Micropollutant biodegradation and the hygienization potential of biodrying as a pretreatment method prior to the application of sewage sludge in agriculture

Abstract

To date, the process of biodrying has been used for the drying of biodegradable waste materials to produce fuel. However, different applications other than incineration also exist for dried materials: e.g., sewage sludge could be used as a fertilizer. This paper investigates the potential of biodrying as a pretreatment method prior to the application of sewage sludge in agriculture. Experiments were performed in two biodrying reactors with operational volumes of 100 dm3 using two different aeration regimes to compare the efficiency of the two process alternatives: i) mesophilic conditions and ii) a full composting temperature profile including the thermophilic phase. The influence of the two different operating regimes on the elimination of pathogens and selected pharmaceuticals and personal care products (PPCPs) was investigated. Phospholipid fatty acid (PLFA) analysis was employed to identify the main groups of microorganisms responsible for the potential elimination of the detected micropollutants. The results showed that antibiotic activity was completely eliminated under both the operational regimes. Bisphenol-A (BPA) and irgasan (IRG) were detected in the untreated sludge, and the elimination efficiency for these substances was greater in the reactor with the mesophilic treatment (max. 88.3% for BPA and 47.7% for IRG) than under thermophilic conditions (60.1% for BPA and no significant removal of IRG). PLFA analysis indicates that Actinobacteria could be responsible for the biodegradation. Of the two indicator microorganisms used for the verification of the efficiency of the improvement in hygiene, only E. coli were completely eliminated under both regimes, while the enterococci content was reduced by 4 orders of magnitude. The results of this study document that the biodrying process possesses the potential for the removal of micropollutants from sewage sludge and might serve as a pretreatment process prior to using the treated sludge in agriculture.