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.

Anaerobic methanogenic thermophilic digestion (AMTD) of the solid fraction of waste from an instant coffee factory was carried out in laboratory scale digesters. Optimal conditions were established for stable fermentation by controlling loading rates, pH, volatile fatty acid concentrations, biogas production and composition. The results of the experiments indicated that steady digestion can be maintained at organic loading rates up to 8.6 g VS L -1 d -1 , at a hydraulic retention time of 20 days, with pH at 7.5-8.2. Biogas production (62% methane) reached 1.8 L L -1 d -1 . The residual slurry of the digestion process was examined as a growth medium in horticulture. Trials of fermented solids, mixed with sphagnum peat-moss in proportions of 1:1 or 3:1 and used as growth media for ornamental plants (Gypsophila, Lysimachia and Phlox), showed a promotional effect on growth in the root systems.

A large amount of solid waste remains after the production of instant coffee. This waste has to be moved to dumps, where it poses a threat of environmental pollution. Treatment of this waste by anaerobic methanogenic thermophilic digestion produced, besides biogas, a digested slurry which was used as a growth medium for horticulture, and proved to be a suitable and economical substitute for peat moss. Biological tests with mung bean cuttings and Grevillea plantlets showed promotional effects on rooting of the slurry and its sieved fraction extract, washed with water (Capul). Green coffee beans, instant coffee waste, its anaerobically-digested slurry and Capul were extracted by various methods and the extracts were analyzed by TLC, HPLC and GUMS. Examinations showed clearly the presence of IAA and IBA in free and bound forms in all the substrates. The values of free and bound IAA were calculated by use of an internal standard and GUMS. The amount of conjugated IAA was found to be much higher than that of free IAA, in both the coffee beans and instant coffee waste (11.1 vs 2.7 nmol g-i, respectively). In the digested slurry and Capul, however, most of the IAA was present as the free form and was approximately 23533.0 nmol g-i , which is almost ten times more than in the waste, and almost twice the total amount of IAA in coffee beans. It is postulated that the high levels of free IAA in the digested instant coffee waste are a result of catabolism of tryptophan by anaerobic bacteria.