We never mind a bit of off-topic. I have a template for looking at these ‘renewable’ sources, anon. First of all we should park the whole (C)AGW controversy because there’s no point: if you reject the AGW concept you don’t go on to discuss the finer details of one scheme vs another.
So: (1) does the source of electricity actually contribute to reducing CO2? Answering this requires looking deeper than just the official line; we have the ghastly example of large-scale biomass burning (wood pellets for electricity by the millions of tonnes) which actually increase CO2 emissions, even vs coal - and the "carbon debt" may not be repaid for decades or even longer. This all happens (and nearly a billion £££ in subsidies paid annually to Drax and the like) because the regulations simply - and erroneously - deem biomass burning to be CO2-neutral at the point of combustion. It's actually pretty outrageous.
(2) Where does the scheme sit on the Marginal Abatement Cost Curve? Logically speaking (*huh!*) there is no point pursuing more expensive approaches when there are cheaper, or indeed self-financing approaches still to be fully exploited. Of course, it happens all the time but it's crazy.
(3) What are the *unintended consequences*? Biomass sources of all kinds are seriously prone to this, from appalling deforestation to direct competition with food crops etc etc. Tree-burning for electricity is among the worst: the 'particulates' emissions are very bad, much worse even (again) than coal. I reckon air quality will be a major new front against biomass soon.
Which brings us to your precise question and biodigestion - which we did once take a pot-shot at before. (1) There is a reasonable case to be made that it can actually reduce CO2 (and methane-escape) though this depends on the detailed counterfactuals, as always. (2) It's not very economically-placed on the MACC - worse than many, but better than some (don't get me started on the Swansea Tidal Lagoon). It's "dispatchable" to some extent, too (i.e. can fire up on demand, more or less), which counts in its favour compared to wind, solar and tidal.
(3) is interesting. What you find is that, even in rural areas, you need to cast your net quite widely to get enough pigshit, soiled straw etc to fire up even a small anaerobic digestion plant. So a large number of diesel-miles are required to round it all up - which ought to be factored into the CO2 equation. (To be fair, they do include a bit of allowance for this in the reporting, though prob not enough.) Secondly, these plants, though not particularly vast to look at, do require extensive foundations to sit on, and a large area of proper concrete hardstanding for the constant arrival and turnaround of shit-laden trucks. Again, this makes it CO2-intensive (and cost-intensive generally) to set them up in the first place - not any old corner of a farmyard will do. But sometimes you can find, e.g., a suitable patch on a disused RAF airbase that fits the bill nicely.
I haven't looked into the particulates in detail, but my general understanding is that while the raw material is foul, it's methane that is being burned (= 0 particulates, of course).
Finally, a cow's stomach is more than 20 times more efficient at producing methane than the most advanced biodigester currently available! This alone suggests the technology is not suitably advanced yet for it to be a Good Idea in the abstract. But when did that ever influence a lobby-pandering politician?
So there you go. I wouldn't allow any of them until the cheaper schemes have been fully exploited - but what do I know?