What is the difference between biogas and RNG?

Biogas is the raw gas produced by anaerobic digestion or landfill decomposition — typically 45–65% CH₄, 30–50% CO₂, with trace H₂S, siloxanes, water vapor, and other contaminants. RNG (Renewable Natural Gas), also called biomethane, is biogas that has been upgraded to ≥96% CH₄ and meets pipeline gas-quality specifications (e.g., SoCalGas Rule 30 Wobbe 1290–1385 BTU/scf). Upgrading involves removing CO₂, H₂S, siloxanes, and moisture via PSA, membrane, amine, or water scrubbing.

How much methane does a landfill produce?

EPA LandGEM v3.03 first-order decay model is the standard estimator: Q_CH4 = Σ k·L₀·(M_i/10)·exp(-k·t_ij). For a typical 100,000 Mg/yr CAA Conventional landfill (k=0.05/yr, L₀=170 m³/Mg), peak CH₄ generation is ~520 MMscf/yr ~25–30 years after closure. The 2024 LandGEM v3.1 (EPA/600/B-24/160) adopts the GHGRP Subpart HH equation HH-1 for current inventory work, but v3.03 remains required for 40 CFR 60 Subpart XXX/Cf applicability determinations.

Why does dairy RNG get a very negative carbon intensity?

Open anaerobic lagoons — the conventional dairy manure-management baseline — emit large quantities of methane to atmosphere (MCF 0.50–0.80 of B₀ × VS, per IPCC 2019 Table 10.17). Diverting that manure to a closed anaerobic digester captures the methane that would otherwise escape. Under CARB LCFS Tier 2 pathway accounting, this avoided-baseline methane gets credited against the project's CI — typical dairy RNG pathway CIs are −250 to −400 gCO₂e/MJ. (CARB 2024 amendments are phasing this credit out over 2025-2040 based on certification date.)

What is the typical methane yield from WWTP sludge digesters?

Per Metcalf & Eddy 5e Ch 14: VS_destruction (%) = 13.7·ln(HRT) + 18.9 for mesophilic, +26.9 for thermophilic. With substrate-specific yield from Table 14-7: primary sludge ~0.45 m³ CH₄/kg VS destroyed; combined primary+WAS ~0.50; FOG-codigestion-boosted ~0.65. A 200 m³/d WWTP at 5% TS, 75% VS-of-TS, mesophilic 20-d HRT typically produces 60–80 MMBtu/d RNG after upgrading.

What are the four primary RNG feedstock categories?

(1) Landfill gas — naturally generated from MSW decomposition; collected via wells per 40 CFR 60 Subpart XXX/Cf. (2) Dairy/livestock manure — anaerobic digestion of collected manure; main LCFS-credit pathway. (3) WWTP digester gas — primary + waste activated sludge digestion; mature technology with biosolids co-product per 40 CFR 503. (4) Food waste — source-separated organics digestion; expanding 2024-2026 with state OWP (Organic Waste Processing) mandates like California SB 1383.

RNG Feedstock Fundamentals

1. Overview: Why feedstock characterization matters

The economics of every RNG project trace back to the methane potential of the feedstock. A 100,000 Mg/yr municipal landfill produces ~25× more annual CH₄ than a 1,000-head dairy herd, but the dairy CH₄ comes with a −300 gCO₂e/MJ CARB LCFS carbon intensity (avoided baseline lagoon credit) that monetizes at $20–50/MMBtu — often more than the gas itself is worth.

This page covers the four primary RNG feedstock categories used in US biomethane production:

Feedstock	Typical project scale	Typical CI (gCO₂e/MJ)	Primary regulatory driver
Landfill gas (LFG)	2,000–10,000 mcfd RNG	−10 to +50	40 CFR 60 Subpart XXX/Cf NSPS
Dairy manure	50–500 mcfd RNG (per 1k cows)	−250 to −400	CARB LCFS dairy pathway
WWTP digester	500–3,000 mcfd RNG	+20 to +50	40 CFR 503 biosolids regs
Food waste	200–1,500 mcfd RNG	−20 to +30	State OWP mandates (CA SB 1383, NY/WA OWBs)

The 80/20 rule of feedstock selection: 80% of US RNG project economics is set by the avoided-baseline methane credit (or lack of it). Dairy and food-waste projects start deeply in the green because their counterfactual (open lagoon, landfill) emits methane that the project displaces. LFG and WWTP projects start near zero because their counterfactual (collected and flared) already destroys the methane.

