I have researched the greenhouse gas emissions from green waste composting. Interestingly, under certain conditions biodegradation can produce greenhouse gases that have a far greater global warming potential (GWP) than carbon dioxide. One example is N20, a nitrous oxide that has a global warming potential equivalent to over 260 times that of carbon dioxide. I would imagine that slurry bioreactors (rich in nitrogenous waste) do run the risk of producing very large quantities of nitrous oxides, and these can equally be released after combustion. Burning of forests has a similar impact because significant quantities of NO and NO2 are released during combustion (which I seem to remember have GWPs in the range of 50 to 150). The real consequences during most biodegradation processes tend to occur when the C:N ratio exceeds 5:1, when both the balance of water and oxygen have a profound influence on the types of nitrous oxides and other greenhouse gases produced. The technology to scrub these gases out from the emissions from slurry bioreactors might not be particularly practical, and as you indicate, would incur an additional carbon footprint. There are additives, monitoring and mitigation strategies that can reduce greenhouse gas emissions during operation, but I would hardly consider training farmers as a reliable to mitigate against the risks of failure (somebody lazy might not care what is produced so long as their tractor runs, and regulation is a farcical). The localised nature of maintenance and production would also add a large carbon footprint I should imagine.
With both bioreactors and composting it is necessary to understand whether there is any value compared against simply locking this "carbon" into landfill. When you landfill most waste, there is good likelihood that at least some of it will be mineralised and not released into the atmosphere. Another study I remember looked at applying waste directly to land, where certain microbial communities do a better job at mitigating greenhouse gas emissions; in reality this is a scientific exercise hard to apply to individual farms. There is a pool of publications available on these topics that is held by the Chartered Institute of Waste Management (CIWM), but few scientists have really engaged with the research. As with many fields of science these days, woke-ism and alienation are very real risks when you do not jump into line and conform with the greenwash. I find it quite amusing that people think that composting is "environmental" or "green", and the same really applies to wood-fuel, slurry bioreactors and most other fads. But the sad thing is that we are talking about really profound greenhouse gas emissions during operation, and not yet even the carbon lifecycle analysis including production, nor methane specifically.
Apologies for not answering your question. Net energy generative I should imagine is quite achievable with slurry bioreactors (case specific), but you would need to track down some whole lifecycle analysis studies to confirm this. Clean methane is easy to fully combust and is not burdened by the environmental issues I have outlined above. As methane often bubbles out of the ground(waters) naturally, there is no reason not to capture and use it. From <
Wikipedia>:
"For instance, burning methane to carbon dioxide would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of methane burned is less than the mass of carbon dioxide released (ratio 1:2.74).[16] For a starting amount of 1 tonne of methane, which has a GWP of 25, after combustion there would be 2.74 tonnes of CO2, each tonne of which has a GWP of 1. This is a net reduction of 22.26 tonnes of GWP, reducing the global warming effect by a ratio of 25:2.74 (approximately 9 times)."
