Processing wastewater and pulp

Where it is practiced

Wherever coffee goes through the wet process, that is, wherever the cherry is pulped before being dried. Measurement and regulation, for their part, have a much narrower geography.

Colombia supplies most of the published figures. Cenicafé, in Chinchiná, stepped up its work on processing water from 1984. The constraint came from the discharge law, decree 1594 of 1984, to which decree 3930 of 2010 has been added since. Cenicafé cites the two texts together as the framework to be respected. Four tools came out of it, in this order:

• tank washing by successive rinses, called tanque tina, in 1993;
• the Becolsub technology, published in 1997 then in 1999;
• the guide to building and operating the Modular Anaerobic Treatment Systems, or SMTA, in 2010;
• the Ecomill mieles dehydration trials, in 2012.

The anaerobic treatment work that led to the SMTA is older than the guide. Cenicafé published on the subject in 1999, then in a technical bulletin of 2006.

Ethiopia supplies the field measurement of what happens when nothing is treated. Two published campaigns in the Sidama region, one in 2021 on the Orsha river, the other in 2023 on four rivers of the Aroresa district, compare upstream and downstream of the processing sites.

For pulp drunk as an infusion, the European Union authorized placing on the market on January 13, 2022. The text is implementing regulation 2022/47, adopted on notification by two companies, Nestlé then Lavazza. It retains a history of food use in Yemen, Ethiopia and Bolivia.

The dry process produces no honey waters (aguas mieles, in Spanish, literally the waters of honey): the cherry dries whole, and the pulp leaves the bean only at hulling, as dry husk. The European regulation admits both origins: its definition provides that the bean be removed before or after drying.

The principle

Wet processing separates the bean from everything else in the fruit, and the rest is the majority. Per 1 kg of cherry, Cenicafé counts 436 g of fresh pulp and 149 g of mucilage. Mucilage is the layer of sweet flesh stuck to the parchment, the rigid envelope that surrounds the bean. The bean sold represents only a small part of the plant matter produced.

These two materials do not pollute through toxicity. They pollute because they are rich and soluble. Pulp and mucilage contain four families of compounds that go into the water:

• simple sugars;
• pectins, the chains of polygalacturonic acid that form the gel of the mucilage;
• organic acids;
• phenolic compounds.

As soon as they touch water, they dissolve in it. The washing waters thus loaded are the honey waters.

From there, the mechanism is microbial. The yeasts and bacteria present consume these sugars. They produce lactic acid and acetic acid, which makes the pH fall. They also consume the oxygen dissolved in the water.

This is exactly what the trade's two indicators measure. COD, chemical oxygen demand, is the quantity of oxygen needed to chemically oxidize all the organic matter. It counts the biodegradable and the rest. BOD5, biochemical oxygen demand over five days, counts only the fraction that microorganisms are able to degrade. Discharged into a stream, this water therefore empties the medium of its oxygen and acidifies it. Aquatic life disappears, not poisoned but suffocated.

One point governs everything else. The pollutant load per kilogram of coffee is almost fixed, since it depends on the substance of the fruit and not on the practice. The volume of water, on the other hand, is a choice. Diluting removes nothing from the load: it only increases the quantity of water to treat, and it dilutes the effluent to the point of making it unfit for anaerobic digestion. The whole strategy developed by Cenicafé comes down to this sentence.

