CSTR (Continuously Stirred Tank Reactor)

A Continuously Stirred Tank Reactor (CSTR) is the dominant biogas digester architecture in Indian CBG plants, characterised by a single fully mixed tank that receives a continuous or semi-continuous feed of slurry, maintains uniform temperature and composition throughout its volume, and discharges digestate at the same rate it receives feed. The CSTR concept comes from chemical engineering; in biogas it is also called a complete-mix digester.

Key design features of a typical Indian CBG CSTR:

• Vertical cylindrical tank — concrete or epoxy-lined steel, height-to-diameter ratio 0.8–1.2, volumes typically 500–5,000 m³
• Mechanical agitation — top-entry or side-entry impeller, intermittent operation at 5–10 W/m³ specific power
• Heating — internal hot-water coils or external heat exchangers maintain 35–40 °C (mesophilic) or 50–55 °C (thermophilic)
• Feed and discharge — typically 8–12 feed events per day from a buffer tank; digestate overflows continuously to a post-storage tank
• Headspace — gas holder integrated as a flexible roof or separated as a downstream double-membrane storage

The CSTR's strength is operational simplicity and robustness to variable feedstock. Because mixing is uniform, local concentration spikes (high VFAs, ammonia, salts) are quickly diluted across the tank volume, buffering the microbial community against acute upsets. Hydraulic retention time (HRT) is the same for liquid and solids — typically 25–45 days at mesophilic temperatures with agri-residue or food-waste feedstock.

The trade-offs against alternative designs:

• UASB and EGSB reactors achieve much shorter HRT (1–3 days) by retaining granular biomass independently of liquid flow, but require diluted soluble feedstock; not suitable for high-solids agri residue
• Plug-flow digesters achieve better solids residence at lower mixing energy, but are harder to operate at scale and less forgiving of feedstock changes
• Two-stage digesters separate acidogenesis from methanogenesis, improving control but adding capex and complexity

For Indian SATAT-scheme CBG plants, the CSTR remains the default because it handles the messy reality of mixed agri-residue, cattle dung, and food-waste feedstocks better than any alternative, even at the cost of larger tank volumes.