Assessment of the toxicity and the fertilizing power from application of gamma irradiated anaerobic sludge as fertilizer: Effect on Vicia faba growth

With increasing sludge produced every day by wastewater treatment plants, new methodologies have been developed to reduce sludge or to valorize them at lower cost and energy. Among these methodologies, agricultural manuring is of great interest as sludge could complement or replace chemical fertilizers. However, sludge pretreatment was necessary prior to its use in order to eliminate the degradable organic contaminants, heavy metals and diverse pathogenic microorganisms. In this study we performed gamma-irradiation of anaerobic sludge coming from wastewater treatment plants at different doses (0.5, 1.5, 2.5, 3.5 and 4.5 kGy), followed by anaerobic digestion. We then used them in addition to soil for Vicia faba cultures. Heavy metal concentrations, toxicological evaluations, and the number of cultivable pathogenic bacteria were also investigated in irradiated and non-irradiated sludge samples to determine the effect of gamma-irradiation on toxicity and pathogens present in the sludge. Results show that 4.5 kGy is the optimal dose, and gamma-irradiation pretreatment of anaerobic sludge added to soil (2 mL/30 g) has significantly improved Vicia faba growth. Furthermore, by eliminating pathogens and by degrading chemical pollutants, gamma-irradiation of anaerobic sludge provides promising insights for re-using sludge as a safe fertilizer.