Challenge

In upgrading the municipal wastewater treatment plant (WWTP) in Zeewolde, The Netherlands, Siemens Water Technologies needed to reduce operational costs for waste activated sludge (WAS) disposal but keep the excellent effluent quality and not impact its design capacity.

Built on reclaimed land, the Zeewolde WWTP is one of five run by the Water board Zuiderzeeland, the local government-controlled water board. Current performance is such that the facility achieves very low phosphate and nitrogen limits, without need for chemical addition.

Still, operational costs of removing excess sludge are very high in The Netherlands and, due to present legislation, all municipal WAS must be incinerated. This leads to high costs for all the Dutch Water boards.

The Water board, however, was concerned that any sludge reduction would release phosphate, which was otherwise removed through sludge disposal and could impact their effluent quality. In addition, chemical dosing also would increase the operational cost of the facility.

Solution

Siemens proposed the Cannibal® Solids Reduction System, a biological batch process aimed at reducing the amount of biological WAS. Based on pilot plant results, it was expected the total yield of the WWTP would be dropped below 0.3 kg/kg BOD, effectively reducing the WAS production by 70%.

In the Cannibal system, a side stream of activated sludge is stressed in the “Interchange BioReactors”under oxygen deficient conditons. The interchange activated sludge (IAS) is returned to the main process where byproducts created in the main interchange bioreactors can be further oxidized, which reduces generated WAS. To deal with phosphates, Siemens added another step to the process for separate chemical phosphate removal (and possible reuse).

In Zeewolde, two 1,200 cubic meter (m³) decommissioned tanks next to the WWTP were converted to Interchange BioReactors. The installation further consists of three small blowers, two mixers, chemical dosing installation and the Smart Cannibal Control System. With assistance of Dutch research partner TNO, the project won a €500,000 grant for combined biosolids reduction and phosphate recovery.

Results

Started up in March 2010, this is the first system of its kind anywhere in the world regarding both biosolids and phosphate reduction. Sludge production at the plant has already been cut 35%, with a goal of 50% by the end of 2010 and an ultimate target of 70% reduction. The plant estimates over 300,000 kg of sludge reduction a year and more than €100,000 annual savings in operational costs as a result.

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