Summary: | 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.
|