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Sea lettuce (Ulva lactuca) — BMP: 2 studies, weighted mean 247 m³ CH₄/t VS

Sea lettuce (Ulva) is a green macroalga found in coastal strandings; two thermophilic tests from different studies are described, and salt and sulphur are the main constraints.

CH₄ ≈ 52 % (estimate)

Published Updated 3 min readFeedstockData Editorial Team

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

Weighted mean
247m³ CH₄/t VS
Range
152–343m³ CH₄/t VS
Records
2studies
Publications
2sources
Temperatures
53 °C, 55 °Cof the test

Methane in biogas

Medium methane content · ≈ 5.2 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)) 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 2 studies

Weighted mean Individual records

BMP curves: Sea lettuce (Ulva lactuca)Cumulative methane production over time for 2 records and the weighted mean of 247 m³ CH₄/t VS. 2 of the curves are model curves (first-order kinetics).01002003004000510152025303540time [d]m³ CH₄/t VSKoçer i Özçimen (2018): 343 m³ CH₄/t VS (model curve)Nielsen i Heiske (2011): 152 m³ CH₄/t VS (model curve)mean247

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 (247); 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
SourceBMP
m³ CH₄/t VS		TemperatureSampleCurve
Koçer i Özçimen (2018) DOI34355 °C · thermophilicUlva lactuca (coastal macroalgal waste)end point (model curve)
Nielsen i Heiske (2011) DOI15253 °C · thermophilicUlva lactuca (Denmark)end point (model curve)
Weighted mean247arithmetic mean 247, SD 135

Process properties

  • seasonal
  • variable composition
  • sulphur-rich
  • editorial classification

Advantages

  • Methane yield in the compendium: median 247 (range 152–343) m³ CH₄/t VS, 2 records from 2 papers (weighted mean 247).
  • Maceration increased the methane yield of Ulva lactuca by 68% (Nielsen et al. (2011))
  • Co-digestion of Ulva with cattle slurry in a CSTR increased the rate of methane production by 48% at 40% VS of Ulva in the feed (Nielsen et al. (2011))

Drawbacks and risks

  • Increasing the share of Ulva from 40 to 50% VS gave no further increase in methane production, which limits use in large biogas plants (Nielsen et al. (2011))
  • The yield of Ulva rises with temperature and falls with increasing particle size (Koçer et al. (2018))
  • After stabilisation the biogas from co-digestion of Ulva sp. with pig slurry (48/52 w/w) contained 3.5% H2S and required cleaning; potassium molybdate lowered H2S only temporarily (Peu et al. (2011))
  • A larger share of Ulva lactuca in a mix with dairy slurry led to inhibition, rapid VFA accumulation and forced a reduction in loading (FitzGerald et al. (2015))

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

Dosing

Indicative limit from the literatureup to 40% VS of the feed[1]

Basis: share of Ulva in the VS of the feed. Nielsen et al. (2011): in a CSTR with cattle slurry, 40% VS of Ulva lactuca increased the rate of methane production by 48%, and increasing to 50% gave no further effect. The value of 40% is therefore a point above which there is no benefit (data from one paper), not a documented toxicity threshold. Peu et al. (2011) in a three-month pilot co-digested Ulva sp. with pig slurry 48/52 w/w without clear disruption of the process, but the biogas contained 3.5% H2S; FitzGerald et al. (2015) described inhibition and rapid VFA accumulation at a larger share of Ulva in a mix with dairy slurry, which forced a reduction in loading. Treat 40% as an upper guide and introduce gradually.

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

Practical requirements

  • Assess the C:S ratio of the feedstock — the Peu et al. model indicates a C:S ratio of at least 40 for H2S below 2% in the biogas; provide for biogas desulphurisation (Peu et al. (2012); Peu et al. (2011))
  • Shred finely (optimally approx. 200 µm in a thermophilic test) (Koçer et al. (2018))

Sources of the process data

  1. Anaerobic digestion of macroalgae: methane potentials, pre-treatment, inhibition and co-digestion
  2. Sulphur fate and anaerobic biodegradation potential during co-digestion of seaweed biomass (Ulva sp.) with pig slurry
  3. Prediction of hydrogen sulphide production during anaerobic digestion of organic substrates
  4. Investigation of the biogas production potential from algal wastes
  5. Methanosarcina Play an Important Role in Anaerobic Co-Digestion of the Seaweed Ulva lactuca: Taxonomy and Predicted Metabolism of Functional Microbial Communities

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

What it is and where it comes from

Ulva lactuca, known as sea lettuce, is a green macroalga with a thin, blade-like thallus. In coastal waters rich in nitrogen and phosphorus it can develop en masse, forming so-called green tides, which require removal once washed ashore. The set contains several records: algae from coastal strandings and material from a Danish study. It is not a typical substrate for Poland.

Properties relevant to digestion

Ulva biomass is poor in lignin and contains cell wall polysaccharides (ulvan), proteins and mineral salts. Good availability to bacteria favours rapid initial degradation, but salt and sulphur pose a risk: high salinity inhibits methanogens, and sulphates are reduced to hydrogen sulphide. Waste from the beach may contain sand, shells and other impurities, which load the tank with sediment and yield no methane.

How to read the spread of results

The records come from several independent studies and both relate to thermophilic conditions, but with different biomass: one study examined algae from strandings, the other material from Denmark. The result is affected by seasonal composition and the place of harvest, salinity, the degree of rinsing, particle size, the method of preservation and the adaptation of the inoculum to salt. The differences between the records should therefore not be interpreted as measurement spread for the same material.

In biogas plant practice

Ulva is sometimes treated as a supplement to the mix in coastal installations. It usually requires rinsing, sand reduction and monitoring of hydrogen sulphide and salt in the system. Supply is seasonal and unpredictable, so it rarely forms the basis of a business plan. Before a batch is used, its content of heavy metals and impurities and its compliance with local waste regulations must be checked.

When assessing profitability, the costs of harvesting and transporting wet biomass, which consists mostly of water, must be taken into account. Literature results are a good reference point for comparisons, but they do not replace testing one's own batch, especially as the composition of algae washed ashore changes from season to season and depends on how long they lay on the beach.

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

  1. Koçer, A.T.; Özçimen, D. (2018). Investigation of the biogas production potential from algal wastes. Waste Management & Research: The Journal for a Sustainable Circular Economy 36(11), 1100–1105. DOI: 10.1177/0734242x18798447 DOI
  2. Nielsen, H.B.; Heiske, S. (2011). Anaerobic digestion of macroalgae: methane potentials, pre-treatment, inhibition and co-digestion. Water Science and Technology 64(8), 1723–1729. DOI: 10.2166/wst.2011.654 DOI

All sources of the site: bibliography.

Other substrates in the group “Algae and aquatic plants”