Coffee wastewater

Processing techniques

The unpicked fruit of the coffee tree, known as the coffee cherry, must undergo a long process to make it ready for consumption. This process often entails the usage of massive amounts of water and the production of considerable amounts of both solid and liquid waste. To determine the type of waste stemming from coffee processing, it is important to know how the coffee cherries are processed. The conversion of the cherry to oro or green bean (the dried coffee bean which is ready to be exported) can be achieved through three different processing techniques: dry, semi-washed, and fully washed.

Dry processing

The coffee cherries are dried immediately after they are harvested through sun drying, solar drying or artificial drying. In sun drying, the coffee cherries are placed on a clean floor and left to dry in the open air. In solar drying, the cherries are placed in a closed cabinet, which has ventilation holes to let moisture out. Artificial drying is used mostly during the wet season, when the low level of sunlight extends the time needed for solar drying and the cherries are prone to mould growth. After being dried, the cherries are hulled. In this process the dried outer layer of the cherry, known as the pericarp, is removed mechanically.

Semi-washed processing

In semi-washed processing, the cherries are de-pulped to remove the pericarp. After this, the removal of the slimy mucilage layer which covers the bean takes place. This is done mechanically by feeding the beans into a cylindrical device which conveys them upward. While the friction and pressure exerted on the beans by this process is enough to remove most of the mucilage, a small amount of it will still remain in the centre cut of the beans. This technique is used in Colombia and Mexico in order to reduce the water consumption from the long fermentation process and the extensive washing. While semi-washed processing requires less time than washed processing and is thus economically advantageous, the quality of the end product is regarded as inferior.

Fully washed processing

This process is mainly used when processing Coffea arabica After de-pulping, the beans are collected in fermentation tanks where bacterial removal of the mucilage takes place over 12 to 36 hours (Von Enden, 2002). The fermentation phase is important in the development of the flavour of the coffee, which is partially due to the microbiological processes that take place. The emergence of yeasts and moulds in acidic water can lead to off-flavours like sour coffee and onion-flavour. However, wet processing is believed to yield higher quality coffee than the other processes since small amounts of off-flavours give the coffee its particular taste and "body" (Calvert, 1998).

When the fermentation is complete, the beans should be washed thoroughly to remove fermentation residues and any remaining mucilage. If they are not removed, these cause decolouring of the parchment and make the beans susceptible to yeasts. After washing, the beans are dried. When the drying process is not rapid enough earthy and musty taints, like Rio-flavour come up (Calvert, 1999).

Water usage

The amount of water used in processing depends strongly on the type of processing. Wet fully washed processing of the coffee cherries requires the most fresh water, dry processing the least. Sources indicate a wide range in water use. Recycling of water in the de-pulping process can drastically reduce the amount needed. With reuse and improved washing techniques, up to 1 to 6 m³ water per tonne of fresh coffee cherry is achievable; without reuse a consumption of up to 20m³/tonne is possible (Table 1).

Table 1. Water use in coffee processing

Country	Process	Water use m³/tonne cherry	Source
India	Semi-washed, wet processing	3	Murthy et al., 2004
Kenya	Fully washed, reuse of water	4-6	Von Enden and Calvert 2002 [2]
Colombia	Fully washed and environmental processing (BECOLSUB)	1-6	Von Enden and Calvert 2002 [2]
Papua New Guinea	Fully washed, recycling use of water	4-8	Von Enden and Calvert 2002 [2]
Vietnam	Semi wet and fully washed	4-15	Von Enden and Calvert 2002 [2]
Vietnam	Traditional, fully washed	20	Von Enden and Calvert 2002 [1]
India	Traditional, fully washed	14-17	Deepa et al. (2002)
Brazil	Semi-washed, mechanical demucilage	4	De Matos et al. (2001)
Brazil	Semi-washed, mechanical demucilage	3.4	Bello-Mendoza and Castillo-Rivera (1998)
Nicaragua	Traditional, fully washed	16	Biomat (1992)
Nicaragua	Fully washed, reuse of water	11	Grendelman (2006)

Coffee wastewater characteristics

When processing coffee using the fully washed principle much wastewater is generated.

