TECHNICAL ADVISORY NOTES

I. Increasing the Productivity and Sustainability of Cassava by Better Soil/Crop Management

Cassava is grown on over 16 million hectares worldwide and is a staple food for over 500 million people. The crop is very tolerant of drought and can produce reasonable yields on very poor soils. For this reason it is a favored crop in areas of infertile soil and with low or erratic rainfall. When other crops fail under these conditions, cassava can stave off famine or provide the farmer with at least some income. However, when grown continuously on infertile soils without inputs of fertilizers or manures, soil nutrients may become depleted leading to a decrease in soil productivity. In addition, when grown on slopes, cassava provides little protection from the direct impact of rainfall during the first 3-4 months after planting, which can result in serious erosion. Better soil/crop management can help to reduce these problems while at the same time increasing yields and income.

Improved nutrient management will increase yields - now and in the future

Although cassava grows better than most other crops in poor soils, it will yield better in more fertile soils and responds well to fertilizer and manure applications. Moreover, with the use of higher yielding varieties, nutrient absorption and removal in the harvested products increases. Unless these nutrients are replaced the soil becomes poorer.

Major nutrient requirements: As a starch-producing root crop, cassava requires mainly potassium (K) for translocation of carbohydrates from leaves to roots. The crop also requires nitrogen (N) to stimulate leaf formation and photosynthesis, but requires smaller amounts of phosphorus (P). In most sandy or sandy loam soils that are low in organic matter the crop responds well to applications of 50-100 kg N/ha (100-200 kg urea), 10-20 kg P₂O₅/ha (50-100 kg simple superphosphate) and 50-100 kg K₂O/ha (80-160 kg potassium chloride). Only in some soils that are extremely low in P (mainly in Brazil and Colombia) does the crop respond to higher rates of P application. In that case high rates of 100-200 kg P₂O₅/ha need to be applied, but only for a few years to build up the available P content of the soil. Simple or triple-superphosphate and rock phosphates are good sources of P.

Lime requirements: Cassava is highly tolerant of soil acidity and seldom responds to applications of lime. In fact, high lime applications can reduce yields by causing more severe deficiencies of micronutrients, particularly zinc (Zn).

Micro-nutrient requirements: In most soils there is no need to apply micro-nutrients. However, in calcareous soils the crop may suffer from zinc and/or iron deficiency. In that case, dipping the stakes in a 2% solution of zinc or iron sulfate for 15 minutes before planting is recommended.

Fertilizer application: Fertilizers should be applied close to but never in direct contact with the planting stakes. Applications of NPK fertilizers in small bands at 5-10 cm from the stake or base of the plant is recommended. Bands are made with a hoe and are covered over after fertilizer application. Fertilizers, especially P, should be applied early in the growing cycle, normally at or one month after planting; N and K applications are sometimes split, half applied at planting and half at two months after planting.

Manure application: Application of animal manure will not only supply nutrients but also organic matter, and may improve the soil's physical condition. Applications of 5 t/ha of chicken or cattle manure in combination with N and K fertilizers is recommended. If no fertilizers are available, wood ash can substitute for K-fertilizers. Manures are generally broadcast and incorporated into the soil before planting.

Integrated crop/soil management will increase yields and reduce erosion

Soil erosion occurs when rainfall strikes the soil surface and dislodges soil particles, which are subsequently carried down-slope with runoff water. The result is a loss of top soil, loss of organic matter, clay, soil nutrients and fertilizers, all leading to a reduction in soil depth and soil productivity. Because of slow initial growth and wide spacing, cassava production on slopes can cause serious erosion. However, research has shown that better crop/soil management practices can markedly reduce erosion, either by providing more rapid soil cover or by reducing the flow of runoff.

