Horse manure a great addition when composting spent coffee grounds – study

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Humans drink a lot of coffee, and it follows that vast quantities of spent coffee grounds are generated.

What to do with this waste product can be a headache – and we’re not just talking about the spent coffee grounds that might be available from local cafes.

Factories processing coffee can produce vast quantities.

Research has shown that composting the grounds can create a product that can partially replace commercial peat and fertilizers in the production of potted plants.

Other well-known applications for spent coffee grounds include fuel in the form of pellets or briquettes, energy production and biofuels. Often, however, they simply go to landfill.

Miguel González-Moreno and his colleagues in Spain set out to learn more about using worms to compost spent coffee grounds and silverskin. Silverskin, also known as coffee chaff, is a thin covering over the coffee seed. During roasting, coffee beans expand and this thin layer is detached, becoming the main by-product of coffee-roasting industries.

The study team set up a laboratory experiment in which they set about composting spent grounds and silverskin in the presence of worms, with the addition of one important ingredient: mature horse manure.

The coffee byproducts used in the experiment were collected from an instant coffee factory and a coffee roasting factory.

In the 60-day study, each coffee byproduct was composted with different proportions of horse manure (25%, 50% and 75%). The researchers also composted the grounds and silverskin with worms, but without any horse manure.

At the end of the composting period, the researchers assessed the quality of the end product, based on its ability to germinate hybrid wheat seeds. They also checked on the mortality of worms, their growth rate, and whether the creatures were successfully reproducing in the various mixtures.

Results varied, depending on whether grounds or silverskin was used, and the ratio of horse manure used.

“Best options were those treatments with a medium-low amount of residue; 25% for spent coffee grounds, and 25% or 50% for coffee silverskins,” the authors reported in the journal Agronomy.

Regardless of the mixture, a lack of toxicity was confirmed after the composting for all treatments, based on the results of the seed germination test.

However, the results when no horse manure was used were poor.

Overall, earthworm mortality was higher in treatments that contained higher coffee waste proportions. Indeed, all the worms died in the attempt to compost silverskin without manure.

“The study reveals that coffee industrial byproducts can be decomposed, obtaining a new value-added material in the form of compost,” they said.

The key is not to have too high a ratio of the coffee byproducts with the manure because of possible toxicity of the grounds and silverskins. Grounds, they noted, had an extremely low pH and silverskin waste had a high ammonia content.

Silverskin might benefit from pre-composting before the use of worms; and the addition of alkaline residues in order to increase the pH level in grounds might prove beneficial, they said.

“Broadly, more trials need to be conducted in the future.”

The study team is variously affiliated with Public University of Navarre in Pamplona and the University of Basque in Vitoria-Gasteiz.

Feasibility of Vermicomposting of Spent Coffee Grounds and Silverskin from Coffee Industries: A Laboratory Study
M.A. González-Moreno, García Gracianteparaluceta, S. Marcelino Sádaba, J. Zaratiegui Urdin, E. Robles Domínguez, M.A. Pérez Ezcurdia and A. Seco Meneses.
Agronomy 2020, 10(8), 1125;

The study, published under a Creative Commons License, can be read here.

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