Conventional and Innovative Hygienization of Feedstock for Biogas Production: Resistance of Indicator Bacteria to Thermal Pasteurization, Pulsed Electric Field Treatment and Anaerobic Digestion

Animal by-products (ABP) can be valorized via anaerobic digestion (AD) for biogas energy generation. The digestate issued from AD process is usually used to fertilize farming land for agricultural activities, which may cause potential sanitary risk to the environment. The European Union (EU) requires that certain ABP be thermally pasteurized in order to minimize this sanitary risk. This process is called hygienization, which can be replaced by alternative nonthermal technologies like pulsed electric field (PEF). In the present study, Enterococcus faecalis ATCC 19433 and Escherichia coli ATCC 25922 were used as indicator bacteria. Their resistance to thermal pasteurization and PEF treatment were characterized. Results show that Ent. faecalis and E. coli are reduced by 5 log10 in less than 1 min during thermal pasteurization at 70 °C. The critical electric field strength was estimated at 18 kV∙cm−1 for Ent. faecalis and 1 kV∙cm−1 for E. coli. “G+” bacteria Ent. faecalis are generally more resistant than “G-” bacteria E. coli. AD process also plays an important role in pathogens inactivation, whose performance depends on the microorganisms considered, digestion temperature, residence time, and type of feedstock. Thermophilic digestion is usually more efficient in pathogens removal than mesophilic digestion.

Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain

An evaluation of the energy and economic performance of thermal hydrolysis technologies is carried out on a theoretical basis. The wastewater treatment plant (WWTP) of Burgos (Spain) was the base scenario of this evaluation. Energy and mass balances were established considering the registered data of primary and secondary thickened sludge in the WWTP for 2011 to 2016. These balances were analysed considering five different scenarios, taking as Scenario 1, the plant operating with conventional mesophilic digestion. The scenarios considered commercially available technologies. The best results were obtained when hydrolysis was applied to digested sludge and sludge from the Solidstream® process. These two scenarios showed the best performance regarding volatile solid removal and lower demand for live steam, achieving a higher amount of biogas available for valorisation using combined heat and power (CHP) units. The main advantage of the hydrolysis process is the decrease in the volume of digesters and the amount of dewatered sludge needing final disposal. The Solidstream® process allowed a 35% increase in biogas available for engines and a 23% increase in electricity production.

Anaerobic mesophilic digestion of sludge with extra-thermophilic and high ph pre-treatment

In this study different methods of sludge pre-treatment with elevated temperatures andpH have been selected for investigation. Five sets of sludge samples were pretreated asfollows: heating at 70°C for 30 min, at 80°C for 20 min, at 90°C for 10 min, at 100°C forWWTP5 min and NaOH-treatment (pH 12 for 4 hours). For comparison a sample from Tallinn(3:1 mixture of primary (dry solids 5.2%) and activated sludge (dry solids 0.5%))was used. Extra-thermophilic pre-treatment increased the degree of hydrolyses of sludge,enabling the following anaerobic digestion process to proceed faster than that of rawsludge. However, extra-thermophilic pre-treatment was insufficient for removal ofnitrogen and phosphorous. Nitrogen can be easily solubilized during extra-thermophilicpre-treatment but solubilization of phosphorous occurs only through digestion. Theproduction of biogas during anaerobic digestion was also dependant on the quality of theinput of pre-treated sludge. The highest cumulative volume of biogas (124 mUg CODadded) was achieved by pre-treatment at 80°C. The percentage of mineralization of pretreated sludges on mesophilic digestion was the highest with 90°C sample - 65.9%.

Biogas production from agricultural and municipal waste

The article presents the results of biogas production from municipal and agricultural waste. The mesophilic digestion of feedstocks was evaluated. Fermentation process was performed according to the DIN 38414-S8. Five types of agricultural waste were used for the research: 100% maize silage, 25% apple pomace – 75% maize silage, 50% apple pomace – 50% maize silage, 75% apple pomace – 25% maize silage and under – size fraction of municipal waste from the sorting drum. Fermentation of waste was carried out for 30 days. In the produced biogas, there were measured the content of: CH4, CO2, O2 and the total yield of biogas. The combination of different groups of waste in the fermentation process causes a higher efficiency of biogas production. Additionally thermal analysis (TG, DSC) of biogas digestate were conducted. Due to the impurities present in the biogas digestate derived from municipal waste, it becomes impossible to use it as fertilizer in agriculture.