Studies on Coupled Photocatalytic and Biological Process for Degradation of Pulp and Paper Mill Effluent

Abstract

The present study facilitated exploring avenues for examining cost effective solutions to treat pulp mill effluents containing chlorinated substituted organics. The research primarily focussed on the degradation of bleach as well as combined pulp and paper mill effluents by coupling of heterogeneous photocatalysis with conventional biological treatment. Photocatalytic degradation of the model compounds such as 2-methoxy phenol (2-MP), 4-chlorocatechol (4-CC) and 4-chloroguaiacol (4-CG) typically found in bleach mill effluent was examined using different photocatalysts (TiO2/ZnO) under UV/solar light irradiation and the degradation efficiency was investigated in terms of changing concentration of the compound by measuring the absorbance. Photocatalytic degradation of all model compounds (2-MP, 4-CC and 4-CG) was observed to be significant as compared to photolysis. The extent of photocatalytic degradation increased with increase in catalyst dose upto an optimum loading and is also pH dependent. ZnO was observed to be better degrading catalyst than Degussa P25 TiO2 for the degradation of the model compounds and effluents. Moreover, ZnO exhibit better photocatalytic activity in the presence of solar light as compared to UV light. Use of sodium hypochlorite (NaOCl) as an oxidant along with TiO2 as photocatalyst enhanced degradation efficiency however, NaOCl showed no effect with ZnO induced degradation. The increase in the concentration of the substrate was observed to decrease the extent of degradation. The reaction rate for the degradation of 4-CC and 4-CG was found to be pseudo-first order with respect to their concentration within the experimental range. Photodegradation kinetics was described by the Langmuir-Hinshelwood model. The degradation of the model compounds was further confirmed by COD/HPLC techniques. Mixed photocatalyst (ZnO and TiO2 in the ratio of 9:1) was used and found to have higher degrading efficacy for degradation of 4-chloroguaiacol (4-CG). The photocatalytic efficiency of synthesized heterostructured ZnO/TiO2 photocatalysts (Z9T) was more than ZnO (Merck) and Degussa P25 TiO2 for the degradation of 4-CG at pH 10. The C and E1 stages bleach mill effluents were also subjected to independent biological, photocatalytic and coupled treatment systems. It was evident from the present study, that the coupling of solar assisted AOP - biological treatment processes or vice versa, based on the type of the effluent, proved beneficial over the either of the treatment systems independently, in reducing COD/TOC load. Qualitative and quantitative analysis of model compounds in C stage bleach mill effluents was evaluated using GC/MS technique and chromatograms confirmed the degradation of model compounds in C stage bleach mill effluent. An attempt was also made to introduce photocatalytic treatment as a part of the existing biological treatment (anaerobic followed by two stage aerobic treatment) being adopted in industrial ETP so as to maximize the degradation efficiency of combined pulp and paper mill effluents with little modifications and minimum input cost. The stage-1 (UASB treated) and stage-2 (sequential UASB-ASP-I treated) effluents were subjected to sequential photocatalytic-biological (aerobic process) treatment. It is concluded from the present study that photocatalytic treatment may be placed after the existing UASB-ASP-1 treatment to achieve integrated anaerobic-aerobic-photocatalytic-aerobic sequential processes for the enhanced degradation of recalcitrant contaminants present in the pulp and paper mill effluents. The fixed catalyst approach was also employed for stage-2 effluents using ZnO under solar mode and the immobilized mode achieved lesser degradation as compared to slurry mode but the former is practically more applicable at industrial level.