TREATMENT OF PULP AND PAPER MILL EFFLUENT

Abstract

Pulp and paper industry is an important basic industry of any country. Paper consumption for any country closely follows the growth economics of that country. Despite its significant contribution, paper industry is one of the major polluting industries and is amongst top twenty-nine industries for which environmental impact assessment (EIA) has been made mandatory. Paper industry consumes large amount of water and generate an equal amount of wastewater having high BOD, COD, pH, turbidity, high temperature, colour etc. The environmental pollution problem for agro based paper mills is, however, different than from the large integrated paper mills due to lack of recovery systems and use of short fibre. Some of the conventional methods of treatments like activated sludge, trickling filters, stabilization lagoon, etc., are well known. However because of the colour and presence of refractory organics, physico-chemical methods are being used in the polishing stage to meet the stringent environmental standards. Amongst various tertiary treatment processes, coagulation using lime, alum etc. and adsorption have been commonly used in the treatment of pulp and paper mill effluents. In the present study, laboratory scale experiments have been conducted using poly aluminum chloride (PAC) as coagulant and low cost bagasse fly ash (BFA), which is generated in sugar mills, as adsorbent for removal of COD and colour from various pulp and paper effluents. Both have been found to be very effective in the COD and colour reduction. Percent COD and colour removal of around 80 and 90% were obtained for Pac whereas for BFA they were about 50 and 55% respectively. Also, effects of various other operating parameters like pH, initial concentration and contact time for coagulation and adsorption have been studied. Frendulich and Langumir adsorption isotherms were foun to fit best for the COD removal process. Experiments were also conducted for COD and colour removal of various effluents using two stages of treatment. Filterate or supernatant or clear solution obtained after first stage of treatment was treated in second stage using optimum dosage and pH of the respective effluents in the respective stages. These two-stage studies included BFA adsorption in both stages; PAC coagulation as first stage and iii BFA adsorption as second stage; and lignin precipitation using sulphuric acid as first stage and BFA adsorption or PAC coagulation or alum coagulation or lime coagulation as second stage. Two stage treatment using BFA in both stages gave combined percentage COD and colour removal of nearly 70% whereas two stage treatment using PAC as coagulant in first stage and BFA as adsorbent in second stage gives combined percentage COD and colour removal of nearly 88% and 92% respectively for different effluents. iv