Effect of Pharmaceutical Sludge Pre-Treatment with Fenton/Fenton-like Reagents on Toxicity and Anaerobic Digestion Efficiency

Sewage sludge is successfully used in anaerobic digestion (AD). Although AD is a well-known, universal and widely recognized technology, there are factors that limit its widespread use, such as the presence of substances that are resistant to biodegradation, inhibit the fermentation process or are toxic to anaerobic microorganisms. Sewage sludge generated by the pharmaceutical sector is one such substance. Pharmaceutical sewage sludge (PSS) is characterized by high concentrations of biocides, including antibiotics and other compounds that have a negative effect on the anaerobic environment. The aim of the present research was to determine the feasibility of applying Advanced Oxidation Processes (AOP) harnessing Fenton's (Fe2+/H2O2) and Fenton-like (Fe3+/H2O2) reaction to PSS pre-treatment prior to AD. The method was analyzed in terms of its impact on limiting PSS toxicity and improving methane fermentation. The use of AOP led to a significant reduction of PSS toxicity from 53.3 ± 5.1% to 35.7 ± 3.2%, which had a direct impact on the taxonomic structure of anaerobic bacteria, and thus influenced biogas production efficiency and methane content. Correlations were found between PSS toxicity and the presence of Archaea and biogas yields in the Fe2+/H2O2 group. CH4 production ranged from 363.2 ± 11.9 cm3CH4/gVS in the control PSS to approximately 450 cm3/gVS. This was 445.7 ± 21.6 cm3CH4/gVS (1.5 gFe2+/dm3 and 6.0 gH2O2/dm3) and 453.6 ± 22.4 cm3CH4/gVS (2.0 gFe2+/dm3 and 8.0 gH2O2/dm3). The differences between these variants were not statistically significant. Therefore, due to the economical use of chemical reagents, the optimal tested dose was 1.5 gFe2+/6.0 gH2O2. The use of a Fenton-like reagent (Fe3+/H2O2) resulted in lower AD efficiency (max. 393.7 ± 12.1 cm3CH4/gVS), and no strong linear relationships between the analyzed variables were found. It is, therefore, a more difficult method to estimate the final effects. Research has proven that AOP can be used to improve the efficiency of AD of PSS.