How the Choice of Primary Treatment Affects the Biogas Potential of Primary Sludge

Sammanfattning: The global heating has resulted in rising temperatures and more extreme weather conditions around the world. In combination with increasing energy demands, this has led to an increasing demand of renewable energy. One option is utilizing the wastewater to produce biogas. There-fore, wastewater can be seen as a resource rather than waste. Biogas production is based on anaerobic digestion where microorganisms transform organic matter into methane. Primary treatment in wastewater treatment plants (WWTPs) remove mostly the particulate or-ganic content, therefore the sludge gathered in this stage is rich in organic matter. The primary treatments available are the conventional primary settling treatment (PST) and filtration, e.g. ro-tating belt filter (RBF), where both can be combined with chemically enhanced primary treat-ment (CEPT). RBF requires less space in comparison to PST and might create a sludge with a higher energy output, due to a different composition of RBF sludge, e.g. it has a higher content of cellulose. RBF sludge has been found to have a higher biogas potential than PST sludge, when comparing primary treatment from different WWTPs. The objective of the project was to determine if RBF has higher biogas potential than PST by using PST sludge and CEPT-RBF sludge from the same WWTP. Further, the biogas potential at mesophilic and thermophilic temperatures were tested. Settled sludge and settled CEPT sludge were produced in order to evaluate the potential of sludge without storage time in settlers. Bio-methane potential (BMP) tests were used to evaluate the biogas potential. An automatic methane potential test system (AMPTS) was used. The AMPTS is a set up where reactors with substrate and inoculum is sealed and placed within a thermostatic water bath and connected through tubes to bottles that absorb water and carbon dioxide. In turn, these bottles are connect-ed to a device that registers the gas that passes through it continuously. The characterization of sludge from PST and RBF sludge indicated that different primary sludge have different compositions. The different types of RBF sludge (the CEPT-fermented RBF, the RBF and the CEPT-RBF) had the highest carbohydrate content, supporting the hypothesis that there is more cellulose in the RBF. That is reasonable since cellulose has a higher buoyancy and does not settle easily, it is more readily removed by filtration as RBF. The PST sludge had the lowest content of carbohydrates, while the protein and fat content could vary. The results showed that thermophilic conditions improved the biogas potential. The BMP had a range from 100 to 250 NmL CH4/g VS at mesophilic conditions for different sludge. In com-parison, the thermophilic BMP test had a BMP range of 150-325 NmL CH4/g VS. In all cases, the BMP of RBF sludge was larger than that of PST sludge. Further, CEPT increased the bio-gas potential when implemented, while the CEPT-fermented RBF sludge had a lower BMP than non-fermented CEPT-RBF sludge. The sludge produced at the laboratory that simulated differ-ent types of PST sludge with a short storage time had a higher biogas potential than both the CEPT-RBF and PST sludge. This data indicates that hydrolysis that occur during storage of primary sedimented sludge can decrease the biogas potential. Therefore, the results indicate that the difference in composition has a significant impact on the biogas yield. Since the content of the different types of sludge have an effect on the biogas pro-duction, it can be concluded that there is a connection between the primary treatment chosen and the BMP.