Volatile fatty acids (VFAs) are important mid-products in the production of methane, and their concentrations affect the efficiency of fermentation. However, their effects on methane yield and methanogenic bacteria growth have been less extensively studied. To address these effects, acetic acid, propionic acid, butyric acid and ethanol were used as substrates and an L9(34) orthogonal table was adopted to design anaerobic digestion tests. When the highest concentrations of ethanol, acetic acid and butyric acid were 2400, 2400 and 1800 mg L−1, respectively, there was no significant inhibition of the activity of methanogenic bacteria. However, when the propionic acid concentration was increased to 900 mg L−1, significant inhibition appeared, the bacteria concentration decreased from 6 × 107 to 0.6–1 × 107 ml−1 and their activity would not reconvert. These effects resulted in the accumulation of ethanol and VFAs, and the total methane yield consequently became very low (<321 ml). The original propionic acid concentration had a significant inhibitory effect on methanogenic bacteria growth (P < 0.01). An optimization analysis showed that ethanol, acetic acid, propionic acid and butyric acid at concentrations of 1600, 1600, 300 and 1800 mg L−1, respectively, led to the maximum accumulative methane yield of 1620 ml and the maximum methanogenic bacteria concentration of 7.3 × 108 ml−1.

Effects of volatile fatty acid concentrations on methane yield and methanogenic bacteria

Anaerobic codigestion of municipal solid waste and biosolids under various mixing conditions-II : microbial population dynamics

The biochemical methane potentials for typical fruit and vegetable waste (FVW) and food waste (FW) from a northern China city were investigated, which were 0.30, 0.56 m3 CH4/kgVS (volatile solids) with biodegradabilities of 59.3% and 83.6%, respectively. Individual anaerobic digestion testes of FVW and FW we re conducted at the organic loading rate (OLR) of 3 k g VS/(m3.day) using a lab-scale continuous stirred-tank reactor at 350C. FVW could b e digested stably with the biogas production rate of 2.17 m3/(m3 .day)and methane production yield of 0.42 m3 CH4/kg VS. However, anaerobic digestion process for FW was failed due to acids accumulation. The effects of FVW: FW ratio on co-digestion stability and performance were further investigated at the same OLR. At FVW and FW mixing ratios of 2:1 and 1:1, the performance and operation of the digester were maintained stable, with no accumulation of volatile fatty acids (VFA) and ammonia. Changing the feed to a higher FW content in a ratio of FVW to FW 1:2, resulted in an increase inVFAs concentration to 1100-1200 mg/L, and the methanogenesis was slightly inhibited. At the optimum mixture ratio 1:1 for co-digestion of FVW with FW, the methane production yield was 0.49 m3 CH4/kg VS, and the volatile solids and soluble chemical oxygen demand (sCOD) removal efficiencies were 74.9% and 96.1%, respectively.

Effects of mixture ratio on anaerobic co-digestion with fruit and vegetable waste and food waste of China.

Anaerobic digestion is an effective waste treatment method both for the pollution control and energy recovery, such as treating many agricultural and industrial wastes which contain high level of easily biodegradable substances. However, there are also many problems to be resolved, such as low methane yield, long retention time and process instability, preventing this technique from being widely applied. Variety of inhibitory substances and intermediate products are the primary cause of anaerobic digester upset or failure, including free ammonia (FA), volatile fatty acids (VFAs) and sulfide/sulfate, though they are essential nutrients for bacterial growth and anaerobic digestion process. Many researches have been conducted to investigate the inhibition mechanisms and the controlling factors during the overall anaerobic digestion process. In this paper, a detailed summary of the researches was discussed on the inhibition of the anaerobic digestion process. Furthermore, the controlling and recovery of the inhibitors as valuable products were also reviewed in detail. Co-digestion with other wastes, acclimation of microorganisms, and incorporation of methods could significantly improve the efficiency of sewage sludge anaerobic digestion.

Progress in inhibition mechanisms and process control of intermediates and by-products in sewage sludge anaerobic digestion

Ammonia increases buffer capacity of methanogenic medium in mesophilic anaerobic reactor thus increasing the stability of anaerobic digestion process. Optimal ammonia concentration ensures sufficient buffer capacity while not inhibiting the process. It was found out in this paper that this optimum depends on the quality of anaerobic sludge under investigation. The optimal concentrations for methanogens were 2.1, 2.6 and 3.1 g/L of ammonia nitrogen in dependence on inoculum origin. High ammonia nitrogen concentration (4.0 g/L) inhibited methane production, while low ammonia nitrogen concentration (0.5 g/L) caused low methane yield, loss of biomass (as VSS) and loss of the aceticlastic methanogenic activity. It was found out that negative effect of low ammonia nitrogen concentration on biomass is caused not only by low buffer capacity but also by insufficiency of nitrogen as nutrient. It was also found out that anaerobic sludge with higher ammonia nitrogen concentration (4.2 g/L) tolerates even concentration of volatile fatty acids (160 mmol/L) which causes inhibition of the process with low ammonia nitrogen concentration (0.2 g/L).

Stability and inhibition of anaerobic processes caused by insufficiency or excess of ammonia nitrogen

The effect of propionate toxicity at different pH values (6.5, 7.0, and 8.0) on methanogen-enriched sludge. Methanobrevibacter smithii, and Methanospirillum hungatii was studied. Organisms were grown in Balch medium 3 in Hungate tubes, and toxicity was characterized by a decrease in production of methane and in bacterial numbers. Propionate inhibited bacterial growth and cumulative methane production at concentrations as low as 20 mM. In the absence of propionate, the methanogen-enriched sludge and M. smithii showed better cumulative methane production at pH 6.5 and 7.0 than at pH 8.0. However, in the presence of propionate, these organisms showed better cumulative methane production at pH 8.0. M. hungatii differed in its behavior; the best values of cumulative methane production for this organism occurred at pH 7.0. Bacterial numbers reflected the microbial response to the presence of propionate. The highest counts of methanogenic bacteria were observed at pH 6.5 and 8.0. The numbers of methanogens were affected by the presence of propionate even at concentrations as low as 20 or 30 mM; at propionate concentrations above 80 mM, the methanogen count was affected by at least 2 orders of magnitude. Upon comparison of the responses of the pure cultures and the methanogen-enriched sludge to increasing propionate concentrations, it was found that the sensitivity of the pure cultures was similar to that of the methanogens in the sludge.

https://aem.asm.org/content/aem/57/6/1764.full.pdf

L M Evison

Papers

Effect of propionate toxicity on methanogen-enriched sludge, Methanobrevibacter smithii, and Methanospirillum hungatii at different pH values.