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REVIEW OF COFFEE WASTE WATER
CHARACTERISTICS AND APPROACHES TO
TREATMENT
Jan C. von Enden1, Ken C. Calvert2
1 PPP Project „Improvement of Coffee Quality and Sustainability of Coffee
Production in Vietnam”. German Technical Cooperation Agency (GTZ).
Khe Sanh, Huong Hoa, Quang Tri, SR Vietnam.
2 CEO Renertech Consulting.
159 St. Andrew Street, Invercargill, 9501. New Zealand
Contact email: office@venden.de
ABSTRACT
Wet processing of Arabica coffee (Coffea Arabica) produces higher quality and
receives higher prices on the world market compared to coffee prepared via dry
method. Behind the background of depressed world market prices, countries with
comparatively low production costs like Vietnam will increasingly switch their
production to high quality and higher priced washed Arabicas in order to
enhance competitiveness and revenues.
However, wet coffee processing requires a high degree of processing know how
and produces large amounts of processing effluents which have the potential to
damage the environment. Characteristics of waste water from coffee processing
is a Biological Oxygen Demand (BOD) of up to 20.000 mg/l and a Chemical
Oxygen Demand (COD) of up to 50.000 mg/l as well as an acidity of below pH 4.
In order to treat coffee processing waste waters, the constitution of waste water
is presented and technical solutions for waste water treatment in a pilot case are
presented.
Keywords: Washed Arabica coffee processing; coffee waste water; waste water
treatment
INTRODUCTION
Coffee is a valuable trading good which is produced in the tropics and mainly
consumed in Europe and the United States. Arabica (Coffea Arabica) and
Robusta (Coffea Canephora) are the two varieties which are internationally
traded. Arabica receives higher prices due to more favourable taste
2
characteristics and makes up 61% of the world production (Deutscher
Kaffeeverband 2001). Robusta coffee is an important component of commercial
coffee blends due to its characteristics of a rich “body”1 (Viani, no date). Brazil is
dominating the world market as it is the biggest Arabica coffee producer. For
Robusta, Vietnam is presently the biggest producer, however, the picture is
expected to change as Brazil is likely to overtake Vietnam during the 2002/3 crop
season (NKG Statistical Unit Quarterly Report 2002).
Coffee world market prices are presently in a severe crisis as the market suffers
from oversupply (Deutscher Kaffeeverband 2001) which is not seen to change in
the near future (NKG Statistical Unit Quarterly Report 2002). Current price
levels make it difficult for many coffee producers to generate profits as their
costs exceed world market prices (FAO 2002). The only way to receive an
optimum price even under the present market scenario, appears to produce high
quality Arabica coffee.
Behind this background, countries with competitive labour costs and feasible
natural conditions like Vietnam, aim to make their marginal profitable coffee
sector more viable by changing production partly to the more profitable washed
(or wet) processed Arabica production (VICOFA 2002). This processing method,
however, requires a high degree on knowledge in processing and has a large
potential of polluting surface waters from processing effluents (Mburu 1999),
especially when processed in centralised manner.
COFFEE PROCESSING
After picking of coffee cherries, the fruit has to undergo several processing steps
in order to remove the outer parts of the fruit, i.e. skin (exocarp), pulp
(mesocarp), the mucilage layer and the endocarpal parchment (see Fig. 1) The
1 Body is the viscosity, fullness and weight in the mouth of a beverage, ranging from
“thin, watery” to “thick, heavy” (Viani, no date).
Figure 1. Morphology coffee cherry (after Rothfos 1979)
Folded endosperm
Skin (exocarp)
Pulp (mesocarp)
Coffee Bean or seed
Parchment (endocarp)
Silverskin (testa)
3
way of processing determines the quality of the end product. In addition, each
processing technique has a different pollution potential.
The most simple and least polluting way of processing is the dry method, which
is mostly applied for Robusta coffee but also for a large amount of Brazil
Arabicas (Adams et al 1987). In this method, cherries are picked and left in the
sun until the whole fruit reaches a moisture content of around 11%. After drying,
the outer flesh and parchment is removed in one step.
In contrast to the dry method, wet processing requires a higher degree of
processing know how and is applied mainly for Arabica coffee (Vincent 1987).
Wet processing is producing a higher quality product, so called “mild coffees”.
The finer quality is due to a pre-sorting step of cherries which only allow ripe
cherries in the process (Fig. 2). During processing, exocarp and coffee pulp
(mesocarp) are mechanically removed before the gelatinous and hygroscopic
mucilage cover, which is coating the parchment, is removed. This is done during
an approximate fermentation time of 36 hours depending an natural conditions
like altitude and temperature (Rothfos 1979). Only after the mucilage layer has
been hydrolysed, all residues are washed off and the clean parchment is ready for
further processing, i.e. drying and hulling (Vincent 1987).
