BMP testingFruit, vegetables and their residues

Water spinach — BMP: 2 studies, weighted mean 141 m³ CH₄/t VS

Kangkong (water spinach) is a fast-growing leafy plant grown in Asia, useful as a light, watery plant substrate for co-digestion with slurry or manure.

rating: neutral CH₄ ≈ 57 % (estimate)

Published 1 January 2025Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Krzysztof Ziarnek, Kenraiz / Wikimedia Commons, licence CC BY-SA 4.0, source file; changes: cropping

Weighted mean: 141m³ CH₄/t VS
Range: 111–172m³ CH₄/t VS
Records: 2studies
Publications: 2sources
Temperatures: 37 °Cof the test

Methane in biogas

≈55–60% CH₄

Medium methane content · ≈ 5.7 kWh/m³ of biogas

estimate from the substrate type The collected studies contain no measurement. The interval follows from the approximate composition (protein from total nitrogen (N 4.6 % DM × 6.25)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

neutral — editorial rating for biogas plantsNo clear advantages — worth charging a fee for accepting it

Potential per tonne of fresh matter: very low (8 m³ CH₄/t); no particular threats to stability. No additional handling requirements.

Why? Rating factors

Potential per tonne −1

−1potential 8 m³ CH₄/t fresh matter (BMP 141 × DM 7.1 % × VS 83.3 %)

Process stability 0

0no factors

Handling and costs 0

0no factors

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

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

Add to the co-digestion calculator CSV data

BMP curves from 2 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 (141); 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
Yan et al. (2017) DOI	172	—	Water spinach (Ipomoea aquatica)	end point (model curve)
Cu et al. (2014) DOI	111	37 °C · mesophilic	water spinach	end point (model curve)
Weighted mean	141	arithmetic mean 141, SD 43

Typical composition

Dry matter (DM): 7.1% (range 7.1–7.1)
Volatile solids (VS): 83.3 % DM (range 80.8–85.7)
Nitrogen: 4.6 % DM
C/N ratio: 7.9

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: BCBPD, MY-D); dry matter: 1 determination, volatile solids: 2 determinations. C/N was calculated as the ratio of C and N content (% DM) from the same source. DM from a single determination – indicative value.

Sources: Yan et al. (2017); Cu et al. (2014).

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

dilutes the feed (low DM)
seasonal
poor in trace elements
variable composition
editorial classification

Low confidence: the description is based on general characteristics of the substrate group, not on studies of this material. Treat it as a hint to be verified.

Advantages

Methane yield in the compendium: median 141 (range 111-172) m³ CH₄/t VS, 2 records from 2 studies (weighted mean 141).

Drawbacks and risks

Water spinach gave less methane than Spirodela duckweed in a BMP test (Cu et al. (2014))

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

combines well
substrates: nitrogen-rich — e.g. Pig slurry, Poultry manure, Waste activated sludge (secondary sludge), Chlorella vulgaris, Nannochloropsis salina
A nitrogen-rich co-substrate balances the low C/N of fruit and vegetable waste (Bouallagui et al. 2009).^[1]

Dosing

No source limits on the dose for this substrate.

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

Kangkong, known as water spinach (Ipomoea aquatica), is a climbing plant of the morning glory family, growing in water or on moist soils. It is widely grown as a leafy vegetable in South-East Asia and China. In Poland it has no productive significance, so data from the literature serve mainly as a reference point for similar leafy plants. It grows very fast, which means it is sometimes considered as biomass for energy purposes in a tropical climate.

Properties relevant to digestion

The plant consists mainly of water. In the dry matter fibre (cellulose and hemicellulose), protein and sugars dominate, and the lignin content is small in young shoots and rises in older stems. A high protein content means a contribution of nitrogen, so the C/N ratio is lower than in woody plants. For digestion, the risk of ammonia accumulation matters only in the case of very large shares of protein-rich materials. Aquatic and semi-aquatic plants may additionally contain sand, silt or contaminants, which sediment in the digester.

How to read the spread of results

The records come from different studies, of which one does not state the test temperature, while the other indicates mesophilic conditions. Results may differ because of the age of the plant at harvest, whether only leaves or whole shoots were tested, and the method of sample preparation: fresh, dried or frozen. The inoculum and the substrate-to-inoculum ratio also matter. With such a small number of records, variability of the material cannot be reliably separated from methodological variability.

In biogas plant practice

In Polish conditions kangkong is not a realistic substrate, because the plant is not grown. For installations in tropical regions it may supplement the feed together with manure or other waste. Size reduction is recommended, and harvesting young shoots gives better digestibility. Watery plant materials have a low energy density and do not pay when transported over longer distances. A plant with invasive properties may be grown only where local regulations allow it.

An additional difficulty is the lack of established practices for storing this plant for biogas plant needs: harvesting fresh mass and using it promptly is the safest solution, and ensiling requires prior wilting, because the material is very watery.

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 = 0.245 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.