The performance of a sequencing batch reactor for the treatment of whitewater at high temperatures
Creators: name: Johnson, Rhiannon
Publisher: name: University of British Columbia
Year: 2009

description: Environmental and economic pressures on pulp and paper mills have prompted the adoption of water-reducing strategies such as Whitewater system closure. Efforts to reduce water use in the Whitewater system increase the Whitewater temperature and cause operational and quality problems in the papermachine through the build-up of dissolved contaminants in the Whitewater. To control the build-up of dissolved and colloidal substances in the Whitewater, an aerobic bioreactor is proposed to treat a substream of the closed Whitewater loop. This research investigated the biological treatability of a synthetic closed-system Whitewater at high temperatures with an aerobic biological sequencing batch reactor (SBR), focusing on the removal of resin and fatty acids, one of the problem compound groups. The bioreactor was operated at a hydraulic retention time (HRT) of 2 days and a solids retention time (SRT) of over 15 days with the intention of maintaining a viable biomass at a mixed liquor volatile suspended solids (MLVSS) level between 2000 and 5000 mg/L. The performance of the bioreactor was assessed at 20, 30, 40, 45, and 50°C in terms of total dissolved solids (TDS), total organic carbon (TOC), chemical oxygen demand (COD), and resin and fatty acid (RFA) removal. The removal of conventional contaminants such as TDS, TOC, and COD was significant at temperatures up to and including 40°C while at higher temperatures, contaminant removal was reduced. Parameters describing reactor operation and performance such as the food to microorganism ratio, the specific substrate utilization rate, and growth yield indicated a reduced conventional contaminant removal capability at temperatures higher than 40°C, along with a decrease in reactor biomass inventories at the higher temperatures. The removal efficiencies of fatty acids (FA) were over 95% at all temperatures, but for resin acids (PvA), near-complete removal was observed only up to 40°C. At higher temperatures, the removal efficiencies of R A were reduced, but still significant. Measurements during the SBR react cycle indicated that FA were mainly associated with the suspended solids, while RA were associated with both the liquid and solid phases. Observed specific removal rates decreased with increasing temperature, while maximum specific removal rates were high for all temperatures studied. For FA, the maximum removal rates were about twice the observed removal rates, while for RA, the maximum removal rates were about four times the observed removal rates. The FA content in the biomass appeared to decrease with increasing temperature, while the R A content appeared to increase. The RFA removed did not accumulate on the suspended solids because the RFA content in the biomass was negligible compared to the overall mass flow through the system. A large non-RFA extractable, chromatographable component of material was removed at all temperatures, though less removal was observed at 50°C. Overall, the bioreactor performed best at temperatures below 40°C for the removal of both conventional contaminants and RFA, especially, RA. These experiments indicated that the biological portion of the membrane bioreactor device would be able to control the concentrations of dissolved and colloidal material using feed from a closed-loop Whitewater application. The problems encountered at higher temperatures such as low sludge growth, solids loss in the effluent, and substantial RA in the effluent would be reduced with the combination of an ultrafiltration unit. Thus, treatment using the membrane bioreactor would probably be effective at temperatures higher than 40°C.