Providing soil cover: Cassava canopy formation can be enhanced by the use of vigorous and high-yielding varieties, good quality planting material, closer plant spacing (0.8x0.8m), and especially, by fertilizer or manure application. Intercropping with maize, peanut, bean, cowpea, melon and pumpkin can also provide a rapid soil cover to protect the soil from rainfall impact. Application of mulch of grass or crop residues can also provide soil cover without causing competition.

Reducing or slowing runoff: Soils with high levels of organic matter, with worm and root channels and with stable soil aggregates provide pathways for rapid water infiltration, thus reducing runoff. Fallowing and the incorporation of green manures, animal manures and crop residues will enhance these favorable physical conditions. Runoff flow can be markedly reduced by maintaining a rough seed bed through minimum tillage, or by contour tillage, contour ridging or the planting of contour hedgerows of grasses (e.g. vetiver grass, lemon grass, Paspalum atratum) or legumes (e.g. Leucaena, Gliricidia, Stylosanthes). Alternatively, farmers can stake out contour lines with an A frame or a simple line level and leave a 50-100 cm strip of unplowed and unweeded land to form natural grass-covered contour strips. Contour hedgerows or grass strips help reduce the speed of runoff and trap eroded soil sediments and fertilizers. With time, natural terraces are formed, thus reducing the slope of the cultivated plots and enhancing water infiltration. Farmers have many options to improve soil fertility and reduce erosion, but some require additional capital or labor. Yields may increase, but can also decrease due to crop competition from intercrops or by reducing the land available for cropping (e.g. by planting grass barriers). Thus, farmers should carefully weigh the pros and cons of each option and choose the best combination of practices for their own conditions.

Supporting References:

1) Developing sustainable cassava production systems with farmers in Asia. R.H. Howeler. In: Systems and Farmer Participatory Research, Developments in Research on Natural Resource Management. CIAT, Cali, Colombia. 1999.

2) Soil fertility maintenance and strategies for cassava production in West and Central Africa. J.A. Okogun, N. Sanginga and E.O. Adeola. IFAD Stakeholder Consultation Meeting. Accra, Ghana. 1999.

II. Minimizing Environmental Impact through Sustainable Cassava Processing Methods: Small-, Medium- and Large-scale Starch Processing

Every year millions of tonnes of cassava are converted to intermediate and higher-value products. For all, in addition to the desired product, by-products are co-generated, which, if incorrectly managed or controlled, will have significant negative impacts on the environment. The cost of treating these products if they are allowed to accumulate can be on a par with that of primary processing.

Choosing a site location

Adequate local land and water resources should be available to supply the required quality and quantity of raw materials and processing inputs (such as water) without causing unacceptable environmental impact. The processor should have access to additional land area to accommodate waste disposal from planned and expanded processing capacity. Minimal land requirements should be assessed on a case-by-case basis, as size of the processing operation, type or sophistication of processing technology used, and local geology will all contribute to the estimate.

The processing site should be in close proximity of receiving land or water capable of handling the effluent discharge without significant impact on the biological environment. These discharge sites are not to include important rivers, streams, or sources of drinking water. Similarly, land should not be densely populated, close to water abstraction points or near to drinking water sources. The processing unit should not be located in an area where it compounds the problems of an already contaminated water source. Facilities should not be sited in areas heavily affected by industrial discharges, because of the risk of product contamination. Nor should the facility be sited in environmentally sensitive areas or at locations where wastes cannot be assimilated without environmental degradation.

Starch processing requires large volumes of water; thus, to prevent conflict with other users or overburdening the supply system, the processing operation should not be within a pollution hotspot zone. These zones are identified as having a large number of factories within close proximity of each other, close to large rural communities and having potential water supply problems (high demand, poor recharge rates).

Water supply must be consistent and not placed under pressure by the entrance of new processing activities; groundwater recharge rates should be sufficient to sustain demand from the processing operation. If surface water is the supply source, seasonal fluctuation should be minimal and water quality constant. Industrial and agricultural activities upstream need to be evaluated and any impact of their discharge on starch quality considered.

