High temperature biological treatment of foul evaporator condensate for reuse

• Main Author: Bérubé, Pierre
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/11377

There is increasing interest in the treatment and reuse of the sewered portion of the evaporator condensate from krafl pulp mills. The treated evaporator condensate could be used in brown stock washing, recausticizing and bleaching, instead of clean water. In addition to reducing the contarninant load to the existing combined mill effluent treatment system, reducing the raw water requirements and potentially reducing the impact of discharging the treated condensate to the environment, reusing the condensate could also result in significant energy savings if the heat content of the evaporator condensate can be recovered. Also, some legislation proposes a number of incentives for treating and reusing the condensate as process water. Methanol and reduced sulphur compounds (RSC) were identified as the primary contaminants of concern contained in evaporator condensate. These contaminants are of concern primarily because they are hazardous air pollutants (HAP) and/or foul odorous compounds. Reusing evaporator condensate in a pulp mill without treatment could result in the subsequent emission of HAP and odorous compounds and generate unpleasant or even hazardous working conditions for mill staff. Some trace organic contaminants contained in evaporator condensate are also of concern primarily because they could disrupt the pulping process and impact pulp quality. A number of conventional technologies have been considered for the treatment of evaporator condensate for reuse. However, the relatively poof treatment efficiencies and/or high costs associated with these technologies provided incentives to investigate and develop a better treatment technology. A high temperature membrane bioreactor (MBR) was selected as the most promising novel technology for the treatment of evaporator condensate for reuse. A preliminary study indicated that the biological removal of methanol from synthetic evaporator condensate using a high temperature MBR was feasible. The results suggested that the specific methanol utilization coefficient was higher during high temperature biological treatment using an MBR, than in a conventional biological treatment system. However, simultaneous biological removal of methanol and RSC from synthetic condensate using a high temperature MBR was not feasible. A low operating pH was required for biological oxidation of RSC to occur at elevated temperatures. In addition, biological removal of methanol was significantly inhibited at the pH required for biological RSC removal to occur. Therefore, a two stage system, with the first stage operating at an acidic pH and the second stage operating at a neutral pH, would be required. This would add significantly to the cost of a biological system to treat evaporator condensate for reuse. Even at an optimal pH for the growth of sulphuroxidizing microorganisms, stripping due to the aeration system accounted for approximately 50 % of the RSC removed from the MBR. The results also indicated that the stability of a mixed microbial culture at a low pH is questionable. For these reasons, the biological oxidation of RSC in a high temperature MBR was not considered to be feasible and simultaneous biological removal of methanol and RSC was not further investigated. Further investigations revealed that it was possible to biologically remove methanol from synthetic evaporator condensate using a high temperature MBR, over the entire expected range of temperatures for evaporator condensate (55 to 70 °C). However, the operating temperature exerted a significant impact on methanol removal kinetics. A maximum specific methanol utilization coefficient and a maximum specific growth coefficient of approximately 0.84 ± 0.08 /day and 0.11 ± 0.011 /day, respectively, were observed at an operating temperature of 60 °C. Above 60 °C, both the specific methanol utilization coefficient and the specific growth coefficient declined sharply, suggesting that at high operating temperatures, the inactivating effect of temperature on the growth-limiting enzyme must be considered. A relatively simple model was proposed and used to accurately estimate the effect of high temperatures on methanol removal kinetics in an MBR over the temperature range investigated. Based on the model, the optimal operating temperature for the biological removal of methanol by a mixed microbial culture was determined to be approximately 60 °C. These results indicated that it is not only possible to operate an MBR at high temperatures, but also that a higher specific methanol utilization coefficient can be achieved at a higher operating temperature. However, care may need to be taken not to exceed the critical operating temperature of 60 °C. The operating temperature was also observed to have a significant effect on the observed microbial growth yield in the MBR. At increasing operating temperatures, a larger fraction of the methanol consumed was converted to energy, reducing the observed growth yield. These results indicate that at high temperatures, less excess sludge may be produced, potentially resulting in lower waste sludge handling and disposal costs. The specific methanol utilization coefficient measured during the treatment of real evaporator condensate was lower than that observed when treating synthetic evaporator condensate. The difference was not due to a direct toxic effect from compounds present in the real evaporator condensate matrix. The reduction was attributed to a shift in the composition of the microbial community present in the MBR. The shift resulted from competition between methylotrophic and partial-methylotrophic microorganisms for the available methanol. Microorganisms that were not capable of growth on methanol as sole substrate, but were capable of consuming methanol in the presence of other organic substrates, were defined as partial-methylotrophic microorganisms. The partialmethylotrophic microorganisms exhibited a lower specific methanol utilization coefficient (0.29/day) than the methylotrophic microorganisms (0.84/day), resulting in a lower overall specific methanol utilization coefficient for the mixed microbial culture of 0.59 ± 0.11 /day. Nonetheless, the specific methanol utilization coefficient observed at 60 °C was still more than 30 % higher than previously reported values from other studies of biological treatment of condensate at much lower temperatures. High temperature biological treatment using an MBR also successfully removed the nonmethanolic contaminants of concern contained in evaporator condensate. Over 99 % of the RSC contained in the evaporator condensate was removed during high temperature treatment using an MBR. The concentrations of hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl sulphide in the evaporator condensate werereduced to below detection limits (approximately 0.4 mg/L) during high temperature operation using an MBR. Approximately 93 % of the organic compounds, measured as TOC, contained in the evaporator condensate could be removed. The concentration of TOC in the evaporator condensate was reduced from 504 ± 137 mg/L to 52 ± 3.6 mg/L. Over 78 % of the reduction in TOC was due to the removal of methanol. Based on assumed removal efficiencies of 99, 90 and 99 % for methanol, TOC and RSC (as hydrogen sulphide and methyl mercaptan), respectively, as well as the characteristics of the evaporator condensate from a local kraft pulp mill, a conceptual design for a fullscale, high temperature MBR to treat an evaporator condensate for reuse was developed. Capital and operating costs were estimated and compared to the costs for a steam stripping system. Depending on the type of ultrafiltration membranes used in the MBR design, the capital cost for the MBR system was 40 to 50 % less than the capital cost of a steam stripping system capable of achieving comparable contaminant removal efficiencies. The operating costs for the MBR system were also approximately 50 % less than the operating costs for a steam stripping system. Therefore, high temperature biological treatment is not only technically feasible, but is also appears to be economically more attractive than the currently favored treatment technology (i.e. steam stripping).