Stricter Requirements for Effluent Threshold Values Lead to Innovative Solutions for the Rotterdam WWTP

The Dokhaven waste-water treatment plant (WWTP) began operating in 1987. The plant was designed to remove COD, BOD and suspended solids and only required nitrification in summer. It was built underground in the middle of the city of Rotterdam with a very compact footprint and little room for expansion. Water is treated in a two-stage process with an extremely high-loaded first stage and a low-loaded second stage, known as the AB process. New legislation regarding the removal of nutrients was introduced in 1987. Because the location did not offer any possibility of a conventional expansion, alternative solutions for improved P and N removal were developed. A system of simultaneous chemical precipitation for P-removal was introduced in the first stage, supported by doses of two types of polyelectrolyte, which makes the process more efficient. P-removal increased from 51% to 82%, and the removal of organics in the first stage also improved. The SHARON and the Anammox processes were developed for N-removal; these treat the reject water of sludge dewatering. The two processes were developed jointly by the water board, universities and consulting companies. Combined with the recirculation of final effluent to the WWTP inlet, the N-removal increased from 35% to 57%.Even stricter requirements are expected in the future, contingent on developments related to the European Water Framework Directive. Several compact techniques were therefore evaluated. Processes which turned out to be feasible are MBBR, MBR, post-denitrification and post-treatment with sand filtration. The costs will be very high, however, and a cost-benefit analysis has yet to take place.