The influence of Pre-treated Waste Activated Sludge on Anaerobic Digestion Efficiency and Microbial Community Structure

Anaerobic digestion is widely applied for the treatment of the continually increasing amounts of waste sludge that are worldwide generated in biological wastewater treatment plants. Its main benefits include (i) the large reduction in sludge volume, which is beneficial to lower the downstream processing costs for sludge treatment, including transportation costs, and (ii) the production of an energy rich biogas (55-70% CH4), which is energetically valorised in gas engines or can be upgraded to natural gas purity by the use of gas membranes, absorption or combined techniques. The mechanisms of anaerobic digestion are very complex, requiring the interaction between various classes of microorganisms to eventually produce biogas. One of the major drawbacks, however, is the limited conversion of organic matter, and the required long retention time (15-20 days), as a result of the presence of rigid and non-biodegradable organic structures. It is generally agreed upon that the first step of the anaerobic digestion, the hydrolysis, is rate limiting for the entire process. During hydrolysis, complex macromolecular structures (such as extracellular polymeric substances) are converted into simple, soluble organics. The application of pre-treatment techniques (e.g., ultrasound, microwave, electrokinetic and electrolysis treatment) facilitates this step by causing a partial disintegration of the sludge (i.e., solubilisation of organic material), which will, in turn, result in higher degradation efficiencies (leading to a higher biogas yield) and shorter retention times during anaerobic digestion. The first part of the thesis is dedicated to the effect of different disintegration techniques on the sludge characteristics and subsequent anaerobic digestion on lab-scale. The investigated pre-treatment techniques are microwave (compared with ultrasound), electrokinetic disintegration, and electrolysis. These techniques will be evaluated towards their ability to both altering the sludge characteristics by means of solubilising particulate organic matter and hence affecting the biogas production in comparison with a (non-treated) control digestion. At lab-scale, it was noticed that the applied treatments do not result in an increased biogas yield for the treatment conditions applied. It also must be noted that the absolute concentration of the soluble organics varied significantly. However, this may represent a misleading picture about the effectiveness of the pre-treatments, since this depends largely on the initial concentrations. The results from the lab-scale experiments were inconclusive. Therefore, experiments on a larger pilot-scale (50 L) were performed. Despite the higher biogas production in the MW and US pre-treated digesters, the outcome of the economic analysis is inconclusive: the energetic value of the extra biogas produced does not balance pre-treatment costs. It appears that only operations applying a low specific energy (< 4.54 kJ/kg sludge), i.e., in this work the electrokinetic and electrolysis pre-treatment seem economically viable. A model to predict the biogas production was built with Partial Least Squares (PLS) regression, based on a dataset (14 data points) consisting of solely non-treated sampling points selected from a second pilot-scale digestion test run. The model was built on 2 components and all 7 variables (analysed for 14 data points) are used. The resulting model is subsequently (after validation) applied on two new datasets: (i) a dataset built from the untreated (control) samples of last test run, and (ii) a dataset constructed from the digesters subjected to various pre-treatments techniques. It can be concluded that the biogas production for (un)treated sludge can be well predicted. The results from 454 amplicon pyrosequencing (Bacteria and Archaea) and qPCR (Archaea) allowed to gain some insights into the structure and dynamics of the microbial community found in anaerobic digesters fed with untreated and pre-treated sludge. The hydrogenotrophic methanobacteriales (Archaea) were the most abundant at the start of the digestion and were thereafter overtaken by the acetoclastic methanosarcinales. From the results based on qPCR, methanosarcinales were shown to prevail from the beginning of the digestion. The largest change in community structure (nMDS ordination) was achieved in the first phase of the digestion test run, before pre-treated sludge was fed to the digesters. Contrary to what was expected, the observations cannot be linked directly to disintegration or solubilisation of the sludge caused by the applied pre-treatments.nrpages: 173status: publishe