The sequence

• Pulping. Two practices are opposed. With water and hydraulic transport of the pulp, or dry with transport by gravity, by auger or by cable-disc conveyor. The choice decides almost everything. The pulp carries 73.7% of the potential polluting power of the by-products. Transported in water, it loses nearly half of its dry weight, which passes into the liquid. Pulping dry prevents this share from going to the river and keeps it usable. Cenicafé gives three other reasons to do without water here. More than half of the process water is saved. The volumes of dirty water to store decrease. The pulp stays less wet, so it is easier to put to use.
• Mucilage removal. By natural fermentation then washing, which gives the honey waters, between 15,000 and 30,000 mg/L of COD. Or by mechanical demucilager, with the concentrated mucilage sent back onto the pulp. One then obtains a leachate, the juice that runs off a heap of wet matter, around 110,000 mg/L. This is not more pollution, it is the same matter in much less water.
• Washing and grading. This is where the volume is decided. Traditional correteo channel, the long trough where the coffee travels down with the water to be washed and sorted: 20 to 25 L per kilogram of dry parchment coffee. Tank washing by four successive rinses, the tanque tina: 4.13 L on average, an 80% saving. Becolsub: less than 1 L. Ecomill: 0.4 to 0.5 L. The COD of the washing water rises accordingly from 6,000 to 26,000 mg/L, and this is desired. A concentrated effluent can be digested, a diluted effluent cannot.
• Balance sheet of the processing site. Conventional: 40 to 50 L of water per kilogram of dry parchment, and about 115 g of COD per kilogram of fruit. Ecological: 0.4 to 5 L, and 10.6 to 30 g of COD per kilogram of fruit. The low figure corresponds to the case where the mucilage is poured onto the pulp, and so taken out of the water and not eliminated.
• Storage of the pulp in a covered pit. The roof keeps rain from leaching the pulp and carrying its components to surface waters. Cenicafé puts the water pollution avoided by this practice alone at 72%, when transport is done without water. The chapter makes it the most important preventive action of the whole process.
• Primary treatment of leachates, when pulp and mucilage are mixed. Four units in series:
• a closed pulp conditioner;
• an insolubles control unit, where hydrolysis and acidogenesis take place;
• a sludge drying bed;
• a filter packed with limestone, which raises the alkalinity.

This last unit is the heart of the matter. Methanogenic bacteria, those that finish the digestion by producing methane, work poorly in an acid medium. Reduction of about two thirds of the COD, service life estimated at 5 years.

• Secondary anaerobic treatment, the SMTA. No one purges the air from these tanks. They are closed, the microorganisms consume the oxygen in them, and the medium becomes anoxic through exhaustion. A pulp trap stops particles larger than 5 mm that would clog the pipes. Next come hydrolytic and acidogenic reactors, then a dosing chamber that sets the flow. The flow ends in methanogenic reactors packed with sections of guadua bamboo or bottles. This packing holds the bacteria and keeps them from being carried away by the water. Inoculum: fresh cattle or pig manure diluted in equal parts in water. Three weeks covered, then ten weeks of gradual load increase, at a flow of 550 mL/min per reactor. Reduction above 80% of the organic load. The tanks are black polyethylene and closed, which raises the internal temperature up to 30 °C and limits odors.
• Sizing. The SMTA assumes washing water of between 4.0 and 5.0 L per kilogram of dry parchment. More water requires a bigger system. Less water gives an organic overload, and this is the most frequent cause of malfunction.
• Post-treatment in a constructed wetland. Length-to-width ratio of 2:1 to 4:1, depth 0.6 m, slope 1%, residence time of 2 to 5 days. Floating plants covering 75% of the water surface and harvested every fifteen days, or emergent plants on gravel of 13 to 38 mm, at about 30 plants per square meter. About 30 m² suffice for a farm producing 1,000 arrobas of dry parchment per year, the Colombian coffee arroba being worth 12.5 kg. The overall reduction goes from 80% to close to 95%.
• Evaporation, when treatment is not possible. On a parabolic solar dryer of 27 m², 2,761 L of Ecomill effluent were placed. 2,538 L were evaporated in 26 days, or 92%, about 3.5 L per square meter per day. On Becolsub leachates, an evaporator connected to the exhaust gases of a mechanical coffee dryer leaves a residue at 295,000 mg/L of COD and 35 °Brix. The Brix degree measures dissolved matter, mostly sugars, in grams per 100 g.
• Measured valorizations, in increasing order of technicality:
• composting of the pulp in a pit, turned every fifteen days, transformed in about four months;
• vermicomposting, about one tonne of fresh pulp per square meter per year, at 5 kg of worms per square meter, yield of 35 to 40% on a wet basis;
• edible and medicinal mushrooms on pulp and trunk sawdust. Average yields above 50% for oyster mushrooms, from 52 to 76% for shiitake on a substrate with a carbon-to-nitrogen ratio of 40;
• pectins, 174 g of low-methoxyl pectin per 10 kg of fresh pulp, 82 g of high-methoxyl pectin per 10 kg of fresh mucilage;
• ethanol, 25.2 mL per kilogram of fresh pulp and 58.4 mL per kilogram of fresh mucilage. The by-products of 100 kg of cherry thus give 1.97 L;
• methanization, 25 L of biogas per kilogram of fresh pulp, and 287 L of methane per kilogram of COD applied at 36 °C;
• cascara, the dried pulp made into an infusion, authorized in the European Union since 2022.

