The methane released (actually only averaging 65% of the biogas by volume) is ultimately non-anthropogenic in the sense that it is the decay product of organic decomposition that has been going on as part of the carbon cycle for millions of years.
It is NOT fossil derived and, like the gases we emit from our own bodies and that of all animals and from the roots of plants and from septic tanks and ponds and lakes and streams and swamps and oceans and forests, most of it is decomposed by methanotrophic microbes who render it back into CO2 and water for the uptake by other organisms (mostly plants) to create more structured carbon which becomes food and food waste and toilet wastes again.
That humans now collect those naturally produced bubbles of CH4 and use them is actually a good thing in that they offset the use of fossil fuels and can contribute to reducing global warming. But if we didn't collect them and burn them (turning them immediately into CO2 and water through combustion) they would be broken down anyway.
The ontribution of domestic biogas to global warming is nil to none.
When people talk about cows belching so much methane that they are a contributor they are talking about unnaturally dense CAFO (Concentrated animal feedlot operations) supported by fossil fuels to concentrate so many animals and grow so much carbon rich food which would have never occurred in a balanced ecosystem. It is only because of the fossil inputs that animal populations can make so much methane.
One could argue that human population is similarly concentrated unnaturally, but this goes on whether we use a biodigester to make it or simply release our organic waste into the "waste stream". And it would be another argument for the wisdom capturing the gas and using it; either way you get the biogas, usually just bubbling out the anus of the person or animal, or bubbling through the leach field, or the land fill or in the garbage can or compost bin. The trick is to somehow get the methane oxidized, and that can be done by burning or by harnessing methanotrophs.
Here is the abstract of an article on methanotrophs;
And here is an article on methane oxidation in rice paddies.
The rice paddy article gives the best clue of how to avoid methane release from your biodigester if you aren't storing or using it:
"Methane-oxidizing bacteria (methanotrophs) are abundant in the oxidized floodwater-soil interface and in the rice rhizosphere. They sequentially oxidize methane to carbon dioxide via methanol, formaldehyde, and formate. Oxygen is essential for the growth of methanotrophs, but the required partial pressure may be low (Cicerone and Oremland 1988). Methane oxidation greatly limits diffusion of methane to the atmosphere. Up to 60% of the methane produced during a rice growing season may be oxidized before it reaches the atmosphere (Holzapfel-Pschorn et al. 1986, Sass et al. 1991). Ammonium ion inhibited methane oxidation in studies with pure cultures of methanotrophs (Hyman and Wood 1983, Whittenbury et al. 1970). Field experiments have revealed no significant effect of ammonium ions, probably because of their immediate uptake by rice plants.
Rice plants supply atmospheric oxygen to the roots for respiration via a special vascular system, the aerenchyma. The aerenchyma has its own openings at the leaf sheath (Nouchi et al. 1991), and the gas supply to and from the roots is independent of transpiration and stomatal gas exchange. Oxygen diffusion from rice roots constitutes an important part of the roots' oxidizing power, aside from enzymatic hydrogen peroxide production. Because of the abundance of methane-oxidizing bacteria present in the rhizosphere, the rhizophere's potential for methane oxidation is high.
De Bont et al. (1978) counted ten times more methane-oxidizing bacteria in the rhizosphere than in the bulk anaerobic soil and one-third more than in the oxidized soil-water interface. They found significant increases in methane emission by the rice cultivar IR36 when methane oxidation was suppressed with acetylene at the soilwater interface. However, acetylene had only a small effect on emission rates when applied to the rhizosphere. De Bont and his colleagues concluded that the use of oxygen by reduced substances and microbes other than methanotrophs at the region of the root-soil interface exceeds the supply of oxygen by the root. Consequently, the aerobic zone surrounding the root of IR36 is too thin to oxidize the diffusing methane, or the rhizosphere is for the most part anaerobic. Nevertheless, variability in root-oxidizing power of rice cultivars is high, and the impact of roots on methane oxidation merits further study.
Methane fluxes in rice fields
Methane is released from anaerobic wetland soils to the atmosphere through diffusion of dissolved methane, ebullition of gas bubbles, and via plants that, like rice, develop aerenchyma tissue. Large portions of methane formed in an anaerobic soil may remain trapped in the flooded soil. Entrapped methane may be oxidized to carbon dioxide when the floodwater is drained during the rice growing season or when the soil dries at the end of or after the rice growing season. But large amounts of entrapped methane may escape to the atmosphere immediately after the floodwater recedes (Denier van der Gon et al. 1992).
The low solubility of methane in water limits its diffusive transport in the flooded soil, and most methane is oxidized to carbon dioxide via methanol, formaldehyde, and formate as it passes the aerobic soil-water interface. The release of methane by diffusion through the wet soil column is negligible in clayey soil, but it may become significant in sandy soils in which bigger pores between soil particles prevail. Most rice soils have high clay contents. Soil fauna, especially aquatic earthworms (Tubificidae), increase emission through diffusion and ebullition when they dig into the topsoil. At the same time, oxidation of methane is enhanced. In deepwater rice fields, diffusing methane may only be oxidized in the upper water column, because the soil-water interface and the lower water column may be anaerobic."
So if you decide you can't currently capture and use the biogas your digester makes, we can follow the natural cycle and work with methanotrophs.
Again, we are talking about extremely small volumes in any case and therefore not a problem, but if you wanted to be sure not to have ANY impact on methane accumulation in the atmosphere and can't burn the gas your digester makes, our suggestion is that you mimic the rhizosphere and allow the methanotrophs to decompose your biogas.
Simply run your biogas hose through a layer of rhizome rich soil. Get a container, fill it with loose biologically active soil or compost and put some weed seeds in there. Let things sprout. Run your gas tube into the bottom of the container and let nature do the rest! Case closed!
I may not be able to use all the gas I produce at times and can't store it. I've read that methane is 28 times stronger than CO2 as a greenhouse gas and that cow farts and belches are possible contributors to climate change. How do I keep my bioidgester from being a part of the problem rather than the solution?