The Continuously Stirred Tank Reactor (CSTR) is the completely-mixed anaerobic digester for high-solids and slurry feeds. Where granular systems cannot form, the CSTR keeps the whole contents fully mixed at a long retention time — the robust, forgiving choice for thick, particulate or variable feedstocks.
The features that define its performance
Mechanical or gas mixing keeps solids in suspension and the biomass in intimate contact with the substrate, so the digester behaves as one uniform reactor.
Hydraulic and solids retention are equal in a CSTR, so the long retention time (typically 15–30 days) gives slow-degrading particulates time to break down.
Because biomass is suspended rather than granular, the CSTR handles thick slurries and high total solids that would blind or wash out a high-rate reactor.
In a completely mixed digester the hydraulic retention time equals the solids retention time: HRT = SRT = V/Q. That single equation is the heart of CSTR design — the volume must give the slow-growing methanogens long enough to both degrade the feed and reproduce faster than they wash out. For thick, particulate feeds the limiting step is hydrolysis of the solids, which is why CSTR retention times stretch to weeks rather than the hours of a UASB or IC reactor. The pay-off is robustness: a CSTR tolerates high total solids, variable load and shock far better than a granular system — at the cost of a much larger, fully mixed tank.
Indicative figures — confirmed by treatability testing on your effluent
| COD Loading | 2 – 5 kg/m³·day |
| COD Removal | 60 – 70% |
| HRT | 15 – 30 days |
| Best For | High total solids, slurry |
Reynolds & Bauhm selects and designs the anaerobic reactor that fits your effluent — confirmed by treatability testing and delivered with biogas handling and energy recovery.
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