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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Ray, Madhumita B.

2nd Supervisor

Dagnew, Martha

Co-Supervisor

Abstract

Pulp and paper (P&P) mills’ wastewater contain high concentrations of resin acids (10–10,000 mg/L) which are toxic to aquatic organisms. In mill’s primary and waste activated sludge treatments reduced concentrations of resin acids in effluent; however, most of the resin acids are ultimately ended up in sludge after primary and secondary clarification due to their extreme hydrophobic nature (log Kow 6.0). Nevertheless, adsorbed resin acids on the surfaces of anaerobes and substrates affect anaerobic digestion (AD) process negatively. The objective of the present work was to determine the effect of pretreatment of thermomechanical pulping mill’s primary sludge (TMP-PS) on AD. The performance of ozone and ferrate pretreatment of TMP-PS on AD was evaluated at their optimum pH around 11.0 and 7.0, respectively. Pretreatment of TMP-PS using 0.150 mg dosages /mg tCOD of ferrate and ozone increased sCOD by around 28.35% and 22.88%, respectively besides reduction of 23.63%, and 37.07% resin acids, respectively. AD showed negligible biodegradation of resin acids by anaerobic consortia. Pretreatment with ferrate and ozone increased the methane yield by maximum of 60.15% and 49.62%, respectively. Compared to ozonation, 11–13% more methane was generated in ferrate pretreated digesters. However, according to cost analysis, ozonation of TMP-PS is economically more feasible than ferrate pretreatment.

Summary for Lay Audience

The worldwide demand of pulp and paper (P&P) products are increasing day by day. The water consumption in P&P mills is also increasing with the demand of paper products. Wastewater produced at P&P mills contains excessive concentrations of toxic compounds. Among them, resin acids have been found in pulping wastewater at higher concentrations. Resin acids generally originate in the resin of tree bark and wood. Resin acids are the most potential inhibitors of microorganisms in anaerobic digestion (AD) of P&P mills’ waste solids (sludge) generated in wastewater treatment plant. Resin acids are mostly found in the solids in sludge due to higher hydrophobicity, which ultimately undergoes AD prior to final disposal. An efficient pretreatment of P&P mills’ sludge is required to remove the toxicity of resin acids to microorganisms prior to AD. The overall research objective of this PhD is to determine the effect of resin acids on anaerobic digestion of P&P mills primary sludge. Pretreatment with ozone and ferrate, which are powerful oxidant to degrade the organic compounds, applied in this study to treat the sludge from P&P mill’s primary sludge. The performance of ferrate pretreatment increased 11-13% more methane yield in comparison with ozone pretreatment in the digester. However, according to cost analysis, the operating cost of ferrate pretreatment of P&P mill’s sludge is significantly higher than ozonation due to higher production cost of ferrate. Moreover, ozone pretreatment can easily be retrofitted in actual P&P mill’s wastewater treatment system to make the process economically viable.

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