Assessment and parameter identification of simplified models to describe the kinetics of semi-continuous biomethane production from anaerobic digestion of green and food waste.

Biochemical reactions occurring during anaerobic digestion have been modelled using reaction kinetic equations such as first-order, Contois and Monod which are then combined to form mechanistic models. This work considers models which include between one and three biochemical reactions to investigate if the choice of the reaction rate equation, complexity of the model structure as well as the inclusion of inhibition plays a key role in the ability of the model to describe the methane production from the semi-continuous anaerobic digestion of green waste (GW) and food waste (FW). A parameter estimation method was used to investigate the most important phenomena influencing the biogas production process. Experimental data were used to numerically estimate the model parameters and the quality of fit was quantified. Results obtained reveal that the model structure (i.e. number of reactions, inhibition) has a much stronger influence on the quality of fit compared with the choice of kinetic rate equations. In the case of GW there was only a marginal improvement when moving from a one to two reaction model, and none with inclusion of inhibition or three reactions. However, the behaviour of FW digestion was more complex and required either a two or three reaction model with inhibition functions for both ammonia and volatile fatty acids. Parameter values for the best fitting models are given for use by other authors.

Effects of green waste addition on waste activated sludge and fat, oil and grease co-digestion in mesophilic batch digester.

Anaerobic digestion (AD) is regarded as an effective method to treat waste activated sludge (WAS) and fat, oil and grease (FOG). Co-digestion of WAS/FOG could promote the methane yield but it will cause acid and salinity inhibition. Green waste (GW) was added into the digesters, and its effects on co-digestion of WAS and FOG in the mesophilic batch digester were investigated. Digestive performances (such as hydrolysis, acidogenesis and methanogenesis) were studied emphatically. The results showed that digester L6 (WAS:FOG:GW = 1:2:1, VS basis) presented the highest specific methane yield (SMY, 341.5 mL/g VS). The results of kinetics study verified that there was a slower hydrolysis rate when GW was applied as a co-substrate, which could reduce the potential of acid inhibition. Volatile fatty acid (VFA) and electrical conductivity analysis showed that GW addition could keep moderate VFA concentrations and alleviate the negative effects of high-salinity substrates on the digestive systems. The microbial community and diversity analysis proved that GW addition was beneficial to keep the balance of hydrolytic bacteria, acidogens and acetogens. The results of this study indicated that GW addition could enhance the energy recovery and system stability in the WAS/FOG co-digestive system.