When an anaerobic digestion plant isn't producing as much biogas as expected, one of the most tempting responses is also one of the most dangerous:
Feed it more.
After more than 30 years working around waste treatment, biogas and environmental engineering, I think this is worth saying plainly. If a digester is already struggling biologically, increasing the organic loading rate can turn an underperforming plant into a seriously unstable one.
The first question shouldn't be, "What can we add to produce more gas?"
It should be:
"What is limiting this digester now?"
More Feedstock Does Not Automatically Mean More Methane
On paper, the logic seems obvious. More biodegradable organic matter enters the digester, so more biogas should come out.
But an anaerobic digester isn't simply a tank into which organic matter disappears and methane emerges. It is a biological system containing interacting populations of microorganisms, and the methane-forming organisms need suitable and reasonably stable conditions.
If organic loading rises faster than the microbial community can process it, volatile fatty acids can accumulate. Alkalinity can be consumed, pH can fall and methanogenic activity can become inhibited.
The operator who increased the feed rate to obtain more methane can therefore end up producing less.
In a badly destabilised digester, recovery can take considerably longer than the initial upset.
Establish the Baseline Before Trying to Optimise
This is why I favour a much less exciting-sounding approach to increasing biogas production:
Measure first. Change second.
Before attempting to optimise a plant, establish what it is actually doing.
That means looking beyond the headline daily biogas-production figure. Feed quantity and composition, organic loading, temperature, pH, alkalinity, volatile fatty acids, retention time, mixing performance, methane concentration and other appropriate process indicators can all help reveal what is happening inside the digester.
Even the gas measurement itself deserves scrutiny. There is little value in optimising against an unreliable baseline.
Only when you understand the present condition of the process can you sensibly ask where the constraint lies.
There Isn't One Universal Trick for Producing More Biogas
I recently substantially updated my guide to how to increase biogas production, and one point I wanted the revised version to make clear is that there is no responsible universal percentage by which every digester can increase its gas yield.
There are certainly many possible improvements.
Feedstock consistency can matter. Carbon-to-nitrogen balance can matter. Co-digestion may help. Better mixing may help. Pretreatment can make some substrates more accessible. Trace nutrients can become limiting in some circumstances. Temperature control, retention time and organic loading can all affect performance.
But the fact that an intervention improves methane yield in one digester does not mean it will do the same in another.
Nor should a percentage improvement achieved during a laboratory study automatically be treated as an achievable improvement at full commercial scale.
Optimise the Constraint, Not Everything at Once
A useful optimisation sequence is surprisingly simple:
Establish the baseline → identify the likely limiting factor → make a controlled intervention → monitor the biological response → measure the net benefit.
The "controlled intervention" part is important.
If several things are changed simultaneously and biogas production improves, it may be impossible to know which change produced the improvement. Worse, if the biology deteriorates, it becomes harder to identify which intervention caused the problem.
There is another consideration that is sometimes overlooked: the objective isn't necessarily to produce the maximum possible volume of biogas.
The objective is normally to improve the useful output and economics of the plant while maintaining a stable biological process.
An intervention that produces a little more gas but consumes excessive electricity, heat, chemicals or operator time may not represent an improvement at all.
Monitoring Comes Before Control
This is perhaps the principle I would most like newer AD operators and students to remember.
You cannot properly control a biological process that you are not adequately monitoring.
A stable digester with good process information gives the operator something to optimise. An unstable digester with poor information gives the operator something to troubleshoot.
Those are very different situations.
For a more detailed discussion, see my updated technical article:
How to Increase Biogas Production – 8 Practical Ways to Improve Digester Yield
Further Practical Reading
For readers who want to go beyond the article, I have also published two more detailed practical guides.
Efficient Biogas Production Techniques & Methods concentrates particularly on improving biogas production, including feedstocks, process conditions, co-digestion, pretreatment and operational optimisation.
For the wider picture, Anaerobic Digestion: A Practical Guide to Feedstocks, Biogas, Digestate and Successful AD Plants looks across the AD process as a whole, from feedstock selection through digestion and biogas production to digestate and successful plant operation.
After a long period in which I haven't posted here as regularly as I once did, I also intend to start using this blog again for shorter observations on anaerobic digestion, biogas technology and some of the practical lessons that can get lost between research papers, equipment claims and day-to-day plant operation.
Steve Last
Chartered Civil & Environmental Engineer (MICE)
Chartered Waste Manager (MCIWM)
Chartered Environmentalist (CEnv)
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