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Wet mills and water use


This is a wet mill. Photo by Kraig Kraft.

This is a wet mill. Photo by Kraig Kraft.

Wet mills.  These are key elements in the coffee landscape.  They are the first step into transforming the cherry into a green bean.  At the heart, these structures can be relatively simple.  They need to receive the cherries, to de-pulp (Some stop here!  We won’t get too much into detail with pulped naturals, naturals or honeyed coffees – we’ll leave that for Paul to explore), ferment and wash.  The set-up of the physical infrastructure depends on the context, the local coffee history, the size of the farm and the distance to market.  In Costa Rica, most of the wet milling is done in central mills, with farmers bringing their cherries to be weighed and processed every day during harvest.  It’s the same in Veracruz, Mexico. However, in most of the Latin American coffeelands, everyone – from estates to small holders – processes their own cherries in their own wet mill.   A wet mill can range from a hand-cranked de-pulper into a plastic barrel or tiled lined tank, to a multi-million-dollar structure with a truck scale to weigh cherries, float tanks to separate impurities and ripe cherries, and multiple lines of de-pulping, fermenting and washing.  Whatever the level of investment, a great majority of wet mills are lot like Frankenstein’s monster.  You start with one piece, add another piece.  A piece gets adapted here, a pipe added here, another piece re-purposed.  You connect the electricity, and… it’s alive!!!  From an engineering or efficiency standpoint, this is the exact wrong way to approach this problem (i.e., quickly and efficiently transforming coffee cherries to green coffee beans). Each piece or step in the mill gets treated like its own entity, with little thought of treating the entire wet mill process as a system, staring with a holistic design that is centered around maximizing the efficient processing of coffee cherries.  Rather, the economic realities of small farmers dictates levels of investment, and therefore, these are almost all built in a piecemeal fashion.

This is a wet mill. Photo by Oscar Leiva for CRS.

This is a wet mill. Photo by Oscar Leiva for CRS.

When it’s done efficiently and well, the wet mill processes coffee cherries with minimal use of water, with systems in place to eliminate ground water contamination. On the quality side, you should not really perceive the milling process in your coffee – unless it’s been processed differently, such as for naturals or honeyed coffees.  Yet, it’s never rarely this simple. Most wet mills use tremendous amounts of water, funneling coffee cherries through inefficient wet mills that end up returning contaminated waste water back into the environment.  Wet mills are an issue that have to be dealt with on an individual basis, yet when you start doing the math and look at the sheer number of wet mills in the Coffeelands, you begin to understand the sheer scale of the problem.  For instance, let’s review some of the numbers of wet mills in some of the countries where CRS works in coffee: In Colombia, there are approximately some 500,000 farmers in Colombia, each with their own wet mill.  There are 40,000 farmers with wet mills in Nicaragua. There are 120,000 farmers in Honduras… you begin get the picture.

This is a wet mill. Photo by Kraig Kraft.

This is a wet mill. Photo by Kraig Kraft.

This past summer, the SCAA published “A Blueprint for Water Security in the Coffeelands.”  In recommendation #3, it addresses wet mills specifically.  The recommendations were to reduce, reuse, recycle water; to treat wastewater and to commit to continuous improvement.  These are the principles, yet for a small farmer it is often difficult to put theory into practice, as this will require making costly investments to change his/her infrastructure.  As the adage goes – in order to manage, you need to measure first.  Our first recommendations to smallholder farmers is to measure their water usage.  How much water does it take to process one bag of green coffee? With this data, we can begin to explore with the farmer where the biggest uses of water are in their process and which is the main source of contamination.

Recently, the Blue Harvest team was in El Salvador, Honduras and Nicaragua with Leonardo Sanchez, a water treatment and wet mill design expert to train technicians on how they could come and “triage” wet mills and look for ways in they can be made more water efficient and more environmentally friendly.

It starts with a process analysis of the wet mills.  For every process, a diagram is created to understand what enters, and what leaves.  Does water enter?  Is it clean water?  Does coffee enter?  In what form?  How does it leave?  How much water and how much coffee? What are the waste products that are produced? This flow diagram helps identify critical points where changes can be made in the future.  In practice, what we encountered was that most wet mills need to start at a much more basic understanding of how much water is actually used during the process. The benchmark of water use efficiency would be to use 1 liter per kg of cherries.  Farmers need to know where they stand against the benchmark before they can identify where the inefficiencies are.  By creating a yardstick, we hope that farmers and the other actors in the value chain, can collaborate to make improvements in mills and to co-investment towards increased water use efficiency and in sensible water treatment systems to ensure the shared stewardship of water resources in the Coffeelands.

1 Comment

  • John says:

    excellent piece, dr. kraft. nice to see theory being put into practice. the issue of on-farm water efficiency is certainly in need of wider attention and resources in these parts.

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