IMPACT OF CASSAVA PROCESSING ON THE ENVIRONMENT

Introduction

Cassava processing, especially in areas where the industry is highly concentrated, is regarded as polluting and a burden on natural resources. Some forms of processing, particularly for starch, have developed beyond traditional methods and are now water intensive yet often sited in areas of water scarcity. By its nature, cassava processing for starch extraction produces large amounts of effluent high in organic content. If untreated this may be displayed in the form of stagnant effluent ponds from which strong odors emanate. Other forms of processing, despite not requiring water, generate very visible dust waste. As a consequence of the visual display of pollution, cassava is often perceived by local populations as contributing significantly to environmental damage and water deficit. Yet, despite this notion, supported mainly by the visual display of pollution, few systematic impact studies have been conducted.

Most studies have tended to focus on the quantity and composition of waste produced by this industry, but do not consider the environmental impact.

Areas and products

Asia

Annual cassava production in Asia is about 48 million tonnes, mainly in Thailand (18), Indonesia (15), India (6), China (4) and Vietnam (2). Diversity is the characteristic of cassava products in Asia, both within and across countries. Between 33 and 60% of the cassava produced in the region is processed for industrial purposes, mainly as pellets for animal feed and as starch. Large amounts of dry chips and pellets are produced in Thailand, Indonesia and China. Cassava flour is produced in some countries in the region, most notably in Indonesia (Figure 15). Thailand, Indonesia, India, China, Vietnam, Malaysia and the Philippines all produce cassava starch. Processing is done at a wide variety of scales, ranging from small family-size units to modern large-scale factories (Table 21). The industry in Thailand, Philippines and Indonesia is large-scale and modernized. China and Vietnam have made considerable advances in modernizing their starch industry, especially in Guangxi province of China and in the southeastern region of Vietnam. Throughout the region, the industry is moving toward larger, more technologically advanced plants, and small less-efficient factories are closing.

In Vietnam, cassava processing has increased in overall scale since the 1970s. Starch processing is of major importance, the development of which is driven by an expanding range of applications for starch. Despite an increase in overall output, processing in north Vietnam is still by small-scale artisan methods (Ha et al., 1996). The process (Figure 16) is typical for many tropical countries and consists of wet-milling washed roots, washing the starch from the milled pulp in mixing tanks, sedimenting the starch in canals or tanks, followed by sun drying. Processing regions such as Cong Hoa village, Quoc Oan district, Ha Tay province, which is close to Hanoi, are characterized by a large number (about 1000) of such enterprises (Viet, 1998). In the six-month processing season, an average family in Cong Hoa village will process 250-300 kg roots each day (producing 50-60 kg starch) and in the peak of the season, this increases to 400-500 kg of roots per day. This equates to a total daily processing load for the village of about 300 tonnes of roots.

Figure 15. Flow chart for small-scale production of cassava flour.
Source: G. Chuzel. (pesonal communication)

Table 21. Examples of three scales of starch processing in Asia.

Source
¹⁾ Tran Quoc Viet, 1998.
²⁾ ERM, 1996.
³⁾ Sriroth et al. (submitted)
⁴⁾ R. Howeler (personal communication)

Figure 16 Flow chart for small-scale production of cassava starch.
Source: G. Chuzel. (personal communication)

Figure 17 Flow chart for small-scale production of cassava starch (Tamil Nadu).
Source: ERM, 1996

India's starch industry centers on Salem district in Tamil Nadu. In this area, 850-1000 processing enterprises operate, each with a daily capacity of about 10-50 tonnes of cassava roots (equivalent to 2-10 tonnes of starch). This area probably represents the greatest concentration of such type of cassava starch processing in the world. The level of technology is small-to-medium scale, with large tanks used for starch sedimentation (Figure 17). The starch extraction process involves hand peeling, washing, mechanicalcrushing over a rotating drum fed continuously with water, followed by sieving/screening of the fibrous pulp. Starch is separated in sedimentation tanks and finally dried in the sun. The total production of starch in the area is estimated to be in the range of 375,000-400,000 tonnes per year (ERM, 1996). The two main products are native starch and sago (starch being a raw material for sago production). The industry is seasonal, usually operating at peak production for a period of 4-5 months per year, starting Oct/Nov.

