EnviDan has implemented the Activated Return sludge Process (ARP) at Bjergmarken WWTP in Roskilde. The ARP is used to increase the organic capacity, and thus prevent an otherwise necessary expansion of the plant. At Bjergmarken WWTP part of the bioconversion takes place in a separate volume where return sludge is added. This makes it possible for both the nitrification and denitrification processes to proceed and still maintain a high level of sludge concentration. The ARP is implemented in the former primary settling tank and has resulted in an increase of the organic capacity by 22 percent or 20,000 PE. At the same time the excess sludge production was reduced by 10-15 percent.

Since the ARP was implemented in 2008, EnviDan and the operating staff at Bjergmarken WWTP has continuously monitored and documented the process, so that we now have an extensive knowledge of the consequences and are able to prove the results using measurements and observations from the plant.

The ARP involves a reorganization of essential parts of the COD conversion in the WWTP to a section of the plant where the biological processes can take place with a higher sludge concentration, thus increasing the efficiency of the plant.

COD is converted in a uniform rate during the sludge retention in the tank. By implementing the ARP important parts of the COD conversion is moved from the main biological tank volumes  to a separate section of the plant in which only return sludge with a high concentration of dry matter is added. Since the predominant part of the COD degradation follows the sludge dry matter in the WWTP the volume capacity in the ARP tank will thus increase significantly compared to the capacity of a conventional sludge treatment plant.

A large part of the nitrogen conversion will also take place in the ARP tank since the tank is run with alternating aerobic and anaerobic conditions.