The second biggest issue with AD is foam; this occurs if the diet of the AD is changed significantly over a short period of time, not allowing the bacteria time to adjust. This is a common occurrence in digesters that process a high percentage of unpredictable food wastes. In an ideal world where the operator has complete control over perfect feedstocks it does not happen. However, with high strength feed stocks, in the real world at some stage it almost inevitably will.
Foaming is extremely problematic to a digestion process for 3 reasons:
- If the foam reaches the methane gas offtake pipe in the digester roof it will shut down the generator; if the foam reaches the generator it will destroy it
- When foaming occurs it is because some of the bacterial activity is accelerated due to acidity and the bacteria needs time to process the material in order for the system to recover; early phase bacteria accelerate and the later phase need time to catch up. This can result in drastically reduced methane gas production over a period of time which can be a long as 2 months, with a consequential loss of associated income.
- In digesters that have gas storage above the digester using an inflatable fabric roof, excessive foaming can result in serious damage to the roof.
Fre-energy anaerobic digesters are designed to reduce the potential for foam to cause a problem. Should foaming occur, foam can be removed from the digester before it reaches the gas pipes. It is then processed enabling the digester level to be maintained and feeding sustained while the digester acclimatises to the new diet.
Note: Foaming can often occur when usable digester volumes are reduced by the accumulation of grit.
The Fre-energy foam alleviation system is incorporated within the thermally insultated fibreglass roof. This system will immediately shut down the generator if foaming occurs, avoiding any foam entering the methane gas offtake pipe. It then allows the foam to be removed from the gas space without the need to reduce the digestate level in the tank.
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