Post – aeration of anaerobically digested sludge: optimization of the process and performance analysis

Sludge stabilization represents one of the most crucial aspects in sludge management. Anaerobic digestion is generally the most widespread method for sludge stabilization in large wastewater treatment plants, mainly because of the high energy recovery obtained by the biogas production. However, the digested sludge produced must be often treated with the aim of quality improvement. Several studies have highlighted the advantages of a post – aerobic stage following the conventional anaerobic digestion to improve the sludge properties. In this study, a post – aeration treatment of anaerobically digested sludge is proposed. This stage could successfully reduce TAN and organic compounds concentration, and improve the dewaterability properties. The sludge utilized for the experiments was collected at the Prague Central WWTP. Two experiments were conducted in a continuously mixed and aerated SBR reactor operating under different conditions, by changing SRT and intensity of aeration rate. In the first experiment, the following operating conditions was set up: during the first phase, the reactor was operated at 4 days SRT and 600 L/h aeration rate, while during the second one, it worked at 2 days SRT and 900 L/h aeration rate. In the second experiment, a 2 days SRT was maintained though all the experiment, while the air flow was increased in three steps: 600 L/h, 1200 L/h and 2000 L/h. As part of the same project, previous experiments were conducted, and their results has been reported in order to have a complete background and a deeper understanding of the topic. The previous experiments were characterized by the following parameters: SRT of 8 and 6 days and aeration rate of 300 L/h; SRT of 4 days and aeration rate of 300, 600 and 900 L/h. The parameters utilized to estimate the treatment efficiency are: Total Ammonia Nitrogen concentration, nitrite and nitrate nitrogen concentration, Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), Capillary Suction Time (CST), pH, Total Solids (TS) and Volatile Solids (VS) concentration. In order to assess the dewaterability properties of the sludge, a new analysis, alternative to the more utilized CST one, was proposed: the centrifuge method. The comparison between the results obtained from these different dewatering analyses was very interesting. Post – aeration significantly reduced TAN concentration in all the conducted experiments and it could be improved by either increasing the SRT or the air flow. As shown from the centrifugation method results, the treatment has the potential to increase the dry matter of the sludge after dewatering. The reduction of organic compounds (COD, TOC and VS) instead, resulted effective only at 8 days SRT.