How to drive a car without a blindfold.

Operating an anaerobic digester can be similar to driving a car with a blindfold on. As long as you are driving in a straight line and you don’t waggle the steering wheel too much you should be ok. The problem comes when you have a bit of play in the steering and you now don’t know which direction your wheels are pointing. If you have a wide enough road to drive on that doesn’t matter so much, you can weave fairly safely from side to side without hitting anything. However, space is limited and you will run out of road eventually, crashing your car and costing you money and perhaps your health!

For a digester, the steering is the feedstock and your direction is the stability of the digester. The quality of your feedstock may vary without you realising, and if you aren’t monitoring the key process intermediates such as the level of volatile fatty acid (VFA) you can’t be  sure which direction you are travelling and you may crash. If the VFA levels increase sharply, this will eventually lead to a drop in the digester pH, which in turn will lead to the digester crashing. What you need to do is lift the blindfold and take a look inside the digester itself.

Traditionally, VFA is measured off line, and sporadically if at all, with operators relying on pH measurements to tell them which direction they are travelling. Unfortunately, by the time the pH of the digester has dropped the damage is done. What is needed instead is a way to measure VFA levels on-line to allow the operator to see which way they are going. Enter near infrared spectroscopy.

NIR Calibration Figure

Figure 1: Performance parameter determination with FT-NIRS (The solid line indicates values derived from standard laboratory analysis, the dashed line indicates values derived using FT-NIRS. Source: Reed et al., 2011)

FT-NIRS is a non-destructive analytical technique that can be used to determine key performance parameters for anaerobic treatment processes. FT-NIRS can be used to gather on-line, real-time data about the operating state of an anaerobic treatment process from feedstock properties to digestate. In-situ probes can be used to measure spectra at different stages of the process; feedstock, digestate, cake, liquor. These spectra can then be used with previously produced calibrations to provide real-time measurements of parameters (Figure 1) such as total and volatile solids, biological methane potential, volatile fatty acids, bicarbonate alkalinity, carbon/nitrogen ratios and more. The measurements gained though the use of FT-NIRS can then be integrated into the plant’s process monitoring system to provide the operator with real-time information about the stability of the digester (Reed et al, 2013). This now means that the operator can see which way they are going and avoid crashing, improving plant flexibility, and allowing optimisation of biogas yields.

References:

Reed, J.P., Devlin, D., Esteves, S.R.R., Dinsdale, R., Guwy, A.J., 2011. Performance parameter prediction for sewage sludge digesters using reflectance FT-NIR spectroscopy. Water Research, 45(8), pp. 2463 – 2472.

Reed, J.P., Devlin, D., Esteves, S.R.R., Dinsdale, R., Guwy, A.J., 2013. Integration of NIRS and PCA techniques for the process monitoring of a sewage sludge anaerobic digester. Bioresource Technology, 133, pp. 398 – 404.

JReedJames Reed

Senior Lecturer in Renewable Energy
Sustainable Environment Research Centre
University of South Wales

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