The cassava industry in Thailand is the most developed in the world. Processing enterprises are large-scale, but are supplied by small-scale farmers. The total annual cassava production (about 18 million tonnes) is converted to 4 million tonnes of chips/pellets, and about 1.6-1.8 million tonnes of starch (Sriroth et al., in preparation). The starch industry in its present form has undergone a series of structural changes, developing from a relatively large number (approximately 170) of medium-scale factories, utilizing simple technology of sedimentation, to a smaller number (approximately 51) of modern automated processing factories (Figure 18). On average, an individual starch processor can process between 400 and 1,200 tonnes of roots each day.

Africa

Annual cassava production in Africa is about 84 million tonnes, mainly in Nigeria (30), the Democratic Republic of Congo (16.8), Ghana (7.1), Tanzania (5.7), Mozambique (5.3) and Madagascar (2.4). Cassava is primarily produced by small-scale farmers and processed at the family- or village-level. Despite the small scale of operation, cassava production in Africa is highly commercialized, with as much as 45% of the total output marketed (Nweke, 1992). A great diversity of products are derived from cassava. The most representative are gari in West Africa (Figure 19), chickwangue in Central Africa (Figure 20) and atap and ugali in East Africa. The range of products in Africa is described in the COSCA study (reported in Nweke, 1992), and summarized in Figure 21.

Traditional processing techniques are flexible in their use of the different processing resources. Retting (soaking) is employed in humid regions (central Africa), while in the dryer regions (Western Africa) a fermentation step is usually included. Cassava granules are commonly produced in areas of high population density, while chips and flour are more widely used in those with low population density (Nweke, 1992). In some regions, techniques for making chips and flour are water-intensive, but in other areas only sunshine is required. A major feature of cassava processing in Africa is that villages in each climatic zone concentrate on making products for which the zone is endowed with the necessary resources (Nweke, 1992). Cassava processing has many technological pathways adapted to the use of locally available processing resources. Where water for fermentation is scarce, heaping or stacking fermentation techniques are used.

Figure 18 Flow chart for large-scale production of cassava starch.
Source: Adopted from Sriroth et al., 2000a

Figure 19 Flow chart for production of gari or atteke.

Source: S. Chuzel. (personal communication)

Figure 20 Flow chart for production of chikwangue.

Source: G. Chuzel. (personal communication)

Figure 21 Unit operations in the processing of various cassava products.

Source: Adapted from Nweke, 1992.

This adaptation strategy has implications for the environmental impact of a processing method, minimizing a drain on natural resources. However, the balance between processing and environmental sustainability is threatened with technology changes from traditional to more automated processes.

Latin America and the Caribbean

Annual cassava production in Latin America and the Caribbean is about 32 million tonnes, mainly in Brazil (24), Paraguay (3.1) and Colombia (1.8). Cassava is used principally as fresh or processed roots for human consumption, but also to a lesser extent for animal feed and starch. In Brazil Farinha is a traditional mechanically dried product derived from cassava (Figure 22); it may also be prepared from sweet or sour (fermented) starch. In the northeastern region of Brazil farinha is processed by farmer-producers on a small scale, processing as little as 100 kg of roots per day. In southern Brazil, cassava is a large-scale industrial crop, and factories process about 10-50 tonnes of roots per day for farinha (Marder et al., 1996). About 3 million tonnes of farinha are produced each year in Brazil, and depending on the maturity of roots, the yield of farinha varies between 30-40%.

In southern Brazil, cassava is also an industrial crop for starch extraction. The enterprises are large-scale, processing 200-500 tonnes of fresh roots per day. Brazilian legislation is strict on waste water management. All the factories have installed water treatment plants (aerobic digestion or lagoons). Two types of starch are produced:

a. Native (sweet) starch. Total production of 200,000 tonnes is processed from 18-24 month old roots by large factories handling 100-900 tonnes of roots per day. The final product is sun dried. Starch yield is 20% when roots are harvested at the correct time, and 18-19% when less mature roots are used. Several starch derivatives are produced from native starch, including glucose syrups and maltose (Henry and Westby, 2000).

b. Sour (fermented) starch. Total production is about 100,000 tonnes. Factories producing sour starch are smaller than those for sweet starch and are less sophisticated. Each processes about 20-25 tonnes of roots per day. Starch is produced in a manner similar to that described previously for sweet starch (Figure 16), except that after final settling, the wet starch is dug out and stored in fermentation tanks for 4-6 weeks. After fermentation, the starch is sun-dried. Starch yields are similar to those for sweet starch manufacture.

