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Anaerobic codigestion (AcoD) is a proven option to significantly boost biogas production while utilizing existing digesters and infrastructure. The aim of the present research was to conduct an exhaustive study regarding anaerobic codigestion of mixed sewage sludge and crude glycerol considering impacts on organic load, hydraulic load, process performance and microbial community. The methane potential of crude glycerol varied from 370 mL CH 4 ·g −1 VS to 483 mL CH 4 ·g −1 VS for different samples tested. The half maximal inhibitory concentration of crude glycerol was 1.01 g VS L −1 , and the primary mechanism of inhibition was through overload from rapid fermentation rather than the presence of toxic compounds in the crude glycerol. In continuous operation over 200 days, feeding glycerol at up to 2% v/v, increased organic load by up to 70% and resulted in a 50% increase in methane production. Glycerol dosing resulted in no change in apparent dewaterability, with both codigestion and control reactors returning values of 22%–24%. Members of the phylum Thermotogae emerged as a niche population during AcoD of sewage sludge and glycerol; however there was no gross change in microbial community structure and only minimal changes in diversity. AcoD did not result in synergisms between sewage sludge and crude glycerol. Actually, at dose rate up to 2% v/v glycerol dosing is still an effective strategy to increase the organic loading rate of continuous anaerobic digesters with minimal impact of the hydraulic retention time. Nonetheless, the dose rate must be managed to: (i) prevent process inhibition and (ii) ensure sufficient degradation time to produce a stable biosolids product. [ABSTRACT FROM AUTHOR]

Inhibition by ammoniacal nitrogen, consisting of free ammonia (NH 3 ) and ammonium ion (NH 4 + ), has been widely investigated for anaerobic digestion. However, despite the large amount of research on the subject, ammoniacal nitrogen inhibition still threatens many anaerobic digesters. This paper presents (i) a method to reliably characterise ammoniacal nitrogen inhibition and (ii) a robust inhibition modelling approach. Results showed that NH 3 and NH 4 + inhibition need to be jointly determined, which can only be done by performing inhibition tests at various total ammoniacal nitrogen (TAN) concentrations and pH values. These test conditions were reliably achieved using the salts NH 4 HCO 3 and NH 4 Cl without pH adjustment, rather than by using NH 4 Cl with pH adjustment. The use of only salts showed a lower pH change during the inhibition test (∼1.5 days), thereby decreasing the uncertainty in TAN speciation and strengthening the test and model outputs. A threshold inhibition function satisfactorily described (R 2  > 0.99) the joint inhibition of NH 3 and NH 4 + on three distinct inocula, and provided a better description of the inhibition testing results than a non-competitive inhibition function (R 2 ∼0.70). The key advantage of the proposed threshold inhibition function is its capacity to identify the inhibition lower limit (concentration where inhibition starts; KI min ) and upper limit (concentration where inhibition is complete; KI max ). The threshold inhibition function also identifies the 50% inhibition concentration (KI 50 ) at the midpoint of KI min and KI max . Finally, experimental and model results show that at pH 7.3–7.7 and TAN concentrations above 2000 mgN·L −1 , both NH 3 and NH 4 + contribute significantly to overall inhibition. [ABSTRACT FROM AUTHOR]

Highlights • Humic acid (HA) inhibition resilience differed with the inoculum source. • Experimental results below 5 gHA L−1 support a threshold inhibition mechanism. • HA < 5 g L−1 led to a time lag, but cellulose hydrolysis rate was unaltered after it. • HA did not affect protein digestion up to 5 gHA L−1. • Methanogenesis was inhibited at 20 gHA L−1. Abstract There is increasing evidence that humic acid (HA) is hampering the performance of anaerobic digesters treating animal manures and thermally-hydrolysed waste activated sludge. In the present study, HA inhibition and inhibition resilience was examined for hydrolysis (carbohydrate and protein) and acetotrophic methanogenesis with four distinct full-scale anaerobic inocula. The aim was to further understand HA inhibition and to explore potential relationships between microbial factors and inhibition resilience. For two of the four tested inocula, cellulose degradation showed a start-up delay that lengthened as HA concentration increased from 0 to 2 g L−1. This inhibition was reversible because, after the initial delay, subsequent hydrolysis rates and methane yields were not significantly influenced by HA concentration. Cellulose hydrolysis results at HA concentrations below 2 g L−1 support a threshold inhibition mechanism, i.e. HA complexes with hydrolytic enzymes preventing them from binding with cellulose, but once all the HA had been complexed, enzymes subsequently released are free to bind with cellulose. Inocula with higher cellulose hydrolytic activity were less affected by HA inhibition, suggesting a potential link between HA inhibition resilience and microbial activity. However, above 5 gHA L−1, cellulose hydrolysis rates decreased with increasing HA concentration; indicating that the mechanisms of inhibition may change depending on some threshold HA concentration. Protein hydrolysis and acetotrophic methanogenesis were less susceptible to HA inhibition than cellulose hydrolysis, since signs of inhibition were only observed above 5 gHA L−1. Acetotrophic methanogenesis was partially inhibited at 10 gHA L−1 and completely inhibited at 20 gHA L−1. These results further support that HA inhibition is selective towards particular enzymes. [ABSTRACT FROM AUTHOR]

The relationship between anaerobic digestion operational conditions and (i) microbial community, (ii) acetoclastic methanogenic activity and (iii) free ammonia (NH 3 ) inhibition resilience was investigated. Thirteen inocula were obtained from full and pilot scale digesters fed with different substrates, digester configurations, operating temperatures and NH 3 concentrations (0.1–241 mgN·L −1 ). Substrate type and temperature were the primary factors influencing microbial community composition. Methanogenic activity ranged from 0.04 to 0.14 gCOD-CH 4 ·g −1 VS·day −1 , and was significantly correlated with archaeal relative abundance and archaeal community PC2. The variability of NH 3 resilience among inocula was moderate, with inhibition threshold values (KI 50 ) ranging between 32 and 175 mgNH 3 -N·L −1 . No microbial or operational factors correlated with NH 3 resilience. However, the slopes of inhibition threshold curves were influenced by some environmental factors, namely substrate type, digester temperature and NH 3 concentration. Overall, these results indicate that low and moderate background NH 3 concentrations is not a key determinant of microbial community nor NH 3 resilience. [ABSTRACT FROM AUTHOR]

Despite the importance of quantifying inhibitory capacity of compounds in anaerobic digestion, there is currently no well-defined method to assess it. Experimental methods in literature are frequently time-consuming and resource intensive. As a result, detailed inhibition testing rarely forms part of anaerobic digestion studies, despite the importance and utility of this information. This study develops and validates a simple and rapid inhibition test protocol, based on relative inhibition of acetoclastic methanogens. The inhibition potential of a compound is determined from the reduction in specific methanogenic activity as inhibitor concentration is increased. The method was successfully performed on two inoculums from different source environments and with both biostatic and biocidal inhibitors. Optimisation work indicated that: (i) sodium acetate is a preferred carbon source compared to acetic acid; (ii) an inoculum to acetate ratio of 5 g VS g −1 acetate is preferred, and (iii) that the inoculum concentration should be normalised to 10 g L −1 VS to reduce mass transfer problems and promote consistency. A key advantage over existing methods is that the sampling strategy has been optimised to three events over 1.5 days while effectively controlling the relative analytical error. [ABSTRACT FROM AUTHOR]