The influence of different temperature programmes on the bioremediation of polycyclic aromatic hydrocarbons (PAHs) in a coal-tar contaminated soil by in-vessel composting

The biodegradation of 16 US. EPA-listed polycyclic aromatic hydrocarbons (∑PAHs), with accompanying humification and microbial community structure changes during simulated in-vessel composting-bioremediation of an aged coal-tar-contaminated soil amended with green waste were studied over 56 days. The experimental design compared one constant temperature profile (TC = 38 °C) with three variable temperature profiles (TP1, TP2 and TP3), including treatment at 70 °C to comply with regulatory requirements. Greatest ∑PAHs removal (75.4 ± 0.1%; k1 = 0.026 day−1, R2 = 0.98) occurred at TC = 38 °C compared to all variable temperature profiles TP1 (62.1 ± 11.0%; k1 = 0.016 day−1, R2 = 0.93), TP2 (71.8 ± 8.2%; k1 = 0.021 day−1, R2 = 0.95) and TP3 (45.3 ± 9.7%; k1 = 0.010 day−1, R2 = 0.91). This study proved that using thermophilic temperatures (70 °C) towards the end of in-vessel composting processes (TP2) resulted in greater ∑PAHs removal than using other variable temperature profiles (TP1, TP3), as long as the increase was stepwise via an intermediate temperature (55 °C). Phospholipid fatty acid (PLFA) signatures indicated that use of thermophilic temperatures towards the end of the in-vessel composting-bioremediation (TP2) resulted in a higher fungal to bacterial PLFA ratio and a lower Gram-positive to Gram-negative (G(+)/G(−)) bacterial ratio. Fluorescence excitation-emission matrix (EEM) showed the presence of peaks typical of humic-like (Ex/Em wavelength pair ∼340/460 nm) and fulvic-acid-like (Ex/Em wavelength pair ∼245/460 nm) substances, indicating mineralization and/or maturation of the compost. Varying the temperature during in-vessel composting to comply with regulatory requirements for pathogen control, promoted contaminant biodegradation, microbial activity and compost maturation.