Elsevier

Biomass and Bioenergy

Volume 16, Issue 6, June 1999, Pages 407-416
Biomass and Bioenergy

Degradation of volatile fatty acids in highly efficient anaerobic digestion

https://doi.org/10.1016/S0961-9534(99)00016-1Get rights and content

Abstract

To improve the efficiency of anaerobic digestion, we examined the effects of C2–C6 volatile fatty acids (VFAs) on methane fermentation, as well as the behavior of VFAs in anaerobic digestion. The VFA concentrations and methane production in anaerobic digestion were increased by pretreatment of waste activated sludge (WAS), such as ultrasonic disintegration, thermal and freezing treatments. The major intermediate products of anaerobic digestion for untreated and pretreated WAS, such as acetate, propionate, isobutyrate, butyrate, isovalerate, valerate, isocaproate and caproate, were used as substrates and the anaerobic degradation of these was carried out under the same conditions. It was found that decomposition rates of the VFAs (C2–C6) with a straight chain (normal form) were greater than those of their respective isomers with a branched chain (iso form). It was shown that the decomposition rates of the iso and normal forms of butyrate were greater than those of valerate and caproate. This was caused by the isomerization between butyrate and isobutyrate which occurred during the digestion process. Anaerobic bacteria in digested sludge converted butyrate to isobutyrate and vice versa by migration of the carboxyl group to the adjacent carbon atom. In addition, inhibition of degradation of the VFAs by acetate in a digester was also examined.

Introduction

Anaerobic digestion is one of the major biological waste treatment processes in use today. This process has been popular in the waste treatment field because it has many advantages such as high treatment efficiency and methane-producing ability. In the anaerobic digestion of waste activated sludge (WAS), complex organic materials are first hydrolyzed and fermented by rapidly growing and pH-insensitive acidogenic bacteria into volatile fatty acids (VFAs) [1], [2]. The VFAs are then oxidized by slowly growing acetogenic bacteria into acetate (HAc), molecular hydrogen, and carbon dioxide that are suitable as substrates for the methanogenic bacteria [3], [4], [5].

Considerable attention has been focused on the relationships between VFAs concentration and anaerobic fermenter performance [6], [7]. It is known that VFAs are important intermediary compounds in the metabolic pathway of methane fermentation and cause microbial stress if present in high concentrations, resulting in a decrease of pH, and ultimately leading to failure of the digester. Therefore, the concentration of VFAs is an important consideration for good performance of a digester. That is, it is necessary to investigate the optimum conditions and efficiencies of digesters by examining VFAs.

It is widely known and accepted that the occurrence of VFAs in anaerobic sludge decreases as the chain length increases. In addition, the presence of formate is usually quite limited due to its rapid conversion to other compounds (e.g. methane and carbon dioxide). Thus, it was reasonable to limit our attention to the C2–C6 VFAs.

In this study, major intermediate products of anaerobic digestion for untreated and pretreated WAS, i.e. HAc, propionate (HPr), isobutyrate (i-HBu), butyrate (n-HBu), isovalerate (i-HVa), valerate (n-HVa), isocaproate (i-HCa) and caproate (n-HCa) were used as carbon sources in the methanogenic experiments.

The purpose of this investigation was to enhance the efficiency of anaerobic digestion, to evaluate the effect of C2–C6 VFAs on the methane fermentation, and to examine the behavior of VFAs in anaerobic digestion.

Section snippets

WAS and pretreatment

The seed sludge (digested sludge) was collected from a mesophilic sewage sludge anaerobic digester in Hiagari, a sewage disposal plant, located in Kitakyushu, Japan, and was acclimatized to the substrate for 2 months at 35±1°C. Waste accumulated sludge (WAS) thickened by flotation was also obtained from the same plant. Percentage of total solids and volatile solids in the sludge were 3.3–4.0% and 2.6–3.1%, respectively.

The conditions for pretreatment of WAS were as follows:

  • 1.

    ultrasonic

Relationship between VFAs concentration and methane production by various pretreatments

WAS with and without pretreatment were respectively put into anaerobic digesters and were digested under conditions described in Experimental. Fig. 1 shows change in VFAs concentration and methane production rate during digestion (trace amount of caproate and isocaproate were also detected, data not shown). In case of pretreated WAS, generation of methane increased drastically during initial 2 days. It reached a maximum value on 2nd day. The maximal rate of methane generation on ultrasonic

Conclusion

The VFAs in anaerobic digestion were produced by pretreatment of WAS such as ultrasonic disintegration, thermal and freezing treatments. When their concentrations did not exceed the inhibition levels for methane fermentation, they were used efficiently as good substrates by methanogenic bacteria. Therefore both the rate and volume of methane production were increased significantly, as compared with those of the untreated WAS digestion.

Under the same conditions, the degradation rates of the VFAs

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