In recent years, biogas production from complex biomass has received great interest. Therefore, many studies have been conducted to understand the anaerobic digestion process and to characterise responsible microbes for the biochemical conversions. Although our knowledge about biogas production in general is rapidly increasing, less information is available about hydrolytic microbes within anaerobic bioreactors. Here, we pinpoint the urgent need for solid fundamental knowledge about hydrolytic bacteria within biogas plants. In this review, current knowledge about anaerobic hydrolytic microbes is presented, including their abundance in biogas plants, and the factors impacting their activity.

Inhibition effect of humic acid (HA) on anaerobic digestion of cellulose and xylan and the mitigation potential of the inhibition were evaluated in controlled fed batch reactors at 30°C and a hydraulic retention time (HRT) of 20 days. Reactor performances were evaluated by biogas production and metabolite measurements for 220 days. Microbial population dynamics of the reactors were monitored with nextgeneration 16S rRNA gene sequencing at nine different sampling times. Our results showed that increasing levels of HA inhibited the hydrolysis efficiency of the digestion by 40% and concomitantly reduced the methane yield. Addition of hydrolytic enzymes helped to reverse the negative effects of HA, whereas calcium addition did not reverse HA inhibition. Microbiological analyses showed that the relative abundance of hydrolytic/fermentative bacterial groups such as Clostridiales, Bacteroidales and Anaerolineales was significantly lowered by the presence of HA. HA also affected the archaeal populations. Mostly hydrogenotrophic methanogens were negatively affected by HA. The relative abundance of Methanobacteriaceae, Methanomicrobiales-WCHA208 and Unassigned Thermoplasmata WCHA1-57 were negatively affected by the presence of HA, whereas Methanosaetacea was not affected.

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Humic compounds are inhibitory to the anaerobic hydrolysis of cellulosic biomass. In this study, the impact of salt addition to mitigate the inhibitory effects of humic compounds was investigated. The experiment was conducted using batch tests to monitor the anaerobic hydrolysis of cellulose in the presence of humic acid. Sodium, potassium, calcium, magnesium and iron salts were tested separately for their efficiency to mitigate humic acid inhibition. All experiments were done under mesophilic conditions (30 °C) and at pH 7. Methane production was monitored online, using the Automatic Methane Potential Test System. Methane production, soluble chemical oxygen demand and volatile fatty acid content of the samples were measured to calculate the hydrolysis efficiencies. Addition of magnesium, calcium and iron salts clearly mitigated the inhibitory effects of humic acid and hydrolysis efficiencies reached up to 75%, 65% and 72%, respectively, which were similar to control experiments. Conversely, potassium and sodium salts addition did not mitigate the inhibition and hydrolysis efficiencies were found to be less than 40%. Mitigation of humic acid inhibition via salt addition was also validated by inductively coupled plasma atomic emission spectroscopy analyses, which showed the binding capacity of different cations to humic acid.

Recently, digestate disintegration gained interest as an alternative strategy to feedstock pretreatment for anaerobic digestion. This study evaluated the effect of three different digestate disintegration methods (hydrogen peroxidation, ozone treatment and ultrasound) on manure digestate, potato waste digestate and mixed organic waste digestate. Lab-scale anaerobic digestion experiments were carried out by adding disintegrated digestate to the related substrate and inoculum with simulated recycle ratios of 0.2 and 0.5. Ultrasound disintegration of potato waste digestate yields 22.5% increase in biogas production. An increase in biogas production was linked to the treated digestate amount and the treatment dosage. First order model was used to investigate the effect of digestate disintegration on the first order reaction rate constant (k). The decrease in k and increase in biogas production were linearly correlated. This correlation was explained by the increased bioavailability of the organic matter and possible negative effects of digestate disintegration on the microorganisms.