In Colombia, production of sour starch is concentrated in the Andean zone. Production ranges from 6,000-10,000 tonnes each year. In Colombia starch accounts for 3% of fresh root production. About 200 factories are capable of processing 1-4 tonnes of fresh cassava roots per day.

A similar small-scale industry exists. Traditional starch is also produced in the western region of Paraguay (about 60 small-scale processors producing a total of 9,000-10,000 tonnes of starch per year). In the northeast of Argentina there are 15 small-scale plants, with a total capacity of 2,000 to 3,000 tonnes of starch per year. The starch is mainly for use in the preparation of traditional cheese bread called chipa, similar to pan de bono or pan de yuca in Colombia and pao de queijo or biscoito in Brazil. The process used is at a low technological level (manual processing, equipment fabricated from wood) and is of low efficiency.

Figure 22 Flow chart for production of farinha.
Source: G. Chuzel. (personal communication)

Table 22. Types of waste and their environmental impact of various unit operations used in cassava processing.

Source: Adapted from Nweke, 1992.

In Venezuela, several medium- to large-scale cassava processing units are operating, mainly for production of native starch and glucose syrups (Henry and Westby, 2000).

Environmental impact

General background

Cassava processing methods are a combination of various operations, each having a different potential impact on the environment. In essence, all cassava products will involve some or all of the unit operations highlighted in Table 22 and Figure 21. The impact of cassava processing should be considered at two levels - broad scale and site-specific. Generally, the maximum impact will be at the site-specific level.

Two categories of methods typify cassava processing: methods that require a lot of water and those that do not require much water. Most traditional products, such as farinha, have modest water input requirements. Water consumption in the production of farinha is a relatively low 5 m³/tonne product. For starch production, however, water is required at all stages irrespective of processing scale. Therefore, large volumes of water are needed - on average 2-6 times more than for farinha production (Table 23). Large factories possess the technological ability to efficiently use water, often incorporating recycling systems (Figure 18). The theoretical maximum water conservation rate is never achieved, as a minimum quantity of water is required for complete washing of starch. If this need cannot be satisfied, starch quality is adversely affected.

Table 23. Water consumption in the processing of various cassava products.

Source:
¹⁾ G. Henry (personal communication)
²⁾ A. Westby (personal communication)
³⁾ ERM, 1996.
⁴⁾ Rojas et al., 1996.
⁵⁾ K. Sriroth (personal communication)
⁶⁾ Tran Quoc Viet, 1998.
⁷⁾ Howeler, 1996b.

Figure 23. Flow chart of hydrogen cyanide during production of 200 tonnes of cassava per day.Source: Wanlaphathit, 1988.

Water use and quality

As a rule, the amount of water used for starch processing varies, depending on the processing scale and the level of technological sophistication; requirements range from 21-76 m³ per tonne of dry starch (Table 23). In some areas, micro- and small-scale processors are highly concentrated, and hence the seemingly modest individual water requirements should be viewed from the perspective of the high concentration of processors in a limited geographic region. For example, in Cong Hoa village, Vietnam, despite a modest requirement of only 2.4 m³/day by individual processors, the total daily water demand for the area is 2,400 m³, required to process 300 tonnes of roots (Viet, 1998). By comparison, to process a similar quantity of roots in Tamil Nadu 1,920 m³ of water are required (ERM, 1996). In Thailand a typical factory processing a similar amount of roots requires 4,500 m³ of water (Sriroth, personal communication), of which only 600 m³ need be fresh water if an efficient recycling system is used (K. Sriroth and A. Annachhatre, personal communications) (Figure 16). The apparent increasing demand for water with scale of operation reflects a need for higher quality of the final product. The only study reviewing the impact of starch processing on groundwater supply suggests that such problems are usually site-specific and are not directly the result of starch processing (ERM, 1996). Despite perceptions to the contrary, starch processing in most areas probably does not consume a significant proportion of the groundwater. In Tamil Nadu, is an area of limited water supply and has a high concentration of starch processors, all of which obtain water from either open ponds or bore holes, Nonetheless, only 1.1-1.2% of the total groundwater recharge is consumed by the starch industry. Tamil Nadu possibly represents the extreme case for direct demand on groundwater supply. In many other cassava processing areas, water is also obtained from a surface water supply, especially streams and rivers.