Process
WashedDry method
Harvesting
Pulping
Drying and Hulling
Long
Fermentation
Products By Products
Inhomogeneous
Coffee Cherries
Demucilated, wet
parchment coffee
Mucilated
parchment coffee
Green coffee
Mechanical
Mucilage
Removal
2
Coffee Pulp
Pulping Water
Liquified or raw
mucilage
Wash Water
Coffee Husk
Semi
washed
Presorting, cleaining, floating Homogeneous
Coffee Cherries
Sticks, stones
Unripe and
overripe cherries
Finish
Fermentation
Washing
1
Figure 2: Coffee processing methods
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The semi-wet or semi-washed process2 is similar to the wet or washed process.
During semi-wet processing, however, the time consuming fermentation step is
reduced as the mucilage layer is removed mechanically. After the mechanical
removal of the mucilage, the wet coffee should ideally undergo a shortened
“finish” fermentation to fully remove remaining mucilage from the parchment
followed by washing/soaking in order to produce an optimal quality. Somewhat
lower taste characteristics have been found when freshly demucilated coffee has
been sent directly into driers (Becker 1999).
WASTE WATER CHARACTERSITICS
The environmental impact of wet and semi-wet processing is considerable.
Problems occur through large amounts of effluents disposed into watercourses
heavily loaded with organic matter rather its than inherent toxicity (Adams et al
1987). Providing the self purification of the watercourse is exceeded, the
microbial degradation reduces the level of oxygen to anaerobic conditions under
which no higher aquatic life is possible.
Water Quantities
Depending on the processing technology applied, quantities of coffee waste
water is varying. Modern mechanical mucilage removal machines producing
semi-washed coffee use only about 1 m3 per tonne fresh cherry (without finish
fermentation and washing) whereas the traditional fully washed technique
without recycling uses up to 20 m3 per tonne cherry (Mburu et al, 1994). In order
to treat waste water properly and at reasonable costs, the amounts of waste water
must be minimised.
Organic Components
The main pollution in coffee waste water stems from the organic matter set free
during pulping when the mesocarp is removed and the mucilage texture
surrounding the parchment is partly disintegrated (Mburu et al 1994). Pulping
water consists of quickly fermenting sugars from both pulp and mucilage
components. Pulp and mucilage consists to a large extend of proteins, sugars and
the mucilage in particular of pectins, i.e. polysaccharide carbohydrates (Avellone
et al, 1999).
2 No clear definition for semi-wet or semi-washed is available. In this context, it will be
used for the process of mechanical mucilage removal. In Spanish the word
“desmucilaginado” is used. Aquapulping has also been used, however, it describes an
entirely different processing method in which pulping and demucilating is done in a one
step process.
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Depending on the processing method applied, further waste water evolves in the
form of hydrolysed pectins from fermentation and washing. During fermentation,
long chain pectins are split by enzymes (pectinase, pectase) into short chain
pectin oligosaccharides. Oligosaccharides are soluble in alkaline and neutral
solutions, but in acid conditions they are thrown out of solution as Pectic acid.
(Rothfos 1979, Treagust 1994). In the presence of calcium or other multivalent
ions, the pectic acid fragments are cross linked into a non-soluble gel of calcium
pectate (Treagust 1994).
Waste water from mechanical mucilage removers contains a certain amount of
sugars (disaccharide carbohydrates), but its apparent gel like texture comes from
the segments of undigested mucilage and pectic substances which have been
removed from the parchment by mechanical means. In order to be biodegraded,
the solid materials have to be fermented, acidified and hydrolysed by natural
fermentation in a later stage.
During fermentation and acidification of sugars in the waste water, pectin oligo-
sacharides get out of solution and float on the surface of the waste water. The
remaining highly resistant materials left in the effluent water are acids and
flavanoid colour compounds from coffee cherries. At around pH 7 and over,
flavanoids turn waste water into dark green to black colour staining rivers
downstream from coffee factories. However, flavanoids do not do any harm to
the environment nor add significantly to the Biological Oxygen Demand (BOD)
or Chemical Oxygen Demand (COD).
