CSTR Anaerobic Digestion

Dry Digestion and CSTR Technology

High-torque, heavy-duty mixing technology built for continuous, reliable biogas production from liquid slurry, cow dung, poultry litter, press mud, spent wash, sewage and industrial effluent.

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Dry Digestion and CSTR Technology

Digester Technologies

Classification Basis

Anaerobic digestion technologies are generally selected based on the total solids (TS) content of the feedstock rather than a single, one-size-fits-all design. CSTR (Continuously Stirred Tank Reactor) and dry digestion sit at opposite ends of that spectrum.

Bharat Bioenergy's Technology

CSTR — Combi Digester™

Pumpable, liquid feedstocks. Total solids roughly 8–15%. Continuous, high-torque agitator or pump mixing.

Niche — Not Offered by Bharat Bioenergy

Dry Digester

Purely solid, fibrous, stackable feedstocks. Total solids roughly 20–40%. Batch cycles, minimal mechanical mixing.

Wet Digestion

CSTR — The Proven Core of Every Bharat Bioenergy Plant

The Continuously Stirred Tank Reactor (CSTR) is the world's most widely deployed anaerobic digestion technology, and it sits at the core of every Bharat Bioenergy CBG and biogas plant as our Combi Digester™ platform. It is engineered for pumpable, liquid organic feedstocks — liquid slurry, cow dung and poultry litter, along with press mud, spent wash, sewage and industrial effluent — diluted to a consistent, pumpable slurry.

Inside the tank, the substrate, bacteria and temperature are kept fully homogeneous through continuous mechanical agitation, so feeding and discharge can run around the clock rather than in stop-start batches. That continuous operation is what keeps gas output steady and predictable — the reason CSTR remains the standard technology of choice for commercial-scale CBG and biogas projects across India.

CSTR digester tank cutaway showing central and lateral agitators, slurry inlet, biogas outlet and digestate outlet

Process Flow

How Combi Digester™ Works

Feedstock Intake

Slurry, cow dung, poultry litter & other liquid organics.

Water Addition

Dilution to a uniform, pumpable consistency.

Agitator Mixing

Combi Digester™ high-torque mixing homogenises the tank.

Anaerobic Digestion

Continuous biogas generation.

Purification & CBG

H₂S scrubbing, upgradation, compression.

Water dilution and agitator mixing happen together at the head of the process — this is what keeps the digester fully mixed and prevents the settling, crusting and dead zones that reduce gas yield in poorly agitated tanks.

The trade-off with any liquid-fed CSTR is that dilution adds volume: every tonne of thicker feedstock that needs water to become pumpable increases the reactor size needed for a given retention time and the heating demand. Combi Digester™'s heavy-duty mixing is engineered to keep that trade-off in check by handling thicker slurries efficiently, reducing how much dilution water is actually needed.

Garage-type dry digester with a loading door showing stacked fibrous feedstock, biogas outlet and access ladder

Dry Digestion

Dry digestion is built for feedstocks that are naturally solid or fibrous, with total solids typically in the 20–40% range — municipal solid organic waste, straw and crop residues, solid manure, silage, and other stackable biomass that cannot easily be pumped. There is no need to dilute the material with large volumes of water, keeping digestate output smaller and heating load lower per tonne of feedstock treated.

Dry systems come in two main configurations. Batch (garage- or box-type) systems load solid material into sealed, gas-tight chambers using a wheel loader, inoculate it by recirculating percolate/leachate collected at the base, and run a fermentation cycle of roughly three to four weeks before the chamber is emptied and reloaded; multiple staggered chambers keep overall biogas output continuous even though each individual chamber operates in batch mode. Continuous plug-flow dry digesters instead move the material horizontally through a long reactor using augers, rams, or gravity, with minimal internal mixing, so fresh feed pushes older, partially digested material along the length of the vessel.

Dry digestion needs less water and produces a smaller, more concentrated digestate, but depends heavily on mechanical handling equipment (loaders, augers, conveyors), runs in multi-week batch cycles rather than continuously, and is a comparatively newer, less standardised technology with fewer long-track-record suppliers than CSTR — which is why it is not part of Bharat Bioenergy's core technology offering.

