Temperature effects on the decomposition of paper mill sludges in soil

Paper mills produce large amounts of sludge from the treatment of process water during the paper making process. These wastes comprise organic materials that differ significantly from municipal and other industrial wastewater sludges. Land-spreading paper mill wastes may be an acceptable alternative to other methods of disposal. More information is needed about these unique sludges in order to develop appropriate management practices for land-spreading programs. Rates of mineralization are key to understanding the dynamics of these soil-applied materials. A laboratory incubation was carried out to determine C and N mineralization of two contrasting paper mill sludges amended to an agricultural soil. The incubation lasted approximately 33 weeks at 12°C and 25°C, and at relatively constant soil moisture content. The combined sludge (a combination of primary and activated paper mill sludges; 0.8% N) was applied at rates up to 160 g kg -1 soil. The total amount of CO 2 produced was similar at 12°C and 25°C, but the 25°C incubation temperature elicited higher rates early in the incubation. The second treatment (activated sludge; 3.3% N) consisted of application rates up to 90 g kg -1 . C mineralization from this treatment was lower at 12°C than at 25°C, but the general shape of the curves was similar. N mineralization from the combined sludge at 12°C was proportional to rate of sludge application and was approximately linear. However, soil amended with combined sludge incubated at 25°C displayed a pronounced immobilization period during the first 80 days of incubation, followed by a high rate of N remineralization. N mineralization from the secondary sludge treatment revealed generally linear rates at 12°C, accumulating mineral N in approximate proportion to the amount of sludge added. The highest rate of addition resulted in approximately 165 mgN kg -1 released. At 25°C, N mineralized at higher rates than at 12°C and resulted in a higher total N accumulation; the highest sludge treatment contained greater than 220 mg mineral N kg -1 at the end of the incubation period. Differences in decomposition characteristics were attributed partly to substrate quality effects and partly to the influence of N availability on C mineralization in these sludges.