Impact of solids management on nutrient ratios for in-line wastewater prefermenters

The full-scale evaluation of in-line prefermentation effects on wastewater nutrient ratios was performed under three different sludge operating conditions, with regards to sludge elutriation rates (0.7, 2.2 and 3.7 kg dry sludge/m3 raw sewage) and the corresponding sludge ages (17.3, 2.5 and 7.5 days). At all three operating conditions the TKN/COD ratio increased (64, 46 and 20% respectively), with an average ratio of 0.057 mg N/mg COD in the raw sewage increasing to 0.082 mg N/mg COD in the settled sewage. These ratio increases can limit the use and performance of certain biological nutrient removal process configurations. This study has further highlighted the importance to counterbalance fermentation, thickening and solids removal requirements in a single tank in-line prefermenter. The minimum suspended solids removal (33%) was achieved at the lowest settled sludge solids content (4.3%), against a corresponding maximum volatile fatty acid generation rate (5.7 mg VFA/l/h). The total solids and volatile fatty acid concentration profiles down the prefermenter tank depth were determined at a high sludge blanket level condition. It was demonstrated that the accumulated constituent mass inventory increase was about constant throughout the water and sludge layers respectively under such operational conditions.