The proportion of water used by the starch processing industry is small compared to overall use (i.e. industrial, agricultural and domestic), and broad impacts are not usually expected. Site-specific problems, however, can occur, especially if processors are clustered and are close to other major water users. Within a cluster of processors, the combined demand for water can have a significant impact on the water level in open wells in the immediate vicinity. This situation is exacerbated if the processors are situated close to domestic users. As a useful strategic planning tool, a model based on local recharge rates and demand by other users should be available to assist the determination of a critical number of processors within a cluster.

Water utilization in the production of other starch products is small and can safely be assumed not to have significant impact. There are no reports of water supply for starch processing being a problem in Brazil.

In Africa, the demand for water tends to be self-regulating when processing methods are traditionally selected and adapted to the amount of available water in a specific geographic location.

Wash water and effluents

Waste water from cassava processing, if released directly into the environment before proper treatment, is a source of pollution. In many areas where traditional processing is practiced, waste water is normally discharged beyond the "factory" wall into roadside ditches or fields and allowed to flow freely, settling in shallow depressions. Eventually this will percolate into the subsoil or flow into streams. In Colombia, starch processors usually return the effluent directly to streams and other surface water sources.

Besides large quantities of soil, discharged waste water contains a number of contaminating substances. Normally, waste water discharged from a cassava starch processing factory is acidic with a high organic matter content (soluble carbohydrates and proteins) and suspended solids (lipids and non-soluble carbohydrates - starch or cellulose fibers). Waste water also contains cyanide as well as sulfur dioxide if this is used during the extraction process.

Cassava roots contain cyanogenic glucosides (the precursors of HCN) in various concentrations depending on the variety and growing conditions. Cyanide is released during peeling, slicing and crushing, such that these operations can reduce the level of cyanide to safe limits (Figure 23). The bound cyanide is converted to free cyanide during the milling operation. Forty to 70% of the total cyanide appears in the water used to wash the starch from the disintegrated tissue, and about 5 to 10% in fibrous residue used in animal feed (Arguedes and Cooke, 1982). Released cyanide, either in expressed juice, wash water or water spray, quickly evaporates. Evaporation of cyanide will occur either during processing or after discharge (Cooke and Maduagwu, 1978).

Source and description of waste water

(1) Squeezing

Water released from cassava during squeezing can have potentially harmful effects on the environment, especially if generated in large amounts. The main products of squeezing cassava are gari and farinha (Figures 19 and 22). Waste water is generated from two operations: pressing and washing. The press water, although produced in relatively low volumes (250-300 liters per tonne of roots), is the main problem because of its high biological oxygen demand (BOD) of 25,000-50,000 mg/l and a typical cyanide concentration of more than 400 mg/l (Table 24). In contrast, the BOD of wash water can be on the order of 500-2,500 mg/l.

Table 24. Comparison of the volume and composition of squeezed water from two types of cassava processing.

Source:
¹⁾ Arguedas and Cooke, 1982.
²⁾ G. Chuzel. (personal communication)

(2) Aqueous extraction methods

Starch is the main product of aqueous extraction. Waste water is generated at three stages (Figure 18):

1. Root washing
2. Primary settling of starch milk
3. Secondary settling of starch milk

In the processing of starch, about 85% of the average 30 m³ of water required to produce one tonne of starch is discharged as liquid waste; the remainder is lost through evaporation. The average amount of waste water discharged from all sources during starch extraction is between 20-40 m³/tonne starch for a modern factory, and about 20-100 m³ water/tonne starch for a medium-scale factory. High daily and within-season variation for waste water generated is normal.

A rigorous review of the literature is difficult because of differences in reporting formats, choice of analytical methods and sampling strategies. However, within these limitations general observations can be made.

The composition of waste water varies among different scales of processors (Table 25) and extraction methods. Total solids in the waste water from small-scale processors in north Vietnam are about 1,500 mg/l and the total nitrogen content about 15 mg/l. These values reflect a tendency to maximize starch yield at the expense of quality. By comparison, waste water released by medium-scale processors (India) contains about 4,100 mg/l total solids, 70 mg/l total nitrogen and has a BOD of 4,900 mg/l. Changing the extraction technology can markedly affect starch quality, as shown by the comparison of waste water measurements from Thai cassava processors, taken 25 years apart - the earlier when sedimentation was the main method for starch extraction and the later one from a fully automated modern factory. Waste water from the more technologically sophisticated processors is purer. For a modern factory, typically found in Thailand or Indonesia, the composition of combined waste waters will be similar to that shown in Table 25.

