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

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Abstract

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.

Introduction

Anaerobic fermentation of organic matter comprises three steps, namely hydrolysis, acidogenesis and methanogenesis. These three steps involve the actions of hydrolytic bacteria, acidogenic bacteria (AB) and methanogenic bacteria (MB), respectively. In traditional anaerobic fermentation, AB and MB, which are two entirely different types of microbes, are grown in the same reactor. However, it is very difficult to maintain the harmony and balance between these two types of microbes because their characters and the demands of ambient culture are very different. Once the balance is broken, the methanogenesis process will be interrupted. Ghosh and Pohland proposed a two-step anaerobic digestion method in 1971 [1], [2]. In this method, the two types of microbes are cultivated under their respective optimal growing conditions in two reactors, and form a two-step anaerobic digestion system with better running stability and higher working efficiency.

The terminal fermentation products produced in the acidogenesis phase are very important for the whole system performance because they can affect the loading, efficiency and running stability of the methanogenesis phase [3]. The conversion rate from volatile fatty acids (VFAs) to acetic acid (HAc) will affect the MB quantity, and subsequently affect the degradation rate of HAc and methane yield.

As shown in many studies, the conversion rates of VFAs to methane vary in the order of HAc > ethanol (HEt) > butyric acid (HBu) > propionic acid (HPa) [4]. Before being degraded to methane, all VFAs are first degraded to HAc, and their conversion rates also vary in the order of HEt > HBu > HPa. Lactic acid, which has the potential to be converted to HPa, is an undesirable terminal fermentation product. Therefore, accumulation of HPa always results in failure of methanogenesis [5].

Owing to the importance of VFAs, many studies about them have been carried out. Dogan et al. [6] fed one type of VFA to a UASB reactor to study the effects of VFA variety and concentration on MB activity, but only studied the inhibition of MB caused by a single VFA. In an actual reactor, there are various VFAs and their cooperative effects need be taken into account. Siegert and Banks [7] considered that VFA concentrations above 2000 mg L−1 led to inhibition of cellulose degradation, while VFA concentrations above 4000 mg L−1 caused only feeble inhibition of glucose degradation.

In addition, many researchers have considered that HPa accumulation would inhibit the activity of MB and lead to cessation of fermentation, but their conclusions have been varied. Barredo and Evison [8] pointed out the MB quantity would fall according to two distinction indexes as the HPa concentration increased. Yeole et al. [9] found that when the pH was 7 and the HPa concentration was 5000 mg L−1, the methane yield decreased to 22–38% and indicated that the inhibition would be greatly strengthened when pH was decreased. Demirel and Yenigun [10] concluded that HPa would inhibit MB growth when its concentration was above 951 mg L−1, while adding HBu could improve the inhibition to some extent. However, Pratap et al. [11] increased the HPa concentration to 2750 mg L−1 and no inhibition appeared. Therefore, the effects of HPa accumulation on the methanogenesis system should be further examined.

Although AB degradation of VFAs and the inhibitory effects of HPa on AB and MB have been examined, the effects of HEt and VFA concentrations on methane yield and MB have been less extensively investigated. To identify the proper substrate types and optimal concentrations suitable for the methanogenesis phase, a set of experiments was designed using various proportions of HEt, HAc, HPa and HBu as substrates to evaluate their correlations with methane yield and MB growth. The findings will increase the capabilities of fermentation reactors and provide us with directions for choosing proper parameters to obtain optimal fermentation efficiencies.

Section snippets

Inoculum and substrates

The inoculum was taken from an anaerobic digester that had been operating for 2 years, and filtered using a 0.0469 inch sieve. The filtrate was placed in an incubator and cultivated for 2 months with constant feeding at a COD of 5000 mg L−1 d−1, a hydraulic retention time of 48 h and a temperature of 35 ± 1 °C. After being deposited and concentrated, the cultivated sludge was used as an inoculant. The characteristics of the inoculum sludge after treatment are shown in Table 1.

HEt, HAc, HPa and HBu at

Degradation rates of VFAs

The concentrations of HEt and VFAs as substrates are shown in Fig. 1. Compared with the VFAs, the degradation rate of HEt was faster and the accumulation was mild. With the exception of experiments 3, 5 and 7, HEt, HAc, HPa and HBu were degraded efficiently with degradation rates of 94.2–99.8%, 96.5–99.15%, 86.9–96.8% and 93.7–99.5%, respectively. However, serious accumulation appeared in experiments 3, 5 and 7. In these experiments, the highest concentrations of HAc, HPa and HBu reached 4125,

Conclusions

  • 1)

    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

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