Digester Gas CHP: Tier I or Tier II? The Classification Question

Wastewater treatment plants are among the most electricity-intensive facilities a municipality operates, and many already produce the fuel to offset a meaningful share of that load. Anaerobic digesters generate biogas; a CHP engine converts it to electricity and heat.

Those systems generate Pennsylvania alternative energy credits. Which tier they generate is a genuine question — and while it is no longer principally a price question, it decides which market the credits are sold into.

The boundary

Act 213 classifies alternative energy resources into tiers. Two classifications potentially reach a digester gas CHP:

• Biologically derived methane gas is a Tier I resource. Digester gas is biologically derived methane. On fuel classification alone, a digester gas system reads as Tier I.
• Distributed generation under 5 MW is a Tier II resource. 52 Pa. Code § 75.1 defines distributed generation as small-scale generation of electricity and useful thermal energy from systems with nameplate capacity not greater than 5 MW. Nearly every municipal digester gas CHP falls under that threshold. On configuration, the same system reads as Tier II.

The system does not change. The classification does.

Why it matters in Pennsylvania

It is tempting to treat this as a price question. At RY2025 prices it is not. The PA PUC weighted average was $29.29 for Tier I non-solar and $26.92 for Tier II — the two tiers are within a couple of dollars per credit. A classification that once looked decisive on revenue alone is close to neutral today.

What the classification actually decides is which supply pool and which set of geographic rules the credits are sold under.

• Tier II is Pennsylvania-only. Act 114 of 2020 restricted Tier II compliance to Pennsylvania-sited generation. Waste coal facilities that supply roughly half the tier continue to retire, and the PA PUC projects a Tier II shortfall beginning in 2028. The pool is small, in-state, and tightening.
• Tier I non-solar is PJM-wide and well supplied. Wind, hydro, and biomass across the PJM footprint can serve Pennsylvania's Tier I obligation, so the tier draws on a much deeper market that has met its obligation comfortably.

So the practical consequences of the determination are forward-looking rather than immediate: the depth of the market the generator sells into, the number of counterparties available, and how the price is likely to behave over the measure's TRM life, capped at 15 claimable years by Act 129 rather than in the current year.

Worked scale

A mid-size municipal wastewater treatment plant treating roughly 12 million gallons per day, running a 1.5 MW biogas CHP at a 75% capacity factor:

1.5 MW × 8,760 hours × 0.75 = 9,855 MWh annually

Verify: 9,855 × 29.29 = 288,653 · 9,855 × 26.92 = 265,296

The plant, the engine, the gas, and the meter are identical in both rows, and at current prices so is the revenue, within about 8%. The determination still has to be right — it governs registration, which market the credits clear in, and the forward outlook — but it is not a five-fold revenue swing.

What determines capacity factor here

Digester gas CHP is fuel-limited rather than demand-limited. The engine runs on whatever gas the digesters produce, which tracks organic loading rather than the plant's electrical demand.

Factors that pull capacity factor down:

• Seasonal loading variation. Influent organic load shifts with population, industrial discharge, and infiltration.
• Gas conditioning. Hydrogen sulfide and siloxane removal is essential to engine life. Conditioning system downtime takes the engine down with it.
• Digester heating parasitic demand. Mesophilic digestion requires holding around 98°F. In winter a larger share of recovered heat goes to maintaining digester temperature.
• Flaring during maintenance. Gas produced while the engine is down is flared, not converted.

A well-run system with reliable gas conditioning typically lands between 70% and 85%. Systems with recurring conditioning problems fall well below that.

Use meter data where the system has operated. The gap between design assumption and actual output is frequently large.

Heat recovery at a treatment plant

Recovered heat maintains digester temperature, which sustains gas production, which sustains electrical output.

That loop matters operationally and it does not generate credits. Under 52 Pa. Code § 75.63, credits are issued per MWh of electricity generated or conserved. Thermal output is not part of the calculation. Useful thermal energy appears in the § 75.1 definition of a distributed generation system, but that provision describes what kind of system qualifies — it is not a credit basis.

Heat recovery belongs in the plant's operating economics. It does not belong in the credit calculation.

Municipal considerations

Attribute ownership. 52 Pa. Code § 75.13(i) provides that a customer-generator eligible for net metering owns the alternative energy credits of the electricity it generates, unless a contract expressly assigns ownership elsewhere. Where a digester gas system was built under a design-build-operate agreement, an energy services agreement, or a third-party ownership structure, attributes may already be assigned. Review the agreement before preparing an application.

• Revenue treatment. How credit revenue interacts with enterprise fund accounting, rate structures, and any bond covenants depends on the authority's specific financing. Direct those questions to the authority's bond counsel and auditor.
• Procurement. Municipal authorities engaging an aggregator may have procurement requirements that apply to the engagement itself. Worth confirming early.

Timing

Under 52 Pa. Code § 75.63(i), credits are issued from the date a complete application is filed with the program administrator forward. Generation before that date does not produce credits.

A digester gas CHP that has operated unregistered for years remains eligible to be certified. Issuance begins from the complete application, not from commissioning. For a system at the scale modeled above, each month before filing is a month of operation that earns nothing — roughly $22,100 in Tier II credit revenue that is never issued rather than deferred.

Verify: 265,296 ÷ 12 = 22,108

Frequently asked questions

Can a system choose its tier?

Classification is determined at application against the resource definitions in Act 213 and Chapter 75, not selected by the applicant. Where a system arguably meets more than one definition, the determination should be sought explicitly rather than assumed.

Does the credit basis differ between tiers?

No. Both are issued per MWh of electricity. The tier affects which market the credits are sold into, not the measurement.

What if the plant consumes all the electricity onsite?

Credits attach to metered generation regardless of whether output is consumed onsite or exported.

Does gas flared during downtime count?

No. Credits are issued on electricity generated. Gas that is flared produces none.

What about landfill gas?

The same classification question applies. Landfill gas is biologically derived methane, and a sub-5 MW landfill gas CHP sits at the same boundary.

Resolve your classification

Digester or landfill gas? At RY2025 prices the two tiers are within a couple of dollars per credit, so this is not principally a price question today. It matters because the determination decides which market the credits are sold into — Pennsylvania-only Tier II under Act 114, with a PUC-projected shortfall from 2028, or PJM-wide Tier I non-solar with a deeper and better-supplied pool. We seek the determination explicitly rather than assuming it.