2. Landfill Gas (LFG)

MSW landfills generate biogas via anaerobic decomposition of buried waste. Active gas collection systems pull the gas to a header for control (flare), beneficial use (engines, RNG), or both. The collected gas is typically 45–55% CH₄ + 35–45% CO₂ + 1–3% N₂ + trace H₂S, NMOC, siloxanes.

EPA LandGEM v3.03 first-order decay model

The EPA LandGEM v3.03 model (EPA-600/R-05/047, May 2005) is the prescribed method for 40 CFR 60 Subpart XXX/Cf applicability determinations:

Q_CH4 = Σ_i Σ_{j=0.1..1.0} k · L₀ · (M_i / 10) · e^(−k · t_ij) Q_CH4 = annual CH₄ generation (m³/yr) i = years of waste acceptance (1..n) j = 0.1-yr sub-interval within each year (10 sub-batches) k = methane generation rate (1/yr) L₀ = methane generation potential (m³ CH₄ / Mg waste) M_i = mass of waste accepted in year i (Mg) t_ij = age of j-th sub-batch of M_i (years)

EPA LandGEM v3.03 default constants (Table 1)

Preset	k (1/yr)	L₀ (m³ CH₄/Mg)	Use case
CAA Conventional	0.05	170	NSPS/EG/NESHAP applicability (default)
CAA Arid (<25"/yr precip)	0.02	170	Arid-climate NSPS sites
Inventory / AP-42 Conventional	0.04	100	General AP-42 §2.4 inventory work
Inventory / AP-42 Arid	0.02	100	Arid AP-42

Regulatory currency: LandGEM v3.1 (Dec 2024) supersession

EPA released LandGEM v3.1 (EPA/600/B-24/160, Dec 2024) and updated AP-42 §2.4 (May 2025) to adopt the GHGRP Subpart HH equation HH-1 (DOC-based, harmonized with §98.343). For current EPA inventory work, use LandGEM v3.1. However:

40 CFR 60 Subpart WWW / XXX / Cf still reference the legacy v3.03 k/L₀ defaults for NSPS applicability determinations.
Most active site-permit submittals continue to use v3.03 through the transition.
For RNG project screening, v3.03 remains the working model.

40 CFR 60 Subpart XXX/Cf applicability triggers

Threshold	Trigger
Design capacity ≥ 2.5 Mt waste	Subpart XXX (post-2014) or Cf (existing) applies
NMOC emission rate ≥ 34 t/yr	Gas collection & control system (GCCS) required per §60.762
NMOC < 34 t/yr	Surface monitoring only; GCCS optional

The 34 t/yr NMOC trigger is the binding economic threshold: above it, gas must be collected and controlled regardless of beneficial use. RNG upgrading converts the cost of compliance (flaring) into a revenue stream.

3. Dairy Manure

Dairy manure RNG is the single most economically attractive feedstock in US biogas — not because the gas is unique, but because the regulatory accounting credits the avoided baseline emissions from open lagoons. A 1,000-cow Holstein dairy in California's Central Valley produces ~20 MMscf/yr RNG and ~10,000 t/yr CO₂e of LCFS credits at ~$60/credit → ~$600k/yr in stacked credit revenue alone.

IPCC 2019 Tier-2 yield method

CH₄ = VS_total · η_collection · B₀ · η_digester (m³ CH₄/d) VS_total = Σ_animal (N_head · m_kg · VS_rate / 1000) VS_rate = 9.2 kg VS / (1000 kg animal mass) / day (ASABE D384.2 R2014 primary; also IPCC 2019 Table 10.13a) B₀ = 0.24 m³ CH₄ / kg VS (IPCC 2019 Table 10.16a NA dairy, ±15%) η_collection = 0.50–0.95 (50% pasture-grazed → 95% freestall + flush) η_digester: Covered lagoon (psychrophilic): 0.65 Plug-flow (mesophilic 35°C): 0.80 CSTR mesophilic (35°C): 0.88 CSTR thermophilic (55°C): 0.92

Avoided baseline (LCFS counterfactual)

The CARB LCFS Tier 2 dairy pathway credits the methane that would have escaped from the conventional open anaerobic lagoon if the manure were not collected and digested. Per IPCC 2019 Table 10.17, baseline lagoon MCF varies by climate zone:

Climate zone (IPCC 2019)	Lagoon MCF	Notes
Cool Temperate Moist	0.60	Upper Midwest, Northeast
Cool Temperate Dry	0.67	Plains states
Warm Temperate Moist	0.73	Southeast US
Warm Temperate Dry	0.76	California Central Valley (most US dairy RNG)
Tropical Moist/Dry	0.80	Hawaii, southern Texas, Florida

For a representative 1,000-cow milking dairy (with 150 dry cows + 800 heifers, 1,950 head total — typical California Central Valley operation) at warm-temp-dry (MCF 0.76):

CH₄_avoided = 8,924 kg VS/d × 0.85 collection × 0.24 × 0.76 = 1,384 m³ CH₄/d × 365 = 504,985 m³/yr = 342 t CH₄/yr × 25 GWP (CARB AR4) = 8,550 t CO₂e/yr avoided baseline credit (VS basis: 1000 milking × 680 kg + 150 dry × 680 kg + 800 heifer × 240 kg = 974,000 kg animal mass × 9.2 g VS/kg-d ≈ 8,960 kg VS/d)

CARB 2024 amendments — avoided methane phase-out

The November 2024 CARB LCFS amendments significantly tighten the timeline for dairy avoided-methane crediting:

Pathways certified before July 1, 2025: Three 10-year crediting periods (30 years total).
Pathways certified July 2025 – December 2029: Two 10-year periods (20 years total).
Projects breaking ground after December 2029: Avoided-methane credits available only until 2040.

This phase-out is the dominant long-term risk factor for dairy RNG project lifetime NPV — a project at the 30-yr cliff has ~50% higher lifetime LCFS revenue than one at the 2040-cliff schedule.

4. WWTP Digester (Primary + WAS + FOG)

Municipal WWTP anaerobic digesters have produced biogas for decades — historically used for in-plant heat/power, increasingly upgraded to RNG. A 50 MGD WWTP produces ~30,000 mcfd biogas at 60–65% CH₄, equivalent to ~20,000 mcfd RNG after upgrading.

Metcalf & Eddy 5e Liu VS-destruction correlation

VS_destruction (%) = 13.7 · ln(HRT_days) + 18.9 (mesophilic, single-stage) VS_destruction (%) = 13.7 · ln(HRT_days) + 26.9 (thermophilic, +8pp shift) Capped at 70% asymptote Example: HRT 20 d, mesophilic → 13.7·ln(20) + 18.9 = 59.9% HRT 25 d, thermophilic → 13.7·ln(25) + 26.9 = 70.0% (capped)

Substrate-specific methane yield (M&E Table 14-7)

Substrate	Y_CH₄ (m³ CH₄ / kg VS destroyed)
Primary sludge alone	0.45
Waste-activated sludge (WAS) alone	0.30
Combined primary + WAS	0.50
Primary + WAS + 5–15% FOG codigestion	0.65
Food waste only	0.55
Food waste + sludge codigestion	0.60

The Buswell-equation theoretical ceiling for pure carbohydrate sludge (C₁₀H₁₉O₃N) is 0.42 m³ CH₄/kg VS at STP — substrates yielding above this implicitly contain significant lipid/FOG fraction. FOG codigestion is the single biggest yield enhancer in municipal WWTP RNG projects.

40 CFR 503 biosolids classification (Subpart D)

Digestate from WWTP digesters is regulated as biosolids under 40 CFR 503. Two Classes:

Class	Pathogen-reduction requirement	Land-application
Class B (PSRP)	HRT ≥ 15 d at 35–55°C OR ≥ 60 d at 20°C (Alt 1)	With site restrictions (no public access, crop harvest delays)
Class A (PFRP)	Alt 1 t-T curve OR equivalent process determination per §503.32(a)(7)	Unrestricted use (sold as EQ — Exceptional Quality biosolids)

Class A via AD alone — important caveat: The §503.32(a)(3) Alt 1 time-temperature curve is the regulatory regime for thermally-treated sludge. AD reaches Class A only via Alt 3 (PFRP-equivalent process determination) or Alt 6 (case-by-case EPA approval), NOT automatically by meeting the t-T curve. Engineers should not assume "AD at 55°C for 22 days" = Class A — the EPA Region biosolids coordinator's approval is the binding determination.