Bharat Bioenergy Proprietary Technology

Combi Digester™ CSTR Technology

Combi Digester™ is Bharat Bioenergy's in-house engineered CSTR platform. Rather than relying on a single mixer type, it matches the right mixing method to the feedstock and tank size — heavy-duty agitators for high-torque, low-speed mixing of thicker slurries, and pump-based recirculation where a lower-solids stream calls for it — all manufactured and serviced through our own Pune and Karnataka facilities.

Agitator-Based Mixing

Central + Lateral Agitator

A vertical shaft mixer mounted through the tank roof at the centre, usually fitted with axial-flow or hydrofoil impellers. On larger tanks, one or more lateral (side-mounted, angled) agitators are added alongside it to eliminate dead zones and prevent scum/sediment build-up at the periphery.

Paddle Type Mixer

Slow-speed, high-torque paddles (rather than propellers) on a shaft, either top- or horizontally mounted. Preferred where the feedstock is fibrous, the biological floc shouldn't be shredded too aggressively, and a high-viscosity substrate needs torque more than speed.

Side Entry Mixer

Motor and propeller enter through the tank's side wall near the base, angled to set up a rolling, circular flow across the floor. The drive unit sits outside the tank, so it can be serviced without opening the roof.

Submersible Mixer

A fully submerged motor-propeller unit, hung from a guide rail or a swivelling wall bracket so it can be raised for maintenance without entering the gas-tight vessel. Avoids gas-tightness and corrosion issues at a roof shaft penetration.

Pump-Based Mixing

Mixing Through Pump

External pumps draw slurry from one zone of the tank and reinject it elsewhere, often through nozzles or eductors, creating mixing purely through recirculated flow with no moving parts inside the tank. This avoids in-tank seals and mechanical wear, suits lower-solids, pumpable substrates, and can double as a way to break surface crust by directing jets across the top of the liquid.

Recommended Technology

Why Combi Digester™ CSTR Is Bharat Bioenergy's Recommended Technology

For the feedstocks Indian CBG and biogas projects actually generate — liquid slurry, cow dung, poultry litter, press mud, spent wash, sewage and industrial effluent — CSTR is the simpler, proven, lower-risk technology, which is why it is the core of every plant we build.

Decades of Proven Operation

CSTR is the most widely deployed anaerobic digestion technology in the world, with a deep base of operating history, equipment suppliers and spare parts. Batch-type dry digestion has a far shorter, thinner track record at commercial scale.

Continuous Output, Not Batch Cycles

Combi Digester™ feeds and discharges continuously, so gas production stays steady day to day. Batch/dry systems load a chamber, run a multi-week cycle, then empty and reload it — adding downtime, manpower and scheduling risk around every cycle.

Matches Real Indian Feedstock Streams

Liquid slurry, cow dung, poultry litter, press mud, spent wash, sewage and industrial effluent are all pumpable, liquid-compatible streams that suit CSTR directly — without the shredding, drying or bulk mechanical handling that purely fibrous dry-digestion feedstock requires.

Simpler, Lower-Risk O&M

Pumps and standard mechanical agitators are easier to source, maintain and service locally than the specialised loaders, augers and gas-tight batch chambers dry digestion depends on.

Backed by In-House Manufacturing

Because CSTR is a standardised, well-understood technology, Bharat Bioenergy manufactures and services the core Combi Digester™ equipment in-house at Pune and Karnataka — giving full control over quality, lead time and long-term support.

Where dry (batch) digestion fits: it has a niche role for purely fibrous, low-moisture solid biomass that can't be pumped at all. For the mixed and liquid-compatible feedstock streams most CBG and biogas projects in India actually generate — including Napier grass, press mud and agri-residue once blended into a pumpable slurry — that pre-treatment step is unnecessary overhead. That's why Bharat Bioenergy builds every plant around Combi Digester™ CSTR technology.

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