Each stage of starch extraction creates waste water with different amounts of contaminating materials. For example, waste water from decanters is high in organic substances and contains starch, fat and protein. The chemical oxygen demand (COD) of waste water from the decanters is 30,000 mg/l, and the combined waste water 15,000 mg/l. The composition of waste water produced at various processing stages for a medium-scale processing operation is given in Table 26. As a comparison, international waste water standards are given in Appendix 3.

Impact

Problems created from waste water occur when this is removed from the factory, especially if handled incorrectly. Waste water generated by starch and farinha processors is sometimes directed into pits (starch processors in Brazil) for conversion, but depending on the soil characteristics, partial leaching may contaminate the groundwater, while overflow may affect surface water. Waste water is also released directly to neighboring land (Vietnam, India), or is returned directly to streams and other surface water sources (sour starch producers in Brazil and Colombia).

Table 25. Quantity and composition of waste water from cassava starch processing factories of various scales.

Source:
¹⁾ Cereda and Takahashi, 1996.
²⁾ Tran Quoc Viet, 1998.
³⁾ ERM, 1996.
⁴⁾ Rojas et al., 1996.
⁵⁾ Tanticharoen et al., 1986.
⁶⁾ K. Sriroth. (personal communication)
Note: NA = data not available

Table 26. Typical waste water composition at each stage of a small- to medium-sized processing factory in Salem district, Tamil Nadu, India.

Source: ERM, 1996.

(1) On groundwater

(a) Magnitude

Contamination of groundwater is often the outcome of seepage of untreated effluent into shallow and deep aquifers. This is a problem in a few areas, such as India (documented) and Vietnam (cited). From the limited information it is clear that open wells and bore holes in close proximity to starch processors can be contaminated by high levels of suspended solids and high BOD (in Tamil Nadu BODs of up to 23 mg/l were recorded (ERM, 1996); US Public Health Department drinking water standard is 5 mg/l). Nevertheless, even in areas of intensive starch processing (e.g. Tamil Nadu), the proportion of contaminated sites is minimal, with the majority not affected. Geology and soil characteristics are important in determining if the groundwater supply will become contaminated; porosity and degree of saturation are the principal factors.

Cyanide contamination, even site-specific, of groundwater seems not to be a problem. In areas close to intensive cassava processing in India the cyanide concentration in the groundwater was found to be below detectable levels. Despite this apparent low incidence of contamination, it is still important to know the frequency of site-specific impacts and be able to predict problem sites. It should be noted that studies are limited and the same conclusions may not be true of a site with geology/soil characteristics that promotes seepage of waste water.

(b) Significance

Depending on the geology, groundwater supplies can remain protected from contamination. However, where physical conditions permit, untreated effluent can reach groundwater in sufficient quantities to pollute open wells and bore holes. If such conditions prevail, groundwater quality may be affected, leading to health problems of nearby communities. This may be the case in Cong Hoa village, Vietnam. However, further surveys are needed to verify the claims of local health officials.

Complaints from the public of stomach ailments are few, but consistent. The possibility of drinking groundwater contaminated with starch effluent, in the areas neighboring starch-processing sites, should not be ruled out. In Cong Hoa village pollution caused by draining cassava waste water to surrounding land is claimed to be the cause of detrimental health effects. Reports by various health authorities claim that the incidence of lung and digestive tract diseases in the village is 22-27% higher than in other villages.

(2) On surface water

Surface water includes all rivers, streams, canals, ditches, lakes, reservoirs, lagoons, estuaries and coastal waters. Environmental impact on surface water may be due to the addition of substances, heat or microorganisms to the water. This leads temperature changes and/or increases in microbial populations. The main problem originating from cassava processing is through the addition of organic substances. This results in eutrophication, which in-turn is responsible for an increase in the number of microorganisms and the growth of algae. This can alter water quality and/or change the aquatic ecology, affecting plants and animals, human health and visual aspects.

(a) Magnitude

The impact to surface water will depend on environmental conditions during the processing season; in the dry months the effects are most apparent. The magnitude depends on the concentration of processors and their distance from water bodies capable of facilitating rapid dispersal. Streams, slow flowing rivers, ponds and lakes are therefore most at risk. Surface water can remain contaminated (high BOD levels) even after dispersal downstream from a factory (Table 27). In extreme cases, surface water can become anaerobic due to the formation of a thick crust forming on the water surface. However, usually the main effect is that water becomes eutrophic from excessive organic matter loading. Contamination of surface water has been reported to be severe in several areas of Vietnam (Viet, 1998). No measurements are available, but reports suggest a high degree of eutrophication.

