Abstract

Over the last two decades, the production of excess sludge has increased rapidly due to a more stringent legislation on effluent quality and a growing number of new plants, becoming an economic and an environmental critical issue. Processing excess sludge could account for half up to 65% of the total operation costs of a wastewater treatment plant. Technologies to reduce the excess sludge had been widely studied. Several studies reported that the technologies integrated in the wastewater handling units should be cost effective and preferable rather than the techniques integrated in the sludge handling units, as they allow to reduce the sludge production rather than treat it. Thus, the development and the optimization of a technology able to reduce the sludge production in the water line is now challenging. A lot of technique have been developed such as biological, thermal, high temperature oxidation, mechanical treatments, ultrasonication, ozonation or by using chemical compounds. Some of these have been proven not energy saving, while others can negatively affect the effluent quality of the process due to the formation of by-products. Among others, biological treatments are a challenging strategy for sludge reduction. In recent years, several studies showed that including an anaerobic bioreactor in the returned activated sludge line of a conventional activated process could significantly enhance the sludge reduction without causing negative effects on operational performances. Today, this configuration is known as anaerobic side-stream reactor (ASSR) process. Several laboratory applications highlighted that the sludge yield of the ASSR process could be reduced up to 60% compared to a conventional activated process. Despite the highest percentage of sludge reduction achieved, the process is still little applied to real scale because its main operating parameters and sludge reduction mechanisms are still unclear. This study focused on the verification of ASSR process, the mechanisms of sludge reduction and the microbial structure of the process. During the first part of the research, a laboratory experimental system was designed and implemented. A sequencing batch reactor (SBR), to simulate the water line of a real wastewater treatment plant, and an ASSR as a sludge treatment unit composed the system. Unlike most of the previous studies, the system was fed with real urban wastewater in order to obtain results that reflect as much as possible what can really happen to a municipal WWTP. Through a critical analysis of the literature, the influence of two important operating parameters, such as the solid retention time (SRT) of the ASSR and the interchange rate (IR), which means the percentage of biomass cycled into the ASSR, had been uncovered Given this, the experimental system was started up and reached a stable condition after 60 days. The research was developed in three different phases that lasted for about 90 days each. The experimental lab system was tested under three configurations: i) 10% sludge interchange rate and SRT in the ASSR of 10 days; ii) 20% sludge interchange rate and SRT in the ASSR of 5 days and iii) 40% sludge interchange rate and SRT in the ASSR of 2.5 days. The observed sludge yield (Yobs) of each phase was evaluated and was equal 0.21 g TSS/g COD, 0.14 g TSS/g COD and 0.12 g TSS/g COD in Phase I, II and III, respectively. These results confirmed that the process could significantly decrease the sludge production and a reduction up to 62% could be achieved. To explain the results obtained in terms of sludge reduction, different tests and analysis were performed. The release of soluble COD and ammonia in the ASSR have highlighted that the endogenous decay and cell lysis mechanism occur in the ASSR. Extraction of EPS, with CER and BASE methods, showed a release of protein and polysaccharides in the bulk solution that increased passing between Phase I and III. At the end of each experimental phase, batch tests were carried out to evaluate the activity of phosphorus accumulating organisms (PAO) and denitrifying phosphorus accumulating organisms (DPAO). Recirculation in SBR-ASSR selects DPAO microorganisms. This was a result of great interest because DPAO could enhance the biological nutrient removal since nitrogen and phosphorus can be simultaneously removed. Furthermore, DPAO has lower cell yield than PAO resulting in lower sludge production. Results showed an activity of PAO, DPAO and other slow growers such as sulfate reducing bacteria. All these results suggested that the high percentage of sludge reduction could be explained as a combination of aspects, such as the cell lysis, the cryptic growth, the selection of slowing microorganisms and EPS destructuration. The SRT and the IR could be considered as main parameters and their variation could significantly affect the performance of the process. Microbial analyses were carried out to investigate the bacterial and archaeal structure of the ASSR sludge during each phase.The results confirmed the presence of several bacteria that are typically heterotrophic responsible of hydrolysis and fermentative process of organic matter. Several slow growers bacteria were also detected. Moreover, according to the batch tests on PAO and DPAO activity, a relevant increase in Phase III of some genera able to enhance the biological phosphorous removal has been observed. In summary, the research found that the ASSR process is a sustainable solution for the sewage sludge reduction due to an efficient and a low sludge production, able to ensure both carbon, nutrients and phosphorous removal applying an extremely simple technology, easy to realize both in new and in existing wastewater treatment plants.