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Primary sludge — BMP: 13 studies, weighted mean 340 m³ CH₄/t VS

Primary sludge from wastewater treatment plants contains organic suspended solids and fats, so it digests better than excess sludge, but results are very widely dispersed.

CH₄ ≈ 63 % (estimate)

Published Updated 3 min readFeedstockData Editorial Team

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

Weighted mean
340m³ CH₄/t VS
Range
175–590m³ CH₄/t VS
Records
13studies
Publications
4sources
Temperatures
35 °C, 37 °Cof the test

Methane in biogas

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 13 studies

Weighted mean Individual records

BMP curves: Primary sludgeCumulative methane production over time for 13 records and the weighted mean of 340 m³ CH₄/t VS. 13 of the curves are model curves (first-order kinetics).02004006008000510152025303540time [d]m³ CH₄/t VSNoh et al. (2026): 303 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 1: 175 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 2: 177 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 3: 180 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 4: 193 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 5: 205 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 6: 212 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 7: 255 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 8: 260 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 9: 302 m³ CH₄/t VS (model curve)Catenacci et al. (2019) · 10: 318 m³ CH₄/t VS (model curve)Chynoweth et al. (1993): 590 m³ CH₄/t VS (model curve)Hao et al. (2022): 239 m³ CH₄/t VS (model curve)mean340

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 (340); 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.

  • Show the remaining records in the legend (1)

Records and sources

Download CSV
SourceBMP
m³ CH₄/t VS		TemperatureSampleCurve
Noh et al. (2026) DOI30335 °C · mesophilicprimary sludge (PS), mono-digestion (R1)end point (model curve)
Catenacci et al. (2019) · 1 DOI17535 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 2 DOI17735 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 3 DOI18035 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 4 DOI19335 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 5 DOI20535 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 6 DOI21235 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 7 DOI25535 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 8 DOI26035 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 9 DOI30235 °C · mesophilicprimary sludgeend point (model curve)
Catenacci et al. (2019) · 10 DOI31835 °C · mesophilicprimary sludgeend point (model curve)
Chynoweth et al. (1993) DOI59035 °C · mesophilicPrimary sludgeend point (model curve)
Hao et al. (2022) DOI23937 °C · mesophilicprimary sludgeend point (model curve)
Weighted mean340arithmetic mean 262, SD 110

Typical composition

Dry matter (DM)
11.4% (range 11.4–11.4)

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: 0 determinations. DM from a single determination – indicative value.

Sources: Hao et al. (2022).

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
  • sand and sediment
  • tendency to foam
  • easily degradable
  • editorial classification

Advantages

  • Methane potential in the Substraty.pl BMP dataset: weighted mean 339.9 m³ CH₄/t VS (SD 110.5; 13 records, 4 sources).
  • More readily degradable than waste activated sludge (Appels et al. 2008).

Drawbacks and risks

  • Excess fats -> LCFA and foaming (Long et al. 2012).
  • Sand settles in the tanks.

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

Dosing

No source-based threshold for the sludge itself; as a base for fats see FOG.

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

Practical requirements

  • 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

  1. Davidsson et al. (2008). Co-digestion of grease trap sludge and sewage sludge. Waste Manag.
  2. Appels et al. (2008). Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci.
  3. Long et al. (2012). Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations. Process Saf. Environ. Prot.
  4. FNR (2010). Guía sobre el Biogás – Desde la producción hasta el uso (hiszp. wydanie „Leitfaden Biogas”, GIZ 2013)

The co-digestion calculator will assess a specific mixture containing this substrate.

What it is and where it comes from

Primary sludge is the fraction settling in the primary clarifiers of a wastewater treatment plant before the wastewater passes to the biological stage. It consists of organic suspended solids from domestic and industrial wastewater: food scraps, faeces, paper, fats and fine sand. It is thickened and sent to digesters, usually together with excess sludge. The dataset contains samples of primary sludge from treatment plants, including in South Korea, tested at mesophilic temperature.

Properties relevant to digestion

Primary sludge contains a lot of readily degradable carbohydrates, proteins and fats and cellulose fibres from paper, but less resistant bacterial biomass than excess sludge. Its potential is therefore higher and degradation faster. Fats increase the yield, but in excess cause VFA inhibition and foaming. The sludge also contains sand and mineral impurities, which settle in the digesters. Its composition changes with the quality of the wastewater.

How to read the spread of results

The range of results in the dataset is very wide because primary sludge is a variable material: it depends on the degree of dilution of the wastewater, the share of industrial wastewater, rainfall, the retention time in the clarifier (a longer time means partial decomposition) and thickening. Storage, the substrate-to-inoculum ratio, temperature and test length also have an effect. Values from different treatment plants are not directly comparable.

In biogas plant practice

Primary sludge is digested in treatment plant digesters and is the best component of sludges for biogas production. In co-digestion, food waste and fats are added to it, which requires load control. Permits are required for accepting waste into treatment plant digesters. Sewage sludge is waste (code 19 08 05 for sludge from municipal wastewater treatment), and the digestate must be managed in accordance with the applicable rules.

Compared with excess sludge, primary sludge contains fewer bacteria and more material that reached the treatment plant in the wastewater, so its composition changes along with the sewer network. This is affected both by the share of kitchen waste in the wastewater and by the presence of industrial wastewater.

A BMP test of your own is therefore recommended.

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

Results after pretreatment (3) — excluded from the mean
SourceBMP
m³ CH₄/t VS		PretreatmentSample
Hao et al. (2022)189chemicalprimary sludge
Hao et al. (2022)174chemicalprimary sludge
Hao et al. (2022)150chemicalprimary sludge

Bibliography

  1. Noh, E.J.; Shin, S.G.; Cheon, J.L.; Ahn, J.H. (2026). Anaerobic Co-digestion of Fish Waste and Primary Sludge: Biochemical Methane Potential and Mixing Ratio. Waste and Biomass Valorization 17(7), 3835–3843. DOI: 10.1007/s12649-025-03344-7 DOI
  2. Catenacci, A.; Azzellino, A.; Malpei, F. (2019). Development of statistical predictive models for estimating the methane yield of Italian municipal sludges from chemical composition: a preliminary study. Water Science and Technology 79(3), 435–447. DOI: 10.2166/wst.2019.063 DOI
  3. Chynoweth, D.; Turick, C.; Owens, J.; Jerger, D.; Peck, M. (1993). Biochemical methane potential of biomass and waste feedstocks. Biomass and Bioenergy 5(1), 95–111. DOI: 10.1016/0961-9534(93)90010-2 DOI
  4. Hao, T.; Liu, Z.; Liang, M.; Liu, X.; Chen, J.; Wu, X.; Varjani, S.; Zan, F. (2022). The effects of sulfite pretreatment on the biodegradability and solubilization of primary sludge: Biochemical methane potential, kinetics, and potential implications. Separation and Purification Technology 297, 121439. DOI: 10.1016/j.seppur.2022.121439 DOI

All sources of the site: bibliography.

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