Abstract: A process for manufacturing liquid methane from a feedstock gas, wherein the feedstock gas is obtained from an alternative gas source generated by anaerobic digestion and comprising methane, carbon dioxide, nitrogen, oxygen, water vapor and hydrogen sulfide, the process comprising the steps of: (i) removing from the feedstock gas constituents which are incompatible with liquefaction, wherein removal is effected by pressure swing absorption, whereby to yield a mixture comprising methane, nitrogen and oxygen; and (ii) liquefying the mixture by cooling, and adjusting the temperature during cooling so-as to remove nitrogen and oxygen, whereby to yield an output consisting primarily of liquid methane.