Traceability of organic contaminants in the sludge line of wastewater treatment plants: A comparison study among schemes incorporating thermal hydrolysis treatment and the conventional anaerobic digestion

The traceability of conventional pollutants and 10 organic microcontaminants in the sludge line of a wastewater treatment plant (WWTP) was evaluated. The application of thermal hydrolysis (TH) as pre-treatment to anaerobic digestion (AD) or as inter-treatment (between two AD stages) was considered and compared with the conventional digestion scheme. TH scenarios reduced the mass flow rate of biosolids (40-60%) as well as the ratio of solids (50-100%), organic matter (5-26%) and nitrogen (8-13%) destined to biosolids. Micropollutants showed a strong tendency to accumulate in the solid phase (more than 90% were sorbed) in spite of thermal and dewatering processes, but TH scenarios exhibited greater removal efficiency (80%) in comparison to conventional AD (50%), reducing the ratio of micropollutants destined to biosolids from a conventional 48% to 7-8%. These findings reveal that TH could increase the value of biosolids from sewage sludge treatment because of greater removal of pollutants and dewaterability.