Biodigester for Coffee Farmers

New Technology Using Bacteria from Dung Helps Coffee Farmers to Purify Water

Coffeegrower Sebastián Mairena admits to being sceptical the first time he heard about the water treatment programme proposed by UTZ, a Dutch certification body. "I thought, ‘I'll believe it when I see it'," he says.

Today, Mairena has all the proof he needs on his farm in Nicaragua's Matagalpa region. It comes in the form of a $20,000 (£12,716) anaerobic biodigester located on the site of an old wet-mill.

The technology, part funded by the Dutch government's Global Sustainable Biomass Fund and development charity Hivos, works by transferring wastewater from the pulping process into a 20m3 collection tank. A mesh gauze filters out the largest pieces of solid waste. "The collection tank is where hydrolysis begins, resulting in the initial decomposition of complex organic and inorganic substances," explains technical advisor Leonardo Sánchez, director of Costa Rican environmental consultancy Aceres.

The wastewater is then transferred to the main biodigester, which contains a thick layer of animal manure. Bacteria produced by the dung gradually decompose any remaining organic matter. "The bacteria feed on the contamination found in the coffee wastewater which they eat and transform into a product they then excrete. Through this process of transformation, the bacteria takes the energy it needs to live, move and multiply," says Sánchez.

After 12 hours, the treated effluent is deposited in a compensation tank and then into the coffee co-operative's existing lagoons. "We are just entering our second harvest using the biodigester, but in the first year our calculations show that the system reduced the levels of contamination by 81.3%", says Marvin Mairena, Sebastián's younger brother and the co-operative's agronomist.

The results far outstrip the organisation's previous attempts to purify the water, which revolved around a drainage filter using sand and volcanic rock. The UTZ pilot programme at the Mairena farm also includes the recirculation of water during the pulping process. "We used to use around 1,500 litres of water per quintal (46 kilograms) of pre-pulped coffee. Now we only use between 240 and 250 litres," says Marvin.

Other benefits include the production of biogas. After around six days, the anaerobic process begins to produce regular amounts of methane gas, averaging around 50 cubic metres per day. The gas is used for cooking food in the farm's communal kitchen, reducing the farm's use of firewood. Another useful byproduct is nutrient rich waste from the bioreactor, which can be used as a natural fertiliser.

A similar, but much larger, pilot biodigester at the milling plant of Nicaragua's largest coffee exporter CISA is capable of generating up to 650m3 of biogas per day. The biogas is used to power the company's wet-mill, generating energy savings worth around £35 per hour.

Coffee contamination

The pilot project responds to the problem of contaminated water discharge that is rife across Latin America's huge coffee industry. Another 18 trials are underway in Guatemala and Honduras, as well as at other Nicaraguan farms.

"The process of separating the beans from coffee cherries generates enormous volumes of waste material in the form of pulp, residual water and parchment," says Vera Espindola, field development coordinator for Latin American at UTZ. Not only is the pulping process water intensive - a 2013 study on Nicaragua's coffee industry estimates that processing one kilo of coffee beans requires as much water as a family of six uses in a single day - but it is highly contaminating too.

"Wastewater from the pulping process has a really high content of organic matter and acidity," says Espindola. "When it's discharged directly into waterways, it can kill aquatic plants and wildlife by depriving them of essential oxygen."

The Chemical Oxygen Demand (COD, a measure of organic matter in water) of wastewater from the coffee pulping process averages between 18,000-30,000 milligrams per litre, she notes. By way of comparison, the EU guidance for industrial discharges say "standard provisions" COD levels should not exceed 125mg/l.

Source: The Guardian

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• Sludge Separation
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