BMP testingSewage and industrial sludges

Waste activated sludge (secondary sludge) — BMP: 12 studies, weighted mean 151 m³ CH₄/t VS

Excess sludge (secondary, activated) is bacterial biomass from wastewater treatment plants, harder to digest than primary sludge; pretreatment improves it.

rating: negative CH₄ ≈ 58 % (estimate)

Published 5 March 2021Updated 29 October 20263 min readFeedstockData Editorial Team

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

Weighted mean: 151m³ CH₄/t VS
Range: 53–183m³ CH₄/t VS
Records: 12studies
Publications: 3sources
Temperatures: 35 °C, 36 °Cof the test

Methane in biogas

≈55–62% CH₄

Medium methane content · ≈ 5.8 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)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

negative — editorial rating for biogas plantsMay endanger process stability — not worth taking even with a large payment

Potential per tonne of fresh matter: very low (2 m³ CH₄/t); process risk: protein / nitrogen, inhibitors. Handling costs: slow degradation. Low potential combined with process risk and handling costs.

Why? Rating factors

Potential per tonne −1

−1potential 2 m³ CH₄/t fresh matter (BMP 151 × DM 2.1 % × VS 48 %)

Process stability −1.5

−1protein / nitrogen: risk of ammonia inhibition at higher shares
−0.5inhibitors: natural or residues (e.g. limonene, polyphenols, antibiotics, metals)

Handling and costs −0.5

−0.5slow degradation — long retention time

Total points: −3 · thresholds: ≥ 2.5 very good, ≥ 1 good, below — neutral; negative with cumulative process risk.

Low potential combined with process risk and handling costs.

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

Add to the co-digestion calculator CSV data

BMP curves from 12 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 (151); 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
Catenacci et al. (2019) · 1 DOI	53	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 2 DOI	95	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 3 DOI	105	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 4 DOI	125	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 5 DOI	133	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 6 DOI	155	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 7 DOI	170	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 8 DOI	175	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 9 DOI	180	35 °C · mesophilic	secondary sludge	end point (model curve)
Catenacci et al. (2019) · 10 DOI	183	35 °C · mesophilic	secondary sludge	end point (model curve)
Erden i Filibeli (2010) DOI	151	36 °C · mesophilic	wastewater sludge	end point (model curve)
Nielfa et al. (2015) DOI	164	—	Biological sludge (BS)	end point (model curve)
Weighted mean	151	arithmetic mean 141, SD 40

Typical composition

Dry matter (DM): 2.1% (range 2.1–2.1)
Volatile solids (VS): 48 % DM (range 48–48)

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. DM from a single determination – indicative value.

Sources: Erden et al. (2010).

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

protein-rich
slowly degrading
nitrogen-rich
contains inhibitors
editorial classification

Advantages

Methane potential in the Substraty.pl BMP dataset: weighted mean 151.0 m³ CH₄/t VS (SD 40.0; 12 records, 3 sources).
Bacterial biomass - a base for co-digestion; improved by pretreatment (Appels et al. 2008).

Drawbacks and risks

Cell walls limit hydrolysis - the limiting step (Appels et al. 2008).
Protein -> ammonia (Rajagopal et al. 2013).
Heavy metal content depends on the wastewater (Chen et al. 2008).

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

combines well
substrates: fat-rich — e.g. Food waste, Spent coffee grounds, Primary sludge, Sunflower seeds, Olive pomace / olive mill solid waste
Sewage sludge is a good base for co-digestion of fats at a wastewater treatment plant (stable operation at FOG 10-30% VS).^[1]

Dosing

No source threshold.

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

Excess sludge, also known as secondary or activated sludge (WAS), is the surplus bacterial biomass removed from the activated sludge system of a municipal wastewater treatment plant. It is thickened mechanically and sent to dedicated digesters, often as a mixture with primary sludge. In Poland it makes up a large waste stream, and digesters at treatment plants are among the most numerous biogas installations. The dataset contains samples of secondary sludge tested at mesophilic temperature.

Properties relevant to digestion

The sludge consists mainly of bacterial cells, protein, extracellular polymers and a mineral fraction. The cell walls are resistant to hydrolysis, which is the limiting step, so degradation is slower and the yield lower than for primary sludge. Because of its high protein content the sludge releases ammonia. The content of heavy metals and micropollutants depends on the wastewater entering the plant. Sludges from plants with biological phosphorus removal may contain compounds that increase struvite formation.

How to read the spread of results

The results depend on the age of the sludge (the older it is, the more mineralised and the harder to digest), the type of treatment plant and nutrient removal, the composition of the wastewater, the thickening method and storage. The test conditions have a strong influence: temperature, inoculum, substrate-to-inoculum ratio and incubation length. If the sludge was disintegrated beforehand (ultrasound, heating), the result increases, but such records are marked separately in the dataset. The records here concern sludge without pretreatment.

In biogas plant practice

Excess sludge is digested primarily in treatment plant digesters, where sludge stabilisation is a required stage. Thermal hydrolysis or disintegration, which increase gas yield, are often applied. The digestate must be dewatered and managed in accordance with the rules on sewage sludge (code 19 08 05), which is costly and regulated. Co-digestion of sludge with wastes rich in carbohydrates or fats increases gas production, but requires a permit and process control.

Compared with primary sludge, excess sludge has slower kinetics, so the retention time in treatment plant digesters is determined, among other things, by its share. With longer ageing of the sludge in the biological system the result falls further, which should be taken into account when interpreting literature data from different treatment plants.

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 (1) — excluded from the mean

Source	BMP m³ CH₄/t VS	Pretreatment	Sample
Erden i Filibeli (2010)	220	physical	wastewater sludge

Bibliography

All sources of the site: bibliography.