Continuous Stirred Tank Reactor (CSTR) (CSTR)
Also known as: Continuously Stirred Tank Reactor · continuous stirred reactor
A biogas digester design where continuous mechanical mixing keeps the slurry homogeneous, ensuring uniform temperature and maximum contact between bacteria and feedstock throughout the vessel.
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What is Continuous Stirred Tank Reactor (CSTR)?
A Continuously Stirred Tank Reactor (CSTR) is the dominant anaerobic digester design used in Indian CBG plants, characterised by continuous mixing of the entire reactor contents and continuous (or near-continuous) addition of fresh feedstock with simultaneous removal of digested material. The defining principle is the assumption of complete homogeneity — at any point inside the tank, temperature, pH, VFA concentration, and biomass distribution are uniform, and the outlet composition equals the bulk reactor composition.
Typical Indian CSTR characteristics include:
- Geometry: cylindrical RCC or steel tank, height-to-diameter ratio 0.5-1.0, partly buried or fully above-ground.
- Volume: 500-5,000 cubic metres per unit; large plants use multiple parallel CSTRs.
- Inlet TS: 8-12%; slurry is pumped continuously from a feed tank.
- Mixing: top-entry mechanical agitator, gas-mixing, or hydraulic recirculation at 5-15 W per cubic metre.
- Heating: internal coils, external heat exchanger, or steam injection to hold 35-40 degC mesophilic temperature.
- HRT: 20-40 days for most feedstocks.
- OLR: 2-4 kg VS/m3/day for stable operation.
CSTRs suit liquid and pumpable slurries — pressmud, cattle dung, food waste, molasses, sewage sludge — and are less effective on coarse fibrous feeds (untreated straw) that float or settle. The continuous-mixing design ensures any inhibitor entering the digester is immediately diluted across the entire volume, providing robustness against feedstock variability. It also avoids the dead zones and channelling that plague unmixed or plug-flow designs.
The principal trade-off is that complete mixing produces a uniform but partially-digested outflow. Some active biomass and undigested feedstock leave with each cycle, reducing yield by 10-15% compared to a plug-flow design where material residence time is more controlled. Capital cost in India runs 3-5 crore INR per 1,000 cubic metres installed, including mixing, heating, and gas collection systems. Operators favour CSTR for its operational simplicity, well-understood scaling, and forgiveness toward feedstock variability — a substantial advantage given the unpredictable quality of Indian agri-residue and municipal organic waste supply chains.
Common questions about Continuous Stirred Tank Reactor (CSTR)
Plain-English answers to what people most often ask.
What is a CSTR biogas digester?
What feedstocks are best suited to a CSTR?
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