BMP testingAlgae and aquatic plants

Chlorella vulgaris — BMP: 9 studies, weighted mean 278 m³ CH₄/t VS

Chlorella vulgaris is the alga most often studied in BMP tests; results are spread widely, because they depend on the strain, cultivation, cell disruption and treatment.

CH₄ ≈ 58 % (estimate)

Published 25 February 2020Updated 29 October 20263 min readFeedstockData Editorial Team

Photo ja:User:NEON / User:NEON_ja / Wikimedia Commons, licence CC BY-SA 3.0, source file; changes: cropping

Weighted mean: 278m³ CH₄/t VS
Range: 91–440m³ CH₄/t VS
Records: 9studies
Publications: 3sources
Temperatures: 35 °C, 37 °C, 38 °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

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 9 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 (278); 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
Jankowska et al. (2017) · 1 DOI	91	35 °C · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review	end point (model curve)
Jankowska et al. (2017) · 2 DOI	131	35 °C · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)	end point (model curve)
Jankowska et al. (2017) · 3 DOI	228	37 °C · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review	end point (model curve)
Jankowska et al. (2017) · 4 DOI	229	— · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review	end point (model curve)
Jankowska et al. (2017) · 5 DOI	230	35 °C · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review	end point (model curve)
Jankowska et al. (2017) · 6 DOI	337	35 °C · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review	end point (model curve)
Jankowska et al. (2017) · 7 DOI	440	35 °C · mesophilic	(orig.: “Chlorella vulgaris”): value quoted in a literature review · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)	end point (model curve)
Lakaniemi et al. (2011) DOI	286	37 °C · mesophilic	Chlorella vulgaris biomass	end point (model curve)
Roberts et al. (2019) DOI	307	38 °C · mesophilic	C. vulgaris	end point (model curve)
Weighted mean	278	arithmetic mean 253, SD 105

Typical composition

Volatile solids (VS): 94.7 % DM (range 94.7–94.7)

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: 1 determination. No DM in the sources – the calculator has to ask for a value.

Sources: Roberts et al. (2019).

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
nitrogen-rich
dilutes the feed (low DM)
slowly degrading
variable composition
editorial classification

Advantages

Methane yield in the compendium: median 230 (range 91-440) m³ CH₄/t VS, 9 records from 3 studies (weighted mean 278).
Methane production from whole microalgal biomass is energetically optimal when the lipid content does not exceed 40%; physicochemical pretreatment, co-digestion or composition control increase conversion (Sialve et al. (2009))

Drawbacks and risks

The high protein content of the cells causes release of ammonia, which can be toxic to the process (Sialve et al. (2009); Rajagopal et al. (2013))
The biodegradability of microalgae may be low: it depends on the biochemical composition and the cell wall (Sialve et al. (2009))

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

combines well
substrates: carbon-rich — e.g. Wheat straw, Maize silage (whole crop), Cereal straw (unspecified), Willow (short rotation), Cardboard and paperboard
The low C/N of algal sludge was balanced by the addition of paper: 50-60% VS of paper increased the rate of methane production, optimum C/N 20-25.^[1][2]
caution
substrates: nitrogen-rich — e.g. Pig slurry, Poultry manure, Waste activated sludge (secondary sludge), Nannochloropsis salina, Lucerne (alfalfa)
Algal protein releases ammonia; additional nitrogen sources increase the risk of ammonia inhibition.^[2][3]

Dosing

The sources do not give a safe share of microalgae in the feed. In the study by Yen and Brune (2007), co-digestion of algal sludge with waste paper (50-60% VS of paper) balanced the C/N to 20-25 and increased methane production.

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

Practical requirements

Do not dry the biomass before feeding: drying lowered biogas production to about 80% (Mussgnug et al. (2010))
Monitor ammonia (high protein) and, for marine species, sodium (Sialve et al. (2009))

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

Chlorella vulgaris is a unicellular green freshwater alga, grown commercially mainly as a dietary supplement and feed, and also studied as a raw material for biofuels and for wastewater treatment. It is grown in open ponds and closed photobioreactors. In Poland and Europe it is not a biogas plant substrate, and the BMP data come from laboratory studies, including literature reviews.

Properties relevant to digestion

Chlorella vulgaris cells have a relatively thick cell wall, which limits hydrolysis, so cell disruption (mechanical, thermal, chemical or enzymatic) usually improves conversion. The biomass contains a lot of protein, so the C/N ratio is low, and the ammonia released at a high load can inhibit methanogenesis. Lipids increase the methane potential, but in excess promote inhibition. The composition depends on the growing conditions: nitrogen deficiency reduces the protein share and increases the content of starch and lipids.

How to read the spread of results

Records for this species cover a wide range, which reflects differences in strains, media, growth phase and light conditions. The form of the sample is of very great importance: fresh suspension, centrifuged biomass, freeze-dried material or material after cell disruption give different results. In addition there are temperature, the substrate-to-inoculum ratio, test length and adaptation of the inoculum to protein. A large part of the records was quoted in reviews, with a simplified description of conditions, which further hinders direct comparisons.

In biogas plant practice

Chlorella vulgaris is not a standard substrate. If it arises in treatment plants or biorefinery installations, it is sensible to combine it with carbohydrate-rich substrates (for example plant waste) in order to lower the ammonia concentration and improve the C/N ratio. Cell disruption increases conversion, but requires energy. Before the alga is included in the mix, it is worth carrying out a BMP test on your own batch, because literature results do not transfer easily. Legal requirements depend on the origin of the biomass and its status as a waste or a product.

It is worth adding that this species is sometimes used as a model organism in algal digestion tests, so its results often serve as a reference point for other species. This does not mean, however, that a value from one study can be transferred to another culture.

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 = 0.102 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.