Evidence suggests that most of the environmental impact of cassava starch processing is site-specific. Rationalizing the concentration of processors within an area will minimize site-specific problems. Distribution strategies will be balanced with a need to contain resources; they also depend on whether or not a communal waste disposal system can be considered.

Choosing a technology

Small- to medium-scale

Upgrading traditional processing technology should be balanced with sustainability. Specific technology that disturbs the balance between processor and environment should not be used. The technology should not place additional strain on the system, either by increased demand for processing inputs or the generation of larger amounts of waste that are not easily disposed of. Technology adoption should be appropriate and tested. Specific technology requirements are:

1. Root grating. Roots must be grated or milled finely, in order to wash out starch granules; fibers still contain starch. Pulp waste with a lower content of starch is easier to handle. Although a hammer mill may be used, there could be a substantial damage to the starch granules.

2. Sieving out fiber. Before sedimentation, the starch slurry should be passed through a screen no coarser than 120 mesh (125 µm). This removes the remaining fiber particles that may be instrumental in the microbial deterioration of the starch. This will also lead to some loss of large starch granules, but this amounts to less than 1%.

3. Settling. Settling tanks should be extensive and shallow, rather than compact and deep. This will ensure a more efficient settling of the starch granules and hence purer discharge water.

4. Storage of wet starch. Starch should not be kept wet for more than 24 hours. If kept wet too long, the starch granules may erode due to residual enzyme activity; this will create waste water high in soluble solids and odor problems at the processing site.

5. Cleaning of factory equipment at stops. Equipment should be cleaned thoroughly after each prolonged factory stop; if this is not done, the new start will be accompanied by increased microbial activity.

Large-scale

The technology for large-scale factories should be new, and of original design. Locally fabricated equipment is difficult to calibrate, this often leading to inefficiencies, either in water use or starch separation (high waste water BOD). The factory design should include a water re-circulation system with appropriate control. A water purification system should also be included. The choice of separator should be considered carefully. Centrifuges using cloth screens result in faster build up of organic material in the re-cycle water, which can be better controlled if hydrocyclones are used. Adequate processing space and ventilation are essential to minimize the risk of cyanide poisoning.

Conserving Water

Starch processing requires 15-76 m³ of water/t starch. Processors should target to be at the lower end of this range. Processors must develop an increased awareness of the importance of minimizing water utilization. Efficient use of water can be promoted by adopting a suitable recycle system.

Small- to medium-scale

The strategy for recycling water will be determined by the size of the processing facility. For the small units simply re-using the water removed from secondary sedimentation for root washing will be of benefit. Through water conservation measures and recycling systems both the volume and loading of waste water are reduced. This has benefits in lowering the cost of effluent treatment as this is related to the quantity of waste water.

For medium-scale processors, incorporation of hydrocyclone units in the cassava starch factory should be promoted. Use of this technology can lead to reduction in the volume of fresh water required for root crushing and starch separation by 50-60%, and reduction in waste water volume by 40-50%. Achieving a reduction in the volume of waste water will allow for a smaller effluent treatment plant.

Large-scale

Large-scale processing units must use water re-circulation systems to minimize their daily demand for water. The water should be purified and its pH adjusted at regular intervals to ensure efficient re-circulation.

Dealing with waste water

Waste water released directly to the environment is a source of pollution and subsequent environmental problems. Simply disposing of the waste water directly to the environment (fields, ditches and surface water) should be discouraged. Proper waste water management is not usually a problem. However, the water originating from settling tanks or decanters is potentially polluting because it can have a high concentration of organic matter, cyanide and processing chemicals.

Small- to medium-scale

In a situation where the only alternative is to discharge to surface water, this should not be to slow moving water bodies, as problems will be compounded; local lakes and ponds are therefore not suitable for receiving waste. This outlet for effluent disposal should be used sparingly and only small-scale processors should avail themselves of this course of action.

