Chemical changes during composting of a paper mill sludge–hardwood sawdust mixture

Abstract

Recycling of paper mill sludge (PMS) by composting is becoming an acceptable practice for converting these chemically complex materials into useful soil amendments, while eliminating negative environmental impacts. The organic composition of a PMS–hardwood sawdust mixture was investigated during composting to better understand the changes in main chemical components. Pyrolysis-field ionization mass spectrometry (Py-FIMS) and cross polarization-magic angle spinning 13C nuclear magnetic resonance (CP-MAS 13C NMR) were employed to characterize the organic composition of the PMS composted materials. The spectroscopic data revealed that the major components of the PMS were lipids, sterols, lignin, N-compounds, and carbohydrates. By the end of composting (at biomaturity), concentrations of carbohydrates and lignin became more prominent, while those of lipids, sterols and proteinaceous components decreased. Increases in carbohydrates and decreases in paraffinic C, proteinaceous C and C in OCH 3 groups appeared to be related to increased microbial activity. Other chemical changes observed during composting were increases in aromatic C, phenolic C, and in aromaticity. While the total C and N contents decreased by about only 12.0%, the compost lost 50% of its initial weight. At biomaturity, the compost consisted primarily of polysaccharide/carbohydrate materials, specifically cellulose and acidic polysaccharides (uronic acids) in combination with smaller quantities of lignin.