Values for Biological Oxygen Demand (BOD) indicating the amount of oxygen
needed to break down organic matter are high in coffee waste water (up to
20.000 mg/l for effluents from pulpers and up to 8.000 mg/l from fermentation
Ether extract 0,48%
Crude fibre 21,4%
Crude protein 10,1%
Ash 1,5%
Nitrogen free extract 31,3%
Tannins 7,8%
Pectic substances 6,5%
Non reducing sugars 2,0%
Reducing sugars 12,4%
Chlorogenic acid 2,6%
Caffeine 2,3%
Total caffeic acid 1,6%
Table 1: Composition of coffee pulp
(Gathuo et al 1991)
Water 84,2%
Protein 8,9%
Sugars
- Glucose (reducing) 2,5%
- Sucrose (non
reducing)
1,6%
Pectin 1,0%
Ash 0,7%
Table 2: Composition of mucilage
(Clifford and Wilson 1985)
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tanks). The BOD should be reduced to less than 200 mg/l before let into natural
waterways.
Resistant organic materials which can only be broken down by chemical means
indicated by the Chemical Oxygen Demand (COD) make up around 80% of the
pollution load and are reaching 50.000 mg/l and more (Treagust 1999). The
material making up the high COD can be taken out of the water as precipitated
mucilage solids. Other substances to be found in small amounts in coffee waste
water are toxic chemicals like tannins, alkaloids (caffeine) and polyphenolics.
However, these toxic substances mainly stay in the disposed solids of the coffee
pulp.
Acidity
During the fermentation process in the effluents from pulpers, fermentation tanks
and mechanical mucilage removers, sugars will ferment in the presence of yeasts
to alcohol and CO2. However, in this situation the alcohol is quickly converted to
vinegar or acetic acid in the fermented pulping water. The simplified chemical
formula for biological fermentation of 6 carbon sugars by yeasts to ethanol is
typified by the fructose to ethanol reaction:
C6H12O6 = 2 CH3 CH2OH + 2 CO2 (1)
Sugar = 2 Ethanol + 2 Carbon dioxide
Ethanol is quickly broken down by bacteria into acetic acids. This complex
enzymatic catalysed reaction is simplified as
2 CH3 CH2OH + O2 = 2 CH3 COOH (2)
2 Ethanol + Oxygen = 2 Acetic acid
The acidification of sugars will drop the pH to around 4, and the digested
mucilage will be precipitated out of solution and will build a thick crust on the
Figure 3: Mass balance coffee processing
6,25 tonnes ripe
cherry
Wet or Semi wet processing
Pulp
2,5 tonnes
Waste water
25.000 litre
1.250 kg COD
375 kg BOD
1 tonne dry
green bean
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surface of the waste water, black on top and slimy orange/brown in colour
underneath. If not separated from the waste water, this crust will quickly clog up
waterways and further contribute to anaerobic conditions in the waterways.
APPROACHES TO WASTE WATER TREATMENT
At the project site in Khe Sanh, Quang Tri, Vietnam, a pilot waste water
treatment system is presently under design and testing for semi-washed coffee
including finish fermentation and washing. At times of peak production, around
100 tonnes of fresh cherry are processed. Average water consumption has been
brought down from over 10 m3/tonne cherry to around 4m3/tonne cherry
processed through recycling and reuse of processing waters. Total effluents reach
400m3 a day at peak times.
The treatment system consist of an acidification pond (200m3), followed by a
neutralisation tank (25m3) filled with ground limestone. After neutralisation of
waste water to pH 5.9 to 6.1., water is treated alternatively in a Upflow
Anaerobic Sludge Blanket (UASB) biogas reactor before entering a constructed
wetland planted with macrophytes for secondary treatment. For tertiary
treatment, waste water runs through a water hyacinth pond for water polishing
before entering the open waterway.
In the acidification pond, effluents from mechanical mucilage removers as well
as the recycled processing (pulping, pre-sorting, washing) water is allowed to rest
at shallow depths for at least 6 hours. During this time, raw mucilage comes out
of solution and will float on top ready to be raked off. The acidity of untreated
acid water below the crust needs to be lifted to at least pH 6 before further
treatment can take place Considering the low cost of natural limestone (CaCO3)
automatically buffering at 6.1, limestone seems the best solution for stabilisation.
In theory, 250 milligrams of limestone is needed to buffer 1 litre of acid water
(Treagust 1999). In the presence of limestone, the acetic acid is converted to
calcium acetate with a radical change in solution pH from 3.8 up to 6.