From larger processing units water should not be released directly into surface water systems. Prior reduction in the organic material and cyanide must be achieved before release. For small- and medium-scale starch processors effluent seepage pits can be constructed and operated. These pits serve the dual purpose of containing wastes and providing a rapid pathway for groundwater recharge. These seepage pits should be sited over permeable soils away from natural water courses and ground water abstraction points. The pits should be suitably designed and carefully managed and maintained.

In certain areas, particularly where factories are clustered or located close to populated areas, more sophisticated waste treatment systems should be considered. These may be individually owned or be a common effluent treatment facility. The simplest technology available is to store the effluent in settling tanks for several days; this will lower BOD and COD and ensure that most cyanide has evaporated. Effluent pits or channels can be used, but the geology of the site should be reviewed and if necessary, the pits or channels lined. Ponding systems and anaerobic reactors are suggested, the choice dictated by available resources (land, technical and financial) and a need to minimize the nuisance factor of open ponds. The ponds should be constructed after due consideration of the site's geology, and all necessary precautions taken that water will not seep into the groundwater supply.

Waste water generated by smaller units may be used directly for irrigation, ideally after treatment, but if this is not possible, after dilution. Cyanide levels should be reduced to less than 0.3mg/l before discharge. Irrespective of whether the waste water is treated or used directly, it must be carefully monitored to ensure that the cyanide level is not too high.

Large-scale

Large-scale processors must treat the waste water before its release to the environment. The simplest technology suitable for this scale of operation is a ponding (lagoon) system requiring as many as 20 ponds. The first ponds should be anaerobic, to reduce odor problems. A more efficient high-rate anaerobic reactor, such as UASB, is recommended.

Dealing with solid waste

The main problem from solid waste is that of foul odor, especially from the final slurry waste. Careful storage, ideally in sealed tanks, will help reduce this problem. Dry waste stored on-site should be stored on concrete paving and under-cover.

Solid waste must be stored in a well-managed manner; it should be covered and protected from the rains. Apart from odor and visual aspect, the main problem from solid waste is that of leachates formed by rain. The solid waste should be stored for a minimum period. Adequate market outlets should be established early in any development process for solid waste products; this may require development of suitable enterprise linkages.

Small- to medium-scale

Solid waste generated by cassava processing should be sold or used quickly. Excess may be land-filled if the necessary precautions have been taken, and if sufficient land area is available. Ideally the waste should be sold or used for composting or animal feed. As a feed, it can be used directly or after partial breakdown through fermentation to improve bioavailability and reduce the cyanide content. The simplest approach would be to ensile the waste, but other possibilities are available. Care should be exercised in the formulation of feed rations, as the waste from cassava processing is deficient in protein, and this should be supplemented from other sources.

Large-scale

Large-scale factories generate huge amounts of pulp waste that must be removed from the factory soon after production. Adequate markets for this waste must be found before processing starts. In line with the quantities of pulp waste generated, it is usually the responsibility of a third party to utilize or re-process this material.

Management measures

Environmental monitoring systems should be in place, adopted and carried out by the processing unit owner. At a minimum, these should include keeping records of water in and out and notes on the environmental impact. This may be gauged by changes in the biological habitat in and around the processing unit.

Monitoring by both the factories and officials of waste piles during the off-season should be incorporated into the management system.

Supporting References:

1) Cassava wastes: Their characterization and uses and treatment in Brazil. M.P. Cereda and M. Takahashi. In: Cassava Flour and Starch: Progress in Research and Development. CIAT publication no. 271, Cali, Colombia. pp. 221-232. 1996.

2) Processing pf cassava waste for improved biomass utilization. Kanarong Sriroth, R. Chollakup, S. Chotineeranat, K. Piyachomkwan and C.G. Oates. Bioresource Technology 71(1):63-69. 1999.

