THE PROCESS IN MOTION

What the effluent does to a river

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01The station returns a loaded water02Lagoons that are too short03The river tips over04Life on the bottom follows the chemistry

The station returns a loaded water

A washing station returns water loaded with what it has dissolved: sugars, pectins, fermentation acids, caffeine and tannins. In Sidama, Ethiopia, two stations measured in 2019 used 15,000 and 20,000 liters a day.

Read the stages without animation

1. The station returns a loaded waterA washing station returns water loaded with what it has dissolved: sugars, pectins, fermentation acids, caffeine and tannins. In Sidama, Ethiopia, two stations measured in 2019 used 15,000 and 20,000 liters a day.
2. Lagoons that are too shortThe stations of the Gidabo basin treat their effluents in lagoons, generally two basins a meter and a half deep. The water stays there only two days on average, too little to break down the load. A constructed wetland, at Wonsho, removed only 16 to 20% of the load.
3. The river tips overAt the discharge point, the Orsha river goes from a pH of 6.9 to 3, frankly acidic water, and its dissolved oxygen falls from 10 to 0.6 milligrams per liter: the microbes in the effluent consume it to digest the organic matter.
4. Life on the bottom follows the chemistryThe benthic invertebrates, the small animals that live on the riverbed, go from 296 individuals upstream to 40 downstream. The sensitive mayflies and caddisflies give way to chironomids, which tolerate this water. These rivers serve the riverside communities.

To remember: Without sufficient treatment, the effluent kills the river not by poison but by suffocation, and the life on the bottom bears the trace.

Read the full article and its sources ↓

Under the pulp, the mucilage: about 22 kg per 100 kg of cherries, a layer of pectins and sugars that the washed process dissolves by fermentation then rinses with plenty of water, that the dry process keeps and that the honey processes keep in part (see the processing). What is drawn from it is technical: pectin, beverage concentrate (the ICO cites NAOX in Costa Rica), foliar fertilizer, natural preservative.

But most stations send it to the water, and that is where the invisible by-product becomes a problem. The washing effluent, which the trade sometimes calls honey water, is loaded with sugars, pectins, fermentation acids, caffeine and tannins; the review by Rattan et al. (2015) gives a biochemical oxygen demand (BOD) up to 20,000 mg/l, a chemical oxygen demand (COD) up to 50,000 mg/l and a pH below 4. In Colombia, the COD of the washing water reaches 24,000 to 30,000 mg/l, and the effluent is judged 60 to 240 times more polluting than domestic wastewater.

ICO 2024, p. 57-61; Rattan et al., Environmental Science and Pollution Research 22 (2015), p. 6461-6472; González-Freire & Martínez-Hernández, Revista Ciencias Técnicas Agropecuarias 31-2 (2022)

Water consumption of conventional washing is of the order of 40 l per kilo of dry parchment coffee in Colombia. The Ethiopian mills of the Gidabo basin, measured in 2022, used about 63 l per kilo of green coffee. The theoretical minimum is however 22 l, provided the water is recycled, and 90% of these mills lacked recycling.

What these volumes do to a river has been measured. In the Wonsho district (Sidama, Ethiopia), in November-December 2019, two stations used 15,000 and 20,000 l a day.

Their raw discharges showed a BOD of 2,410 to 3,770 mg/l and a COD of about 4,100 to 4,300 mg/l. The pH fell between 2.4 and 2.7, so frankly acidic water, and the dissolved oxygen was almost nil. The Ethiopian discharge limits are 80 mg/l for BOD, 250 mg/l for COD and a pH between 6 and 9: the effluent came out far beyond them.

Washing water

González-Freire & Martínez-Hernández 2022; Genanaw et al. 2021; Ulsido et al. 2024; Rattan et al. 2015

The Orsha river records the difference between upstream and the discharge point. Its pH goes from 6.9 to 3. Its dissolved oxygen falls from 10 mg/l to 0.6. Its BOD rises from 28 mg/l to 2,090.

Life on the bottom follows the chemistry, because a river without oxygen no longer feeds much. The benthic invertebrates, the small animals that live on the riverbed, fall from 296 individuals upstream to 40 downstream. Mayflies and caddisflies, sensitive insects, give way to chironomids, which tolerate this water.

One of the two stations did, however, have a constructed wetland, supposed to filter the effluent before the river. It removed only 16.4% of the BOD and 20.1% of the COD. The equipment existed; it was not enough.

Genanaw et al., Environmental Health Insights 15 (2021), PMC8842449; Ulsido et al., Environmental Health Insights 18 (2024), PMC11181887

At the scale of a basin, the Gidabo basin (southern Ethiopia) has more than 285 washing stations concentrated along the river and its tributaries. During the 2022 processing season (September-November), the BOD of the watercourses was 254 to 600 mg/l and the COD 1,100 to 3,830 mg/l, dissolved oxygen close to zero, turbidity up to 516 NTU; the water quality index rated all sites “bad” except one upstream tributary.

The stations treated their effluents in lagoons, generally two basins 1.5 m deep with an average residence time of 1.99 days, insufficient to break down the load. A neighboring study (Aroresa, 2023) likewise finds that most effluent and downstream samples clearly exceed the limits of the Ethiopian environment agency. These rivers serve the riverside communities.

Ulsido et al. 2024, PMC11181887; Amare et al., 2023, PMC10068982