The pilot trials at the Ruhleben wastewater treatment plant proved that the microsieve technology combined with chemical pre-treatment achieves good and reliable phosphorus removal with effluent values < 80 µg/L TP. The first three months of pilot operation confirmed the general process performance observed during the pre-trials in 2009 but also revealed a need for process optimization with regard to the removal of suspended solids and the reduction of coagulant breakthrough. An improved performance was achieved through change from ferric chloride (FeCl3) to polyaluminum chloride (PACl). In the presented case, PACl gave clearly better results for the removal of phosphorus and suspended solids than FeCl3. Additionally, the occurrence of coagulant residues could be noticeably reduced. In contrast to FeCl3, dosing PACl led to an improvement of the water transmittance simplifying disinfection with UV irradiation. Load proportional dosing of PACl and polymer was introduced in order to avoid under as well as over dosing of the chemicals. The dose of cationic polymer had a significant impact on water quality and backwash time: With the initial process configuration 1.5 to 2 mg/L cationic polymer were recommended for a safe and stable operation with adequate backwash time resulting in an average polymer dose of 1.7 mg/L. However, latest results showed that a polymer dose of only 0.6 mg/L is possible without losses in water quality and filtration performance when mixing conditions were optimized. During the constructional modifications the hydraulic retention time of the coagulation was reduced from 4 to 1 min at peak flow. Due to the installation of a TurbomixTM short-circuiting could be avoided. Furthermore, the turbulence in the flocculation tank was increased. Despite the noticeable reduction of the hydraulic retention time and the polymer dose the rebuild resulted in improved reduction of suspended solids (2.2 mg/L) and coagulant residues in the microsieve effluent. The operation regime of the chemical treatment prior to the microsieve filtration showed to be a trade-off between the energy demand for mixing and the polymer consumption. Due to the continuous operation over more than 20 months important operational experience was gained with regard to backwash behavior and cleaning intervals. The backwash time mainly correlates with the influent flow (1030 m3/h), the influent water characteristics and the properties of the formed flocs. Due to progressing fouling of the filter panels chemical cleaning was necessary every 4 to 7 weeks. A shorter cleaning interval (e.g. every 4 weeks) might be beneficial as the backwash time and thus the energy demand could be kept on a lower level. In this application the microsieve produced on average 1.8 % of backwash water. The backwash water showed excellent settling properties (SVI << 50 mL/g) and might be easily treated via returning to the primary clarifiers. The UV disinfection plant behind the microsieve was operated with a fluence of 730 J/m2. Good disinfection could be provided for a continuous operation of 7 months. During this period there were always less than 100 MPN/100 mL of E. coli and Enterococci in the effluent of the UV disinfection. Overall, the microsieve in combination with dosing of coagulant and polymer is a robust technology with low phosphorus effluent values (< 80 µg/L) and a low energy demand of about 21 Wh/m3 (+ site-specific energy demand for water lifting). Microsieving, together with UV disinfection, can be an option for applications targeting phosphorus removal and disinfection, e.g. effluent polishing for sensitive areas or landscape irrigation.

Im Projekt OXERAM wurden verschiedene Technologien im Hinblick auf die Anforderungen an die 4. Reinigungsstufe, vor allem Phosphorentfernung, in Pilot- und Laborversuchen untersucht. Ferner wurden die Leistungsfähigkeit der Verfahren sowohl durch eine Ökobilanz als auch eine Kostenrechnung bewertet. Der vorliegende Bericht fasst diese Ergebnisse aus den Jahren 2010 bis 2013 zusammen. Die Vorgehensweise und eine ausführliche Ergebnisdiskussion sind in den Kapiteln 2 - 6 beschrieben.

In this study the applicability of the microsieve technology together with coagulation and flocculation for advanced phosphorus removal was investigated. A pilot unit including a microsieve with 10 µm mesh size is operated continuously with secondary effluent. By applying a pretreatment of 0.07-0.09 mmol/L coagulant and 1.5-2 mg/L cationic polymer total phosphorus values below 80 µg/L were achieved. 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. Also the transmittance of UV radiation through the water is improved by using PACl. The amount of backwash water was very low (< 3 %). Results after rebuilding the chemical pre-treatment showed that under optimized mixing conditions polymer doses << 1 mg/L are possible without losses in water quality and filtration performance. In total microsieving with chemical pretreatment is a viable option for high quality effluent polishing.

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.

In this study the applicability of the microsieve technology together with coagulation and flocculation for advanced phosphorus removal was investigated. A pilot unit including a microsieve with 10 µm mesh size is operated continuously with secondary effluent. By applying a pretreatment of 0.036 – 0.179 mmol/L coagulant and 2 mg/L cationic polymer total phosphorus values below 100 µg/L were easily achieved. Values below 50 µg/L were possible at high metal dosing, but the higher suspended solid load reduced the capacity of the pilot unit. 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. Also the transmission of UV radiation through the water is improved by using PACl. The amount of backwash water was very low (< 3 %). In total, if combined with UV disinfection, microsieving with chemical pretreatment is a viable option for high quality effluent polishing.

In this study the applicability of the microsieve technology together with coagulation and flocculation for advanced phosphorus removal was investigated. A pilot unit including a microsieve with 10 µm mesh size is operated continuously with secondary effluent. By applying a pretreatment of 0.036 – 0.179 mmol/L coagulant and 2 mg/L cationic polymer total phosphorus values below 100 µg/L were easily achieved. Values below 50 µg/L were possible at high metal dosing, but the higher suspended solid load reduced the capacity of the pilot unit. 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. Also the transmission of UV radiation through the water is improved by using PACl. The amount of backwash water was very low (< 3 %). In total, if combined with UV disinfection, microsieving with chemical pretreatment is a viable option for high quality effluent polishing.