General

Water used in processing coffee leaves the coffee processing unit with high levels of pollution. The main component is organic matter, stemming from de-pulping and mucilage removal. The majority of organic material in the wastewater is highly resistant and COD values make up 80% of the pollution load, with values as high as 50,000 mg/l. The BOD, coming from biodegradable organic material can reach values of 20,000 mg/l.

With a (rough) screening and removal of the pulp COD and BOD values become considerably lower. Values in the range of 3,429 – 5,524 mg/l for COD and 1,578 – 3,242 mg/l for BOD₅ were found by (De Matos et al., 2001). Recorded values of 2,480 mg/l for COD and 1,443 mg/l for BOD₅ (Bello-Mendoza et al., 1995 in Bello-Mendoza and Castillo-Rivera, 1998).

A large part of the organic matter, pectins, precipitates as mucilated solids and could be taken out of the water (Von Enden and Calvert, 2002 [2]). When these solids are not removed and pH values rise and an increase in COD can be observed.

In order to optimize the anaerobic processing of the wastewater pH values should be between 6.5 and 7.5, instead of the generally present values of pH = 4, which is highly acidic. This is obtained by adding calcium hydroxide (CaOH₂) to the wastewater. This resulted in a regained solubility of the pectins, raising COD from an average of 3,700 mg/l to an average of 12,650 mg/l.

The water is further characterised by the presence of flavanoid compounds, coming from the skin of the cherries. Flavanoid compounds result in dark colouration of the water at a pH = 7 or higher, but they do not add to BOD or COD levels of the wastewater, nor have major environmental impacts. Lower levels of transparency, however, can have a negative impact on photosynthetic processes and growth and nutrient transformations by (especially) rooted water plants. Many efforts in olive and wine processing industries, with relatively large funds for research, have been trying to find a solution for this problem. Calvert (1997) mentions research done into the removal of polyphenolics and flavanoid compounds by species of wood digesting fungi (Basidiomycetes) in a submerged solution with aeration using compressed air. These complex processes seemed to be able to remove the colour compounds, but simplified, cheaper techniques using other types of fungi (i.e. Geotrichum, Penicillium, Aspergillus) only thrived in highly diluted wastewaters.

Coffee wastewater is not a constant flow of water with uniform loadings of contamination. The processing of coffee cherries is a batch process and regarding water flows, two processes can be determined: de-pulping and fermentation/washing.

De-pulping

The water used for de-pulping of the cherries is referred to as pulping water. It accounts for just over half of the water used in the process. According to Von Enden and Calvert ”pulping water consists of quickly fermenting sugars from both pulp and mucilage components. Pulp and mucilage consists to a large extent of proteins, sugars and the mucilage in particular of pectins, i.e. polysaccharide carbohydrates” (Von Enden and Calvert, 2002 [1], pp.4). These sugars are fermenting using the enzymes from the bacteria on the cherries. Other components in pulping water are acids and toxic chemicals like polyphenolics (tannins and caffeine). Pulping water can be reused during the de-pulping of the harvest of one day. This results in an increase in organic matter and a decrease in pH. Research in Nicaragua (Grendelman, 2006) showed COD averages rising from 5,400 mg/l up to 8,400 mg/l with most of the pulp removed. The drop in pH can be attributed to the start of fermentation of the pulping water. This drop continues until fermentation is finished and pH levels of around 4 are reached. The nutrient content of the pulping water at the maximum COD load, which was considered to reflect maximum pollution, was determined during this research. Total nitrogen (TN) concentration in the samples ranged from 50 to 110 mg/l with an average over all samples of 90 mg/l. Total phosphorus (TP) concentration in the samples ranged from 8.9 to 15.2 mg/l with an average over all samples of 12.4 mg/l.

Washing

Washing of the fermented beans leads to wastewater containing mainly pectins from the mucilage, proteins and sugars. The fermentation of the sugars (disaccharide carbohydrates) into ethanol and CO₂ leads to acid conditions in the washing water. The ethanol is converted in acetic acids after reaction with oxygen, lowering the pH to levels of around 4. The high acidity can negatively affect the treatment efficiency of treatment facilities treating the coffee wastewater like an anaerobic reactor or constructed wetlands and is considered to be detrimental for aquatic life when discharged directly into surface waters.