5. Food Waste (SSO & Codigestion)

Source-separated organics (SSO) — restaurant food waste, grocery waste, FOG (fats/oils/grease) — has the highest specific methane yield of common RNG feedstocks (0.55–0.65 m³ CH₄/kg VS destroyed) and is driven by a wave of state OWP (Organic Waste Processing) mandates:

California SB 1383 (2016, enforced 2022): 75% reduction in organic waste landfilling by 2025; ARB-tracked compliance.
New York Organic Waste Ban (2022): Generators > 2 t/wk within 25 mi of a digester must divert.
Washington OWB (2024): Phased generator thresholds 2024–2030.
Connecticut, Massachusetts, Vermont: Various tonnage-threshold bans 2020-present.

These mandates create a feedstock-pull on RNG project economics that's distinct from the carbon-credit-driven dairy/LFG economics: food-waste projects often have a tipping-fee revenue stream ($30–80/t accepted) in addition to gas + RIN + LCFS.

Feedstock composition variability

Food-waste source	Typical CH₄ yield (m³/kg VS)	Notes
Restaurant food scraps	0.40–0.55	High variability; pre-consumer typically higher
Grocery / produce waste	0.45–0.65	Cellulose-heavy lowers yield
FOG (fats / oils / grease)	0.85–1.05	Approaches Buswell triolein ceiling
Slaughterhouse / DAF	0.65–0.90	Protein-heavy; nitrogen inhibition risk
Brewery / distillery waste	0.40–0.60	Carbohydrate-heavy

Codigestion of food waste with municipal sludge (1:5 – 1:10 by VS) is the most common implementation — leverages existing WWTP digester capacity while boosting yield 20–40% over sludge-only baseline. Excessive food-waste fraction (> 30% of VS) can cause ammonia inhibition or VFA accumulation.

6. Feedstock Comparison Summary

Metric	Landfill	Dairy	WWTP	Food waste
Biogas CH₄ content	45–55%	55–65%	60–65%	55–70%
H₂S typical (ppmv)	100–2,000	1,000–3,000	200–1,500	2,000–5,000+
Siloxane typical (mg Si/m³)	5–50	< 2	2–20	5–30
Avoided-baseline credit	Modest (flared counterfactual)	Major (lagoon counterfactual)	Small (existing digester)	Modest (landfill diversion)
RFS2 D-code	D3 (cellulosic)	D3	D5 (advanced) — or D3 if cellulosic	D5
Typical CAPEX intensity	$2–5M per MMscfd RNG	$5–15k per cow	$30–50M turn-key (large)	$15–40M for 50–150 t/d
Lifecycle CI (gCO₂e/MJ)	−10 to +50	−250 to −400	+20 to +50	−20 to +30

All four feedstocks share the same downstream cleanup-and-upgrading train (H₂S removal → siloxane removal → CO₂ removal via PSA/membrane → pipeline injection), but the inlet contaminant concentrations differ materially — siloxanes drive landfill-gas pretreatment design, H₂S drives dairy/food-waste pretreatment design.

7. Standards & References

EPA LandGEM v3.03 User Guide, EPA-600/R-05/047 (May 2005)
EPA LandGEM v3.1 / EPA/600/B-24/160 (Dec 2024) — supersedes v3.03 for inventory
EPA AP-42 §2.4 (May 2025 update — adopts GHGRP Subpart HH equation HH-1)
40 CFR 60 Subpart WWW (NSPS for landfills constructed 1991–2014)
40 CFR 60 Subpart XXX (NSPS for landfills constructed/modified after Jul 2014)
40 CFR 60 Subpart Cf (Emission Guidelines for existing landfills)
40 CFR 98 Subpart HH §98.343 (GHGRP — modern landfill inventory methodology)
40 CFR 503 Subpart D (biosolids pathogen reduction Class A/B)
IPCC 2019 Refinement to 2006 Guidelines, Vol 4 Ch 10 (livestock and manure)
EPA AgSTAR Project Development Handbook, 3rd ed (EPA 430-B-20-001, 2020)
ASABE D384.2 (March 2005, R2014) — Manure Production and Characteristics
Tchobanoglous, Stensel, Tsuchihashi, Burton — "Wastewater Engineering: Treatment and Resource Recovery" 5e (Metcalf & Eddy, McGraw-Hill 2014)
WEF Manual of Practice No. 8, 6th ed (2018) — Design of Municipal Wastewater Treatment Plants
EPA Process Design Manual for Sludge Treatment and Disposal, EPA 625/1-79-011 (1979)
California SB 1383 — Short-Lived Climate Pollutant Reduction Strategy (organic-waste diversion)
CARB LCFS Regulation 17 CCR §95480–95503 (2024 amendments)
IPCC AR6 — CH₄ GWP100 = 29.8

RNG Feedstock Characterization