Chemical and leaching properties of paper mill sludge

Paper mills produce large amounts of paper mill sludge in the treatment of process water. According to overburden studies, the hydraulic conductivity of the paper mill sludge originating from a paper mill in Northern Finland at a pressure of 30kPa was 4.4x10(-10)m s(-1), and 1.7x10(-10)m s(-1) at a pressure of 100 kPa. These values well meet the generally required values of between 1.0 x 10(-7)m s(-1) and 1.0x10(-9)m s(-1) for a geological barrier in landfill base and sides at landfills in the European Union. Paper mill sludge can also be used in the artificially constructed geological barrier layer of landfill cover structures. The angle of internal friction of the paper mill sludge was 34.8 degrees and the cohesion of inter-particle adhesion 23 kPa, which are important measures for assessing the shear strength of paper mill sludge and thus the stability of the landfill layer in which it is used. During a 28-day period, the biodegradability of the paper mill sludge in soil was ca 0.4% and in ground water under 1%, whereas according to the OECD 301F standard concentrations (BOD28), it was ca 8%. For the determination of total element concentrations in the paper mill sludge, the dried sample was digested using USEPA method 3052. A five-stage sequential leaching procedure was also used to fractionate trace elements in the paper mill sludge between the water-soluble (H2O), exchangeable (CH3COOH), easily reduced (HONH3CI), oxidizable (H2O2+CH3COONH4), and (5) the residual (HF+HNO3+HCI) fractions. This paper covers also examples of case studies how paper mill sludge is utilized in Finland.