20 yrs or so ago the factory maint guys at Peterboroughs British sygar corp made an amazing digester using 300 litre blue food grade barrels , two barrels , each had a pigtail stirrer sealed inside , they ran on 12 volts .
All charging material was liquidised , produced gas was self propelled into one of two barrels that each had two antiflashback flame arrestors on the gas outlets pipes.
The digester barrels were inverted and clamped into a bigger barrel ( like a gasometer . each gas barrel had weight of water filled containers put on the top to give a working pressure for a small burner set up which they used to heat a 10 x 12 glasshouse .
Last time I spole to Alvin the system had been upgraded with a drain cock sustem so that the gas was used to propel the fully digested matter out the pipe . The gas outlets to the gas ometer had one way valves fitted to allow all generated gas to help make up the supply.
He said that they were working on the Mk 2 system that would have a simple sealed plunger charging tube to each digester barrel and that they would be using some old fuel oil tanks of 2000 litres each for the actual digesters.