Feasability

To calculate the profitability of a biogas plant, it is important to have enough feedstock to make biogas. From organic matter, bacteria produce biogas; a mixture of methane and carbondioxide.

The retention time in the digester and the organic loading rate (that is how much food is given to the digester each day per m3 volume) are important design criteria. For some feedstockit may take longer to digest than other. Also certain feedstock will need a treatment before it can be used; eg. cutting it to smaller pieces like grass and straw, removal of excess nitrogen or dilution with water.

Also the quality of the feedstock is important. Fresh manure gives a higher biogas yield than manure that is already a week or so old. 

For the biogas plant it is important to look at a possible usage of the surplus heat from the CHP unit. You can separate and dry the solid fraction of the digistate and pelletise this. 

Also important is to look how to use the digistate. it is a Potent fertiliser with good properties and could replace chemical fertiliser needs if properly used.