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This paper presents a mathematical framework that can be used to determine the flow distributions for a step-feed activated sludge process that result in maximum nitrogen removal. The model indicates that nitrogen removal efficiency in a step-feed activated sludge process is highly dependent on the ultimate biochemical oxygen demand (BOD L )-to-total Kjeldahl nitrogen (TKN) ratio of the wastewater. For typical domestic wastewater, which has a relatively high BOD L -to-TKN ratio, the step-feed process will outperform the Modified Ludzack-Ettinger process for nitrogen removal, when the flow to each step is optimally distributed. Using plantspecific water quality data and operating conditions from a 1-year period, nitrogen removal performance for four step-feed activated sludge plants operated by the Sanitation Districts of Los Angeles County (California) was calculated using the developed model. The calculated nitrogen removal efficiencies match well with the actual plant performance data. These results validate the model as a useful tool for predicting nitrogen removal in a stepfeed activated sludge process. Other analyses revealed that improvements in nitrogen removal at existing facilities are achievable by adjusting the split of primary effluent flow to each anoxic zone several times during the day. The timing of the adjustments and the optimal flow splits can be determined from data on diurnal fluctuations in BOD L and TKN concentrations. An example is provided to illustrate the application of such an operating strategy and the potential enhancement of nitrogen removal. Water Environ. Res., 79, 367 (2007).

This paper presents a mathematical framework that can be used to determine the flow distributions in a step-feed activated sludge process for achieving the greatest nitrogen removal efficiency. The model indicates that the nitrogen removal efficiency in a step-feed activated sludge process is highly dependent on the BOD to TKN ratio of the wastewater. For typical domestic wastewater, which has a relatively high BOD to TKN ratio, the step-feed process will outperform the Modified Ludzack-Ettinger (MLE) process for nitrogen removal, if the flow to each step is optimally distributed. Nitrogen removal performance of four step-feed activated sludge plants operated by the Los Angeles County Sanitation Districts is calculated with the developed model using plant specific water quality data and operating conditions, and is compared with actual observations over a one-year period. The calculated nitrogen removal efficiencies match closely with the actual data at these four plants indicating the model is a useful tool for predicting nitrogen removal in a step-feed activated sludge process. To achieve the greatest nitrogen removal efficiency in a step-feed activated sludge process, the operator may consider the use of two or three daily adjustments of primary effluent flow to each step based on diurnal fluctuations of BOD and TKN. An example is provided to illustrate the application of such an operating strategy and the potential enhancement of nitrogen removal.

In the planning phase of the extension of the Main Treatment Plant of Vienna, special effort and emphasis were put on the conception of the tender procedure. The project tender was divided into several tender units in order to achieve optimum quality standards by specialised workmanship. Important parameters for operation conditions, especially energy consumption and maintenance costs, were considered and valuated according to the tender guidelines. Suppliers were required to prove the guaranteed quality standards of the tender documents by means of preliminary installation units and extensive performance check procedures. Only after fulfilment of all requirements were suppliers allowed to apply the design of the preliminary installation works to the entire installation. If necessary, extensive optimization works were carried out. Thus favourable operation conditions were achieved and, in particular, a highly efficient aeration system was implemented. Process parameters were optimized by means of a follow-up project during the regular operation phase, taking into consideration the results of the performance check procedures. Further optimization of energy consumption was thereby achieved.