BMP testingFats, oils and animal by-products

Fats, oils and grease (FOG) — BMP: 5 studies, weighted mean 488 m³ CH₄/t VS

Fats from separators (FOG) are a waste from the treatment of catering and food-industry wastewater. They have a high energy potential, but require careful dosing and infrastructure.

CH₄ ≈ 63 % (estimate)

Published 28 April 2021Updated 29 October 20263 min readFeedstockData Editorial Team

Photo SuSanA Secretariat / Wikimedia Commons, licence CC BY 2.0, source file; changes: cropping

Weighted mean: 488m³ CH₄/t VS
Range: 350–688m³ CH₄/t VS
Records: 5studies
Publications: 3sources
Temperatures: 35 °Cof the test

Methane in biogas

≈59–68% CH₄

High methane content · ≈ 6.3 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); 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

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 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 (488); 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
Rico et al. (2014) · 1 DOI	350	35 °C · mesophilic	sludge from the fat separator of a dairy factory (SL2)	end point (model curve)
Rico et al. (2014) · 2 DOI	388	35 °C · mesophilic	sludge from the fat separator of a dairy factory (SL1)	end point (model curve)
Holliger et al. (2017) DOI	688	—	Oil-fat remover sludge	end point (model curve)
Labatut et al. (2011) · 1 DOI	402	35 °C · mesophilic	FOG	end point (model curve)
Labatut et al. (2011) · 2 DOI	413	35 °C · mesophilic	FOG	end point (model curve)
Weighted mean	488	arithmetic mean 448, SD 136

Typical composition

Volatile solids (VS): 87 % DM (range 86–88)

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: 2 determinations. No DM in the sources – the calculator has to ask for a value.

Sources: Labatut 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
tendency to foam
forms a floating layer
physical contaminants
variable composition
ABP cat. 3
editorial classification

Advantages

Methane potential in the Substraty.pl BMP dataset: weighted mean 488.3 m³ CH₄/t VS (SD 136.1; 5 records, 3 sources).
Sludge from grease separators fed into a wastewater treatment plant digester increased methane yield by 9-27% at a share of 10-30% VS (Davidsson et al. 2008).
A waste that is available regularly and often collected for a fee - an economic benefit given suitable infrastructure (Long et al. 2012).

Drawbacks and risks

As the sole substrate it did not give stable methane production in continuous pilot digestion (Davidsson et al. 2008).
LCFA from hydrolysis and detergents/cleaning agents in the waste inhibit methanogenesis and promote foaming (Long et al. 2012; Chen et al. 2008).
Composition highly variable between deliveries (Long et al. 2012) - the dose must be adjusted after batch analysis.

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/sewage sludge as a base dilutes LCFA; DAF/FOG was digested stably in a mix with pig slurry.^[1][2]
combines well
Primary sludge
Sewage sludge was the matrix in which FOG was digested with a positive effect.^[2][3]
avoid
substrates: fat-rich — e.g. Food waste, Spent coffee grounds, Primary sludge, Sunflower seeds, Olive pomace / olive mill solid waste
Adding a further fat stream (oil, animal fat, flotation sludge) to the same dose accumulates LCFA.^[4]

Dosing

Indicative limit from the literatureup to 30% VS of the feed^[2]

Basis: share of separator sludge in the VS of the wastewater treatment plant digester feed. Davidsson et al. (2008): adding 10–30% VS of separator sludge to sewage sludge increased methane yield by 9–27% at pilot scale. 30% is the highest of the shares tested, not an inhibition threshold; it is not stated what happens above that. The data relate to sewage sludge as the base; in an agricultural biogas plant start many times lower and increase gradually.

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).
Do not feed hygienised substrate into the digester hot (FNR 2010, Ch. 3: hygienisation).
Provide for removal of sand and heavy contaminants: sand is often strongly bound to organic matter and is released only in the digester, and sediments reduce its usable capacity, so a scraper or bottom drain is needed (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

FOG (fats, oils and grease) is fats, oils and grease retained in grease separators in restaurants, catering establishments, dairies, slaughterhouses and other food plants. They arrive there with wastewater and separate as a layer on the surface, which is periodically removed. The dataset contains sludge from the grease separator of a dairy, sludge from grease traps and a material labelled generally as FOG.

Properties relevant to digestion

The material is a mixture of fats, water, food residues and contaminants. The high lipid content gives a large gas potential, but the long-chain fatty acids released in hydrolysis can inhibit methanogenesis and lead to flotation of biomass and foaming. Moreover, depending on the source, the sludge contains salts, detergents and cleaning agents, and sometimes also odorous decomposition products. At low temperature the fat solidifies, so heating and good mixing are needed. It is usually dosed slowly and in a mixture with other substrates.

How to read the spread of results

The records compiled come from different suppliers: a dairy, a plant with a grease trap and other sources, and in some studies the temperature was not given. The composition, water content, fat share and degree of contamination differ between batches, and even within a single separator over the course of a year. The test result also depends on the emulsification of the sample, the substrate-to-inoculum ratio, adaptation of the microflora to fats and the length of the test. Conversion to volatile solids does not remove these differences.

In biogas plant practice

FOG is an attractive substrate for co-digestion, because it gives a lot of biogas, and its suppliers usually pay for collection. It requires a heated reception tank, removal of solid contaminants and very gradual dosing, with control of volatile fatty acids and foaming. As a waste from catering or food processing it is subject to the regulations on wastes or animal by-products, depending on origin; this concerns classification, documentation, hygienisation and the permits held. The rules must be checked for the specific plant and type of supplier. Regardless of the attractive reception fee, the handling costs must be calculated: heating, cleaning of tanks, removal of contaminants and possible downtime after overloading, because it is these that often decide the profitability of this substrate.

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.