Generation of chlorinated aromatic acids during sludge-composting and their fate in soils

Abstract

Changes in the concentrations of selective legume-seed germination inhibitors in raw and composted sewage sludge were analyzed to clarify their generation process. Their fate in soils was also investigated by the determination of the inhibitors which remained after incubation in two soils, a regosol and an andosol amended with sludge compost. The inhibitors, i.e. 3,4-dichlorophenylacetic acid (3,4-DCP), 3,4-dichlorobenzoic acid (3,4-DCB), 3,4,5-trichlorophenylacetic acid (3,4,5-TCP), and 3,4,5-trichlorobenzoic acid (3,4,5-TCB) were extracted efficiently with water and a larger amount was extracted with alkaline methanol from the composted sludge. Higher concentrations of 3,4-DCP and 3,4,5-TCP were extracted with alkaline methanol from the sludge than from the composted sludge, but the inhibitors were hardly extracted with water from the sludge. After composting of the raw sludge for 2 weeks, the concentrations of water-soluble forms of 3,4-DCP, 3,4,5-TCP increased remarkably. Heating of the raw sludge also increased the concentrations of water-soluble 3,4-DCP and 3,4,5-TCP. These results indicate that a large amount of 3,4-DCP and 3,4,5-TCP is present in the form of alkali-hydrolyzable conjugates in the raw sludge. These conjugates could be hydrolyzed into water-soluble free acids by heating and/or microorganisms during composting. These chlorinated aromatic acids were degraded more rapidly in the regosol than in the andosol. This phenomenon may be associated with a stronger sorption of the compounds in the andosol than in the regosol. Free acid forms could be degraded more easily than conjugate forms because the amounts of 3,4-DCB and 3,4,5-TCB in free acid forms were significantly reduced by incubation.