Anaerobic Digestion Plant Odour & Air Quality Assessments
Combined air quality and odour assessments for AD plants across the UK — biogas CHP stack dispersion modelling, digestate and feedstock odour, MCPD registration support and EA permit packages. Delivered personally by a chartered specialist.
One Assessment, Two Pollutant Streams
Anaerobic digestion is one of the few sectors where air quality and odour assessments sit so closely together that they are best treated as one piece of work. On the one hand, biogas combustion in the CHP engine releases combustion pollutants — NOx, SO2, CO and particulates — through an elevated stack, which require a quantitative air dispersion modelling exercise against national air quality objectives. On the other, feedstock reception, depackaging, digestate storage and tankering generate odour at ground level from open and semi-enclosed sources, which require an IAQM- and H4-compliant odour impact assessment. Both pollutant streams reach the same receptors. Doing them together saves cost and produces a much more coherent narrative for the LPA and the Environment Agency.
Malcolm Pounder CEnv MIAQM leads every AD assessment personally. We use ADMS for CHP stack modelling, calibrated against PCM background concentrations and the relevant air quality objectives (NO2 40 µg/m³ annual mean, SO2 15-minute and hourly objectives, PM10/PM2.5 annual means). Odour is assessed to IAQM (2018) and EA H4 methodology using feedstock-specific emission rates from sector datasets and our own benchmarking. Where MCPD registration is required for engines >1 MWth, we draft the relevant emission limit comparisons directly into the report.
We are particularly experienced with food-waste AD in rural settings — the most challenging combination of high-odour feedstock and high-sensitivity receptors. We are pragmatic about CHP stack height, biofilter sizing and digestate storage cover technology, and recommend solutions that work operationally as well as on the page.
Our AD Plant Air Quality & Odour Services
Planning, permitting and ongoing compliance support across the full AD pipeline — from feasibility through to MCPD registration.
CHP Stack Dispersion Modelling
ADMS modelling of biogas CHP engine emissions for NOx/NO2, SO2, CO and particulates with stack height optimisation. We model against the relevant short-term and long-term air quality objectives and Environment Agency Environmental Assessment Levels, and present results so the LPA, Environmental Health and EA permitting officer can all see the headroom (or lack of it) on each parameter.
IAQM & H4 Odour Assessment
Full odour impact assessment to IAQM (2018) and EA H4 methodology covering feedstock reception, depackaging, pasteurisation venting, digestate storage and tankering operations. Feedstock-specific emission rates, sensitive rural receptor identification, and conclusions framed for both planning significance and permit acceptability.
EA Permit & MCPD Support
Technical input to EA Environmental Permit applications and Medium Combustion Plant Directive (MCPD) registrations. We provide the emission inventory, dispersion modelling, Odour Management Plan and engine emission limit value compliance demonstration in formats the EA recognises — reducing the number of EA RFI rounds and accelerating determination.
Biofilter & Mitigation Design
Sizing and specification of biofilters for feedstock-reception extract, advice on digestate storage cover options (floating, fixed, gas-tight retrofits), and operational mitigation through tankering windows and rapid clear-down protocols. We help the design team and operator land on a mitigation package that is both effective and operationally sustainable.
Common Scenarios
AD plants attract air quality and odour conditions from almost every LPA in the UK, and the Environment Agency expects substantive evidence at permit application. We are typically engaged at the points below, and combining the air quality and odour pieces upfront usually delivers the best value.
We have particular depth in food-waste AD in rural settings, on-farm AD in mixed-use locations, and biomethane-to-grid plants where biogas upgrading replaces (or runs alongside) the CHP engine.
- New AD plant planning applications
- EA Environmental Permit applications
- MCPD registration support
- CHP engine upgrades & additions
- Feedstock changes (food waste / ABPR)
- Biomethane-to-grid conversions
- Digestate storage expansion
- Odour complaint investigations
Frequently Asked Questions
Do anaerobic digestion plants need an air quality assessment?
Yes — almost always. Anaerobic digestion (AD) plants with biogas combustion in CHP engines emit NOx, SO2, CO and particulates, and the engines are usually classed as Medium Combustion Plant under the MCPD regulations. A combined air quality and odour assessment is normally required to support the planning application, to discharge planning conditions, and to support the Environmental Permit application. The IAQM and EA both expect dispersion modelling of CHP stack emissions and an H4-compliant odour impact assessment for feedstock reception, digestate storage and tankering operations.
What pollutants come from the CHP engine at an AD plant?
Biogas CHP engines emit primarily oxides of nitrogen (NOx, mostly NO converting to NO2 in the atmosphere), sulphur dioxide (SO2, mainly from residual hydrogen sulphide in the biogas), carbon monoxide (CO), particulate matter (PM10 and PM2.5) and minor unburnt hydrocarbons. NOx is usually the dominant pollutant for receptor impacts. Emission rates depend on engine specification, biogas H2S content, the level of biogas pre-cleaning and the abatement applied (oxidation catalyst, SCR). The MCPD sets emission limit values for engines over 1 MWth — typically 190 mg/Nm³ NOx and 100 mg/Nm³ SO2 for new spark-ignition gas engines.
What odour sources are typical at an AD plant?
The principal odour sources at an AD plant are the feedstock reception area (especially for food waste and food-processing residues), the depackaging line (if installed), the pasteurisation and digestate storage tanks (where headspace gas can be released during loading), digestate tankering operations and any open storage of fibre. Biogas leaks from digesters and gas-handling equipment are a secondary source. We model these as area and point sources to IAQM odour methodology, with feedstock-specific emission rates from H4 datasets and benchmark facilities.
Does an AD plant need an Environmental Permit?
Most AD plants require an Environmental Permit from the Environment Agency. Anaerobic digestion of biowaste over 100 tonnes per day is a Schedule 1 Part A(1) activity under the Environmental Permitting Regulations 2016; smaller plants may operate under a Standard Rules permit if eligible. Where a CHP engine over 1 MWth combusts biogas, MCPD registration is also required. Our air quality and odour assessments are written to dovetail directly with the EA permit application — same source list, same emission rates, same dispersion model results.
How much does an AD plant air quality and odour assessment cost?
Fees depend on scale, the number of CHP engines and the complexity of feedstock and digestate handling. A combined air quality and odour planning assessment with CHP stack dispersion modelling and IAQM odour methodology typically falls in the mid-to-upper four-figure range. Where the same assessment can be re-used to support an EA permit application with only minor adaptation, the combined fee is usually 15–20% lower than buying the two pieces separately. We provide fixed-fee quotes following a short scoping conversation.