۱- Introduction

۲- Requirements for sensors to determine the state of an anaerobic digester

۳- Algorithm-based monitoring

۴- Utilisation of colourimetric sensors in anaerobic digestion

۵- Concluding remarks

References

Abstract

Although biogas is not a new approach to producing renewable fuel, it could further be developed to improve its potential as an alternative energy source. To achieve this, vast improvements in the efficiency and cost of biogas production are essential. These enhancements require detailed systematic monitoring to attain a near-optimal biogas production process. To date, there is a striking imbalance between the inherent biological complexity of anaerobic digestion, and the minimal information currently measured on-line. The objective of this review is to discuss how improvements in availability and cost of sensor technology used for determining the key compounds and their dynamics within the biogas processing plant will facilitate the further understanding of the biogas production process, preventing the biological process failure. In particular, colourimetric assays (sensor assays based on coloured dyes) for variable detection in anaerobic digestion provide a stable, multivariate system for the detection of Volatile Fatty Acids (VFAs), but also provide a much deeper insight into the process by assessing other parameters, which, to date have never been measured on-line. These sensor improvements will allow the biogas production, even on a small scale, to be guided in the optimum direction, avoiding the biological process from collapsing. This will result in improved efficiency, at a reduced operational cost. The potential of colourimetric assay methods for use in anaerobic digestion as a sensor technology with associated data analysis methodologies has not previously been observed. Here, a 23-dye colourimetric sensor array was experimentally assessed to exhibit the differentiation of 10 mM acetic acid, 5 mM propionic acid and 0.3 mM butyric acid. The feasibility of on-line, cost-effective, rapid, and efficient detection of VFAs together with other key parameters by these colourimetric sensor arrays is intended to be assessed to advocate their usage in AD.

Introduction

One route to production of renewable, clean energy is through biogas production, where biogas is derived from biological waste materials. Biogas is produced through anaerobic digestion (AD) of organic waste materials by a variety of microorganisms [1]. The inherent complexity of this ecosystem resulting from the dynamical interaction of several hundreds of bacterial and archaeal species [2], developing on a mixture of substrates, results in significant difficulty in determining the state of the digestion process [1]. The most straightforward information is provided by monitoring of gases (H2, CH4 and CO2) measured in the gas-phase of the anaerobic digester. Their fluctuations can provide information about the digesters’ productivity [3]. Measurement of pH is also important. A pH outside the range of 6–۸ often indicates process deterioration, limiting methane production and eventually collapse of the entire biological process [4]. Concentrations of volatile fatty acids (VFAs; mainly acetic, butyric and propionic acid) have been suggested as useful control parameters, as these acids are indirectly indicative of the activity of the methanogenic consortia [3,4]. VFA accumulation can be interpreted as organic overload or inhibition of the methanogenic microbial communities [5]. Acidogenic microorganisms transform hydrolysis products into VFAs, while acetogenic microorganisms then convert VFAs into acetate, H2 and CO2. Methane is then produced by the methanogens [6], with the majority being produced by acetotrophic methanogens, transforming acetate into methane and CO2 [7]. VFA concentrations are a required control parameter for biogas production monitoring [8,9], and it is essential that VFAs are monitored to understand the biological process. Additionally, the ratio of VFAs over total alkalinity is often suggested as an indicator of the processes stability, with the objective of this ratio remaining below 0.3 [10].