BMP testingMilk, whey and dairy waste

Milk and buttermilk — BMP: 5 studies, weighted mean 522 m³ CH₄/t VS

Milk and buttermilk from waste batches (out of date, withdrawn) are a substrate rich in fat, lactose and protein, with a high yield and a risk of inhibition.

rating: neutral CH₄ ≈ 66 % (estimate)

Published 29 October 2021Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Ukko-wc / Wikimedia Commons, licence CC BY-SA 3.0, source file; changes: cropping

Weighted mean: 522m³ CH₄/t VS
Range: 458–563m³ CH₄/t VS
Records: 5studies
Publications: 2sources
Temperatures: 36 °Cof the test

Methane in biogas

≈60–69% CH₄

High methane content · ≈ 6.6 kWh/m³ of biogas

estimate from the substrate type The collected studies contain no measurement. The interval follows from the approximate composition (protein-rich (process classification); fat-rich (process classification)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

neutral — editorial rating for biogas plantsNo clear advantages — worth charging a fee for accepting it

Potential per tonne of fresh matter: high (60 m³ CH₄/t); process risk: fats, protein / nitrogen, foaming. Handling costs: ABP cat. 3.

Why? Rating factors

Potential per tonne +2

+2potential 60 m³ CH₄/t fresh matter (BMP 522 × DM 11.5 % × VS 99.2 %)

Process stability −2

−1fats: risk of accumulation of long-chain fatty acids (LCFA), flotation and overloading
−0.5protein / nitrogen: risk of ammonia inhibition at higher shares
−0.25foaming: tendency to form foam in the digester
−0.25acidification: rapid degradation / acidic pH, risk of VFA accumulation, needs buffering

Handling and costs −0.5

−0.5ABP cat. 3: pasteurisation at 70 °C / 60 min (dairy products — derogations possible)

Total points: −0.5 · 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 5 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 (522); 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
Luna-delRisco et al. (2011) · 1	458	36 °C · mesophilic	milk 2.5 % fat	end point (model curve)
Luna-delRisco et al. (2011) · 2	466	36 °C · mesophilic	milk 3.5 % fat	end point (model curve)
Luna-delRisco et al. (2011) · 3	489	36 °C · mesophilic	butter milk	end point (model curve)
Luna-delRisco et al. (2011) · 4	512	36 °C · mesophilic	raw milk	end point (model curve)
Koch et al. (2020) DOI	563	—	Whole milk	end point (model curve)
Weighted mean	522	arithmetic mean 498, SD 42

Typical composition

Dry matter (DM): 11.5% (range 10.9–12.8)
Volatile solids (VS): 99.2 % DM (range 99.2–99.3)

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 4 determinations, volatile solids: 2 determinations.

Sources: Luna-delRisco et al. (2011).

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

fat-rich
protein-rich
easily degradable
ABP cat. 3
tendency to foam
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 522.1 m³ CH₄/t VS (SD 42.2; 5 records, 2 sources).
Readily available lactose and milk fat (Prazeres et al. 2012; Long et al. 2012).
Milk and dairy products in co-digestion with slurry are described as a substrate of high potential (Hagos et al. 2017).

Drawbacks and risks

Excess milk fat gives LCFA and foaming (Long et al. 2012).
Milk protein releases ammonia (Rajagopal et al. 2013).

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
^[1]
caution
substrates: fat-rich — e.g. Food waste, Spent coffee grounds, Primary sludge, Sunflower seeds, Olive pomace / olive mill solid waste
Milk fat + other fat: LCFA accumulation.^[2]

Dosing

No source-based threshold. Start with a low share in a mix with slurry; shared limit for lipids.

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

Practical requirements

ABP cat. 3 (Art. 10 of Reg. 1069/2009; admitted to biogas plants by Art. 14): hygienisation before the digester in a mandatory unit: shredding to a maximum of 12 mm, temperature throughout the mass of at least 70 °C for at least 60 min without interruption (Reg. 142/2011, Annex V Ch. I Section 1 and Ch. III Section 1); the installation requires approval by the competent authority (Art. 24(1)(g) of Reg. 1069/2009).
Milk, dairy products and colostrum may be used without hygienisation unless the authority considers there to be a risk of spreading a serious transmissible disease (Reg. 142/2011, Annex V, Chapter III, Section 1, point 1).
Do not feed hygienised substrate into the digester hot (FNR 2010, Ch. 3: hygienisation).
Remove foreign contaminants before dosing: material from biowaste may contain interfering substances, and the co-substrate should be free of them (FNR 2010, Ch. 3).

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

Milk and buttermilk in the context of a biogas plant are batches that are not fit for consumption or sale: out of date, withdrawn from the market, rejected at the dairy, or arising from the rinsing and start-up of a line. The dataset includes samples of milk of different fat content, raw milk, whole milk and buttermilk, tested mainly at mesophilic temperature. Milk is a product of animal origin, so once it is no longer fit for consumption it becomes an animal by-product.

Properties relevant to digestion

The dry matter of milk is a dozen or so per cent, and consists of lactose, protein (casein and whey proteins) and fat. Lactose and some of the proteins are readily degradable, so the feed quickly acidifies, and if the dose is excessive there is a risk of accumulation of volatile fatty acids. Milk fat has a high methane yield, but in excess it causes inhibition by long-chain fatty acids and foaming. Protein releases ammonia. Buttermilk has less fat than whole milk, and more phospholipids.

How to read the spread of results

The results differ in fat content (e.g. milk with 2.5% and 3.5% fat), in the type of product (raw, whole, buttermilk), and in the degree of acidification or spoilage of the batch. The substrate-to-inoculum ratio (milk is easily overloaded), the adaptation of the inoculum to fat, the temperature and the test length also have an effect. At a high loading, a test may show a delayed lag phase or a drop in the result because of inhibition. The origin of the samples from different studies further differentiates the methods.

In biogas plant practice

Milk and buttermilk can be a valuable co-substrate, dosed carefully and as a small part of the mix, because of the easy acidification and the fat. They are best given in a mixture with slurry or another buffering material. As animal by-products they go to an installation subject to Regulation (EC) No 1069/2009 and its implementing rules: the installation must be approved for processing such materials and apply the required procedures, including hygienisation, unless the competent authority permits a derogation. The rules should be confirmed with the county veterinary officer.

For comparison with whey, milk contains considerably more fat and protein, and so is higher-yielding, but also harder to dose. With a sudden increase in the dose, pH and volatile acids must be monitored.

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 = 0.270 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.