During the washing process the research in Nicaragua (Grendelman, 2006) showed a clear decrease in contamination of the wastewater. The COD values drop from an average of 7,200 mg/l to less than 50 mg/l. Despite the fact that wastewater with COD values below 200 mg/l is allowed to be discharged in the natural waterways in Nicaragua it is advisable to redirect all the wastewater to the treatment system. This is because COD levels cannot be determined onsite during the washing process and discharge of the wastewater into surface waters is based on visual inspection. When the water is "clear" it is considered to be clean enough but the COD values measured during the research showed that discharge generally was to soon, resulting in wastewater with higher levels of COD than permitted. Another positive effect of diverting the wastewater to a treatment system is the dilution of the wastewater which enables better treatment by anaerobic bacteria due to more favourable pH values and better post-treatment due to lower concentrations of ammonium. TN concentration in the samples of wastewater stemming from washing ranged from 40 to 150 mg/l with an average over all samples of 110 mg/l. TP concentration in the samples ranged from 7.8 to 15.8 mg/l with an average over all samples of 10.7 mg/l.

References

• Bello-Mendoza, R. and Castillo-Rivera, M.F. 1998. Start-up of an Anaerobic Hybrid UASB⁄Filter Reactor Treating Wastewater from a Coffee Processing Plant. In: Anaerobe Environmental Microbiology vol. 4, pp. 219 – 225.

• BIOMAT 1992. Estudio y diseño de la Planta de Tratamiento de los Desechos del Café en la finca “San Luis”. Alcaldía de Matagalpa and Oficina Biogás y Saneamiento Ambiental. Matagalpa, Nicaragua.

• Calvert, K.C. 1997. The treatment of Coffee Wastewater. The Biogas Option. A Review and Preliminary Report of Ongoing Research. Coffee Research Report no. 50. Coffee Industry Corporation ltd. Kainantu, Papua New Guinea.

• Calvert, K.C. 1998. The Microbiology of Coffee Processing, part 1. PNGCRI Coffee Research Newsletter

• Calvert, K.C. 1999 The Microbiology of Coffee Processing, part 3. PNGCRI Coffee Research Newsletter

• Deepa, G.B., Chanakya, H.N., de Alwis, A.A.P., Manjunath, G.R. and Devi, V. 2002. Overcoming Pollution of Lakes and Water Bodies Due to Coffee Pulping Activities With Appropriate Technology Solutions. In: Proceedings “Symposium on Conservation, Restoration and Management of Aquatic Ecosystems”, paper 4. Centre for Ecological Sciences, Indian Institute of Science (IIS) and the Karnataka Environment Research Foundation [KERF], Bangalore and Commonwealth of Learning, Canada.

• De Matos, T, A., Lo Monaco, P.A., Pinto, A.B., Fia, R. and Fukunaga, D.C. 2001. Pollutant Potential of Wastewater of the Coffee Fruits Processing. Federal University of Viçosa, Department of Agricultural Engineering, Viçosa-MG, Brazil.

• Droste, R. L. 1997. Theory and Practice of Water and Wastewater Treatment. John Wiley & Sons, Inc. Hoboken, Canada.

• Grendelman, E.R. 2006. Tratar las Aguas Mieles. Wageningen University, sub-departments: Irrigation & Water Engineering Group and Environmental Technology. Unpublished internship paper. Wageningen University, Netherlands.

• Murthy, K.V.N., D’Sa, A. and Kapur, G. 2004. An effluent treatment-cum-electricity generation option at coffee estates: is it financially feasible? Draft version. International Energy Initiative, Bangalore.

• Von Enden, J.C. 2002. Best practices at wet processing pay financial benefits to farmers and processors. GTZ-PPP Project “Improvement of coffee quality and sustainability of coffee production in Vietnam”.

• Von Enden, J.C. and Calvert, K.C. 2002 [1]. Review of Coffee Waste Water Characteristics and Approaches to Treatment.

• Von Enden, J.C. and Calvert, K.C. 2002 [2]. Limit Environmental Damage By Basic Knowledge of Coffee Waste Waters. GTZ-PPP Project “Improvement of coffee quality and sustainability of coffee production in Vietnam”.