Experimental Research on the Total Hardness Control over Wastewater Treatment Recycling Process for Pulp and Paper Mill

In this paper, we presented an approach to control on the total hardness over the wastewater treatment recycling process for pulp and paper mill. The main problem, however, is the effects of Ca2+ concentration on the recycling treatment process. The experiments were conducted by using coagulation method with reagents of Ca(OH)2 addition alone and Ca(OH)2 +PAC addition respectively to control on total hardness. The samples were collected from the effluent and the influent of the biological treatment tank respectively of the wastewater treatment plant in a pulp and paper mill. The experimental results identified that the optimum reagent addition position was in the effluent of the biological treatment tank. When addition Ca(OH)2 alone, the optimum Ca(OH)2 addition amount was 2g/L, as a result, 70.1% of total hardness was removed ( from 1088.05mg/L down to 325.16mg/L), pH value was from 7.23 to 8.61, and sludge volume ratio was 9.9%. When addition PAC+ Ca(OH)2, the optimum Ca(OH)2 addition amount was 1g/L, the optimum PAC addition amount was 300mg/L respectively, as a result, 71% of total hardness was removed( from 1088.05mg/L down to 315.25 mg/L), pH value was from 7.23 to 7.66, and sludge volume ratio was 6.5%. The effluent-quality can meet the National Industrial Wastewater Recycling Standard on the total hardness of 450 mg/L requirement. The reagents of Ca(OH)2 +PAC had many advantages over Ca(OH)2 alone, such as less sludge quantity, high density of sludge, small volume of sludge, and less addition of reagents and so on.