BMP testingBy-products of the agri-food industry

Walnut residues (shell, skin, green husk) — BMP: 3 studies, weighted mean 148 m³ CH₄/t VS

Walnut waste (shell, skin, green husk) comprises three fractions from several studies; the green husk is wet and rich in juglone, and the shell strongly woody.

rating: neutral CH₄ ≈ 52 % (estimate)

Published 31 August 2023Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Ivar Leidus / Wikimedia Commons, licence CC BY-SA 4.0, source file; changes: cropping

Weighted mean: 148m³ CH₄/t VS
Range: 46–282m³ CH₄/t VS
Records: 3studies
Publications: 2sources
Temperatures: 37 °Cof the test

Methane in biogas

≈51–53% CH₄

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 (protein from total nitrogen (N 0.9 % 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: moderate, but limited by the poor degradability of the organic matter (123 m³ CH₄/t); process risk: inhibitors. Handling costs: fibres, floating layer, slow degradation.

Why? Rating factors

Potential per tonne +1

+1potential 123 m³ CH₄/t fresh matter (BMP 148 × DM 90.6 % × VS 91.9 %); low degradability of VS (BMP 148 m³ CH₄/t VS) — points capped

Process stability −1

−1inhibitors: natural or residues (e.g. limonene, polyphenols, antibiotics, metals)

Handling and costs −1

−0.5fibres, floating layer — shredding and intensive mixing
−0.5slow degradation — long retention time

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 3 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 (148); 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
Czubaszek et al. (2022) DOI	131	—	Walnut husks	end point (model curve)
Shen et al. (2018) · 1 DOI	46	37 °C · mesophilic	walnut shell · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)	end point (model curve)
Shen et al. (2018) · 2 DOI	282	37 °C · mesophilic	walnut skin · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)	end point (model curve)
Weighted mean	148	arithmetic mean 153, SD 119

Typical composition

Dry matter (DM): 90.6% (range 90.1–91)
Volatile solids (VS): 91.9 % DM (range 84.6–99.2)
Nitrogen: 0.9 % DM
C/N ratio: 74.3

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 2 determinations, volatile solids: 2 determinations. C/N was calculated as the ratio of C and N content (% DM) from the same source. Samples were tested dried/air-dry – the DM refers to the material in the test, not to fresh material.

Sources: Shen et al. (2018).

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
slowly degrading
low in nitrogen
carbon-rich
high dry matter
contains inhibitors
seasonal
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 potential in the Substraty.pl BMP dataset: weighted mean 147.6 m³ CH₄/t VS (SD 119.2; 3 records, 2 sources).
Walnut shell and husk: a dry, durable material, no risk of acidification - does not burden the VFA system (Hendriks and Zeeman 2009).

Drawbacks and risks

The green husk contains juglone with antibacterial activity (Chen et al. 2008, as an inhibiting phenolic compound).

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

combines well
substrates: high alkalinity (buffer) — e.g. Cattle manure, Pig slurry, Cattle slurry, Duck slurry, Rumen and digestive tract content
Slurry supplies N and alkalinity, balancing the high C/N of the material.^[1][2]
combines well
substrates: protein-rich — e.g. Food waste, Municipal sewage sludge (mixed), Poultry manure, Organic fraction of municipal solid waste, Grass clippings
The high C/N of the material balances protein substrates, limiting the ammonia risk.^[1][3]

Dosing

No source limits on the dose for this substrate.

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”).
Control floating crust and sediment: when solids are fed directly into the digester, the risk of their formation depends on the substrate (FNR 2010, Ch. 3).

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

The walnut (Juglans regia) grows in Poland in gardens and orchards, and worldwide it is grown on a large scale in China, the USA and Iran, among others. The fruit has a green, fleshy covering (the outer husk), a hard woody shell and a thin skin covering the kernel. The dataset includes three fractions from several studies: the green husk, the shell and the skin. The green husk is a seasonal waste, arising at harvest in autumn.

Properties relevant to digestion

The green husk contains a lot of water and phenolic compounds, including juglone, which has antibacterial activity and can inhibit microorganisms in the system. The kernel skin is rich in polyphenols and dietary fibre. The shell is a woody material with a high lignin content, resistant to decomposition. For these reasons the fractions behave differently: the green husk decomposes fastest, but may inhibit, and the shell very slowly. The material is poor in nitrogen.

How to read the spread of results

The records describe three different fractions from several studies, so the spread has its source in differences of composition. Laboratory differences are hard to separate, because each fraction is described in a different study. The variety, the harvest date (husk fresh or dried), the share of shell and skin, comminution and, in the case of the husk, the juglone concentration and adaptation of the inoculum all have an effect.

In biogas plant practice

Walnut green husk is sometimes available seasonally in orchards and farms, but rarely in industrial quantities. If it goes into the digester, it should be dosed carefully and microbial activity observed, while the shell should be shredded or directed to other uses (fuel, bedding). Before processing larger batches, a BMP test should be carried out and it should be checked that the phenolic compounds do not reduce the yield.

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

Bibliography

All sources of the site: bibliography.