Anaerobic treatment to improve sludge recovery at a deinked fiber pulp and paper mill

Investigations were conducted to explore the use of anaerobic treatment options that may allow complete recovery of primary and waste activated sludge streams for incineration from a graphic paper mill with an integrated deinked pulp plant. The effective reduction of waste activated sludge quantity (55%-65%) was obtained by the anaerobic treatment of wastewater from the deinked pulp plant or by the anaerobic stabilization of the mill effluent from primary treatment, where chemical oxygen demand (COD) removal efficiency of 67.6% and 64.8%, and biogas potential of 440 and 496 m(3)/t CODremoved were achieved, respectively. The anaerobic treatment of wastewater and alkali hydrolysis of waste activated sludge decreased the dewatering properties of solids residue in treated effluent resulting in a negative impact on the sludge recovery energy balance. The addition of municipal wastewater to anaerobic treatment in the amount of 18 vol%, representing 10% of the total COD, increased the reduction of waste activated sludge to 78%, with a positive impact on the treatment costs and efficiency. Anaerobic treatment of the studied effluents may enable incineration of all mill sludge with a positive energy balance, a reduction of biological wastewater treatment operational cost by up to 83%, and mill conventional energy demands by up to 2.7%, resulting in total cost savings of up to EUR 5.2 (USD 5.60) per ton of paper.