technologies with regard to their efficiency and applicability in tertiary treatment. In the course of the project membrane and microsieve filtration are tested in pilot scale at the Ruhleben STP. In this thesis the optimization of coagulation and flocculation prior to microsieve filtration for advanced phosphorus removal (< 80 µg/L TP; total phosphorus) was investigated. For the optimization of the coagulation/ flocculation several test series were conducted with the aid of jar test and the mircosieve pilot plant. A direct comparison of jar tests and the pilot plant showed that jar tests are an appropriate method to predict the approximate outcome of optimization steps (e.g. variation of chemical doses) in the pilot plant. The pilot trials were able to demonstrate that the microsieve technology (10 µm pore size) in combination with chemical pre-treatment of 0.036 - 0.179 mmol/L coagulant (Fe or Al) and 2 mg/L cationic polymer could easily achieve good and reliable TP removal. The phosphorus removal was comparable to dual media filtration (< 80 µg/L TP) and partly even to membrane filtration (< 50 µg/L TP). The reduction of the residual coagulant contents in the filtrate was identified as the main challenge of this technology. High iron contents of about 1 mg/L were accompanied by floc formation behind the mircosieve in filtrate tank and pipe. In a microsieve the formed flocs have to endure high shear forces. Thus, the so-called post-flocculation was most probably caused by re-flocculation of floc fragments. Very low phosphorus values < 50 µg/L were possible at high metal dosing. But the higher suspended solid load reduced the filtration capacity of the microsieve. Coagulation with polyalumium chloride (PACl) produced better effluent quality compared to FeCl3 as less suspended solids and less residual coagulant were found in the microsieve effluent. Furthermore, the transmission of UV radiation through the water was improved from 47 up to 66 % by using PACl which is favorable if a downstream UV disinfection is considered. When using FeCl3 the transmission was not improved or even reduced. Due to the influence on the performance of the microsieve cationic polymers were preferred to anionic polymers. However, the tested anionic polymer proved to be not applicable in the given process configuration due to very low filtrate flows. When cationic polymer was applied the polymer dose had a high impact on the particle removal and moreover on the contents of phosphorus and coagulant residuals in the effluent. In most cases 2 mg/L polymer was necessary. In total, the microsieve technology in combination with chemical pre-treatment is a suitable option for advanced phosphorus removal. Through a dynamic adjustment of the chemical dosing to the influent water quality (e.g. ortho phosphate and turbidity online measurement) and the choice of polymer the process could be optimized in the future with regard to efficient chemical application.

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