Two-Stage High Rate Digestion and Phosphorus Recovery

Improved sludge disposal can be achieved by intensified digestion. With the reduction of the hydraulic retention time and by higher dried solid matter of the sludge, the volume load increases. In the WWTP Waßmannsdorf the digestion could be intensified by higher DSM of the mixed sludge. Furthermore, by application of a two-stage high rate digestion process in first tests in 2002 a degree of degradation of around 50 % organic DSM was achieved. A high organic volume load of around 3.2 kg/m3 per day and a degradation rate of around 50 % were results of first pilot plant trials over a time period of more than 3 month with the two-stage high rate digestion process without usage of a rotating membrane disk filter and showed a stable and safe operation. By using the RMD in the next phase of the pilot plant trials more water will be extracted from the digested sludge and so the digestion time can be extended and the organic volume load can be increased, in order to achieve further improvements (degree of degradation and improved gas production). Furthermore, there is a high potential for P-recovery in the highly digested (mineralised) sludge. First trials show in principle that precipitated MAP could be separated by using the hydro-cyclone technique.