HYDROLYS AV RETURSLAM FÖR FÖRBÄTTRAD BIOLOGISK FOSFORAVSKILJNING / Hydrolysis of return sludge for improved biological phosphorus removal

High demands for phosphorus removal in Sweden result in extensive use of chemicals for precipitation. This has a negative environmental impact. A way to reduce or eliminate the use of chemicals is to use biological phosphorus removal. However, municipal wastewaters normally contain too low concentrations of easily biodegradable
organic substances to make this process work satisfactory.
One way to increase the supply of easily biodegradable organic substances is to use sludge hydrolysis. Fullscale tests were performed at the Källby wastewater treatment plant in Lund, where two biological volumes were rebuilt for hydrolysis of the return sludge and two identical volumes were kept unchanged for comparison. The
outcome of the first volume being rebuilt was a bio-P process resulting in a lower effluent phosphorus concentration compared to the volume not being rebuilt. The same difference could not be seen for the second rebuilt volume, which could not be explained. These two volumes were however less loaded, had a somewhat different physical form and only a few samples been taken. Later observations showed a difference, although smaller than for the first volume being rebuilt.
The results of the tests show that hydrolysis of the return sludge can be an alternative when biological phosphorus removal does not work because of too low concentrations of easily biodegradable organic substances. No operational problems occurred, the return sludge hydrolysis process “took care of itself” and no odour problems were observed.