Recent studies have shown that anaerobic co-digestion (AnCoD) is superior to conventional anaerobic digestion (AD). The benefits of enhanced bioenergy production and solids reduction using co-substrates have attracted researchers to study the co-digestion technology and to better understand the effect of multi substrates on digester performance. This review will discuss the results of such studies with the main focus on: (1) generally the advantages of co-digestion over mono-digestion in terms of system stability, bioenergy, and solids reduction; (2) microbial consortia diversity and their synergistic impact on biogas improvement; (3) the effect of digester mode, i.e., multi-stage versus single stage digestion on AnCoD. It is essential to note that the studies reported improvement in the synergy and diverse microbial consortia when using co-digestion technologies, in addition to higher biomethane yield when using two-stage mode. A good example would be the co-digestion of biodiesel waste and glycerin with municipal waste sludge in a two-stage reactor resulting in 100% increase of biogas and 120% increase in the methane content of the produced biogas with microbial population dominated by Methanosaeta and Methanomicrobium.

The efficiency of ultrasonication as a pretreatment method for hog manure prior to anaerobic digestion is evaluated at specific energies of 250-30,000 kJ/kgTS. This study confirmed that COD(solubilisation) from particulates correlated well with the more labor and time intensive degree of disintegration test. The particle size distribution for hog manure was bimodal (0.6-2500 μm), while ultrasonication primarily impacting particles in the 0.6-60 μm range. Hog manure was found to be more amenable to ultrasonication than waste activated sludge, as it took only 3000 kJ/kgTS to cause 15% more solubilization as compared to 25,000 kJ/kgTS for waste activated sludge. Bound protein degradation during sonication was 13.5% at 5000 kJ/kgTS and remained constant thereafter for higher energy input. It was noted that biomass cell rupture occurred at specific energy of 500 kJ/kgTS. An economic evaluation indicated that only a specific energy of 500 kJ/kgTS was economical, with a net energy output valued at $4.1/ton of dry solids, due to a 28% increase in methane production.

Batch experiments were performed to investigate the effect of particulate protein size on the hydrolysis of casein in anaerobic degradation. Whereas particle size did not affect the ultimate protein degradation efficiency, the hydrolysis rate coefficient increased from 0.034 to 0.298 day–1 with a change in specific surface area from 0.01 to 0.192 m2/g. The maximum rate of methane production increased from 6 to 14 mL of CH4 per gram of COD added per day with the same change in specific surface area, although the ultimate volume of methane produced was approximately the same despite the change in specific surface area. A mathematical relationship between the hydrolysis rate coefficient and the specific surface area was developed, and a new hydrolysis equation was proposed and verified.

Ultrasound treatment of wastewater sludge prior to anaerobic digestion disrupts the flocs and causes lysis of the bacterial cells, releasing both inter-and intracellular materials. Primary and waste-activated sludge (WAS) were treated with different ultrasonic intensities, varying sonication time and amplitude at a constant frequency. Results showed that the gas production, volatile fatty acids, ratio of soluble chemical oxygen demand to total chemical oxygen demand, and soluble protein increased, while the particulate protein and particle size of the sludge decreased, with sonication time. An empirical model was developed to determine the economic viability of ultrasound based on electrical energy input and energy obtained from enhanced methane production. It has been found that ultrasonic pretreatment is only economically viable for primary sludge at low sonication doses. The Anaerobic Digestion Model No. 1 (ADM1) was applied to the batch anaerobic digestion for sonicated and non-sonicated sludge. In almost all cases, the model successfully simulated the experimental trends.

Degradation of known odor precursors in sludge during anaerobic digestion was systematically studied and simulated using the Anaerobic Digestion Model Number 1 (ADM1). The degradation of various protein fractions (particulate, soluble, and bound), volatile fatty acids (VFAs), lipids, and amino acids of primary sludge (PS) and waste-activated sludge (WAS) were monitored during anaerobic digestion. The degradation kinetic constants of the odor precursors namely, protein, lipid, and VFAs were determined. Relationships between degradations of protein fractions and volatile suspended solid were established; a strong relationship between bound protein, a major odor precursor, and volatile suspended solid degradation was found. No statistically significant difference in bound protein reduction was observed between PS and WAS. ADM1 was successfully used to simulate the lab scale continuous anaerobic digestion; model results with optimized parameters showed good agreement with the experimental data for methane production and several other sludge parameters including odor precursors such as lipids, VFAs, and proteins.