Mid Sweden University

The production of biogas is important for the environment as it is a renewable

energy source. To ensure economical and efficient production, it is important to

monitor the anaerobic digestion (AD). AD is a natural process which uses microor-

ganisms to break down the organic material in the absence of oxygen, producing

biogas and digestate. A well controlled process must be able to detect an imbal-

ance at an early stage, thus both gradual and sudden changes are important. Many

parameters have been tested as indicators and it has been revealed that they are

often too slow. However, volatile fatty acids (VFA) are one of the first indicators

of deterioration and may therefore have the ability to act as an indicator to de-

tect sudden changes. Many different sensor technologies have been produced for

monitoring and control of biogas production and the existing systems and meth-

ods have their associated advantages and disadvantages. Therefore, endeavors for

improved sensor technologies will increase the efficiency and productivity of the

biogas production.

An emerging technology for chemical sensing is the use of an optical sensor ar-

ray. The sensor array mimics the human olfactory system, by having highly cross

reactive dyes, which change optical properties due to changes in the chemical envi-

ronment. Together, the dyes constitute a unique pattern response to the different

chemicals. Experiments with colorimetric sensor arrays have provided accurate

results on different areas such as food and beverages, amino acids, cancer and pro-

teins. It has also provided a differentiation of 100 volatile organic compounds. The

optical sensor array provides a new approach to biogas sensing, and with the highly

complex environment within an anaerobic digester, the sensor array may be ideal

as they can provide superior sensitivity and specificity to very similar substances.

The primary objective of this master project was to develop and test a colorimetric

sensor array ideal for assessing VFAs in biogas. For this, 23 dyes including Brønsetd

acid-base, redox and metalloporphyrings was tested by immobilizing them into a

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TLC plate. The sensor arrays were exposed to known VFA concentration dissolved

in effluent extracted from a cow manure fed reactor. A program was constructed

resulting in the acquisition of high dimensional data for multivariate statistical

analysis using PCA and a difference map for visual verification. The results from

the experiments suggest that it is possible to utilize colourimetric sensor arrays

for assessing VFAs in biogas. The sensor array were able to differentiate between

the VFAs with low concentrations, down to a fifth of what starts inhibiting the

AD process. Thus the sensor array may provide early warnings on the state of the

AD process to avoid cessation. Although its the total response of the array that

constitutes the response, a set of ideal dyes was collected based on their significant

colour respons.