Kiyohiko Nakasaki*, Tomotaka Tsukada*, and Akihito Ohtaki*

* Department of Materials Science & Chemical Engineering, Faculty of Engineering, Shizuoka University

+ Correspondence should be addressed to Kiyohiko Nakasaki:
(3-5-1 Johoku, Hamamatsu city 432 Japan)

Abstract

The composting rate of coffee grounds was determined by measuring the CO2 evolution rate and the conversion of carbon with the use of a bench-scaled autothermal composting reactor. The C/N ratio of the coffee grounds was 24.2, thus existing around the optimum C/N range for composting known hitherto. It was quantitatively confirmed, however, that the degradation of organic materials during composting proceeded only at a low rate when the coffee grounds were employed alone since the coffee grounds contained water-soluble nitrogen compounds in only an extremely low amount (1.73X10-3g/g-dry coffee grounds), lower than 10% of total nitrogen in the coffee grounds. Water-soluble nitrogen-containing compounds (ammonium sulfate and peptone) were added to the coffee grounds to accelerate the composting. In the case of each additive, the decomposition of the coffee grounds was accelerated, and peptone (i.e., an organic nitrogen-containing compound) was superior to ammonium sulfate, the result being accelerated composting. In addition, it was clarified that the growth of thermophilic bacteria, which largely contributed to the composting at high temperatures, was also promoted when composting was accelerated by adding a nitrogen-containing compound. Furthermore, using waste shellfish obtained from drainage canals in a thermal power plant as nitrogen-containing compounds for composting coffee grounds was also attempted. As a result, it was confirmed that these shellfish were effective in elevating the composting rate of coffee grounds, shortening the composting period to 32days.

Key words: coffee grounds, waste treatment, composting, C/N ratio, microorganisms