Water entered coffee processing when Arabicas began being planted in areas that were hotter and/or more humid than the highlands of Ethiopia. Recently harvested coffee cherries fermented rather quickly in these new areas, with adverse impacts on cup quality. The solution at the time was to remove the sugar-rich, fermentation-prone pulp and mucilage in what was to be called ‘wet milling,’ thus giving birth to washed coffee in opposition to the dry system that produced naturals at the birth place of Arabica coffee. It was also found that coffee processed by the two methods had cup features that were different, with body and sweetness stronger in natural coffees as opposed to the aroma and acidity in washed coffees.
The processing of natural Arabicas require little or no water at all, and where water is used – in Brazil, for example, for flotation – it can be recycled for several days and the degree of contamination is very low, if any.
For well over a century, the wet milling of coffee was a water-intensive process, as the name indicates, and there was little concern for water consumption and contamination. In the conventional technology then prevailing and still used in many areas today, water is used in flotation, to separate over-ripe and dry cherries from the ripe ones, in pulping, to remove the pulp, in mucilage removal, carried out by natural fermentation or friction in machines, and in the transport of coffee and by-products (e.g., pulp). The growing concern for the environment in the last quarter of the last century led to questioning the use and contamination of so much water – often 10,000 m³ per ton of green coffee – in the wet processing method. The pollution load in the wastewater from the wet milling of coffee can be 30 to 40 times greater than the one found in urban sewage!
The technological reactions to the environmental concerns came in different ways. The first and most obvious reactions were to recycle waste water and to avoid the transport of coffee with the help of water in channels and pipes; mechanical means – conveyors and elevators – should be used instead. Next came the drastic reduction and even the elimination of water in pulping; since the pulp is rich in water, it was found that the friction or tearing process that removes the pulp from around the coffee bean could take place with the addition of little or no water. The last frontier of water reduction and/or elimination remains in the removal of the mucilage that sticks to the parchment skin that involves the coffee bean. The original technique of breaking down the mucilage layer by means of natural fermentation in tanks with or without water followed by washing was progressively combined with, and even replaced by, the removal of mucilage by friction in several types of machines that were also water intensive in the past, but are now quite water efficient.
Whereas there is little debate today about the impacts on coffee quality of water recycling and the reduction of its use in transport, floating, and pulping – even though bean damage may increase with dry pulping – the debate is still far from settled in what regards water conservation in mucilage removal, which is the wet processing step that today consumes and contaminates the most water by far. There are expert cuppers who insist that fermentation is the only way to develop specific cup features, (e.g. acidity), especially in high-grown coffee. They add that mechanical mucilage removal has yet to produce the same results in these cases, although the cup differences may subsidize and even disappear as altitude drops. To complicate matters further, there are studies that show that given the difficulties to control the natural process of fermentation, mechanical removal may, on average, produce better quality coffee. Yet other studies show that natural fermentation for a few hours followed by mechanical removal, which causes water consumption to fall substantially, may yield the same cup quality in most altitudes. It seems, however, unavoidable that mainstream, high quality, and even specialty washed coffee processing will progressively resort in full or partial mechanical removal of mucilage with natural fermentation remaining the option for less and less coffee, whose price will have to eventually reflect the cost of treating the wastewater produced in the process.
Even though the trend is for water consumption and contamination to fall progressively in years to come, albeit at a faster pace, the need remains to address the treatment and disposal of wastewater produced by the wet milling of coffee. A basic tenet of wastewater processing is that it is cheaper to treat a small volume of wastewater that is highly contaminated than a large volume with little contamination. Therefore wastewater recycling and reuse should deserve the highest priority in any wet mill. Other than in the wet mill itself, reuse can take the form of irrigation of coffee or other crops planted nearby. Volumes, degree of contamination and ground-water level conditions allowing, infiltration into the soil may be the easiest form of disposal, but it finds little practical application, mostly for very small growers.
Technology exists to treat the wastewater from coffee processing in order to discharge it into water bodies in an environmentally sound way. However, the barrier is the cost of such treatment, which has triggered research on ways to do it in a more cost-efficient manner for small growers and millers who constitute the majority around the world. Space allowing, the most cost-efficient treatment system is ponds, anaerobic and aerobic. Because space is usually scarce in the case of small growers, research is focusing on biodigestors and low-cost systems to build them, with very promising results that produce bio-gas for cooking and even coffee drying.
The speed of conversion of wet mills into ecological water conserving systems depends primarily on the strictness of local legislation and the efficiency of the monitoring of its implementation in coffee producing countries. Sustainability standards, e.g. Utz Certified and Rainforest Alliance, also play an important role, as do coffee importers who demand sustainable coffees that must be produced with concern for the environment, water conservation included. Equipment suppliers contribute to the process by researching and offering ecological systems that consume and contaminate less water. Finally, coffee quality experts and cuppers may also help the conversion process to the extent that they properly understand how water can be conserved with little or no impact on coffee quality, and advise growers accordingly. An important barrier to conversion is the message that growers often receive that they should stick to conventional, traditional wet processing techniques in order to preserve coffee quality. It is seldom the case, if ever, that this message is applicable bearing in mind all the recent developments to optimize the use of water in coffee processing.
Carlos Brando is a partner at P&A, a consulting and trading company with coffee projects around the world, in areas as diverse as marketing, strategy, technology, and sustainability. He is now sharing his 30 years of coffee experience in consulting, presentations, and the boards of Utz Certified, CQI, Ipanema Coffees, and the Santos Coffee Museum.