Solid Waste from the Instant Coffee Industry as a Substrate for Anaerobic Thermophilic Digestion

Abstract

In the process of producing “instant coffee”, large quantities of relatively solid waste (20%) are left, causing ecological harm to the area by polluting ground water, and which therefore have to be carted from the factory and dumped, at considerable expense. Several solutions and alternative uses of the coffee wastes have already been examined (as fertilizers, livestock feed, compost) without giving economically viable results. The aim of this research was to develop biogas technology for the treatment of coffee wastes and the evaluation of the digested material as a growth medium for horticulture. The study included anaerobic, thermophilic, methanogenic digestion of solid coffee wastes in laboratory scale digesters. Optimal conditions for the process in loading rates, retention time, solids concentration and chemical parameters were examined. The results of these experiments showed that digestion of instant coffee waste is a feasible process, not requiring the expected addition of nitrogen, nor prior grinding of the coffee waste, though pH control was necessary. The continuous anaerobic digestion process can achieve a steady state of fermentation at loading rates up to 4.7 g VS/l/d. The overall qualities of the digested slurry were determined, with a view to their suitability for horticulture. It was found that there is a clear similarity in both root and plant growth using peat-moss or digested coffee slurry, after thermophilic digestion, as soil growth media for growing Gypsophila. Growth promotion effects on Phlox plants were found, as well as a positive effect on the growth-rate of rootlets sprouting in Lysimachiafontuni.