Design, construction and testing of a lab-scale membrane distillation bioreactor for water purification

Abstract

The pulp and paper industry produces a significant amount of wastewater that contains a variety of organic and inorganic contaminants. This makes it impossible to discharge untreated wastewater directly into a water source. As a result of the condition of the untreated mill effluent and strict environmental regulations, significant pressure has been placed to develop suitable technologies capable of treating and reusing this wastewater. In light of this, this study evaluated the potential of a membrane distillation bioreactor (MDBR), a technology which combines a conventional membrane bioreactor (MBR) with membrane distillation (MD) for the treatment and reuse of pulp and paper mill effluent. In this study, effluent from a paper mill was analyzed for its most significant components. The analysis identified sulphate, sodium, chloride and calcium as the main components of the feed wastewater with concentrations of 391 mg/L, 300 mg/L, 160 mg/L and 157 mg/L respectively. A high TDS of 1 394-1 566 mg/L, TSS of 496-876 mg/L and COD of 397-496 mg/L were found for the feed wastewater, typical of pulp and paper mill effluent. MDBR performance characteristics including permeate water quality, permeate flux as well as membrane fouling and membrane wetting were investigated systematically. The performance of the MDBR was also evaluated at three different operating temperatures of 45ºC, 55ºC and 65ºC. Experimental results showed that the MDBR achieved 99.4% removal of all compounds and a high salt rejection rate of 86.3%, regardless of the operation temperature. However, the MDBR was only able to achieve an organic rejection rate of 78.0%. It was found that the effects of bioreactor temperatures had strong impacts on both the permeation performance and fouling behaviour. The permeate flux dropped by 88.1% over the duration of the experimental program at the relatively low operational temperatures due to membrane fouling. SEM analysis showed a compact fouling layer on the membrane surface from the bioreactors operated at the temperatures of 55ºC and 65ºC while only a few depositions were found on the membrane from the 45ºC bioreactor. EDS results indicated that the deposits formed on the membrane surface mainly consisted of calcium. A loss in membrane hydrophobicity of 66.0% at increased temperatures was detected with contact angle measurement due to the partial wetting of the membrane. In the present study, the optimal MDBR temperature was found to be 65ºC as it showed better process performance and treatment efficiency. Overall, the MDBR in this study was effective in remediating pulp and paper mill effluent.