The biogas comes off the digester by its own pressure and comes through the Sulphide stripper, and is collected in a gas bag which has a air pressure switch gotten from an automatic washing machine, which clicks in at about 12" water gauge and out at about 2-3". When the gas bag is full the switch clicks in and starts a simple air compressor which only needs to pump the gas up to around 50-60psi. although the compressor can go much more than that.
The compressed gas is then bubbled into the base of a vertical standing length of 6-10" pipe about 4-6metres high and 2/3 full of water. The pipe can be anything though PVC looks nice and clean. If you are lucky enough to have a high mains pressure for your supply, you just feed that in through a partially opened tap. The water level is controlled by an exit float valve which bleeds off the highly gas charged water when the level gets beyond 2/3 to 3/4 way up the pipe. That water, when the pressure is released, will fiz just like soda water.
The gas then comes off the top of the vertical pipe through a pressure release valve which controls the whole operation, and goes into a pressure cylinder for storage at what we call intermediate pressure. If your water supply is at low pressure, then you will have to use an old piston water pump or any type of positive displacement pump which can work at its limit, say 150psi. It should be switched off and on along with the gas compressor. This means that the gas compressor can also work up to that figure, and the whole system shifts up a cog in terms of its efficiency and the gas comes out the stripping column for storage at that much higher a pressure (150psi) and correspondingly less volume.
We liked to get hold of old 500 litre LPG cylinders from hotels and the like. One farmer I knew got hold of an old railway locomotive boiler and used that. He actually had two. One alongside the biogas plant and another alongside his grain drier which was about half a mile away. They were connected together by 3/4" pvc water pipe, which gave adequate gas transfer when the grain drier was working. Just make sure that you have a water drain tap on the bottom of your storage cylinder because everything gets warm and the gas will drop a lot of moisture as it cools.