What Is a CSTR Reactor?
The CSTR is a continuously stirred, fully mixed reactor, the most common digester design in anaerobic digestion for thick substrates.
A CSTR (Continuously Stirred Tank Reactor) is a fully mixed anaerobic digester in which substrate is fed and discharged continuously while a stirring system keeps the contents homogeneous. It is the most widespread digester design in biogas plants that process thick substrates such as manure, slurry, agro-industrial residues and energy crops.
The core idea of the CSTR is simple: inside the reactor, temperature, bacterial concentration and substrate are (ideally) uniform throughout the volume thanks to mixing. That promotes stable contact between bacteria and organic matter and even biogas production.
How it works
The CSTR operates continuously and at controlled temperature. Its typical elements and parameters are:
- Continuous or semi-continuous feeding: substrate is loaded several times a day and digestate is discharged in equal measure.
- Mixing: mechanical (submersible or shaft agitators) or by gas/liquid recirculation, to prevent sediment and crusts.
- Heating: usually mesophilic (~35–40 °C) or thermophilic (~50–55 °C).
- Hydraulic retention time (HRT): typically 20–40 days for manure and co-digestion.
Because it is fully mixed, HRT (the time the liquid stays in the reactor) equals the solids retention time, which distinguishes it from retained-biomass reactors such as the UASB.
Typical reference parameters
| Parameter | Orientative range (CSTR) |
|---|---|
| Temperature | 35–40 °C (mesophilic); 50–55 °C (thermophilic) |
| Retention time (HRT) | 20–40 days |
| Organic loading rate (OLR) | ~2–5 kg VS/m³·day |
| Substrate total solids | ~4–12% (thick substrates) |
| Typical substrates | Manure, slurry, agro-industrial residues, crops |
These are general engineering values and vary with substrate and design; they are always tuned with tests and the plant's mass balance.
CSTR vs UASB: when to use each
They do not compete: they solve different problems. The CSTR is ideal for substrates with solids (manure, co-digestion), while the UASB shines with diluted liquid effluents of high soluble load (breweries, dairies).
- CSTR: thick substrates, high biogas output, tolerates solids; large, heated reactors.
- UASB: liquid effluents, BOD removal on the order of 65–75%, compact reactors with low HRT (hours).
Where it applies in Tech Tank projects
The CSTR is the basis of many of our biogas and biodigester plants for agribusiness, dairy farms and feedlots. We integrate it with a double-membrane gas holder, desulfurization and cogeneration in a turnkey package. To understand the general concept, see what an industrial biodigester is; and if your effluent is liquid and diluted, look at what a UASB reactor is and how it works. To define the right technology for your substrate, contact us.
Frequently Asked Questions
What does CSTR mean?
CSTR stands for Continuously Stirred Tank Reactor. It is a fully mixed anaerobic digester fed continuously and kept homogeneous by stirring.
What substrates is a CSTR good for?
Thick substrates with solids: manure, slurry, agro-industrial residues and energy crops, often in co-digestion. It is not the best choice for very diluted liquid effluents, where a UASB is preferable.
What is the retention time of a CSTR?
Typically 20 to 40 days, depending on substrate, temperature and organic load. Because it is fully mixed, the liquid and solids retention times coincide.
What is the difference between a CSTR and a UASB?
The CSTR mixes the whole volume and retains substrate for days; the UASB retains biomass in a granular sludge blanket and treats liquid effluents in hours. CSTR handles solids, UASB handles liquids.
Why is the reactor stirred?
To keep temperature and substrate homogeneous, prevent settling and crusts, and ensure contact between bacteria and organic matter, which stabilizes biogas production.