In the Netherlands a development programme for the application of the MBR technology in municipal wastewater treatment has started. This paper briefly describes this development programme and goes into detail for the first step: a large pilot plant study at the Beverwijk WWTP. Under the supervision of DHV Water four different membrane systems from Kubota, Mitsubishi, X-Flow and Zenon were extensively tested. The objectives set for the study were more than achieved. The available world knowledge regarding municipal MBR was extremely limited and often very specific for a particular country’s wastewater and wastewater flow characteristic. The conditions set by the Dutch situation were the first of its kind and lead to new methods of MBR optimisation. After almost two years of intensive research and further development of the technological feasibility of the MBR for the Netherlands, it has been decided to extend it to demonstration scale.

The Membrane Bio Reactor (MBR) in Varsseveld is the first full-scale plant in the Netherlands and functions as a demonstration plant for MBR technology. It has been in operation for 5 years and many lessons have been learnt in that time. To gain experience with this new type of treatment and to demonstrate that MBR is suitable for purification of domestic wastewater in a real situation a vast research programme was undertaken. Based on the operational experiences and the research program it can be concluded that the MBR Varsseveld shows promising results. Compared to conventional activated sludge systems with sand filtration (CAS-SF), the MBR produces superior effluent quality. Energy consumption remains up to 30% higher than the CAS-SF system but there is possibility for further improvements. The cleaning philosophy, that combines a regular maintenance cleaning with 2-yearly intensive cleanings, is effective at maintaining the membranes in optimal condition. Based on the experiences with the Zenon ZW500d membranes, a membrane lifetime of up to 10 years is expected.

The (low loaded) biological nutrient removing activated sludge process is the generally accepted and applied municipal wastewater treatment method in the Netherlands. The hydraulical and biological flexibility, robustness and cost efficiency of the process for advanced removal of nutrients like nitrogen and phosphorus without (too much) chemicals results in a wide application of the activated sludge process within Dutch waterboards. Presumably, wastewater treatment plants will have to contribute to the improvement of the quality of the receiving surface waters by producing cleaner effluent. In this perspective, the Dutch research organisation STOWA initiated a research project entitled "The Boundaries of the Activated Sludge Process" to investigate the possibilities and limitations of activated sludge processes to improve the effluent quality. It is concluded that the activated sludge process as applied and operated at WWTP's in the Netherlands has the potential to perform even better than the current effluent discharge standards (10 mg Ntotal/l and 1 mg Ptotal/l). Reaching the B-quality effluent (<5mg Ntotal/l and <0.3 mg Ptotal/l) will be possible at almost all WWTPs without major adjustments under the conditions that: the sludge load is below 0.06 kg BOD/kg TSS.d the internal recirculation is above 20 the BOD/N ratio of the influent is above 3. Complying with the A-quality effluent (<2.2 Ntotal/l and <0.15 mg Ptotal/l) seems to be difficult (but not impossible) and requires more attention and insight into the activated sludge process. Optimisation measures to reach the A-quality effluent are more thorough and are mostly only achievable by additional construction works (addition of activated sludge volume, increasing recirculation capacity, etc.). It is furthermore concluded that the static HSA-results are comparable to the dynamic ASM-results. So, for fast determinations of the limits of technology of different activated sludge processes static modelling seems to by sufficient.

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From 1 January 1995 in The Netherlands 75% of the phosphorus in the influent has to be removed in the waste water treatment plants (wwtp's). At this moment 40 – 50% is reduced by biological incorporation and some chemical dosing. By additional measures an extra 35% reduction has to be accomplished. A further reduction of total nitrogen to 75% has to be realised from 1 January 1998. Further extension of the activated sludge system is not always possible and in most cases rather expensive. Therefore optimization of the existing treatment plant is considered. For this purpose the advanced simulation model STREAM• is used. With STREAM• almost every waste water treatment plant can be simulated with the computer. In this way, for example, the effect of different control mechanisms and other measures can be studied without costly research. Computer simulation has been applied at the Kralingseveer wwtp of the HH van Schieland (Waterboard in the Netherlands) to optimize aeration control and to evaluate a number of technological process adjustments. Attempts are being made to reduce nitrogen drastically using equipment currently available, supplemented where possible with other limited measures. It can be concluded from the results of simulation, that the use of integrated aeration control, whereby influent flow, and nitrate and ammonium concentrations in the activated sludge are measured on-line, further improves the treatment process. Furthermore, when the sludge content in the aeration tank is raised and nitrogen is removed from the internal return flows, it seems that an average annual effluent quality of 10 mg Ntotal/1 is feasible. Verification of these factors will be carried out in the near future using practical measurements. The Kralingseveer project demonstrates that computer simulation is more than simply a ‘game’. It has become a real means of guiding or even limiting the need for extensive and costly (practical) investigation.