Design WWTP

A section of the wastewater treatment plant at Varsseveld, designed by DHV.
On the left the sand and grease trap (6), the low-oxygen section (11) and the section where the water is purified by bacteria using oxygen (12).
On the right the membrane supply pump (14) and the membrane tanks (15).

Click the picture for a complete cut-away diagram of the membrane bioreactor wastewater treatment plant at Varsseveld (PDF file, 789KB).

Load
The load of the Varsseveld wastewater treatment plant will total 23,150 population equivalents and 755 cubic metres per hour in 2015. The design is based on stricter effluent requirements than stipulated by law, namely nitrogen < 5 mg Ntotal/l and phosphate < 0.15 mg Ptotal/l. Because weather conditions in the Netherlands lead to a high level of rain drainage via the sewer system, this aspect played an important part in the design. As a result, the maximum hydraulic load of the MBR plant is almost three times the average supply level. The treatment of wastewater from a nearby cheese factory also played an important role in the design.

Pre-treatment
Because the membranes are vulnerable to contamination and damage, a great deal of attention was paid in the design to the pre-treatment of raw wastewater. Leaves, plastic, sand, grease, hairs and the like must all be removed from the water supply. To achieve this, the wastewater is first led over a screen with a rod distance of 6 mm. It then flows through an aerated sand and grease trap and finally through microsieves with a perforation of 0.8 mm.
Even when this pre-treatment procedure is followed, solid content (e.g. encrusted sludge or falling leaves) may end up in the activated sludge installation and impair the operation of the membranes or damage them. To keep the activated sludge clean, it is therefore continually recirculated over the microsieves from the activated sludge tank. When the membrane tank is emptied during a purification cycle, the entire content of the tank is also circulated over the microsieves.

Biological purification
In principle, the biological processes taking place in an MBR plant do not differ from those in a conventional activated sludge plant. Because a lower dry matter content can be used and no final sedimentation tanks are needed, an MBR plant can be constructed in a much more compact manner. The design of the biological purification process and the control of the biological processes require a different approach because of shorter accumulation times and a different oxygen balance. A dynamic simulation model of the MBR plant was therefore developed for design and process control purposes. The model was based in part on the IAWQ-1 model in the SIMBA software application.

Membrane filtration
Following a European tender procedure, the contract for membrane filtration was awarded to the membrane supplier Zenon. Positive experiences were gained with capillary membranes in a number of treatment plants, including one in Beverwijk, a town in the west of the Netherlands. The key design parameters for the membrane filtration process are listed below in Table 1.

table 1 – Design data for the membrane installation

All the required membranes will be installed in four separate compartments. To ensure that the membranes function properly, the load must be distributed as evenly (i.e. symmetrically) as possible. To this end, special attention was paid to the design of the inflow and outflow facilities of these tanks.

The undersides of the membrane modules have been fitted with coarse bubble aeration. The air bubbles cause turbulence, so that activated sludge particles cannot attach themselves to the membrane surface. The inclusion of a back-flush step in the process cycle prevents membrane pore blockage. The division into compartments makes it possible to take membranes out of service in case of low supply levels, when full capacity is not needed. This option saves energy because the membranes do not have to be continually aerated and are given sufficient time for relaxation. Periodic chemical cleaning is also required, in addition to the continuous cleaning of the membrane surface.

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