BMP testingBy-products of the agri-food industry

Molasses — BMP: 2 studies, weighted mean 410 m³ CH₄/t VS

Molasses is a thick, sugar-rich by-product of sugar factories that ferments almost immediately; it threatens acidification and salinity, so it is dosed in small portions.

rating: good CH₄ ≈ 52 % (estimate)

Published 20 November 2020Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Badagnani / Wikimedia Commons, licence CC BY 3.0, source file; changes: cropping

Weighted mean: 410m³ CH₄/t VS
Range: 363–457m³ CH₄/t VS
Records: 2studies
Publications: 2sources
Temperatures: 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

good — editorial rating for biogas plantsStable, usually accepted without a fee

Potential per tonne of fresh matter: very high (307 m³ CH₄/t); process risk: salt / sodium, acidification, sulphur. No additional handling requirements.

Why? Rating factors

Potential per tonne +3

+3potential 307 m³ CH₄/t fresh matter (BMP 410 × composition per FNR (2013), Guía sobre el Biogás, Tabla 4.9: molasses (DM 85 %, VS 88 % DM))

Process stability −1.75

−1salt / sodium: inhibition at high shares
−0.5acidification: rapid degradation / acidic pH, risk of VFA accumulation, needs buffering
−0.25sulphur: H₂S in the biogas (toxicity to bacteria, corrosion, desulphurisation)

Handling and costs 0

0no factors

Total points: 1.25 · thresholds: ≥ 2.5 very good, ≥ 1 good, below — neutral; negative with cumulative process risk.

Editorial rating per the methodology · confidence of the rating: low · excluding market prices

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 (410); 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
De Vrieze et al. (2015) DOI	363	—	molasses	end point (model curve)
Dubrovskis et al. (2019) DOI	457	38 °C · mesophilic	Molasses	end point (model curve)
Weighted mean	410	arithmetic mean 410, SD 66

Process properties

easily degradable
carbon-rich
saline
sulphur-rich
acidic
editorial classification

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.

Advantages

Methane potential in the Substraty.pl BMP dataset: weighted mean 410.0 m³ CH₄/t VS (SD 66.5; 2 records, 2 sources).
Very readily available sugars - rapid increase in biogas production (Hagos et al. 2017).

Drawbacks and risks

Potassium and sodium in molasses promote salt inhibition (Chen et al. 2008).
Sulphates -> sulphate reduction and H2S, competition with methanogens (Hilton and Archer 1988).

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

combines well
substrates: high alkalinity (buffer) — e.g. Cattle manure, Pig slurry, Cattle slurry, Duck slurry, Rumen and digestive tract content
Slurry supplies N and alkalinity, balancing the high C/N of the material.^[1][2]
avoid
substrates: easily degradable — e.g. Maize silage (whole crop), Food waste, Sorghum, whole crop, Whey, Beet leaves and tops
Molasses + whey/bread/sugars: overlapping acidification.^[1]

Dosing

No source-based threshold. Dose in small portions, with slurry (buffer, N).

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

Molasses is a dark, viscous syrup remaining after the crystallisation of sucrose in a sugar factory. In Poland it arises in the processing of sugar beet, and in other countries in the processing of sugar cane. It is a valued raw material for distilleries, yeast plants, citric acid production and feed, so in a biogas plant it occurs less often and usually under favourable price conditions or as an energy reserve for quickly raising gas production.

Properties relevant to digestion

Molasses contains a great deal of sugars (mainly sucrose), as well as betaine, amino acids, mineral salts (potassium, sodium), sulphates and colour compounds. The sugars are immediately available, so the risk is abrupt acidification at too large a dose, and the high content of potassium and sodium can cause salt inhibition. Sulphates favour sulphate-reducing bacteria, which compete with methanogens and produce hydrogen sulphide. The material is thick and viscous, requires heating or dilution for pumping, and tolerates storage in tanks well.

How to read the spread of results

The dataset contains several records from several sources, including an average from several different inocula from mesophilic biogas plants (35 days, Belgium) and a result from a project at 38 °C. Differences may result from the molasses feedstock (beet or cane), the production batch, the content of dry matter and salts, and from the inoculum and test length. For such an easily fermentable material, the substrate-to-inoculum ratio is very important.

In biogas plant practice

Molasses is used as a yield-boosting additive, in small portions well spread over time and with monitoring of pH and volatile acids. It combines well with slurry, which has buffering capacity. When planning, the salt and sulphate content and the efficiency of desulphurisation must be taken into account. Its market price is sometimes volatile, which determines cost-effectiveness. As a by-product of the sugar industry, its status depends on how it is managed.

An additional advantage of molasses is its durability: in a closed tank it can be stored for a long time, which allows it to be used as a reserve for quickly correcting the yield or restoring production after disturbances, provided the doses are small and controlled.

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 = 0.091 d⁻¹ was assumed (no kinetic data for the group – the median of all available rate constants k (93 values) was used).

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.