What the pulping waters contain, and a complete treatment chain

This sheet describes the pollutant load of processing water through its overall quantities. A whole chapter of a 2025 book gives its qualitative composition and a complete treatment chain, from the receiving basin to the finishing basin. Both are missing here, and the chain is usable as is.

What is in it

After pulping, the wastewater sludge carries microorganisms and a mixture of sometimes toxic substances: nitrate, phosphate, caffeine, phenols, melanoidins, lignins and tannins. Melanoidins are the brown compounds born of the Maillard reaction, that is, the meeting of a sugar and an amino acid, and their presence is a reminder that pulp heats up and browns as soon as it is piled. The medium is acidic, around a pH of four, it has a high biological oxygen demand and it contains suspended solids.

A word on this biological oxygen demand, since this sheet uses chemical demand elsewhere: the first measures the quantity of oxygen that microorganisms would consume to degrade the organic matter of the water, the second measures the same load by total chemical oxidation. The chemical one always gives a higher value, since it also oxidizes what microbes cannot digest. The two cannot be converted into one another by a fixed factor, and a figure given without specifying which of the two it is is worth nothing.

A recent Peruvian study is cited, according to which eighty-five percent of small producers do not treat their wastewater at all. This is the figure that gives the problem its measure: the question is not optimizing installations, it is that there are almost none.

The treatment chain, step by step

The authors give one, borrowed from Enden and Calvert, and its logic is to treat first what is cheapest to remove.

• Acidification basin: It removes the settleable solids, that is, what falls by itself. Removing solids first is what makes everything else possible, since they are what clog the following stages.
• Lime treatment: Applied to the clarified effluents, it raises the pH from four to six. Without this rise, the microorganisms of the next stage do not work: a medium at pH four is not one where a bioreactor flora thrives.
• Anaerobic bioreactor: A fermentation, run this time on purpose, that degrades the dissolved organic matter in the absence of oxygen. The authors usefully recall that a bioreactor is simply a closed vessel fitted with inlets and outlets whose parameters are controlled, and that a plastic fermentation bag is one: the word implies neither stainless steel nor probes.
• Constructed wetlands: Planted basins, in several stages, where plants and the microorganisms of their substrate reduce the phosphate and the remaining organic matter by the aerobic route. They take advantage of the high temperatures of the producing countries, which speed up biodegradation. Vetiver is the plant most cited there, a tufted grass with very dense roots, proven as a filter in several countries, inexpensive and needing no strict maintenance, on condition that the water has first been rid of the organic matter likely to clog the system.
• Water hyacinth basin: Placed last, it treats the residual bacterial load and heavy metals. Water hyacinth is a floating plant of very fast growth, which is both its quality here and its fault elsewhere, since it is a dreaded invasive in tropical waterways: it must not escape from the basin.

The authors finally mention screens to remove pectins and cellulose fibers upstream, which lightens the following stages. And on valorization, they point out that the mucilage is mainly composted, but that successful field trials exist for producing biohydrogen and bioethanol from mucilage and other residues, and that laboratory experiments have produced alcoholic drinks from pulp and mucilage, subject to optimization still to be done.

Sources of this passage: Hackler and Särchen, chapter 7, p. 102 for the composition of effluents and the Peruvian study; pp. 103 and 104 for the treatment chain, after Enden and Calvert; chapter 5, p. 59 for the valorization of mucilage. Glossary for biological oxygen demand and the bioreactor.

What it gives in the cup

This process takes place beside the coffee, not in it. Once the bean is separated, what is done with the water and the pulp no longer changes the cup. None of the works read here measures an effect of effluent treatment on the coffee's sensory score.

A single claim, in these sources, touches the cup. Cenicafé's Avance Técnico 187 credits transport without water with a faster and more homogeneous fermentation. It says so of the café en baba, the bean still covered with its mucilage, and of the pulp. It is written as an expected advantage of the process, with no published sensory trial in support in this document.

