Pig manure as a co-composting material for biodegradation of PAH-contaminated soil

Pig manure at three different ratios of 12.5%, 25% and 50% (w/w dry weight basis) was amended with a soil spiked with 100 mg kg(-1) each of three polycyclic aromatic hydrocarbons (phenanthrene, anthracene, and pyrene) to investigate its effect on the biodegradation of these PAHs in a bench-scale composting system. An increase in pig manure amendment was effective in enhancing the amounts of soluble organic carbon, ammoniacal nitrogen, and soluble phosphorous in the composting mass. It could also increase the populations of total thermophilic and mesophilic bacteria as well as PAH-degrading bacteria, but this pattern was restricted only to the early stage of the composting process. High amounts of pig manure in the composting mass reduced the seed germination or root growth of cress seeds, but the composting process was effective in reducing the phytotoxic effects of the compost. Amendment of pig manure was beneficial to PAH removal during composting treatment and maximum removal rate at the end of composting accounted for 90% of the initial concentrations of PAHs. A pig manure application rate of 25% showed the most efficient removal of 3-ringed PAHs (phenanthrene and anthracene), while no significant difference in pyrene removal for those receiving 25 or 50% pig manure amendment. Taking into consideration the effects of pig manure on seed germination and available nutrients in the composting mass, this study suggested that a pig manure amendment of 25%, i.e., 3:1 ratio of contaminated soil: pig manure, is recommended to co-compost with PAH-contaminated soil.