BMP testingFruit, vegetables and their residues

Onion peels and residues — BMP: 2 studies, weighted mean 462 m³ CH₄/t VS

Onion skins and waste are a dry, fibrous waste from peeling, rich in flavonoids and sulphur compounds; at a high share they can limit digestion.

CH₄ ≈ 52 % (estimate)

Published 6 August 2023Updated 29 October 20263 min readFeedstockData Editorial Team

Illustration: FeedstockData (AI, Google Gemini)

Weighted mean: 462m³ CH₄/t VS
Range: 400–523m³ CH₄/t VS
Records: 2studies
Publications: 2sources
Temperatures: 35 °C, 38 °Cof the test

Methane in biogas

≈51–54% CH₄

Medium methane content · ≈ 5.2 kWh/m³ of biogas

estimate from the substrate type The collected studies contain no measurement. The interval follows from the approximate composition (carbohydrate-dominated (no protein or fat characteristics)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

not rated — editorial rating for biogas plants

Not rated: no dry matter or volatile solids in the data. We calculate the rating when the dry matter and volatile solids of the material are known.

Editorial rating per the methodology

Add to the co-digestion calculator CSV data

BMP curves from 2 studies

Weighted mean Individual records

Dashed lines are a model curve (first-order kinetics, B(t) = B₀·(1 − e^−kt)); the end point comes from the publication. All curves are model curves.

Cumulative methane production over the test [m³ CH₄/t VS]. The thick line is the weighted mean of all records (462); dots mark the end values given in the publications. Point at the chart to read values; click a legend item to hide or show a record.

Records and sources

Download CSV

Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Dubrovskis i Plume (2022) DOI	523	38 °C · mesophilic	Onion residues and peel	end point (model curve)
Gunaseelan (2004) DOI	400	35 °C · mesophilic	onion exterior peels	end point (model curve)
Weighted mean	462	arithmetic mean 462, SD 87

Typical composition

Volatile solids (VS): 88.2 % DM (range 88.2–88.2)

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 0 determinations, volatile solids: 1 determination. No DM in the sources – the calculator has to ask for a value.

Sources: Gunaseelan (2004).

The composition of the raw material varies between batches — for plant calculations use an analysis of your own sample. The calculator inserts typical values automatically.

Process properties

Low confidence: the description is based on general characteristics of the substrate group, not on studies of this material. Treat it as a hint to be verified.

No published process studies

The literature we use does not describe the advantages or drawbacks of digesting this substrate beyond the BMP test results. Below are process characteristics assigned on the basis of the type of material — an editorial classification, not a study result.

fibrous: lignocellulose, long fibres → floating layer, harder mixing, slow degradation
slowly degrading: slow degradation, requires a long retention time
seasonal: seasonal availability — requires storage
variable composition: high variability of composition between batches

Co-digestion — what to combine it with, what to watch out for

combines well
substrates: easily degradable — e.g. Maize silage (whole crop), Food waste, Sorghum, whole crop, Whey, Beet leaves and tops
In BMP studies, easily degradable substrates increased the yield of the mixture with slurry (synergy in some mixtures).^[1]

Dosing

No source limits on the dose for this substrate.

Sources of the process data

The co-digestion calculator will assess a specific mixture containing this substrate.

What it is and where it comes from

The onion (Allium cepa) is an important vegetable crop in Poland, and waste arises in packing houses and sorting plants (damaged, overgrown and sprouting bulbs), in processing (peeling, cutting) and in catering. A separate stream is the dry outer skins removed during peeling. The dataset describes both outer skins and onion waste including skins.

Properties relevant to digestion

The skins contain a lot of fibre (cellulose, hemicellulose, pectins), as well as flavonoids, including quercetin, which give them their colour. Onion flesh contains soluble sugars, fructans and sulphur compounds such as alliin and its breakdown products. These compounds have antimicrobial activity and can inhibit microorganisms, especially methanogens, if the share of onion in the feed is high. In addition, sulphur promotes the formation of hydrogen sulphide in the biogas. The material is drier than typical vegetable waste if skins dominate, and wetter with fleshy onion.

How to read the spread of results

The dataset contains several records from several sources (38 and 35 °C): onion waste and skins, and outer skins. The results are similar, but they describe different fractions, so they should not be equated absolutely. The variety, the proportions of skins and flesh, the degree of comminution, sample preparation and the adaptation of the inoculum to sulphur compounds and phenols all matter. In a short test without adaptation, the result may be underestimated.

In biogas plant practice

Onion waste is added at a small share together with slurry or silage, after shredding, while monitoring H₂S. Dry skins are light and bulky, so the transport cost is high relative to their energy value. Composting or flavonoid extraction is sometimes an alternative. Waste from processing and trade is subject to waste legislation, while a farm's own residues are treated as agricultural products.

In the case of regular deliveries, it is good to determine the share of skins and flesh in the batch, because both the dry matter content and the concentration of sulphur compounds depend on it, and hence the risk of inhibition and hydrogen sulphide. Shredding dry skins usually requires moistening so that the material does not generate dust.

Methodology in brief

BMP is the amount of methane obtained from volatile solids (VS) in a batch test, given in m³ CH₄ per tonne of VS (numerically the same as NmL CH₄/g VS in the publications) and converted to normal conditions (0 °C, 101.325 kPa). The weighted mean gives each publication a total weight of 1, split equally between its results for the substrate — a paper testing twenty varieties does not outweigh a paper with a single sample. In inter-laboratory programmes the unit is the laboratory (total weight 1), and within it the weight of a test grows with the number of bottles (replicates). Results after pretreatment (e.g. thermal, chemical, enzymatic) are shown separately and are not included in the mean.

Solid lines are curves measured in the tests. Where a publication gives only the final result, we draw a first-order kinetic model, B(t) = B₀·(1 − e^−kt): only the end point comes from the publication, and the shape of the curve serves purely as visualisation — do not read the production on a particular day from it. For this substrate k = 0.245 d⁻¹ was assumed (median of rate constants k reported in the sources and/or fitted to the mean measured curves).

The statistics describe the spread of published results — a descriptive compilation, not a meta-analysis or a guarantee of plant yield.

Full methodology and correction policy

Bibliography

All sources of the site: bibliography.