Biogas is created when organic material is fermented in anaerobic conditions. Specialized bacteria thrive in anaerobic (without oxygen) environments, consuming organic acids and respirating combustible methane. The bacterial activity inside a biogas digester is very similar to the processes occurring in a compost pile. The bacteria break down carbon and require a certain amount of nitrogen to sustain their bodies. Thus, a biogas digester needs to maintain a specific balance between carbon and nitrogen. This balance is expressed in a carbon to nitrogen ratio (C/N). The ideal C/N for a pile of aerobic compost is 25-35. Similarly, the optimal carbon to nitrogen ratio in which organic mater is decomposed inside a biogas digester is 25-35.

Because this C/N is similar to aerobic compost, many people have assumed that the finished product of biogas digestion is a natural fertilizer. They have suggested that the effluent of a digester can be diluted with water an added directly to the soil or to plants. However, research by Dr. Elaine Ingham, a soil biology expert puts this practice into question. She has studied the microbial life of the soil in her laboratories in Corvallis, Oregon at both the Sustainable Studies Institute and Oregon State University. She states in her Compost Tea Brewing Manual 5^th ed. that teas formed in anaerobic conditions contain high levels of natural alcohols that poison and overwhelm other soil microbes, thereby destroying a portion of the life in the soil and harming plants. At present, only aerobic (oxygenated) compost tea can be guaranteed to actually benefit and stimulate life in the soil.

Given this research we must be careful with the ways in which we choose to use the effluent slurry left over after the biogas digestion process. Although there are some harmful alcohols, the slurry may contain a C/N of 30 and be full of water soluble nutrients. This is indeed useful. We need not throw it away as waste, and instead we might build a separate compost pile and add the effluent to it in order to create a healthier environment for soil organisms. The aerobic composting process may do away with some of the natural alcohols and add a diversity of aerobic microbes to the effluent. This extra step may be a way to make good on the promise of biogas—the creation of combustible energy and a high quality fertilizer.