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For proper treatment, recycling, or disposal of the pulp and paper mill secondary sludge qualitative and quantitative determination of its characteristics are necessary. Chemical extraction, quantitative characterization, and spectroscopic experiments have been performed to determine the molecular composition and chemical functionality of a pulp and paper mill secondary sludge. In order to extract the low-molecular-weight substances, soxhlet extraction with polar and non-polar solvents was performed where most of the target substances (17±1.3%.) were extracted after 2 hours. Over time, this extraction followed a first-order kinetics. Fiber analyses have shown 12±3% lignin, 28±3% cellulose, and 12±4% hemicelluloses content. The ash content was about 17±0.5%. In this work, 7 and 16% intra- and extracellular polymeric substances, respectively, were extracted from the secondary sludge. EPS and mixture of intra- and extracellular biopolymers have shown similar chemical functionalities. These analyses confirmed that the paper secondary sludge consisted mainly of wood fiber, i.e. lignocellulosic substances, along with proteins and polysaccharides originated from microorganisms.

Secondary sludge from pulp and paper mills can be considered as potential filler for composite industry. Enzymatic modifications of the waste secondary sludge from pulp and paper mills to reduce the hydrophobicity and increase the molecular weight have been carried out by lipase and laccase, respectively. The enzymatic modification was performed to enhance the reinforcing capability of the secondary sludge for further composite production. The lipid content of the secondary sludge, which was measured to be 6 AE 0.5%, was hydrolyzed by lipase from Candida rugosa and the structural changes were followed by Fourier transform infrared (FTIR) spectroscopy. Laccase from Trametes versicolor was tested for its activity and reaction rate in the secondary sludge and the alkali-extracted lignin. Characterization of the sludge before and after the laccase treatment was carried out by FTIR spectroscopy. High pressure size exclusion chromatography (HPSEC) was applied to determine the molecular weight distribution of the lignin samples and also as a means for comparing modified and unmodified samples. Biokinetic parameters for the Michaelis-Menten kinetic model as a function of dissolved oxygen concentrations were determined the K m values to be 3.491 and 2.318 g/m 3 for sludge and the alkali extract, respectively. The FTIR results on the laccase-treated secondary sludge showed clear changes in the molecular structure, which was mainly attributed to the crosslinking reactions and generation of new bonds. Moreover, the HPSEC results revealed that laccase modifies the sludge by increasing the molecular weight. The manufactured nylon/sludge composites showed lower tensile strength for the lipase treated sludge/nylon composite. However, the laccase-treated sludge/nylon composite showed a statistically significant increase in the mechanical strength, which is attributed to the increase in the components molecular weight.

Secondary sludge from pulp and paper mills can be considered as a potential filler for composite industry. The surface thermodynamics of the secondary sludge from two different pulp and paper mills, processing capability, and material characteristics of biocomposites filled by secondary sludge were studied in this study. Inverse gas chromatography (IGC) has been employed to study the surface characteristics of the secondary sludge. Also, the cellular biopolymers were extracted and their surface energy determined by IGC. Based on the surface thermodynamics and the chemical structure of the secondary sludge, Nylon 11 was selected as the polymeric matrix. The dispersive component of surface energy for the secondary sludge samples was obtained in the range 60—42 mJ/m2 measured at 313—373 K, which is high enough to allow the biosolid to be coupled with conventional polymeric resins. The manufactured nylon/sludge composites showed acceptable, yet not improved, mechanical strength. Also, 10% of dried sludge as filler proved to be an effective amount, which is sufficient to fill but not deteriorate the tensile and flexural strengths of the composite. Sludge-filled composites compounded by a twin-screw extruder exhibit considerably better tensile properties than those compounded by the K-mixer. Maleated polyolefins used as coupling agents also improved the composite’s mechanical properties significantly.