Composting of organic waste for enhanced bioremediation of PAHs contaminated soils
Polycyclic Aromatic Hydrocarbons (PAHs) are organic pollutants widely distributed in the environment and very frequently detected in soils as they are mainly produced from an incomplete combustion of organic matter. They are toxic and even carcinogenic, therefore their removal from soils has been massively studied in the past years. Among all treatments suitable for treating soils contaminated by PAHs, biological ones are promising and challenging as they have a low or even no impact on the environment as well as their efficiency is tightly dependent on many factors not easy to control. The objective of this thesis has been to get more accurate knowledge on bioremediation of PAHs contaminated soils, by defining, through a series of experiments, the most suitable conditions for their biological remediation mainly in terms of physical and chemical soil characteristics, PAHs type and concentration, microbial density and composition, pH value, moisture content and availability of nutrients. The bioremediation experiments carried out in this thesis are based on landfarming as well as composting soil treatments and aimed at promoting the simultaneous biodegradation of PAHs and fresh organic wastes under controlled conditions. The expected result of this approach has been the conversion of organic pollutants into less harmful compounds, due to the activity of the microorganisms present in soil as well as in the organic waste added to soil. With the aim of deeply understanding the influence of the aforementioned factors on the bioremediation of PAHs contaminated soil, experiments were conducted on a synthetic soil, artificially contaminated, as well as on a real contaminated soil. In details, four different types of fresh organic wastes were selected to be added to PAHs artificially contaminated synthetic soil and the results showed that centrifuged activated sewage sludge (SS) was the most effective organic amendment compared to buffalo manure (BM), food and kitchen waste (FKW) and fruit and vegetable waste (FVW). The removal efficiency of total PAHs reached with SS was actually a little higher than 60%. Furthermore, this set of experiments proved that mesophilic conditions were more performing than thermophilic conditions, as well as the content of nitrogen, soluble fraction and proteins played an important role in the PAHs removal. Based on the previous results, the next set of experiments was conducted on a real contaminated soil amended with different amounts of centrifuged activated SS (e.g. contaminated soil to SS mass ratios were 1:2, 1:1, 1:0.5 and 1:0 as wet weight basis). In contrast to results obtained with synthetic soil, in a real contaminated soil, SS amendment resulted in being not beneficial to PAHs removal. The best results were actually achieved where no SS was added (total PAHs removal efficiency of 32%, whereas with SS the best results showed a value of 14%), proving that the adaptation of microorganisms to PAHs is the key factor for the success of the bioremediation process coupled with setting favorable environmental conditions.