Primary sludge (PS), waste activated sludge (WAS) and a mixture of WAS and PS were fermented at 21 degrees C. The sludge was collected from two plants: the biological nutrient removal (BNR) West End Water Pollution Control Center (WEWPCC) and carbon-removal only South End WPCC (SEWPCC). The PS fermentation predictably generated a significantly higher amount of soluble COD than WAS. WAS fermentation released considerable amounts of phosphate and ammonium. Co-fermentation of WAS with PS enhanced soluble COD production and increased the release of phosphate and ammonium. The semi-continuous flow fermenters showed that regardless of the sludge source, with a similar total COD load, there was no significant difference in soluble COD production observed during co-fermentation between the two plants. Volatile fatty acids were the primary components of the soluble COD generated during fermentation. 20-22% volatile solids destruction was achieved due to sludge fermentation. The WEWPCC sludge released a higher concentration of phosphate than the SEWPCC sludge - the latter originating from a non-BNR process. Fermentation of combined PS and WAS sludge generated a concentration of phosphate high enough to allow phosphorus recovery as struvite at both plants.