What is really drunk here is cascara.

What is measured. A cascara drink produced in Switzerland from dried pulp assays 226 mg/L of caffeine and 283 mg of gallic acid equivalent per liter of polyphenols. The six dried pulps analyzed in the same study give from 4.9 to 9.2 mg of gallic acid equivalent per gram of dry matter. The two most caffeinated, a Maragogype and a Congo Bourbon, reach 6.8 and 6.5 mg of caffeine per gram of dry matter. A 2025 Polish study compares two extracts prepared at 20 mg/mL. Cherry husk gives 53 µg/mL of caffeine, ground arabica bean 450 µg/mL, eight times more.

What is claimed. Four descriptors circulate in the cascara trade:

• red fruits;
• raisin;
• hibiscus;
• rosehip.

No published blind panel was found in this research to support them. Beware also of a widespread confusion. The Polish drink cited above assays 280 mg/L of caffeine. But it is a mix in equal parts of coffee infusion and husk infusion. This value is not that of cascara alone.

What goes wrong

The first risk is direct discharge into a stream, and it is documented by measurement.

In the Wonsho district, Sidama region, in November and December 2019, the raw effluent of a processing site stands at pH 2.38. It carries 4,302 mg/L of COD, 2,410 mg/L of BOD5 and 0.11 mg/L of dissolved oxygen. Upstream of the discharge, the Orsha river is at pH 6.9 and 10 mg/L of dissolved oxygen. Downstream, it falls to between pH 3.56 and 4, and between 0.96 and 1.33 mg/L of oxygen. The second campaign, published in 2023 on four rivers of the Aroresa district, finds the same pattern. At the discharge point, dissolved oxygen is divided by more than two and the pH falls between 3.6 and 4.3.

Note in passing that the COD of these Ethiopian effluents is much lower than that of Colombian washing waters. This is not a better mill, it is more water for the same matter.

The other risks are distributed by step.

• Toxicity of the already treated effluent. Cenicafé measured three thresholds on SMTA outflows, expressed in COD. The concentration that kills half of the individuals is 700 mg/L in the microcrustacean Daphnia pulex. It is 490 mg/L in the fish Lebistes reticulatus, the guppy, which the study gives as the most sensitive indicator. The one that kills half of the cells of the alga Chlorella vulgaris is 890 mg/L. Compliant with the law therefore does not mean without effect.
• Organic overload of the SMTA. Washing with less than 4 L per kilogram of dry parchment concentrates the effluent beyond what the reactor digests. Acids accumulate, the methanogens fail, the reduction collapses.
• Sending a non-pretreated pulp and mucilage leachate to an SMTA. It is presented with 110,000 mg/L when it is designed for washing water at 15,000 or 30,000 mg/L.
• Inhibition of the finishing plants. Water hyacinth fails at 1,281 mg/L of COD, water lettuce at 1,222, salvinia at 903 and cattail at 508. An effluent that is too loaded kills the stage meant to polish it.
• Pulp left in the open air. Rain leaches its components, which changes the soil pH and loads surface waters.
• Poorly dried cascara. This is a risk of mold, hence of mycotoxin. The European regulation sets four limits on dried pulp:
• water, less than 18%;
• water activity, 0.65 at most;
• ochratoxin A, less than 5.0 µg/kg;
• aflatoxin B1, less than 2.0 µg/kg.

Pulp dried too slowly or taken up again by humidity does not pass these thresholds.

• Cascara caffeine underestimated. Above 150 mg/L, the label must carry the mention of high caffeine content and advise against the product for children and for pregnant or breastfeeding women. The drink measured at 226 mg/L shows that the case is not theoretical.

Benchmarks

• Conventional processing water40 to 50 L per kg of dry parchment coffee
• Water with Ecomill0.4 to 0.5 L per kg of dry parchment coffee
• Organic load, conventional versus ecological115 versus 10.6 to 30 g of COD per kg of fruit
• COD of honey waters15,000 to 30,000 mg/L; of the pulp and mucilage leachate: 60,000 to 120,000 mg/L
• Share of potential pollution carried by the pulp73.7%
• Reductionmore than 80% with the SMTA alone, close to 95% with a constructed wetland