III. Minimizing Environmental Impact through Sustainable Cassava Processing Methods: Pastes, Chips and Flour

Every year millions of tonnes of cassava are converted to intermediate and higher-value products. For all, in addition to the desired product, by-products are co-generated, which, if incorrectly managed or controlled will have significant negative impact on the environment. The cost of treating these products if they are allowed to accumulate can be on a par with that of primary processing.

Choosing a site location

Adequate local land and water resources should be available to supply the required quality and quantity of raw materials and processing inputs (such as water or fuel wood) without causing unacceptable environmental impact. The processor should have access to additional land area to accommodate waste disposal from planned and expanded processing capacity. Minimal land requirements should be assessed on a case-by-case basis as size of the processing operation, type or sophistication of processing technology used and local geology all contribute to the final estimate.

The site, if it generates waste water, should be in close proximity to either receiving waters or land capable of handling the effluent discharge without significant impact on the biological environment. Water bodies should not be important rivers, streams, or sources of drinking water; similarly, land should not be densely populated, close to water abstraction points or near to drinking water sources. Waste discharge from the processing unit should not compound any existing problems of an already contaminated water source or site. Processing units should not be sited in areas heavily affected by industrial discharges because of the risk of product contamination. The facility should not be sited in environmentally sensitive areas or at locations where wastes cannot be assimilated without environmental degradation.

Some forms of cassava processing require moderate quantities of water. If sited in a region of water scarcity care should be taken to minimize the impact. Ideally, the technology/process should be adapted to suit the natural resources available. Processing units should be located away from environmental hotspot zones, regions characterized by a large number of factories within close proximity of each other, close to large rural communities and having potential water supply problems (high demand, poor recharge rates).

Choosing a technology

Upgrading traditional processing technology should be balanced with a continued need for sustainability. Specific technologies that disturb the balance between processor and environment should be avoided. Particular care is required if automation of a traditional process is planned. The added impact to the environment must be carefully determined. Adopted technology should not place additional strain on the system by an increased demand for processing inputs or generation of larger amounts of waste. The added waste volume will be more difficult to disperse through traditional outlets. Technology adoption should be appropriate and tested.

Cyanide is released during peeling, processing and cooking. This can be a problem if the processing facility is not of sufficient volume, or if the airflow is too low. Precautions should be made to protect the operators. Adequate ventilation should be provided. Better roasting equipment should also be adopted.

Conserving water

The production of many types of cassava products does not require water, and those that do, require only a modest amount of water. Water is used for either washing, retting or soaking cassava. The problem often lies with the water-scarce location of many cassava-processing regions.

Processors must develop an increased awareness of the importance of minimizing water utilization. For small-scale processing, this can contribute significantly in lowering overall water demand. Close adherence to local practices is recommended. In many areas, especially Africa, these have become highly adapted to local conditions. Water should be conserved, either by re-using soaking water or adopting procedures that require less water, such as retting in dry conditions, i.e. in bags, or solid state fermentation.

Dealing with wash water

At the family-scale, waste water volumes are small. However, because of high cyanide content, this water can be poisonous to small animals. The scale of the problem increases with the scale of production. In regions where many processing units are sited in close proximity of each other the problems are magnified.

Waste water released directly to the environment is a source of pollution and subsequent environmental degradation. Simply disposing of the waste water directly to the environment (fields, ditches and surface water) should be discouraged. Wash water is not a problem, but that from retting or squeezing can be. Water from both sources will contain a high amount of cyanide and organic matter.

In a situation where the only alternative is to discharge to a surface water body, slow moving water should not be used for receiving the waste; local lakes and ponds are therefore not suitable for receiving effluent. This approach to effluent treatment should only be used sparingly and only small-scale processors are to avail themselves of this course of action.

