OPTIMIZATION OF BIOLOGICAL ACTIVITY FOR ANAEROBIC SLUDGE DIGESTION

Two pilot projects were carried out aimed at increasing the processing capacity of municipal anaerobic sludge digesters by accelerating the substrate utilization rate of anaerobic biomass in digesting sludge. The focus of this paper is on the first pilot project as the second project is the subject of another paper at this conference.The first pilot project was carried out during the first half of 2001 at the municipal sewage treatment plant of New Bedford, Massachusetts. In this project the impact of shear on biomass activity in anaerobic sludge digesters was analyzed. Data from the six-month pilot project yielded a correlation between substrate utilization rate of biomass and shear power input per unit digester volume. Periodic viscosity measurements as function of shear rate enabled us to estimate the velocity gradient in anaerobically digesting sludge. Depending on shear power input, biomass activity could be increased up to four times in the absence of thickening polymers. One thickening polymer was quite toxic while another was only mildly inhibitory. Sludge digesters typically are designed on the basis of retention time. Therefore their volumetric capacity is determined by the degree to which sludge is thickened prior to anaerobic digestion. Even though increased biomass activity would make it possible to design significantly smaller digesters operating at reduced retention time, it is not possible to fully capitalize on that because biomass might be inhibited by certain thickening polymers while sludge feed would become too dilute in the absence of polymer. Details on this pilot study will follow in the body of this paper.Attempts to circumvent the above identified process limitation led to the concept of simultaneous digestion and membrane thickening of sewage sludge in the absence of thickening polymer. This recuperative thickening concept was evaluated in a second pilot project, which was carried out at our facility in Camden, New Jersey with sludge obtained from the Gloucester County Utilities Authority in Paulsboro, New Jersey. Two membrane systems were evaluated, namely an oscillating cross flow system with a teflon sheet membrane and a tubular cross flow system with a titanium oxide and stainless steel membrane. Both membrane systems performed quite well. Under optimal conditions the oscillating system yielded consistent flux rates of around 2.0 m/d (50 gpd/ft2) up to almost 3 % TS solids while requiring virtually no chemical cleaning. The tubular system yielded flux rates exceeding 3.6 m/d (90 gpd/ft2) up to about 2 % solids, which declined to 1.2 m/d (30 gpd/ft2) at about 3 % solids while requiring regular chemical cleaning. Details on this study can be found in Jackivicz, Lanting and Kirschbaum (2003). Some highlights will be provided at the end of this paper.