Treatment of Evaporator Condensate Using a High Temperature Membrane Bioreactor: Determination of Maximum Operating Temperature and System Costs, 1999 Environmental Conference Proceedings

Pierre R. Bérubé & Eric R. Hall

Department of Civil Engineering and the

Pulp and Paper Centre

The University of British Columbia

The feasibility of biologically treating the foul fraction of the evaporator condensate from a kraft pulp mill for reuse as process water, using a high temperature membrane bioreactor (MBR), was investigated. Treatment is required because of the high concentration of odorous compounds contained in the foul fraction of the evaporator condensate. Also, increasingly stringent legislation, such as the Cluster Rule, prohibits the reuse of the foul fraction of the evaporator condensate without treatment because of the high concentration of hazardous air pollutants (HAP) the condensate contains. The HAP can volatilize to the atmosphere causing hazardous working conditions. Methanol is the main contaminant of concern. A high temperature MBR was selected for the treatment of evaporator condensate for reuse since it is potentially more efficient and less expensive than steam stripping and conventional biological treatment.

It was possible to develop a mixed culture of thermotolerant methanol-utilizing bacteria, on synthetic foul condensate containing methanol, dimethyl sulphide and dimethyl disulphide, at temperatures ranging from 55 o C to 70 o C and a sludge retention time of 20 days. The maximum feasible operating temperature for the biological removal of methanol from condensate using a membrane bioreactor was approximately 60 o C. A maximum methanol removal rate of 1.4 mg/l . min (specific utilization rate of 0.8/day) was observed at 60o C. Over 99 % of the methanol was removed from the condensate at operating temperatures of 55 o C and 60o C. Above 60 o C, the methanol uptake rate declined sharply significantly reducing the methanol removal efficiencies. The specific methanol utilization rate at 60 o C was higher than previously reported for conventional biological treatment systems operating at lower temperatures.

Based on the kinetic information collected and the characteristics of the evaporator condensate at a local kraft pulp mill, an MBR system was designed. Capital and operating costs were estimated for two operating scenarios. Scenario 1 consists of treating only the fouler fraction of the evaporator condensate. Scenario 2 consists of treating the fouler fraction and approximately 50 % of the cleaner fraction of the evaporator condensates. Capital and operating cost estimates were lower for an MBR system compared to a steam stripping system capable of achieving similar treatment efficiencies.