As renewable natural gas (RNG) and biomethane move from pilot to mainstream, developers are forced to pick winners among biogas upgrading technologies. Two routes dominate new plants in Europe and Asia: water scrubbing systems and polymer membrane separation. Both can reach grid-spec biomethane, but they differ sharply in capex intensity, specific energy use, uptime, and scaling behaviour. In this brief, Energy Solutions compares membrane vs water scrubbing costs using normalised €/Nm3 upgraded and highlights where each technology makes the most sense in 2026 project pipelines.
What You'll Learn
- 1. Market Context: Why Upgrading Choice Matters Now
- 2. Technology Basics: How Water Scrubbing and Membranes Work
- 3. Cost & Performance Snapshot: Membrane vs Water Scrubbing
- 4. Scale & Electricity Price Sensitivity
- 5. Case Study: 500 Nm3/h Biogas Plant
- 6. Project Economics: IRR & NPV Sensitivities
- 7. Revenue Stack & RNG Pricing
- 8. Regional Comparison: EU vs US vs Asia
- 9. Operating Reality: Uptime, Failures & IRR Impact
- 10. Devil's Advocate: Where Upgrading Projects Go Wrong
- 11. Outlook to 2030: Technology Roadmap
- 12. FAQ: Practical Questions from Developers and Financiers
1. Market Context: Why Upgrading Choice Matters Now
RNG and biomethane are moving from niche climate projects to mainstream gas portfolio assets for utilities and midstream operators. In many markets:
- Feed-in tariffs and renewable gas certificates are tightening and becoming more performance-based.
- Developers are pushed to deliver lower specific upgrading costs while respecting grid gas quality rules.
- Investors increasingly ask for benchmarking across technology routes rather than OEM marketing claims.
Upgrading as the Margin Maker
For many AD plants, upgrading OPEX is now one of the biggest levers after feedstock cost. A poor technology fit can erase most RNG premium.
Power-Price Exposure
Electricity-intensive routes (e.g. multi-stage membranes with high compression) are more exposed in markets with volatile wholesale power prices.
Methane Slip & ESG
Even small differences in methane slip and flare practices convert directly into climate impact and regulatory risk.
2. Technology Basics: How Water Scrubbing and Membranes Work
Water scrubbing uses physical absorption of CO2 (and some H2S) into water under pressure, then regenerates the water in a desorption column. Membrane separation uses selective permeability of gas components through polymer layers, usually in 2–3 stages with recycle loops.
High-Level Technical Comparison
| Attribute | Water Scrubbing | Membrane Separation |
|---|---|---|
| Main separation driver | Solubility of CO2, H2S in water under pressure | Different permeation rates through polymer membranes |
| Key utilities | Electricity (pumps), water make-up, sometimes cooling | Electricity (compressors), small water use for pre-treatment |
| Typical plant size sweet spot | 300–2,000 Nm3/h | 100–5,000 Nm3/h |
| Methane slip potential | Low with good design and stripping; some loss in off-gas | Managed via recycle; risk of higher slip if stages undersized |
| Complexity | More columns and liquid handling equipment | More compression, but skid-mounted and modular |
For a broader view of how different feedstock families shape project risk and returns, see our anaerobic digestion (AD) market outlook, and for landfill-based RNG opportunities compare with the landfill gas to RNG economics brief.
6. Project Economics: IRR & NPV Sensitivities
From an investor's perspective, upgrading technology is judged ultimately on its contribution to equity IRR and project NPV, not only on €/Nm3 metrics. Using the same 500 Nm3/h farm plant, we can build a simplified view of returns under a 10-year offtake contract.
Illustrative 10-Year Project Cash Flow Metrics (Post-Tax, Levered)
| Scenario | Upgrading Route | Assumed RNG Price (€/MWh) | Equity IRR | NPV @ 7% (per kWth biogas) |
|---|---|---|---|---|
| Base case | Water scrubbing | 95 | ˜ 11–12% | €190–220 |
| Base case | 2-stage membranes | 95 | ˜ 12–13% | €210–240 |
| Low power price | 2-stage membranes | 95 | ˜ 13–14% | €240–270 |
| High power price | 2-stage membranes | 95 | ˜ 10–11% | €160–190 |
IRR Sensitivity to RNG Price and Power Cost
Illustrative equity IRR for membrane vs water scrubbing under different RNG prices and power cost environments.
3. Cost & Performance Snapshot: Membrane vs Water Scrubbing
The table below shows an illustrative comparison for a mid-scale 700 Nm3/h raw biogas plant (55–60% CH4) in a European market with moderate power prices.
Indicative Cost and Performance (700 Nm3/h Biogas, 2025–2026)
| Metric | Water Scrubbing | Membrane Separation |
|---|---|---|
| Turnkey capex (upgrading island) | €2.2–2.6 million | €1.9–2.3 million |
| Specific capex (€/Nm3/h biogas) | ˜ €3,100–3,700 | ˜ €2,700–3,300 |
| Electricity use (kWh/Nm3 upgraded) | 0.18–0.22 | 0.22–0.28 |
| Water consumption (l/Nm3 biogas) | 0.5–2.0 (depending on recirculation) | Very low |
| Typical methane recovery | 96–98% | 96–99% (with 3-stage design) |
| Estimated total upgrading cost* | €0.095–0.12/Nm3 upgraded | €0.085–0.11/Nm3 upgraded |
*Upgrading only; excludes feedstock, digesters, and grid connection. Ranges combine capex amortisation and opex under reference assumptions.
