Assessment of anaerobic treatment of select waste streams in paper manufacturing operations
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The most common strategy for handling paper mill solid waste is typically disposal in landfills. However, several drawbacks are associated with this type of solid waste management, such as increasing costs due to oil price rise, governmental restrictions on land use, and environmental concerns such as leaching of disposed contaminants into groundwater, as well as methane generation of and release to the atmosphere, thus contributing to global warming. An alternative to reduce solids prior to disposal and to recover methane as a renewable fuel is anaerobic digestion, but it is not yet clear whether such an approach is feasible in paper mills. In this study, the anaerobic digestion of paper mill waste streams was evaluated for a paper plant located in Belen, Costa Rica, to investigate up to what extent certain waste streams can be anaerobically digested, to what extent energy can be produced in the form of methane for implementation in a wastewater treatment plant and to evaluate the conditions that will favor methane generation from select waste streams. Batch assays were performed to evaluate the biodegradability of single and combined waste samples under ideal, laboratory conditions. Samples were obtained from the manufacturing plant as well as the wastewater treatment plant at the paper mill under study. The ultimate biodegradability ranged 25 to 85% in terms of volatile solids destruction, corresponding to the waste activated sludge (WAS) and Flotation Cell rejects, respectively. The COD destruction of single samples ranged from 45 to 63%, corresponding to WAS and wastewater treatment plant (WWTP) dissolved air flotation (DAF) skimmings, respectively. Methane generation ranged from 80 to 190 ml at 35oC/g COD added for all single samples (excluding underflows). In combination Feed 1 was reduced by 46 and 52% and Feed 2 by 27 and 38%, respectively. Two combinations of two single samples each (Feed 1 and 2), formulated according to plant operational data, and their solids and COD destruction as well as methane generation in semicontinuous flow anaerobic digesters were evaluated at different solids retention times (30, 20, 15, and 7 days). Nutrients (N, and P) availability as well as alkalinity in the plant waste streams were evaluated and minimum supplements were used to support an efficient anaerobic digestion process. The reactors reached stable operation at all retention times evaluated. Methanogenesis was the predominant, terminal metabolic process under anaerobic, mesophilic conditions, but the overall process rate was determined by the hydrolysis of the particulate substrate. Reactors fed with Feed 1 achieved the highest level of destruction, which amounted to 85% of phosphorus that is typically present in paper mill wastes. Alkalinity addition to the feed (3.5 mg NaHCO3/L) was necessary to maintain the reactors pH above 6.9.