1% to 2% (1–2%)

The 1% to 2% range refers to methane slip — the fraction of input methane that escapes a biogas upgrading system as part of the vent gas rather than being captured in the product biomethane stream. It is the most important environmental and economic performance indicator after methane purity itself, because methane has a Global Warming Potential roughly 28 times that of CO₂ over a 100-year horizon and 84 times over a 20-year horizon.

Different upgrading technologies have characteristic methane slip ranges. Amine scrubbing achieves the lowest slip (under 0.1%) because CO₂ is selectively chemisorbed while methane passes through. Membrane separation delivers 0.5–2% in single-stage configurations and under 0.5% with multi-stage recycle. Water scrubbing typically operates at 1–2% slip. Pressure Swing Adsorption (PSA) ranges from 2–4% in single-stage units, dropping below 1% with recycle of the regeneration gas. The 1–2% band represents the acceptable industry performance window for SATAT-scheme plants and is increasingly the maximum allowed under emerging CPCB guidelines for biogas plants.

The economic and environmental implications are significant. A 5 TPD CBG plant produces around 1,800 tonnes of methane per year as product. A 2% slip means 36 tonnes of methane vented to atmosphere — equivalent to about 1,000 tonnes of CO₂e annually. Revenue lost at SATAT pricing of ₹54/kg is ₹19 lakh per year. Vent gas treatment options include regenerative thermal oxidisers, which destroy methane to CO₂ (reducing GWP impact by 28× but losing the energy content), or routing the vent stream back to the CHP engine or boiler for thermal recovery. Under emerging carbon-credit methodologies, claimable credits are typically discounted by the methane slip percentage, creating a direct financial incentive to invest in lower-slip technologies.