BMP testingFats, oils and animal by-products

Flotation sludge (DAF skimmings) — BMP: 3 studies, weighted mean 600 m³ CH₄/t VS

Flotation sludge is a fat-rich sludge skimmed from the surface in the treatment plants of meat and dairy plants. Highly energetic, but requires careful dosing and hygienisation.

rating: neutral CH₄ ≈ 61 % (estimate)

Published 4 February 2021Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Stefan.p21 (Maciej Preś) / Wikimedia Commons, licence CC BY-SA 3.0, source file; changes: cropping

Weighted mean: 600m³ CH₄/t VS
Range: 409–787m³ CH₄/t VS
Records: 3studies
Publications: 2sources
Temperatures: 35 °C, 37 °Cof the test

Methane in biogas

≈57–64% CH₄

High methane content · ≈ 6.0 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 1.3 % DM × 6.25); 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: moderate (41 m³ CH₄/t); process risk: fats, protein / nitrogen, inhibitors. Handling costs: ABP cat. 2, fibres, floating layer.

Why? Rating factors

Potential per tonne +1

+1potential 41 m³ CH₄/t fresh matter (BMP 600 × DM 7.8 % × VS 88 %)

Process stability −2.25

−1fats: risk of accumulation of long-chain fatty acids (LCFA), flotation and overloading
−0.5protein / nitrogen: risk of ammonia inhibition at higher shares
−0.5inhibitors: natural or residues (e.g. limonene, polyphenols, antibiotics, metals)
−0.25foaming: tendency to form foam in the digester

Handling and costs −2.5

−2ABP cat. 2: pressure sterilisation before digestion (Reg. 1069/2009, 142/2011)
−0.5fibres, floating layer — shredding and intensive mixing

Total points: −3.75 · thresholds: ≥ 2.5 very good, ≥ 1 good, below — neutral; negative with cumulative process risk.

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

Add to the co-digestion calculator CSV data

BMP curves from 3 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 (600); 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
Browne et al. (2013) DOI	787	37 °C · mesophilic	dissolved air flotation waste from cheese processing · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)	end point (model curve)
Kouas et al. (2017) · 1 DOI	409	35 °C · mesophilic	skimmings of aeroflotation	end point (model curve)
Kouas et al. (2017) · 2 DOI	417	35 °C · mesophilic	skimmings of aeroflotation	end point (model curve)
Weighted mean	600	arithmetic mean 538, SD 216

Typical composition

Dry matter (DM): 7.8% (range 7.8–7.8)
Volatile solids (VS): 88 % DM (range 88–88)
Nitrogen: 1.3 % DM
C/N ratio: 50.1

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 1 determination, volatile solids: 1 determination. C/N was calculated as the ratio of C and N content (% DM) from the same source. DM from a single determination – indicative value.

Sources: Browne et al. (2013).

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
tendency to foam
forms a floating layer
ABP cat. 2
variable composition
contains inhibitors
editorial classification

Advantages

Methane potential in the Substraty.pl BMP set: weighted mean 600.0 m³ CH₄/t VS (SD 216.0; 3 records, 2 sources).
Slaughterhouse wastewater flotation sludge (DAF) in co-digestion at 1:1 (DM) with pig slurry gave a stable and efficient thermophilic process (Creamer et al. 2010).
The combination of fat and protein gives a large share of the biogas, and the slurry base supplies buffer (Creamer et al. 2010).

Drawbacks and risks

DAF sludge alone did not give stable thermophilic digestion, presumably because of LCFA or a nutrient deficiency (Creamer et al. 2010).
Protein releases ammonia, fat releases LCFA; both mechanisms add up (Palatsi et al. 2011; Chen et al. 2008).
May contain residues of coagulants and cleaning agents, which inhibit microorganisms (Chen et al. 2008).

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
Pig slurry enabled stable digestion of DAF.^[1]
caution
substrates: protein-rich — e.g. Food waste, Municipal sewage sludge (mixed), Poultry manure, Organic fraction of municipal solid waste, Grass clippings
Flotation fat + blood/slaughter waste: high N load, ammonia risk, especially under thermophilic conditions.^[2][3]
avoid
substrates: fat-rich — e.g. Food waste, Spent coffee grounds, Primary sludge, Sunflower seeds, Olive pomace / olive mill solid waste
Additional fats in a mixture with flotation fat accumulate LCFA.^[4]

Dosing

Creamer et al. (2010): DAF sludge from pork processing gave stable thermophilic digestion at a ratio of 1:1 (w/w, on dry matter) with pig slurry at an HRT of 10 d and an OLR of 4.68 g VS/(L·d); DAF sludge alone did not give stable operation, and higher substrate concentrations lengthened the lag phase. The share is given on dry matter, not on fresh matter or VS, so max_share was not determined. Start from a low dose.

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

Practical requirements

ABP cat. 2 (Art. 9 of Reg. 1069/2009): with the exception of manure, digestive tract content, milk, colostrum and eggs, admitted to a biogas plant only after pressure sterilisation and permanent marking (Art. 13(e)(i)); method 1: shredding to a maximum of 50 mm, core temperature above 133 °C for at least 20 min without interruption, absolute pressure of at least 3 bar (Reg. 142/2011, Annex IV Ch. III); cat. 1 material has no route to a biogas plant (Art. 12 of Reg. 1069/2009).
Do not feed hygienised substrate into the digester hot (FNR 2010, Ch. 3: hygienisation).

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

Flotation sludge (sludge from flotation, DAF skimmings) arises in industrial wastewater treatment plants in which particles of fat and suspended matter are carried to the surface by air bubbles and skimmed off. It most often comes from meat plants, slaughterhouses and dairies. The compilation contains flotation sludge from cheese processing and sludges skimmed from flotation in another study. Each plant produces flotation sludge of a somewhat different composition, because it depends on the production process, the coagulants used and the retention time.

Properties relevant to digestion

Flotation sludge is a mixture of fats, proteins and water, often with the addition of chemicals used in treatment, such as iron or aluminium salts or polymers. Lipids have a high energy potential, but their breakdown releases long-chain fatty acids, which in excess inhibit methanogenesis and cause flotation of biomass and foaming. The dose must therefore be introduced gradually and in a mixture with other substrates. Proteins, in turn, release ammonia, and its concentration must be controlled. The material can be variable and may contain contaminants, for example residues of cleaning and disinfecting agents.

How to read the spread of results

The results come from plants with different profiles: one is cheese processing, another reflects sludges skimmed from flotation of a different kind. The composition of flotation sludge differs in the share of fat, protein and water and in the presence of coagulants, so conversion to volatile solids does not remove these differences. The method of sample preparation (skimming temperature, phase separation), the substrate-to-inoculum ratio and adaptation of the sludge to fats also have an effect. Tests of fatty substrates are particularly sensitive to too high a load on the inoculum.

In biogas plant practice

Flotation sludge is valued in biogas plants for its high biogas production per unit of mass, but it requires infrastructure: a heated tank so that it can be pumped, and gradual dosing. The share in the feed should be limited and controlled, and volatile fatty acids, pH and the tendency to foam should be observed in the process. Origin in the animal-based food industry means that the regulations on animal by-products apply, including requirements for the category of material, hygienisation and documentation. The specific rules depend on the category and the type of installation, so they should be checked against your own permit.

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.