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Small scale, multi-stage biodigesters

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I've been studying biodigesters for a couple of years now, reading everything I can find online, including academic papers, and while there's a fair amount on small single stage digesters, there's not so much about two- or multi-stage designs until you're looking at larger scale, farm or village digesters. Specifically, I can't find much information on a continuous-flow design, showing how the stages are separated chemically and environmentally from each other. I get that the first stage is relatively fast, requiring a retention time of only a few days, while the second [and subsequent, if any] stage takes longer and produces the methane, but how do you actually connect the tanks so that the oldest, most processed material flows to the second stage, while isolating the different pH levels between the two stages? It can't be as simple as sizing the tanks for proper retention times, connecting them up to feed one to the next, and letting the bacteria regulate themselves, can it?

My digester will be fed with blackwater [two people], kitchen scraps and garden/yard waste, possibly a few chickens or other small livestock. The quantity of gas we get is secondary; I'm approaching this as an alternative to installing a septic system, with fertiliser as the primary output. I intend to separate the blackwater/greywater streams, treating the greywater with this type of system: and using it for irrigation purposes. To simplify things, I thought I might be able to treat the effluent from the digester as greywater, or should I sterilise it [as I plan to do for the sludge] before use on the veggie garden?
[Thumbnail for waste.jpg]
simplified two stage digester?
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