ELRCTROCHEMICAL TREATMENT AND RECYCLING OF PULP AND PAPER MILL WASTEWATER

Abstract

The pulp and paper industry is among one of the largest sources, which discharge enormous quantity of pollutants including gaseous, liquid, and solid waste into the environment. Despite these, it is also the third largest consumer of fresh water after primary metals and the chemical industries. Higher consumption of water is one of the key environmental concerns in paper industry, since it produces a large volume of wastewater as well. The wastewater from paper industry contains a significant amount of organic matter, with low biodegradability, high color, AOX and toxicity values. Various wastewater pollutants, including toxic organic compounds at a high concentration, are difficult to treat with conventional treatment processes. Hence, treatment technologies require continuous advancements in order to meet the rigorous discharge standards and to improve the cost effectiveness of the processes. The electrochemical (EC) method is a treatment process that can be a promise as an effective treatment alternative for high strength organic wastewater containing toxic substance. The pulp and paper mill wastewater from primary and secondary clarifier was characterized in terms of environmental parameters; COD, TOC, BOD, color, solids, Cl- and AOX. The electrochemical (EC) treatment using stainless steel anode was used to treat the present effluent. Optimization of the process variables i.e., initial pH of the wastewater, current density (CD), treatment time (t), and initial organic load, for the treatment of paper mill wastewater by EC was carried out. This was done in terms of COD and color removal efficiency. The removal efficiency for other parameters such as BOD, TOC, AOX, and chlorophenols were determined under optimized conditions. Both primary and secondary clarifier wastewater was subjected to electrochemical treatment under the optimized conditions. The electrochemical treatment method was found to be efficient for the color removal. BOD5/COD ratio of the effluents improved after the treatment to a significant extent. The biodegradability of the effluent improved substantially after the EC treatment, indicating the easy removal of pollutants by biological process. The generated sludge was characterised for various physico-chemical parameters. The results on detection and quantitative determination of various chlorophenols in the combined pulp and paper mill effluent are reported. The GCMS analysis of both primary and iv secondary clarifier effluents samples leads to the identification of 20 low molecular weight chlorophenolic compounds in primary clarifier effluent and 18 compounds in the secondary clarifier wastewater in various concentrations. Six types of chlorophenolics compounds i.e., chlorophenols, chloroguaiacols, chlorocatechols, chlorovaillin, chlorosyringols and chlorosyringaldehyde, were identified in the effluent. The results indicate that, among the total compounds detected, the higher contribution to the chlorophenols comes from di-substituted compounds in the paper mill effluent. The effectiveness and performance of the stainless electrodes for the degradation of chlorophenols is also examined. In case of chlorophenols, monochlorophenols were oxidized to the maximum extent followed by di- and trichlorophenols. For chlorophenol treatment, 68.7 % organic removal was achieved after 2h on stainless steel anode at current density of 15 A/cm2 for primary clarifier effluent and 85.3 % removal was achieved in secondary clarifier effluent. Analysis with the High Performance Liquid chromatography (HPLC) revealed many intermediate products during the EC process, from aromatic compounds to aliphatic acids, which were oxidized by SS electrode. Based on the EC degradation performance and the HPLC results, the treated samples were also analyzed on GC-MS for the qualitative and quantitative analysis of the chloro-organic intermediates. The EC treated secondary clarifier wastewater was used as wash water to study the recycling of this wastewater in pulping and bleaching sections of paper mill. The pulp was bleached to 84% ISO target brightness using DEpD bleach sequence in the laboratory. D and EP, refer to chlorine dioxide and H2O2 reinforced NaOH extraction stages, respectively. For DEpD sequence a Kappa Factor of 0.28 was found to be optimum for attaining the target brightness (84% ISO). The two DEpD sequences were done in parallel using fresh water (DEpD) as well as EC treated water (RC_DEpD) for washing to achieve the target brightness. The two bleaching sequences were compared in terms of bleach chemical demand, pulp brightness, and change in bleach chemical dose to get the same target brightness. The generated bleach effluent was characterized for environmental load. A higher COD and color was generated in RC_DEpD sequence as compared to DEpD as a higher kappa factor of 0.29 (RC_DEpD) was required to achieve the same target brightness (84% ISO). There was no effect of wastewater recycling on pulp and paper properties i.e. viscosity, tear, tensile, and burst indexes.