Anaerobic digestion produces methane. The thinking is recently produced carbon and will be presumably immediately re-absorbed by the surrounding plants in the next growing season. This acting as carbon balanced energy source.
Biogas is a mixture of methane and CO2. It's been discussed to separate out the CO2 (easy because it's at such a high concentration) and sequester it. This makes biogas net carbon negative. Alternately, hydrogen can be added to convert the CO2 into additional fuel.
The downside is the large land area needed to make the biomass that gets digested. In a fossil fuel free future, carbon-containing waste streams will be valuable as feedstocks (for chemicals, for liquid fuels that cannot be electrified easily), so there will be competition for these streams.
A smaller amount of hydrogen is also produced.