A novel anaerobic membrane bioreactor for wastewater treatment and bio-energy production

Abstract

A novel anaerobic bioreactor integrated with membrane microfiltration was developed to study the treatment of prehydrolysis liquor (PHL) from a dissolving pulp production facility located in Nackawick, New Brunswick, Canada. The PHL with total chemical oxygen demand (COD) of approximately 100 g/L mainly consisted of sugars, acetic acid, furfural, and lignin. This high organic strength PHL poses a serious disposal problem for the pulp industry leading to environmental concerns if not treated adequately. The current treatment method of the PHL (evaporation and burning) used by the industry is highly energy intensive. A submerged membrane-based bio-treatment system called sludge-bed anaerobic membrane bioreactor (SB-AnMBR) was designed to provide a possible alternate treatment solution for the PHL. It consists of a granular anaerobic sludge-bed at the bottom followed by a submerged flat-sheet microfiltration membrane module with a biogas scouring system placed above the sludge bed zone in an upflow reactor. Biogas scouring helped in reducing the fouling of the membranes from excessive deposition of sludge layer on surfaces of the flat-sheet membranes. The experimental study was designed and conducted for about 800 days using two 50 liter SB-AnMBRs made of steel at mesophilic (35°C) and thermophilic (55°C) temperatures in an environmental chamber. The reactors were operated in a continuous mode applying organic loading rates ranging from 0.8 to 10 kg-COD/m³-d at mesophilic and thermophilic temperature conditions. A series of batch experiments was conducted prior to the continuous operation to evaluate the anaerobic treatability of the PHL. The effect of factors such as biological activity of the seed sludge, organic loading rate, and temperature on anaerobic digestion of the PHL was investigated through batch respirometric experiments. Results from characterization and batch studies showed that the PHL have potential to be a suitable substrate for anaerobic degradation in continuous SB-AnMBRs. SB-AnMBRs’ overall performance was found to be good with greater than 82% removal of the total COD at mesophilic (35°C) and thermophilic (55°C) temperatures. Methane yield was found to be more than 0.33 m³-CH4/kg-COD removed/day at the both the temperatures studied. Biokinetic coefficients were determined using the data obtained from the continuous and batch studies at mesophilic and thermophilic conditions. The coefficients were analyzed using Monod and Arrhenius models. The results of biokinetic coefficients were found in the agreement with the effluent characterization from the continuous reactors. Health of the bioreactor in terms of the measurement of cellular adenosine triphosphate (cATPtm) and its correlation with COD removal, methane production, and active biomass concentration was also investigated. Membrane fouling was not found to be significant at the sustained flux rate of 0.1 m/d during the continuous operation. Membrane autopsy and characterization studies of the foulants suggested that fouling occurred mostly due to organic foulants like carbohydrates and proteins. Chemical cleaning of the membranes resulted in more than 80% recovery of the permeate flux. This novel SB-AnMBR treatment system exhibited good potential for a sustainable biological treatment option for the PHL from the dissolving pulp production industries. In general SB-AnMBR can be adopted as an efficient treatment alternative for industries producing very high COD (80-100 g/L) biodegradable waste stream. In addition, it has also good potential of producing methane for a possible production of bioenergy.