BMP testingFruit, vegetables and their residues

Onions — BMP: 2 studies, weighted mean 305 m³ CH₄/t VS

Onion (whole bulbs, market waste) has plenty of sugars, but contains sulphur compounds and oils which at larger doses inhibit microorganisms.

CH₄ ≈ 52 % (estimate)

Published 26 May 2022Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Pascal Kings / Wikimedia Commons, licence CC BY 4.0, source file; changes: cropping

Weighted mean: 305m³ CH₄/t VS
Range: 269–342m³ CH₄/t VS
Records: 2studies
Publications: 2sources
Temperatures: —of 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 (protein from total nitrogen (N 1.2 % DM × 6.25)) 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 (305); 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

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Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Yan et al. (2021) DOI	269	—	Onion (Allium cepa) – Whole	end point (model curve)
Aybek et al. (2017) DOI	342	—	Onion	end point (model curve)
Weighted mean	305	arithmetic mean 305, SD 52

Typical composition

Volatile solids (VS): 95.1 % DM (range 95.1–95.1)
Nitrogen: 1.2 % DM
C/N ratio: 33.3

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: BCBPD); dry matter: 0 determinations, volatile solids: 1 determination. C/N was calculated as the ratio of C and N content (% DM) from the same source. No DM in the sources – the calculator has to ask for a value.

Sources: Yan et al. (2021).

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.

easily degradable: sugars, starch, glycerol → fast hydrolysis and acidification (VFA), needs buffering
acidic: low pH of the substrate (silages, stillage, whey, fruit)
seasonal: seasonal availability — requires storage
poor in trace elements: poor in trace elements — mono-digestion requires supplementation
carbon-rich: high C/N → nitrogen deficiency; combine well with nitrogen-rich substrates
low in nitrogen: high C/N → nitrogen deficiency; combine well with nitrogen-rich substrates

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

combines well
substrates: nitrogen-rich — e.g. Pig slurry, Poultry manure, Waste activated sludge (secondary sludge), Chlorella vulgaris, Nannochloropsis salina
A nitrogen-rich co-substrate balances the low C/N of fruit and vegetable waste (Bouallagui et al. 2009).^[1]
combines well
substrates: high alkalinity (buffer) — e.g. Cattle manure, Pig slurry, Cattle slurry, Duck slurry, Rumen and digestive tract content
Co-digestion of easily degradable substrates with dairy slurry increased the methane yield relative to slurry alone.^[2]

Dosing

No source limits on the dose for this substrate.

Introduce gradually, starting with a small dose and monitoring the process (e.g. FOS/TAC, pH, gas composition).

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

Onion (Allium cepa) is one of the basic vegetables grown in Poland, and waste arises in sorting plants, stores and processing plants: damaged, frost-damaged and diseased onions, rejects that are too small, and also skins and ends. The dataset contains whole bulbs and onion waste from a market, with no test temperature stated. Vegetable processing plants also generate highly concentrated onion waste.

Properties relevant to digestion

Onion consists of water (about 90%), sugars (including fructans) and fibre, so carbohydrates are readily available and lignin is not a problem. The challenge is posed by organosulphur compounds and essential oils, which have antimicrobial activity and at higher concentrations can inhibit bacteria and archaea. The breakdown of sulphur compounds increases the hydrogen sulphide content of the biogas. The skins are rich in flavonoids and fibre and degrade less readily. Rapid acidification is possible at large doses.

How to read the spread of results

The records differ in variety (sweet and pungent onions have a different content of sulphur compounds), the condition of the waste (fresh, stored, spoiled), the share of skins and size reduction. Adaptation of the inoculum to sulphur compounds, temperature, the substrate-to-inoculum ratio and test length are also significant. Low values may result from inhibition and not from a low potential of the material. A small number of records is too few to determine a typical range.

In biogas plant practice

Onion waste should be dosed in a small share and in a mixture with other substrates, which dilute the inhibitors. Size reduction and control of hydrogen sulphide in the biogas (for example biological desulphurisation) are advisable. Storage should be short. Vegetable waste is usually classified as waste (group 02 or 20), and its acceptance requires installation permits; plant raw material is not subject to the regulations on animal by-products.

For comparison with other vegetables, onion requires greater caution because of its content of sulphur compounds. In practice, onion waste is mixed with slurry or other substrates of high buffering capacity, and trial dosing at the start of the season makes it possible to check the response of the biogas plant.

Because the dataset is small, it is best to confirm the result with your own BMP test on the specific batch.

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.