Optimization of an Aerobic Selector for Bulking Control in a Pulp & Paper Wastewater Treatment Plant

At a pulp and paper wastewater treatment plant in China, sludge bulking was found to be severe, with an average SVI of 233 mL/g on average, leading to a reduction in capacity for flow through the secondary clarifiers and an overall reduction in capacity of the wastewater plant. As this is a toll facility, reduced capacity lead to a substantial reduction in revenue. Despite the presence of aerobic selectors, which are designed to reduce bulking, bulking persisted. In order to control bulking, the microbial basis for bulking was first determined and then an operating strategy was developed based on addressing the root cause leading to the overgrowth of that filament. The microbial analysis showed that the sole filamentous organism present at abundant levels in the mixed liquor was Type 021N, whose growth is promoted by specific levels of septic substrates such as volatile fatty acids, particularly acetic acid, and hydrogen sulfide. To eliminate this organism and promote the growth of flocculating organisms, a range of operating conditions were evaluated in a 25 m3/d pilot plant that receives the same wastewater as the full scale plant and has selectors of the same scale. Optimization consisted of varying the selector soluble COD levels through variation of the selector F/M (controlled for equivalent F/M at 20°C) and correlating that F/M with the settling index (SVI). Control of the selector F/M was enabled by the installation of a recycle sludge (RAS) bypass line which enabled variable flow of RAS directly to the main aeration tank, bypassing the selector. This bypass allowed controlled reduction of the biomass concentration (M) in the selector, independently of the sludge age (SRT). Higher bypass flow rates relative to normal RAS rates lead to higher selector F/M. Natural variations in temperature were another mechanism by which the sCOD levels in the selector were influenced with higher temperatures leading to lower sCOD levels in the selector(lower F/M equivalent at 20°C). Optimum settling was found to occur at a selector F/M of 5-7 g sCOD/gVSS/d at 20°C.