It is possible to use co-fermentation to increase biogas production considerably in digesters with free capacity. Also, the energy production increases by increasing biogas production. Many co-substrates such as energy crops, biodegradable waste from the food industry, restaurants and public parks, etc. are on the market. Water consumption is decreasing because of population movement and a decreasing population size because of a drop in the birth rate. Therefore, capacities in digesters will increase and new uses for this capacity are needed. Co-fermentation could be one solution. Useful co-substrates should be identified by material characteristics. It is important to assess suitability of materials for fermentation in small-scale tests, and to gain experience particularly with regard to the handling, the compatibility of the biocoenosis in the digester and the effects of returning effluent to the activated sludge process. A test period with fat separator contents has started at the wastewater treatment plant, Waßmannsdorf, Germany. Biogas production and methane content increased compared to the reference cascade without co-substrate input.