Windrow composting of paper mill by-products: scale-up and seasonal effects

A significant portion of by-products generated at pulp and paper mills are biodegradable organics. Presently, over 70% of these by-products are disposed in landfills. Composting can be an effective process to stabilize and reuse them in value-added applications. Laboratory research has shown that adding nitrogen amendments to a mixture of paper mill by-products to achieve a CN ratio of approximately 130 is sufficient to compost these organics. This paper describes evaluating the laboratory-developed mixes and amendments in a full-scale pilot. Two trials, one in fall and one in winter, were conducted to quantify seasonal variations. Each trial consisted of four windrows of approximately 85 tons each; two were amended with chicken litter and two with ammonium nitrate. Temperature, moisture, C:N ratio, volatile solids, pH, soluble salt contents, stability and maturity were monitored over the 76-120 days of composting. Data collected were analyzed using a general linear model repeated measures design. Results indicate that seasonal variations in process were significant at the 10% level for temperature, moisture content, volatile solids content, soluble salts and stability. Although differences in process performance existed between fall and winter, the results of this study clearly showed that mill solids composted in this process reached a satisfactory level of stability within 76 days in winter and 120 days in the fall trial. Final product stability and maturity ranged between stability indices of 0.05 to 0.26 mg02/gSolids/hr and germination indices of 78.4% to 100%, respectively. Although the compost product from the winter trial was more stable (mg(02)/ g(Solids)/hr basis), the germination indices were lower indicating phytotoxicity resulting from a less desirable composting process. Addition of low levels (5 to 6 g (Ammoniurn nitrate)/kg(Cornposting blend)' dry basis) of nitrogen amendment was sufficient to develop an active composting environment, thus confirming that laboratory developed mixes perform similarly at full-scale. A key limitation in scale-up was the oxygen availability within windrows, which affected the duration of composting required to achieve stability. Stability levels of 0.10 mg(02) / g(Solids)/ hr were achieved only after a minimum of 11 weeks (76 days in winter trial) in full-scale, compared to four weeks during laboratory trials.