Illustrative Specific Upgrading Cost (€/Nm3 Upgraded)
Mid-point specific costs derived from the ranges above, for a 700 Nm3/h plant.
4. Scale & Electricity Price Sensitivity
Both technologies benefit from scale, but in different ways:
- Water scrubbing sees capex per Nm3 fall sharply as columns and pumps are scaled, but water management (cooling, make-up, treatment) can be a limiting factor.
- Membranes scale by adding modules; this is inherently modular, but compression stages dominate at very high flow rates.
Scale Effect on Specific Capex (Illustrative)
Indexed capex per Nm3/h for small vs mid-scale vs large plants; 100 = baseline at 700 Nm3/h.
5. Case Study: 500 Nm3/h Biogas Plant
Case Study – Farm-Based AD Plant Upgrading to Grid Biomethane
- Location: Northern Europe, moderate climate.
- Feedstock: Manure + energy crops, 500 Nm3/h raw biogas.
- Grid gas spec: > 96% CH4, H2S < 5 ppm, dew point controlled.
| Option | Capex (upgrading) | Opex (€/yr) | Specific cost (€/Nm3) | Notes |
|---|---|---|---|---|
| Water scrubbing | €1.7 million | ˜ €115,000 | ˜ €0.11 | Higher water use, robust to H2S peaks with polishing. |
| 2-stage membranes | €1.5 million | ˜ €105,000 | ˜ €0.10 | Compact skid, more sensitive to high O2/N2 content. |
6. Revenue Stack & RNG Pricing
Choosing an upgrading technology is not only about €/Nm3 costs; it must be seen against the revenue stack of the RNG project:
- Base energy value of biomethane (linked to wholesale gas price).
- Renewable gas certificates (e.g. Guarantees of Origin, RINs, LCFS-type credits).
- Premiums for carbon-negative or manure-based pathways.
- Digestate valorisation and potential CO2 off-take.
Illustrative RNG Revenue Stack vs Upgrading Cost
Indicative €/MWh breakdown for a European RNG offtake contract; upgrading cost is one slice of the cost stack.
8. Regional Comparison: EU vs US vs Asia
While technology physics are the same, policy and energy price environments make membrane vs water scrubbing economics look very different in Europe, North America, and Asia.
Illustrative Regional Differences (Mid-Scale Agricultural Plant)
| Region | Typical RNG Support Mechanism | Power Price Signal | Upgrading Route Often Favoured | Key Constraint |
|---|---|---|---|---|
| EU (NW Europe) | Feed-in premiums + GO certificates | Moderate to high, often volatile | Mix of membranes and water scrubbing | Grid connection costs, permitting |
| US (RNG into LCFS/RIN markets) | RIN + LCFS-style credits, CI-driven | Relatively low wholesale prices | Membranes (especially at smaller plants) | Credit price volatility, offtaker concentration |
| Asia (selected markets) | Emerging certificates, pilot subsidies | Very heterogeneous, some high tariffs | Water scrubbing or hybrid with PSA | Policy stability, currency and FX risk |
Regional Benchmark: Indicative Upgrading Cost vs RNG Price
Illustrative comparison of average RNG offtake prices and upgrading cost contribution across regions.
9. Operating Reality: Uptime, Failures & IRR Impact
Real-world returns are extremely sensitive to availability. A plant modelled at 8,200 full-load hours but operating at 7,200 h loses more than 10% of expected upgraded gas output – often enough to flip IRR from attractive to borderline.
- Typical contracted availability: 95–97% for well-engineered membrane and water scrubbing plants.
- Common unplanned downtime causes: foaming and flooding in scrubbers, fouling of membrane pre-filters, compressor trips, and control system glitches.
- Impact on IRR: every 1 percentage point loss in availability can reduce equity IRR by ~0.3–0.5 points, depending on leverage and tariff level.
Membranes and water scrubbing can both be reliable workhorses if pre-treatment, operator training, and spare-part strategies are properly funded. Under-investing in these "soft" elements is a more common failure mode than choosing the "wrong" core technology.
10. Devil's Advocate: Where Upgrading Projects Go Wrong
Despite strong policy support, many upgrading projects underperform or fail to reach expected returns:
- Feedstock volatility: Lower biogas production than planned pushes specific upgrading costs up and can breach offtake minimums.
- Under-designed pre-treatment: Siloxanes, high H2S, or foaming cause membrane damage or scrubber downtime.
- Underestimating methane slip: Small leaks and vents add up, damaging the carbon intensity profile of the fuel.
- Over-optimistic utilisation: Assuming 8,000+ full-load hours without realistic maintenance and grid outage assumptions.
From a financier's perspective, technology choice is only one risk layer. Contracts, feedstock, and O&M capability often matter more than whether the OEM offers membranes or water scrubbing.
11. Outlook to 2030: Technology Roadmap
Looking ahead to 2030, upgrading technology choice will be shaped by:
- Module cost decline: Membrane module prices are expected to fall as volumes grow and new materials enter the market.
- Water stress: Regions facing water scarcity will favour membrane, PSA, or hybrid concepts over classic high-water scrubbing.
- Integrated solutions: OEMs are bundling upgrading with feedstock logistics, digester packages, and CO2 liquefaction into standardised project templates.
Our base case at Energy Solutions: by 2030, membranes and water scrubbing will both remain relevant, but membranes are likely to dominate small and mid-scale, modular plants, while water scrubbing and hybrid routes hold ground in larger centralised hubs where waste heat and water management are optimised.