Sustainable Utilization of Coffee Processing Wastes through Biogas Technology

Introduction

In the 21st century and beyond, bio-resource based economy is envisaged globally as a strategy towards achieving social, economic and environmental sustainability in addressing climate change.

The purpose of this project is to demonstrate innovative technologies with a potential to utilize agro-industrial wastes currently seen as low value materials as bio-resource to produce value added products at the same time reducing environmental pollution burden.

Coffee Processing Waste

In the Eastern Africa’s economy, agriculture is the mainstay and currently accounts for about 80% of the rural incomes, and coffee is among the important cash crops. EA is currently producing about 600,000 tons of green coffee beans, of which Ethiopia share is 400,000 tons annually. Crop production and processing activities are generating huge quantities of organic waste particularly to the growers’ environment.

Coffee processing done by wet and dry methods discard away 99% of the biomass generated by the coffee plants at different stages from harvesting to consumption. This includes cherry wastes, coffee parchment husks, sliver skin, coffee spent grounds, coffee leaves, and wastewater. Wet processing uses up to 15 m3 of water to produce one ton of clean beans and for every ton of beans produced, about one ton of husks are generated. It is estimated that coffee processing is generating about 9 million m3 of wastewater, and 600,000 tons of husks annually in the EA region.

Agro-industrial wastes generated in EA are mostly underutilized, untreated and thus in most cases disposed of by burning, dumping or by unplanned landfilling. These practices are wastage of bio-resources apart from contributing to emission of greenhouse gases (GHG) leading to climate change.

Coffee waste contains high amounts of organic substrates including carbohydrates proteins, pectins, fibres and fat for bioconversion into value added bio-products.

One unit weight of ripe fresh coffee cherry yields 16.5% (on average) of its weight as clean green coffee beans, while a unit weight of dry cherry (dry processed 12% moisture content) yields 44%.

The biogas yield potential of different coffee processing wastes (the pulp, husk and mucilage) and the spent husk (after mushroom cultivated) is experimented in AAiT lab with bench and pilot scale (single and two stages) digesters.

From bench scale batch experimentation done in a controlled mesophilic temperature range (at about 37oC) it was revealed that 831m3, 465m3 and 184m3 of biogas can be recovered from a ton of organic dry matter of coffee pulp, husk and spent respectively. The average methane content of the biogas produced was about 51%, 58% and 41% for the pulp, husk and spent wastes.

Biogas manure (BgM) is a byproduct obtained from biogas plants after anaerobic digestion of organic matter. BgM is rich in organic matter, supplies essential nutrients, trace elements and other active substances. It can sustainably substitute artificial fertilizers. It is rich in humic acid that enhances water holding capacity, soil aeration, accelerates root growth and inhibits growth of weed seeds.