For lab scale batch study, water displacement method was utilised to measure biogas production per day. But how do I measure biogas production per day in a pilot scale continuous reactor ??
You can put a Coriolis mass flow meter with totalizer in the biogas line, its a costly proposal. Or a vortex flowmeter with compensation for temperature and pressure could be used.
A rotameter or orifice meter with flow totalizer can also help.