Table 27. Impact of release of starch factory waste water on the quality of surface water.

Source: A. Westby (personal communication)

The water used in aqueous extraction methods will help to dilute the cyanide. Hence, there is little risk of water contamination from cyanide on a broad scale. However, site-specific effects can be significant. For example, high cyanide content in waste water discharged by sour starch processors in Brazil (representing 50-60% of the total cyanide content of the roots) was not present in river water downstream of the factory; in fact, no cyanogens could be detected. In comparison, when microtox and tropical duckweed were exposed to untreated effluent from a cassava starch factory in Thailand, the waste was found to be toxic to the plants. The toxicity was thought to be associated with the high cyanide content and to a lesser extent other components (Bengtsson and Triet, 1994). Cyanide content in by-products and wastes also depends on the cassava variety. The high yield varieties used throughout Asia generally contain high amounts of HCN. For production of 200 tonnes of starch/day, about 800 tonnes of roots are required, which will contain about 32 kg of HCN. Generally, the HCN concentration of fresh peeled roots varies from 6-250 mg/kg fresh weight. The factory with a process discharge of 2,323 tonnes of waste water will also discharge about 30 kg HCN, equivalent to 13 mg HCN per liter of waste water (Figure 23). In Vietnam, small-scale processors released 28 mg HCN per liter of waste water.

(b) Significance

The significance of surface water contamination should be interpreted from the perspective of seasonal variation. It is most acute in an extended dry season. Care should be exercised to identify all possible sources of contamination, as starch processing may only be a minor contributor. This is especially difficult when examining the impact of processing on river water quality. It is also not easy to determine the number of lakes and ponds affected by starch processing because of problems in distinguishing between starch effluents, agricultural run-off and domestic sewage. Reports suggest that in some cassava processing areas, where increasing agricultural intensification has taken place, many water bodies have become progressively more eutrophic. The contribution to this problem from cassava processing is difficult to assess, except for a few areas where cassava processing is the only source of effluent to the body of water.

Environmental problems from cyanide in wash water or expressed liquid can occur if the water is used, without dilution, for irrigation (e.g. young stages of rice, vegetables) (Bengtsson and Triet, 1994). Negative effects on local agriculture and sensitive stages of fish and crustacean populations in receiving water bodies can also be expected if the concentration of cyanide increases above 0.3 mg/l. However, this has never been documented.

Starch effluents may have significant environmental effects on lakes and ponds, ultimately affecting local communities. Effluent from the Brazilian sour starch processors is claimed to kill fish and other animals. Reports concerning contaminated water courses in processing areas increase towards the middle and end of the dry season. Further research is required to substantiate such claims, as there has not been any systematic monitoring.

Waste water generated by small-scale processors, unless they are highly concentrated, has minimal impact on the environment. In contrast, the much higher volumes generated by larger factories can have a significant and serious impact on the environment.

Solid wastes

Solid waste is created by all forms of cassava processing. For example, in the artisan production of starch about 60-66% of the fresh root is liberated as waste material (including water). Solid waste from cassava starch processing is divided into three categories:

1. Peelings from initial processing
2. Fibrous by-products from crushing and sieving (pulp waste)
3. Starch residues after starch settling

(1) Magnitude

An indication of the proportion of solid waste produced during cassava processing is shown in Figures 15-21. In starch processing, pulp waste is the main problem, especially for the bigger factories, which produce massive quantities (each year the 51 starch processors in Thailand will generate about 1 million tonnes of pulp waste). Dealing with this waste is difficult, as it is not easily dried, due to its high moisture and starch contents (Sriroth et al., 1999a). Smaller scale processors have less of a problem as the smaller volumes are more easily disposed of. For example, starch processors in Cong Hoa village, Vietnam, produce from each 100 kg of roots: 14.5 kg peel, 23.5 kg pulp and 3.6 kg residue. During the processing season, 130-160 tonnes of residue are produced daily (equivalent to 200 tonnes of residue per family each year). Yet despite such quantities it is not locally regarded as a source of pollution (Viet, 1998). Another report for a similar scale of processing suggests that, on a dry weight basis, pulp constitutes about 20% and residue 0.8-1% of the original roots (Preston and Maurgucito, 1992).

