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Membrane bioreactor treating kraft evaporative condensate at a high temperature under different operational conditions and turpentine shockloads Alsuliman, Abdullah

Abstract

This study was initiated to assess the feasibility of biological treatment and reuse of kraft pulp mill evaporator condensate, using a high temperature MBR operated at a short solids retention time (SRT) and low hydraulic retention time (HRT). In addition, the mechanisms responsible for the fouling of the membrane were examined. The decline in the biomass concentration associated with short SRT operation affected membrane fouling. Furthermore, the impact of short-term turpentine shock loads on the overall removal kinetics of the contaminants of concern present in the evaporator condensate was investigated. To obtain as much data as possible within a short time frame, a lab scale system consisting of two bench scale high temperature MBRs was operated at different operational SRTs and FIRTs. The SRT conditions selected were 15 and 10 days. The associated HRTs set at either 12, 10, or 8 hours. Other operating parameters were selected according to recommendations from a membrane manufacturing company and from a previous study (Berube, 2000). Methanol, monoterpenes, and organic compounds represented by total organic carbon (TOC) were identified as the primary contaminants of concern contained in the evaporator condensate. Methanol and monoterpenes are of concern primarily because they are hazardous air pollutants (HAP) and/ or foul odorous compounds. They contribute 60 to 75% of the biochemical oxidation demand (BOD) in the evaporator condensate (Hough et al, 1977). The results of the study indicated that the biological removal of the contaminants of concern using a high temperature MBR operated at a short SRT (as short as 10 days) and a low HRT (as low as 10 hours) was feasible. However, when the HRT was decreased from 10 to 8 hours (organic load increased), the mixed microbial culture responsible for the removal of methanol was inhibited. The potential toxic contaminants present in the evaporator condensate exhibited an immediate influence on the biotic removal kinetics and removal efficiencies of methanol. For monoterpenes and TOC, the significant decline in the values of overall removal kinetics and efficiencies, when HRT was lowered to 8 hours, was due to the short contact time imposed. Investigation of the membrane performance indicated that the reduction in the permeate flux with time and the resulting cleaning intervals were a function of the operational SRTs and HRTs. Membrane fouling was found to be a function of the mixed liquor volatile suspended solids (MLVSS) concentration in the MBR and the operational transmembrane pressure (TMP). Membrane fouling was mainly controlled by pore plugging resistance (R[sub pp]) rather than concentration polarization (R[sub cp]) and membrane resistances (R[sub m]). R[sub pp] composed from 60 to 80% of the total foulant resistance. Turpentine shock loads exerted a significant impact on the overall removal kinetics of the main contaminants of concern in evaporator condensate. Overall removal rates decreased significantly as the spiked monoterpenes accumulated in the MBRs. The overall removal rates decreased more significantly when the spiking concentrations of monoterpenes were increased from 300 to 1500 mg/L.

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