Lifting the lid on process optimisation for anaerobic digestion

Reed, J.P., Devlin, D., Esteves, S.R.R., Dinsdale, R. and Guwy, A.J., University of Glamorgan, UK



A number of benefits can be gained from operating anaerobic digesters with improved knowledge of feedstocks characteristics, digestion chemistry and stability of digestates.   However, this requires the measurement of a significant number of parameters, many of which are labour intensive and can only be performed off-line so producing a delay in response. This paper provides an overview of progress in the process monitoring of anaerobic digesters using Fourier transform near infrared spectroscopy (FT-NIRS). The use of FT-NIRS with multivariate statistical techniques enables the possibility of real time process monitoring of anaerobic digesters allowing the optimisation of their operating conditions. Results from a recent study into the use of FT-NIRS with sewage sludge digesters are also presented, that demonstrate it is possible to distinguish between different mixtures of primary and secondary sludges as feedstocks, and monitor total and volatile solids, volatile fatty acids and bicarbonate alkalinity.

Key words

Anaerobic Digestion, Sewage Sludge, Reflectance FT-NIR, Solids, Alkalinity, VFA, Multivariate Analysis


Anaerobic digestion is a multi-stage bioprocess that requires the interaction of at least 4 trophic groups of bacteria. A stable process is characterised by the case where the bacterial consortia are able to successfully utilise the intermediate by-products and reach equilibrium. If this equilibrium is not maintained then failure of the process can occur. The acetigenic bacteria and obligate-anaerobes (methanogens) are unable to metabolise the volatile fatty acids (VFA) produced by the acidogenic bacteria fast enough, which then leads to a build up of VFAs and a corresponding drop of bicarbonate alkalinity (BA) and eventually of the pH inhibiting the methanogens further and eventually leading to a collapse of the process.  As such, a key issue in the successful operation of anaerobic digestion plants is the extent to which timely measurements of key process parameters such as VFA, volatile solids (VS), and BA can be made. Typically these measurements are made sporadically off-line with little opportunity to use the resulting data for any type of closed loop control of the system. It is clear however that online monitoring of such parameters can be used to optimise the performance of the digester and improve gas yields (Steyer et al., 2002; Lardon et al., 2004).

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