Water Scrubbing (water scrubbing)

Water scrubbing, more precisely Pressurised Water Scrubbing (PWS), is a biogas upgrading process that exploits the selectively higher solubility of CO2 in water compared to methane to separate them. Raw biogas at 6-10 bar is contacted counter-currently with chilled water in a packed tower; CO2 dissolves into the water while methane passes through largely unabsorbed, yielding an upgraded gas stream at 95-98% methane purity. The CO2-rich water is regenerated in a flash tank at low pressure (1-2 bar), releasing the CO2 to atmosphere and recycling the water. PWS is the most widely deployed upgrading technology in India's SATAT-aligned CBG sector because it is robust, scalable, and tolerant of variable raw biogas composition.

Typical PWS operating parameters for an Indian 5-15 TPD CBG plant include:

• Operating pressure: 6-10 bar.
• Water-to-gas ratio: 0.18-0.30 m3 of water per Nm3 of raw biogas.
• Water temperature: 5-15 degC (chilled to maximise CO2 solubility).
• Methane purity: 95-98%.
• Methane slip: 1-2% (lost to atmosphere with CO2 vent).
• Electricity demand: 0.20-0.30 kWh per Nm3 of biomethane.

The same packed tower also removes most of the hydrogen sulfide, since H2S has similar solubility characteristics to CO2 in water. However, H2S accumulation in the recirculating water reduces CO2 absorption capacity over time, so a small fraction of water (5-10%) is continuously bled off and replaced with fresh water — typically 50-150 L per Nm3 of CBG produced.

Trade-offs against alternative technologies are well understood. PWS scores low on capex (1.5-3 crore INR per 1,000 Nm3/hr capacity), simple operation, and no consumable chemicals — but it requires significant water resources (a constraint in arid Indian regions), 1-2% methane slip that represents both lost revenue and GHG emissions, and bulky chilled-water infrastructure. PSA, by contrast, eliminates water demand and slightly reduces methane slip but doubles capex and requires periodic adsorbent replacement. Membrane systems offer the smallest footprint but lose 2-4% methane slip. For mid-size Indian SATAT plants (5-15 TPD CBG), PWS remains the default choice; for water-stressed regions or plants under 3 TPD, membrane systems are gaining share.