Zone 2 - Equipment function and application - Core Reactor Technologies
Five anaerobic digester reactor technologies compared by feedstock type, total solids percentage, key design features, and the core advantage of each — a reference for selecting the right reactor for a specific feedstock mix.
| Reactor Type | Feedstock Application | Solids Content (TS%) | Key Features | Why Choose It? |
| CSTR (Vertical) | Cow Dung, Food Waste, Slurries | 8% - 12% | Vertical tank; constant mechanical agitation. | Universal & Reliable. Best for consistent, pumpable organic mixes. |
| Plug Flow (Horizontal) | Agri-waste, Napier Grass, Straw | 15% - 40% | Horizontal tunnel; "First-in, First-out" flow. | Water Saver. Handles dry solids without needing massive dilution. |
| UASB (Up-flow) | Distillery/Food Industrial Wastewater | < 3% | Upward liquid flows through a "bacterial blanket." | High Speed. Very high gas production rate per m^3 of tank volume. |
| Fixed Film (Bio-Filter) | Thin Wastewater, Dairy Effluent | < 1% | Bacteria stick to plastic/stone media inside. | Stability. Prevents "washout" of bacteria in very thin liquids. |
| Two-Stage | High-Acid Waste (Fruit/Sugar waste) | Variable | Separate tanks for Acidification and Methanation. | Efficiency. Higher methane purity (>70%) directly from the digester. |
Beyond definitions
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How to read this table
- Each row is one reactor technology; columns show feedstock fit, TS operating range, design principle, and the key selection reason.
- TS% (Total Solids percentage) is the critical matching criterion — a reactor designed for 8–12% TS will fail or underperform if operated outside that range.
- The CSTR, Plug Flow, and Two-Stage rows are the most relevant for typical CBG project feedstocks. UASB and Fixed Film are relevant primarily for wastewater-attached projects.
- "Why Choose It?" summarises the primary advantage — not an exhaustive pros/cons analysis. Read this column to narrow down the shortlist before detailed technical review.
About this table
Selecting the right anaerobic digester reactor design is one of the most consequential technical decisions in a Compressed Biogas (CBG) project. The wrong reactor for a feedstock leads to poor gas yield, unstable digestion, and costly retrofits. This table compares five reactor technologies across feedstock type, Total Solids (TS) content, design features, and the primary reason to choose each.
The CSTR (Continuously Stirred Tank Reactor) is the most widely deployed reactor type in India's CBG sector. Its vertical tank with mechanical agitation handles the 8–12% TS range typical of diluted cow dung, food waste, and MSW slurries — the feedstock types most commonly available near urban and peri-urban project sites. Its reliability and the availability of local fabrication and service support make it the default choice for first-time developers who do not have a specific feedstock reason to choose otherwise.
The Plug Flow Reactor is fundamentally different: a horizontal tunnel where material moves from inlet to outlet in a first-in, first-out pattern. This design handles 15–40% TS — the typical range for baled agricultural residue and chopped Napier grass — without requiring the massive water dilution that a CSTR would need to process the same material. For projects in Punjab, Haryana, or Maharashtra where paddy straw or Napier grass is the primary feedstock, the Plug Flow design reduces daily water consumption significantly. The UASB (Up-flow Anaerobic Sludge Blanket) is the choice for projects attached to distilleries or large food processing facilities — the wastewater input is already at below 3% TS, and UASB's upward-flow bacterial blanket design achieves exceptional gas production per cubic metre of reactor volume, making it the most space-efficient reactor for high-volume liquid waste streams.
The Fixed Film reactor handles very thin liquid waste — below 1% TS — where the bacterial community would be literally washed out of a conventional reactor by the fast hydraulic flow. Attaching bacteria to plastic or stone media inside the reactor keeps the microbial population stable even at high flow rates. The Two-Stage reactor is the most complex option but achieves the highest methane purity directly from the digester (above 70% CH₄) by separating the acidification stage from the methanation stage in two tanks. For high-acid feedstocks like fruit processing waste or sugar industry effluent, the separation prevents the acid-forming bacteria from inhibiting the methane-forming bacteria — a common cause of digester failure in single-stage reactors processing these feedstocks.
Key insights
- CSTR is the right default for mixed organic feedstocks at 8–12% TS — it handles the widest range of pumpable materials and has the largest base of Indian fabricators and service providers.
- Plug Flow handles high-solids agricultural feedstock (15–40% TS) without the water dilution penalty of a CSTR — critical for water-scarce regions where daily water input is a cost driver.
- UASB achieves the highest gas output per cubic metre of tank volume for liquid waste streams below 3% TS — making it the economic choice for distillery-attached CBG projects.
- Two-Stage reactors pre-treat high-acid feedstocks in a separate acidification tank, protecting the methanation bacteria from acidic shock and directly achieving above 70% methane purity at the digester outlet.
Methodology & sources
Reactor type characteristics are based on established anaerobic digestion engineering literature and typical Indian CBG project designs as of 2024. TS% ranges are operating design windows; actual performance depends on hydraulic retention time, organic loading rate, and temperature management. The Two-Stage methane purity figure (>70%) refers to output from the digester before gas upgrading.
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