Why long retention times can lead to bigger floating layers
The biogas sector has always been dominated by engineers. They started tinkering decades ago. And whenever they had a problem with the biology, the degradation rate or biogas production, the answer was: “You need to increase the retention time.”
That is complete nonsense.
Long retention times and low organic loading rates lead to poorer degradation, less biogas per tonne of organic dry matter and more feedstock for full load.
That’s where the vicious circle begins. Feeding more with poor degradation leads to floating layers. That leads to more agitation, disruption of the biology and even poorer degradation. Anyone who has ever had their digestate store dredged out at a cost of €50,000 knows what I’m talking about.
The solution is high-load digestion.
Research by Prof. Scherer (from 1999!!!), my own experience and the latest research results (Biogas Journal) prove it.
I run the hydrolysis at an organic loading rate of 50 kg oDM/m³*d. Single-stage biogas plant operators shake their heads in disbelief, because they don’t even dare to go near 4 kg.
With high-load digestion, the floating layers disappear. At some biogas plants the separator no longer runs, because there are no more solids to separate.
A high organic loading rate, the right temperature, optimal trace element supply and proper agitation lead to an organic dry matter degradation rate of 95% and more.
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In this video you’ll find a nice anecdote. A single-stage biogas plant operator can’t believe that the digestate is so liquid without separation.
