Feasibility of using ultrasonic irradiation to recover active biomass from waste activated sludge

Under typical operating conditions, the microbial fraction of activated sludge flocs is approximately 40% by weight. The objective of this research is to evaluate the feasibility of using ultrasonic irradiation to disrupt activated sludge flocs allowing for the subsequent separation of active and inactive fractions. If separation of floc components is possible, then methods may be incorporated into wastewater treatment plant operations whereby only the inactive fraction of floc is wasted (i.e., of waste activated sludge, WAS), which in turn could increase the overall effective biological solids retention time, leading to increased process robustness with no net increase in reactor size. The results indicate that ultrasonic irradiation of WAS at 800 Wl(-1) followed by 30 min of settling can produce a supernatant with heterotrophic specific oxygen uptake rates (SOURs) of over two times the SOUR measured in the bulk mixed liquor. Under these conditions 26% of the initial heterotrophic activity was recovered within only 11% of the initial volatile mass. Similarly, autotrophic analysis revealed that nitrifying organisms, while sensitive to the effects of ultrasonic irradiation, can be separated from the activated sludge floc and recovered. An irradiation density of 200 Wl(-1) with an exposure time between 1 and 2 min produced a supernatant with a specific ammonia removal rate of over two times the initial mixed liquor rate.