Ermittlung prozessbestimmender / -begrenzender Parameter bei MW-Zufluss auf Kläranlagen. Prüfung der Übertragbarkeit auf die Kläranlagen Berlins
In combined sewer systems domestic, trade and industrial waste water and in case of rainfall events significant volumes of storm water are collected and transported to wastewater treatment plants (wwtp). In the frame of this study a literature review on the impact of combined water inflow on wastewater treatment and the identification of critical system processes and parameters has been carried out. The objective of this work was to study the possibilities of an optimal charging of wwtps during rain and to analyse in how far those concepts could be transferred to Berlin plants. When the inflow of combined sewage to the wwtp increases, screening waste increases, too. However, this is no problem if the screening capacity is sufficient. In the primary settler the formation of primary sludge increases due to the higher inflow load. Concerning dissolved substances there can be an adverse effect when the highly concentrated content of the primary settler is pushed into the biological tanks. To ensure a reliable separation of the primary sludge a hydraulic residence time of 30 minutes is recommended. In literature, the processes of nitrification, biological P removal and the separation of the activated sludge in the final clarifier have been identified as being most critical during combined water inflow. Usually, effluent concentrations of the critical parameters increase only at the beginning of a rain event. Due to the dilution effect that typically can be observed after a maximum duration of 2 hours the concentrations then keep constant again. The process of biological P elimination can be supplemented by chemical P precipitation to avoid critical effluent concentrations during combined water inflow. In the aerated zone the oxygen content can be regulated to improve P incorporation. In the past, in Germany combined water inflow to the wwtp was limited to 2*Qwastewater+Qinfiltration (according to the standard ATV-A 131, 1991). However, the treatment capacity of wwtps that have been designed according to ATV-A 131 (1991) may exceed this value. According to the standard ATV-DVWK-A 198 an inflow of 3-6 times the average dry weather flow may be possible. In literature it can be found that factors of 3 to max. 4 have been realized successfully. Hence, in praxis the peak factor for combined water inflow is rather in the lower range given in ATV-DVWK-A 198. In Berlin the approach given in ATV-DVWK-A 198 will not be applicable. Since wastewater transport is realised via long pressure mains the dilution by stormwater reaches the wwtps only after 5-10 hours. In contrary to wwtps that are directly connected to a gravity sewer system, this Berlin situation leads to long-lasting disturbances of the processes in the activated sludge tanks and in the final clarifiers. However, it must be mentioned that only few information was available on the behavior of the Berlin wwtps during combined water inflow (mainly inflow data). At some wwtps (Stahnsdorf and Ruhleben) an adapted sporadic increase of the inflow rates during rain may be possible. The “bypass process” and an adapted oxygen regulation may be further interesting options for the management of combined water at the wwtp. However, the “bypassprocess” has not yet been tested in situations with long-lasting (5-10 hours) high load situations. The most important options for the reduction of combined water overflows in Berlin will still be the unsealing of currently impervious surfaces, the reconfiguration of the combined sewer system into a modified system (preventing stormwater to enter the combined sewer), the prevention of stormwater inflow into the sanitary sewers of the separate system and the construction and (real-time) control of storage capacities within the combined sewer system. In the future it would be desirable to charge the Berlin wwtps in accordance to their actual capacities based on measurement information. Thus, an optimisation between combined water treatment (reduction of combined water overflows) and the capacity and resilience of the plant could be realised. Therefore, a system for the assessment of the actual capacity of a wwtp (nitrification, final clarification) would be needed.