Waste water generated by larger processing units should not be released directly into surface water. Prior reduction of the organic material and cyanide must be achieved before release. For small- and medium-scale processors, effluent seepage pits can be constructed and operated. These pits serve the dual purpose of containing wastes and providing a rapid pathway for groundwater recharge. The seepage pits should be sited over permeable soils away from natural water courses and ground water abstraction points. The pits should be suitably designed and carefully managed and maintained.

In certain areas, particularly where factories are clustered or located close to populated areas more sophisticated waste treatment systems should be considered; these may be individually owned or be a common effluent treatment facility. The simplest technology available is to store the effluent in settling tanks for several days; this will lower BOD and COD and ensure that most cyanide has evaporated. Effluent pits or channels can be used, but the geology of the site should be reviewed and if necessary, the pits or channels lined. Ponding systems and anaerobic reactors are suggested, the choice dictated by available resources (land, technical and financial) and a need to minimize the nuisance factor of open ponds. The ponds should be constructed after due consideration of the site's geology and all necessary precautions taken that water will not seep into the groundwater supply.

Waste water generated by smaller units may be used directly for irrigation, ideally after treatment, but if this is not possible, it must be diluted. Dilution should be to the extent that the cyanide level is reduced to less than 0.3 mg/l before discharge. These precautions are particularly important when treating "squeeze water" waste, because of its high cyanide content. Also, if high cyanide varieties of cassava are processed, waste water treatment is necessary. Irrespective of whether the water is treated or used directly, it must be carefully monitored to ensure that the cyanide level is not too high.

Dealing with solid waste

Solid waste must be stored in a well-managed manner; it should be covered and protected from the rains. The only problem from solid waste is that of leachates formed during heavy rain. The solid waste should be stored for a minimum period. Adequate market outlets need to be established early in the development process for solid waste products; this may require development of suitable enterprise linkages.

The main problem from solid waste is that of foul odor, especially from the final slurry waste. Careful storage, ideally in sealed tanks will help reduce this problem. Dry waste stored on-site should be stored on concrete paving and under-cover.

Solid waste should be sold or used shortly after its production. Excess may be land-filled, following the necessary precautions and if sufficient land is available. Waste could be used as an animal feed, either directly or after partial breakdown through fermentation; this improves bioavailablity and reduces the cyanide content. The simplest approach would be to ensile the waste, but other possibilities are available. Care should be exercised in the formulation of feed rations, as the waste from cassava processing is deficient in protein, and this should be supplemented from other sources.

Dealing with cyanide

Further details of methods for dealing with cyanide waste are discussed in the main text.

Dealing with dust

The production and use of cassava chips is associated with a significant dust problem. For small-scale industries the problem is minimal, but as process capacities increase and the operation becomes semi-automated significant dust discharge is expected. Precautions are limited to care in processing, and locating chipping factories away from populous areas. Workers should be appropriately protected from skin contact with the dust that can cause dermatological problems and from inhalation that can trigger a variety of respiratory diseases. Dust can be minimized by spraying a fine coat of vegetable oil (0.3 - 0.5%) to bind dust. This should be done after a screening stage. Screening will remove the major proportion of dust particles and broken fragments. Equipment for screening should be covered and able to contain the dust.

Management measures

Environmental monitoring systems should be adopted and carried out by the processing unit owner. At a minimum, these should include keeping records of water in and out, and notes on the environmental impact of the processing unit. This may be gauged by biological changes in the environment and around the processing unit.

Monitoring by both the factories and officials of waste piles during the off-season should be incorporated into the management system.

Supporting References:

1) Cassava wastes: Their characterization and uses and treatment in Brazil. M.P. Cereda and M. Takahashi. In: Cassava Flour and Starch: Progress in Research and Development. CIAT publication no. 271, Cali, Colombia. pp. 221-232. 1996.

2) Processing of cassava waste for improved biomass utilization. Kanarong Sriroth, R. Chollakup, S. Chotineeranat, K. Piyachomkwan and C.G. Oates. Bioresource Technology 71(1):63-69. 1999.