As for the fixed film technique, it has certain merits over the above mentioned methods. Once the material to be used as carrier has been installed in the reactor, the reactor can be operated under normal conditions without any daily addition or monitoring. Different materials (stone chips, iron mesh, clay, wood chips, etc.) had been tried for fixed film reactors depending on their local availability and mechanical strength in long run. It would help to reduce HRT considerably resulting in cost reduction of biogas plant, without compromising on quantity and quality of biogas. Surprisingly in this promising research area, using high solids content substrate like cow dung slurry most of the studies have been carried out at laboratory level only, whereas these techniques have been successfully tried in the treatment of wastewater under field conditions. An extensive study on this aspect is warranted.