Han, Ngoc (2021) Improving Water Reuse For A Commercial Paper Mill. Research Master thesis, Victoria University.

Abstract

The pulp and paper industry traditionally consumes high levels of water and energy and the reduction of fresh water use with emphasis on process water management and wastewater recycling are key factors for the growth of this industry. The use of fresh water has reduced significantly during the last decades. The main reasons for this include increased environmental legislations relating to effluent discharge, and hefty costs arising from either the supply of fresh water or treatment of wastewater thus effecting the marketing potentials. Although using recovered paper as raw material has an advantage of saving energy, water and landfill space, attention needs to be paid to treating the wastewater due to discharge regulations and standards. Wastewater sent to a treatment facility is regarded as waste. It is treated only with the purpose not to cause a negative impact to the environment. Although the level of impurities and toxic substances in treated wastewater satisfies the discharge standards, it often limits recycling potential because it adversely affects manufacturing processes and paper quality. Methods for removing these impurities and toxic substances include various physical and biological methods and the intended reuse of the water determines type and level of treatment required. This study aims to explore the possibility of introducing advanced technologies such as enhanced coagulation and flocculation, and membrane separation to improve wastewater recycling in a commercial paper mill. A commercial plant water circuit was first analysed and a water balance was proposed to reduce the water consumption from 10m3 to 6m3/tonne product. The quality of treated wastewaters and associated parameters from commercial individual processes were analysed to forecast the plant influent flows and predict pollutants concentrations in wastewater. Although organics in wastewater were mostly processed through biodegradation, non-biodegradable recalcitrant compounds limit its potential for reuse, thus a tertiary treatment is proposed. Enhanced coagulation and flocculation yielded very promising results, especially for colour removal. The treatment was able to achieve the target colour value. The desirable COD output values were also achieved by varying coagulation/flocculant ratio. For membrane separation, different units of operation and pollution parameters were examined. Results indicate that membrane technology produced higher colour removal efficiencies (96% for NF and 87% for UF) compared to enhanced coagulation and flocculation (61%). However, when it comes to removing COD, enhanced coagulation and flocculation was able to achieve a removal efficiency of 46% COD, compared to 43% for NF and 20% for UF. Amongst the three methods, UF, if used as a stand-alone method, failed to reduce the COD level to the target value. However, when feedwater was pre-treated with coagulant, UF was able to produce a similar COD removal rate to NF. Pre-treating feedwater also reduced fouling of membranes. Between the two membrane cleaning agents used, alkaline was shown to be more effective in reducing fouling. Backwashing was also investigated and found to be effective in prolonging membrane lifespan. However, despite the reduction in fouling, irreversible fouling still happened. Although membrane technology performed better in the removal of colour, retrofitting a membrane system into an existing plant can be difficult, requires the implementation of appropriate pre-treatment technology to control fouling resulting in higher capital and operating costs compared to enhanced coagulation and flocculation.

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