Buffer requirements for enhanced hydrogen production in acidogenic digestion of food wastes

Abstract

The requirements for pH buffer addition for hydrogen production and acidogenesis in batch acidogenic digestion of a food waste (FW) feedstock with limited alkalinity was studied at various initial pH conditions (6.0–8.0). The results showed that, without buffer addition, hydrogen production from this feedstock was insignificant regardless of the initial pH. With buffer addition, hydrogen production improved significantly if the initial pH was greater than 6.0. Substantial hydrogen production occurred when the pH at the end of the batch digestion was higher than 5.5. The maximum hydrogen production was found to be 120 mL/g VS added when the initial pH was 6.5 and buffer addition was in the range of 15–20 mmol/g VS. The effect of pH buffering on the formation of volatile fatty acids (acetic acid, propionic acid and butyric acid) was similar to its effect on hydrogen production. The results of this study clearly indicated shifts in the metabolic pathways with the pH of fermentation. The changes in metabolic pathways impacted upon the dosage of buffer that was required to achieve maximum hydrogen generation.

Introduction

Two stage anaerobic digestion (i.e. acidogenic–methanogenic) is a promising technology for generating biogases (i.e. hydrogen and methane) from concentrated organic substrates such as food wastes. If correctly operated, the first stage of these systems can achieve several objectives including hydrolysis, acidification, and hydrogen gas production. Enhancing production of hydrogen gas is of interest because of its value in the alternative energy economy. In addition, the performance of the acidogenic reactor in a two stage system can impact on the design and operation of the downstream methanogenic reactor. Enhanced hydrolysis and acidogenesis in the first stage will reduce residence time requirements in the downstream reactor.

The performance of acidogenic digesters is known to be a function of pH (Fang and Liu, 2002). The pH conditions impact on the rate of hydrolysis, the types and quantities of acidogenic products and rate and extent of H₂ generation (Li and Fang, 2007). The establishment of appropriate pH conditions in acidogenic reactors that are operated for H₂ gas production is made complex by the fact that these digesters are often operated in a batch or semi-batch mode to minimize the establishment of hydrogenotrophic methanogens in the digester and hence increase hydrogen yields (Chen et al., 2002). The transient nature of batch operations presents a challenge with respect to establishing the appropriate pH range during the digestion. Acid formation during fermentation will act to depress the pH; however, the pH affects the types and quantities of acids that are generated. Hence, there is a natural feedback between digester pH and acid generation. On-line pH monitoring with addition of acid and base into operating biological reactors is challenging to implement in practice. An alternate approach is to supplement the feedstock with sufficient buffer to counteract pH decreases that result from the generation of organic acids during batch digestion. The latter operating procedure was explored in this study.

This study focused on identifying the impact of the initial pH and feedstock buffer supplementation on enhanced hydrogen production from batch acidogenic digestion of food waste. The impact of these variables on the properties of the digester effluent was also characterized to facilitate an assessment of the impact of the acidogenic stage operation on the design and operation of a subsequent methanogenic stage.