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Eggplant stalks and leaves — BMP: 4 studies, weighted mean 218 m³ CH₄/t VS

Aubergine stems and leaves are residues left after fruit harvest from greenhouse and field crops. Fibrous and of variable composition, they are suitable as an additive to a digestion mix.

CH₄ 56 %

Published Updated 3 min readFeedstockData Editorial Team

Photo Karen and Brad Emerson / Wikimedia Commons, licence CC BY 2.0, source file; changes: cropping

Weighted mean
218m³ CH₄/t VS
Range
94–374m³ CH₄/t VS
Records
4studies
Publications
3sources
Temperatures
35 °C, 36 °Cof the test

Methane in biogas

Medium methane content · ≈ 5.6 kWh/m³ of biogas

measured mean of 1 study BMP; in a batch test the CH₄ share tends to be higher than in a plant.

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 4 studies

Weighted mean Individual records

BMP curves: Eggplant stalks and leavesCumulative methane production over time for 4 records and the weighted mean of 218 m³ CH₄/t VS. 4 of the curves are model curves (first-order kinetics).0100200300400051015202530time [d]m³ CH₄/t VSCai et al. (2019) · 1: 174 m³ CH₄/t VS (model curve)Cai et al. (2019) · 2: 196 m³ CH₄/t VS (model curve)Gunaseelan (2004): 374 m³ CH₄/t VS (model curve)Li et al. (2018): 94 m³ CH₄/t VS (model curve)mean218

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 (218); 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		CH₄ in biogas
% vol., measured		TemperatureSampleCurve
Cai et al. (2019) · 1 DOI174——Eggplant (Solanum melongena) – Stemend point (model curve)
Cai et al. (2019) · 2 DOI196——Eggplant (Solanum melongena) – Leafend point (model curve)
Gunaseelan (2004) DOI374—35 °C · mesophilicBrinjal Stalkend point (model curve)
Li et al. (2018) DOI9456.4 %36 °C · mesophilicEggplant crop residues (EGR)end point (model curve)
Weighted mean21856.4 %arithmetic mean 210, SD 118

Typical composition

Volatile solids (VS)
84.2 % DM (range 74.7–94.1)
Nitrogen
3.5 % DM
C/N ratio
12.4

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: BCBPD, MY-D); dry matter: 0 determinations, volatile solids: 3 determinations. C/N was calculated as the ratio of C and N content (% DM) from the same source. No DM in the sources – the calculator has to ask for a value.

Sources: Cai et al. (2019); Gunaseelan (2004).

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

  • fibrous
  • seasonal
  • dilutes the feed (low DM)
  • variable composition
  • physical contaminants
  • editorial classification

Advantages

  • Methane potential in the database: weighted mean 218 m³ CH₄/t VS (median 185; range 94-374; 4 records from 3 sources).

Drawbacks and risks

  • Spread of results in the database: standard deviation 118 at a mean of 210 m³ CH₄/t VS (4 records from 3 sources).

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

Dosing

No source dose limits; add together with other vegetable residues and manure, initially in a small share.

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

Practical requirements

  • Shred: shredding increases the surface available for degradation and speeds it up, but does not necessarily raise the final gas yield (FNR 2010, Ch. 3 “Triturado”).
  • Remove foreign contaminants before dosing: material from biowaste may contain interfering substances, and the co-substrate should be free of them (FNR 2010, Ch. 3).

Sources of the process data

  1. Li, P.; Li, W.; Sun, M.; Xu, X.; Zhang, B.; Sun, Y. (2018) Evaluation of Biochemical Methane Potential and Kinetics on the Anaerobic Digestion of Vegetable Crop Residues. Energies 12(1), 26
  2. Shin, K.S.; Kim, C.H.; Lee, S.E.; Yoon, Y.M. (2011) Biochemical Methane Potential of Agricultural Byproduct in Greenhouse Vegetable Crops. Korean Journal of Soil Science and Fertilizer 44(6), 1252–1257
  3. Cai, F.; 1. Biomass Energy and Environmental Engineering Research Center, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China; Yan, H.; Zhang, R.; Liu, G.; Chen, C.; 2. Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, USA (2019) Prediction of methane production performances based on determination of organic components for different vegetable wastes. International Journal of Agricultural and Biological Engineering 12(3), 154–159
  4. Gunaseelan, V. (2004) Biochemical methane potential of fruits and vegetable solid waste feedstocks. Biomass and Bioenergy 26(4), 389–399
  5. Hendriks, A.T.W.M.; Zeeman, G. (2009) Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresource Technology
  6. Weiland, P. (2010) Biogas production: current state and perspectives. Applied Microbiology and Biotechnology
  7. 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

Aubergine (Solanum melongena) is grown in the open field and under cover. After fruiting has finished, whole plants remain: woody stems, leaves and fragments of shoots. The producer has to remove them from the facility, so a one-off, seasonal batch of plant waste arises. Some research studies treat stems and leaves separately, others the whole crop residue, so the compilation contains records for the fractions and for the mixture.

Properties relevant to digestion

Leaves usually have higher digestibility and a greater share of protein than stems, which contain more cellulose and lignin. The moisture content of fresh residues is high, but after wilting and partial drying the dry matter content rises. The material is fibrous and, without size reduction, may form scum or block feeding systems. As a vegetable residue it may contain residues of plant protection products, and in greenhouses also artificial items such as twine, clips and films, which must be removed before feeding into the digester. The C/N ratio is usually higher than in kitchen waste and lower than in straws, so it combines well with nitrogen-rich substrates.

How to read the spread of results

The records concern different fractions: stems separately, leaves separately and a mixture of crop residues, which in itself explains a large part of the spread. In addition, the variety, the growth stage at the time the plants were removed, the method of drying and comminuting the sample and the test temperature differ (in some sources it is not stated directly). The studies come from different regions and cropping systems, so the composition of the plants is also different. With such a small number of records, it must be remembered that they do not cover the whole variability that will be encountered on a specific farm.

In biogas plant practice

Aubergine residues are available once, at the end of the season, so they are not suitable as a regular component of the ration but as a supplement, best after collection from several horticultural holdings. They require size reduction and removal of artificial contaminants. They can be ensiled together with other horticultural residues if the dry matter content is suitable, or fed as they arise. The origin of the waste (a horticultural holding) means that on acceptance at the biogas plant it is necessary to check whether the material falls within the range of substrates permitted under the permit held or the register entry. The energy value is best confirmed by a BMP test on the specific batch.

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 = 0.136 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. Cai, F.; 1. Biomass Energy and Environmental Engineering Research Center, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China; Yan, H.; Zhang, R.; Liu, G.; Chen, C.; 2. Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, USA (2019). Prediction of methane production performances based on determination of organic components for different vegetable wastes. International Journal of Agricultural and Biological Engineering 12(3), 154–159. DOI: 10.25165/j.ijabe.20191203.4705 DOI
  2. Gunaseelan, V. (2004). Biochemical methane potential of fruits and vegetable solid waste feedstocks. Biomass and Bioenergy 26(4), 389–399. DOI: 10.1016/j.biombioe.2003.08.006 DOI
  3. Li, P.; Li, W.; Sun, M.; Xu, X.; Zhang, B.; Sun, Y. (2018). Evaluation of Biochemical Methane Potential and Kinetics on the Anaerobic Digestion of Vegetable Crop Residues. Energies 12(1), 26. DOI: 10.3390/en12010026 DOI

All sources of the site: bibliography.

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