Effect of Thermal Hydrolysis Pretreatment on Anaerobic Digestion of Protein-Rich Biowaste: Process Performance and Microbial Community Structures Shift

To reveal the effects of thermal hydrolysis (TH) pretreatment (THPT) on anaerobic digestion (AD) of protein-rich substrates, discarded tofu was chosen as the object, and its batch AD tests of tofu before and after being subjected to TH at gradually increasing organic loads were carried out and the AD process characteristics were compared; furthermore, its continuous AD tests without and with THPT were also conducted and the difference of the microbial community structures was investigated. The results showed that, during AD of protein-rich tofu with increase in the organic load, inhibition from severe acidification prior to accumulation of ammonia nitrogen (AN) occurred. THPT helped overcome the acidification inhibition present in batch AD of tofu at such a high TS content of 3.6%, to obtain the maximum methane yield rate of 589.39 ml·(gVS)−1. Continuous AD of protein-rich tofu heavily depended on ammonia-tolerant hydrogenotrophic methanogens and bacteria. The continuous AD processes acclimated by HT substrates seemed to be resistant to severe organic loads, by boosting growth of ammonia-tolerant microorganisms, above all methylotrophic methanogens such as the genera RumEnM2 and methanomassiliicoccus. The process response of continuous AD of HT tofu was hysteretic, suggesting that a sufficiently long adaptation period was required for stabilizing the AD system.

Energy Balance of Turbocharged Engines Operating in a WWTP with Thermal Hydrolysis. Co-Digestion Provides the Full Plant Energy Demand

The energy balance of lean-burn turbocharged engines using biogas as fuel is reported. Digestion data were obtained from the wastewater treatment plant (WWTP) of the city of Burgos (Spain), operating with a thermal hydrolysis unit for sludge pre-treatment. Operational performance of the plant was studied by considering the treatment of sludge as a comparative base for analyzing global plant performance if co-digestion is implemented for increasing biogas production. The calculation methodology was based on equations derived from the engine efficiency parameters provided by the manufacturer. Results from real data engine performance when evaluated in isolation as a unique control volume, reported an electrical efficiency of 38.2% and a thermal efficiency of 49.8% leading to a global efficiency of 88% at the operating point. The gross electrical power generated amounted to 1039 kW, which translates into 9102 MWh/year, with an economic value of 837,384 €/year which was completely consumed at the plant. It also represents 55.1% of self-consumption regarding the total electricity demand of the plant. The analysis of the system considering the use of the total installed capacity by adding a co-substrate, such as cheese whey or microalgae, reveals that total electrical self-consumption is attained when the co-substrate is directly fed into the digester (cheese whey case), obtaining 16,517 MWh/year equivalent to 1,519,160 €/year. The application of thermal hydrolysis as pre-treatment to the co-substrate (microalgae case study) leads to lower electricity production, but still attains a better performance than a mono-digestion baseline scenario.