Comparison between Composting of Untreated and Anaerobically Treated Paper Mill Sludges

Manufacture of pulp or paper results in the production of several byproducts including pulp sludge, wood yard debris, bark, and ash. Mill sludges are either landfilled or anaerobically digested by placing them in treatment ponds. Aerobic composting is a technically feasible and environmentally sustainable alternative that is being explored by the pulp and paper industry. This article reports on aerobic composting of seven different virgin Kraft mill sludges (three untreated, three anaerobically treated for 6 months, and one anaerobically treated for 18 months) and compares these treatments using dry matter degradation, compost stability, and compost maturity at the end of 30 d of composting as indicators. Results confirm that the type of sludge (i.e., degree of pretreatment) significantly affects the amount of degradation during aerobic composting and the stability of the product after 30 d of composting. Average dry matter loss during composting of untreated primary sludges, 6–month treated secondary sludges, and 18–month treated tertiary sludges were 20.5%, 11.4%, and 0.4%, respectively. Average stability indices at the end of test were 0.39 and 0.25 mg–O2/g–VS/h for primary and 6–month treated secondary sludges, respectively. Maturity indices of compost products at the end of 4 weeks of aerobic treatment were found to be independent of the type of sludge (primary, secondary, or tertiary).