Windrow Composting of a Pulp and Paper Mill Sludge: Process Performance and Assessment of Product Quality

Abstract

Pulp and paper mill sludges (PMS) are a significant by-product of the paper making industry worldwide, and composting with mineral nutrients in Tasmania is viewed as the most environmentally acceptable technology to convert this material into a useful horticultural or agricultural product, thereby eliminating the need for landfilling. The objective of this study was to determine the feasibility of composting PMS in large-scale periodically turned windrows based on optimal conditions previously determined in laboratory-scale reactors. Performance of the composting process and quality of the composted PMS is reported. PMS was composted with mineral nutrients at an initial C:N:P:K ratio of 35:1:0.6:0.1 in three windrows of 25 m in length, 3 m in width and 2 m in height with weekly turning for 21 weeks, following which the trial was terminated. Temperature at the pile center remained above 50°C for the duration of the trial, with pile pH peaking at 7.46 after 11 weeks. At termination, phytotoxicity was absent and pile volume had decreased by 45 percent, partly due to a 31.6 percent increase in bulk density and a decrease in gravimetric water content from 71.4 percent before to 63.7 percent after composting. Due to the moderate level of aluminum in the PMS compost, substitution of the aluminum sulfate flocculant used in primary clarification and sludge dewatering with a nonaluminum based flocculant may be required to reduce the potential for aluminum toxicity in plants. The C:N ratio was reduced from 218:1 prior to mineral nutrient addition to 23:1 after composting with a concomitant rise in electrical conductivity from 0.59 dSm−1 to 2.78 dSm−1, possibly making the material unsuitable for direct contact with plants sensitive to moderately saline conditions. The electrical conductivity could be reduced to less than 2 dSm−1 by substituting some of the nitrogen and potassium amendments with nonsalinizing nutrient forms, thereby substantially improving the quality of the composted PMS.