2CH3COOH + CaCO3 = Ca(CH3 CO2)2 + CO2 + H2O (3)
Acetic Acid + Limestone = Calcium Acetate + Carbon dioxide + Water
During primary water treatment, neutralised waste water is used as feedstock in
an UASB biogas digester working on a special strain of methanogenic bacteria
from coffee plantation soils. The bacteria are active at a pH of around 6 at
ambient temperatures. In the process of anaerobic decomposition, bacteria
metabolise dissociated acetate ions which is the reaction product of Calcium
Carbonate (CaCO3) and acetic acids (2HAc) in the neutralised waste water.
2 CH3 COOH = 2CH4 + 2 CO2 (4)
8
2 Acetic acid = 2 Methane + 2 Carbon dioxide
During biogas operation, a reduction of 70 to 90% of BOD content can be
achieved in as little as 4-6 hours retention time (Calvert 1999, Vinas et al 1988)
delivering around 5 m3 methane per tonne cherry processed (Calvert 1999).
Presently, the prototype biogas digester in use has a capacity of only 5 m3 and is
able to process about 20 m3 neutralised waste water per day leaving an access
amount of acetate effluent be lead directly into the constructed wetland. Methane
resulting from UASB digestion can be reused for fuelling coffee driers and
contributes to the reduced energy costs for post harvest processing costs
Acid Pond
(In)
Neutrali-
sation Pond
UASB
Digster
Settling
Tank.
Wet-
land
Hyacinth
Pond
Out-
let
PH 3,8 6.1 6.1 6.5 6,5 7 7
BOD mg/l 20,000 10,000 1,000 800 <400 200 <200
Reduction
in BOD 50% Minor 90% 20% 50 % 50%
Secondary treatment is done in a constructed wetland planted with rushes and
reeds (Phragmitis australis) following the design of an emergent macrophyte
treatment system with subsurface flow (Vymazal et al 1998). In this treatment
method, dissolved oxygen levels in the water are increased through diffusion of
oxygen in the root zone of the macrophytes growing in the flooded gravel bed.
The additional oxygen supplied is speeding up the aerobic decomposition of
remaining organic matter. The water levels in the wetlands may also be
artificially raised and lowered to assist the oxygen flow. In addition to aerobic
Table 3: Estimated efficiency of waste water treatment system
Fi
g
ure 4: Planned
p
ilot waste water treatment setu
p
Mechanical
mucilage remover
Recycled
Processing Water
2 Waste Effluents
Biogas
Digester
Acid Pond Neutralisation
Tank
Constructed
Wetland
Water Hyacinth
Pond
Fresh Water Lake
Fresh
water inflow
Outflow
Methane
Fresh
water inflow
1 Fresh water
Constructed
Wetland
Settlement
Tank
Settlement
Tank
9
bacteria active close to the roots of the plants, anaerobic decomposition can also
take place in a wetland. A construction of wetland is able to remove up to
between 49 and 81% BOD loadings and lower the amount of suspended solids
between 36 and 70% depending on initial BOD loadings and retention time
(Biddlestone et al 1991). In addition, macrophytes remove nutrients and salts
from biogas digester effluents.
Tertiary treatment and final cleanup will be done by water hyacinth (Eichornia
crassipes) ponds. Water Hyacinth are particularly active in the removal of both
bacteria and heavy metals. In addition, fresh water inflow into the water hyacinth
pond dilutes the organic loadings.
SUMMARY
Coffee waste waters are high in organic loadings and exhibit a high acidity.
When washed or semi washed coffee is processed in large quantities, untreated
effluents greatly exceed the self purification capacity of natural waterways. In
order to overcome the pollution potential of processing waste waters, a clear
understanding of waste water constitution in inevitable to design a feasible
treatment system. Especially when expanding wet coffee processing or setting up
new large scale processing operations, treatment of waste waters needs to be
considered.
Firstly, the amount of sedimentable solids contributing to COD loading of waste
water need to be lowered. This is achieved during a sufficient time of
acidification of sugars present in the waste water during which solids get out of
solution. After full acidification, the clear, acid waste water is treated by natural
limestone to lift the pH from around 4 pH to a pH to around 6. Only at this pH
levels, UASB digestion and constructed wetland will achieve optimal results.
The UASB technology is central in the treatment process as the highest reduction
of BOD levels in relatively short times are achieved. Effluents from the UASB
digester are high in phosphates and still reveal a BOD which needs to be treated
in secondary treatment. Secondary treatment and consumption of phosphates is
accomplished in a locally adopted constructed wetland using macrophytes to alter
aerobic bacterial decomposition of organic matter. Before disposed, waste water
tertiary clean up and dilution of BOD loadings is achieved by leading waste
waters through a pond of water hyacinths. Only after this multi step clean up,
water is safe to re-enter natural waterways.
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