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Strategies for an Inexpensive Wet Mill Makeover

protect water sources wastewater sign

A sign on a farm in Nicaragua reads, “Protect water sources.” Photo by Kraig Kraft.

Makeover: The desire to fix flaws, to transform, to improve, to become the “2.0.” It is a sexy concept, one that feeds tons of industries, catering to our most basic desires for progress.

Yet unlike the kind of makeover you might see on reality television, real transformation doesn’t happen overnight. It is the result of hard work and investment over time.

For the Blue Harvest project in Nicaragua, we recently completed six wet mill makeovers, improving water use efficiency and wastewater treatment just in time for this year’s harvest, while requiring lots of hard work on the part of the farmers involved. Before we get into the details of what was changed, let’s look at why these wet mills needed a makeover.

Why is It a ‘Wet’ Mill

The wet mill is one of the ways in which coffee cherries are transformed into parchment coffee, which is then dried and milled into green coffee (I wrote about wet mills last year, while my colleague Paul Hicks had a great piece on honeys and naturals). While there are many ways to put together a wet mill, the main functions of the wet mill are straightforward:

  1. Receive the coffee cherries
  2. Depulp the cherries
  3. Ferment
  4. Wash
  5. Classify coffee by density.

These are called wet mills because these processes use water, or water is used to move the coffee from one process to another. In a traditional wet mill, water is used to sort the ripe cherries, to help lubricate the depulping machine and to ferry the coffee to the fermentation tank before the coffee is washed with water. Finally, water is used to classify the coffee by density.

The water use in many traditional wet mills is excessive and wasteful. The wastewater that comes from the washing process and the discarded coffee pulp are untreated. This wastewater — which is loaded with organic matter, is extremely acidic, and has high biological oxygen demand — is returned to streams and rivers. With the aid of Leonardo Sanchez, a Costa Rican expert in wet mill wastewater treatment, we designed our ideal small wet mill and wastewater treatment system.

We focused on a couple of key transformative issues:

Reducing Water Use During Depulping and Washing

As I mentioned in a previous post, each wet mill is slightly different, as there are myriad factors that determine its unique design. Farmers have to consider the landscape in which it is constructed; the access to a water source, and how much water is available; the volume of coffee processed; the topography of the land; the distance from the wet mill to a surface water source; and the operational layout.

For our makeover, each wet mill required its own improvement plan based on an assessment of these factors. We started by measuring how much water is used during the traditional processes and made producers aware of this.

The efficiency improvements are relatively simple. We eliminate the use of water in depulping by harvesting and processing only ripe cherries and using gravity to convey the cherries to the depulper. This change also contributes to the improvement in the quality of the coffee. If the wet mill is small, the pulped coffee can fall straight into the fermentation tank. Larger wet mills require moving the pulped coffee into the fermentation tanks. Rather than using water to convey the coffee, we encourage farmers to use a screw conveyor to move the coffee.

A screw conveyor takes coffee into the fermentation tank. Photo by Kraig Kraft.

After fermentation, farmers often wash the coffee up to four times to remove the mucilage and prepare the coffee for drying. Coffee gets placed into a longer canal, and water is added to the coffee, and the mix is agitated until the water runs clear and the coffee separates by density in order for it to be classified. The worst quality coffee is the lightest and runs the farthest through the canal, while the best quality coffee stays in the first parts of the canal.

Tradition has farmers leaving the water on for the duration of the classification. Blue Harvest trains farmers to be more conscientious of their use, using just enough water for the washing and classifying.

Treating Wastewater

The wastewater generated from this process is substantial. Untreated, it harms animal life and is unsuitable for human consumption. Many previous treatment systems have consisted of a series of filtration tanks that are poorly designed, insufficiently sized and do not actually treat the wastewater. The Blue Harvest wastewater treatment system consists of the following:

  1. Separation of solids
  2. Separation of water
  3. Sediment separation
  4. pH balancing
  5. Sediment secondary tank
  6. Bio-garden filter/mini reactors/superficial runoff

The final twist is that we encourage farmers to treat the water from their first two washes, which produce the most contaminated water. When the water starts to clear, there is no need to treat it and it can be safely returned to other water sources. The water from the first two washes is sent through a series of screens to eliminate any solids — pulp, coffee, sticks — that might be in the wastewater.

This water is then sent into a separation tank. While in the separation tank, the wastewater’s pH is balanced through the addition of agricultural lime. The solids are then sent to another holding pond, while the liquid is sent to a final filter, a bio-garden, where edible plants are growing in a mix of volcanic rocks and porous soil. The water that is not used by the plants is then allowed to run off into a surface water system.

From right to left: Washing canal empties into 2 filters, then into a first sedimentation tank. The solids go into the lower lined pit, while the water is pH balanced at square concrete tank with an agricultural lime solution from the blue barrel. The water from here goes into the last filter on the left-hand side, filled with volcanic rocks and soil, where malanga and other root crops are planted. Photo by Kraig Kraft.

The best news of all is that these improvements do not require expensive investments. Blue Harvest has done this by investing $1,500 into some of the materials for the system and the farmers have contributed about $800 of their own cash towards the infrastructure. The farmers have put in the labor needed to build the new canals and the ponds.

The final wastewater product meets all of the Nicaraguan national laws on wastewater treatment. Today, these farmers are now able to quantify the volume of water that is used for processing their coffee and they are proud of the fact they can process coffee using less water and in an environmentally responsible way.


1 Comment

Jeri Idso

I love that the wastewater is channeled through a final bio-filter where farmers can plant and harvest a food crop! Wastewater is changed from a problem to an opportunity to grow a crop. Good design.

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