Peel waste is also generated in the production of farinha, gari, and chikwangue. Farinha factories in Brazil generate 2,000-5,000 tonnes of peel each day. Only in the case of chikwangue are there reports of a possible problem. Inappropriate storage of solid waste for long periods is the main issue, especially with heavy rainfall. This culminates in the production of leachate that can contaminate groundwater. In the dry season there is little problem, but if stored through the monsoon season the problem can be quite significant.

(2) Significance

Usually even the small-scale processors manage their solid waste well. There is therefore little or no problem. In many areas, such as Vietnam, despite the large amounts of residue produced, it is usually disposed of quickly - sold as animal feed or dried and used as fuel. In India solid residue is dried and sold as cattle feed or soil conditioner.

Under most conditions, solid waste will not create an environmental problem. However, if conditions for storage are inappropriate, problems can occur during periods of heavy rainfall. In the dry season, there is little problem except for a foul odor.

Visual impacts

(1) Magnitude

Most factories discharge untreated effluent to the land, often in quantities that exceed the field capacity of the soil. This results in stagnant ponds and ditches that increase in number and area as the processing season progresses. These ponds are unsightly and have a negative visual impact on the environment.

Figure 24. Flow diagram of sulfur dioxide used for production of 200 tonnes of cassava starch per day.
Source: Wanlaphathit, 1998.

(2) Significance

The major significance is the nuisance of this impact to a large number of people. This type of pollution reinforces the negative image of starch processing.

Chemical use

Almost all large-scale factories use sulfur dioxide as a bleaching and anti-microbial agent - in Thailand only one of the 59 factories does not use sulfur dioxide (Sriroth et al., 1999b). Sulfur dioxide is discharged mainly to the atmosphere (Figure 24). If used, 1.6 kg of sulfur is burned to sulfur dioxide for each tonne of starch. For production of 200 tonnes of starch, resulting in 2,323 tonnes of waste water, about 38 and 237 kg of sulfur dioxide were found in the starch and waste water, respectively. The rest is released to the atmosphere. Release of sulfur dioxide to the atmosphere by a cassava processor is assumed to be safe, but no systematic study has been undertaken to substantiate this assumption.

Atmospheric pollution

Hydrogen cyanide

As already mentioned, most of the cyanide released during cassava processing ends up in the waste water and will eventually evaporate into the atmosphere without detrimental effects. However, the effect of volatile HCN on the health of cassava processors may be a problem that requires further study, especially in the design of toasters and boiling equipment. Processors (mainly women and children) producing gari in ill-ventilated sheds are often exposed to high levels of HCN liberated during frying. Design of the processing facility with adequate ventilation is critical. In Thailand, there are reports (K. Sriroth, personal communication) that high-speed grating machines produce levels of HCN in dust and water sprays that effect the health of workers if located in a not well-ventilated space. This needs further study.

Dust

Dust can also be a problem in areas of intensive production of cassava; as high as 10% of the dry weight of cassava can be lost as dust. Reference This is mainly a problem in large-scale chipping and drying operations (Thailand).

Odor

Odor is generated from an uncontrolled fermentation of the organic matter in cassava processing waste. This is a cited problem in Vietnam, India, and Thailand, especially during the processing season. Odor is the most noticeable and widespread impact. It affects the greatest number of people and increases in impact as the processing season progresses. Despite being unpleasant and widespread, the significance of this impact is small. The biggest problem is its contribution to the image of cassava processors.

Effect of cassava processing on forest resources

Wood is the principal energy source where heat is required in small-scale cassava processing, e.g. boiling, drying or toasting. The use of wood is common in Africa (gari, attieke, chikwangue), in the Caribbean (starch) and Brazil (farinha). Wood supply is an increasing problem in many cassava-growing areas.

Conclusions

The main problems of cassava processing are its unattractive visual display and odor. The impact of other forms of pollution is generally not as large as one might imagine given the magnitude of the visual display. Different types of products of cassava processing will each impact the environment differently; these are summarized in Tables 28 and 29.

Table 28. Summary of the magnitude and significance of the broad-scale and site-specific impacts of cassava starch factory wastes on the environment.

Source: ERM, 1996.

Table 29. Processed cassava products and their impact on the environment.

¹⁾ Assuming no re-cycle system
Source: C. Oates, unpublished