BMP in Ethiopia: coffee husks, pulp and mucilage and water hyacinth as biogas substrates

Residues from coffee processing in Ethiopia have a varied methane potential: in the 2018 study the average yield ranged from about 31 NmL CH₄/g VS for parchment to about 295 for mucilage, and pulp reached about 245. The results show that the fractions differ significantly from one another, so suitability is decided by which part of the residues is available.

What was studied

Chala and co-authors published in the journal Sustainability (2018) the results of BMP tests on residues from primary coffee processing: husks (produced by the dry method) and pulp, parchment and mucilage (by the wet method). The tests were carried out at 37 °C, i.e. under mesophilic conditions. The average methane yields were 159.4 ± 1.8 (husks), 244.7 ± 6.4 (pulp), 31.1 ± 2.0 (parchment) and 294.5 ± 9.6 (mucilage) L/kg VS. The differences reflect composition: mucilage had the least hemicellulose (0.8%) and lignin (5%), and parchment the most lignin (32%) and neutral detergent fibre (96%), and therefore decomposes very poorly; the authors found it unsuitable for digestion. They also estimated that, nationwide in Ethiopia, digestion of husks, pulp and mucilage could yield as much as 68 million m³ of methane a year, which converted to cogeneration corresponds to 238 thousand MWh of electricity and 273 thousand MWh of heat. This is a theoretical potential, not a production forecast.

How to read such results

Laboratory BMP values do not transfer directly to a biogas plant. The result is affected by the variety, the processing method (wet or dry), the storage time of the residues, particle size and the inoculum. We discuss these principles in the articles Scatter of BMP results between laboratories and BMP versus full-scale yield. In addition, waste from wet coffee processing is acidic and may contain phenolic compounds as well as caffeine and tannins, which at higher concentrations inhibit digestion, so in practice it is used in mixtures (co-digestion), not as the sole feedstock.

Water hyacinth: a second Ethiopian and East African feedstock

Water hyacinth is an invasive aquatic plant that overgrows lakes in East Africa. As biomass it is attractive because it "grows" for free, but it has high moisture and a lot of fibre. Keche and colleagues (RSC Advances, 2022) studied hyacinth from Lake Abaya in Ethiopia; according to the abstract, pretreatment with urea increased the biomethane potential and improved the quality of the digestate as a biofertiliser. Oduor et al. (Heliyon, 2022) studied in Kenya the co-digestion of food waste (in that country food waste is, according to the authors, 57% of municipal waste) with hyacinth under mesophilic conditions (37 °C), in order to mitigate the acidification typical of digesting food waste alone.

Other tropical wastes from the region

A conference abstract from Tropentag 2017 (Awiszus, Latif, Müller) assessed cassava peels from Tanzania and cassava pulp: the methane yield was about 0.225 m³N per kg of dry matter (DM) for peels and 0.224 for pulp. The authors pointed out that the content of micronutrients is low and that for a stable process they need to be supplemented, e.g. by mixing with manure. This is only an abstract, not a full peer-reviewed paper, so we treat the values as indicative.

Practical conclusions

1. Residues from coffee processing have significant potential, but they work best in a mixture with a buffering substrate (e.g. manure).
2. Every biogas project based on such a feedstock requires its own BMP tests on samples from the plant, not values from the literature.
3. Biomass with high moisture (hyacinth) has to be dewatered or partly dried, and the cost of harvesting and transport may outweigh the energy gain.
4. Results should be presented with the unit (NmL CH₄/g VS) and a description of the test conditions, so that they can be compared. Broader context: What is BMP.

Analogies for Poland

In Poland, a role similar to that of coffee processing waste in Ethiopia is played by residues from fruit processing: apple pomace, pulp or stillage. Their characteristics are seasonality (produced in short campaigns), acidic pH and rapid biodegradation. Operating practice is similar: storage or ensiling, gradual dosing and control of pH and FOS/TAC.

A note on comparability

Results from different publications can be compared only when the temperature, the duration of the test, the inoculum-to-substrate ratio and the method of conversion to standard conditions are given. Therefore, when using literature data it is always worth going back to the original article and checking these parameters.