Irrigation in Developing Countries Using Wastewater - Institute

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Irrigation in Developing Countries Using Wastewater - Institute

Transcript Of Irrigation in Developing Countries Using Wastewater - Institute

International Review for Environmental Strategies
Vol. 6, No. 2, pp. 229 – 250, 2006 © 2006 by the Institute for Global Environmental Strategies. All rights reserved.
Special Feature on Groundwater Management and Policy
Irrigation in Developing Countries Using Wastewater
Blanca Jiméneza
Wastewater is an important source of water and nutrients for irrigation in developing countries, particularly but not restricted to those located in arid and semi-arid areas. The use of wastewater is widespread and represents around 10 percent of the total irrigated surface worldwide, although varying widely at local levels. While the use of wastewater has positive effects for farmers, mainly related to their income level, it also has negative effects on human health and the environment. The negative effects impact not only farmers but also a wide range of people. Because wastewater reuse is currently necessary, it is important for governments to put in place wise but feasible management practices, such as the ones discussed in this paper, to improve the benefits while reducing and controlling the drawbacks. In order to implement sustainable reuse of wastewater and to contribute to food security, reuse projects need to be planned and constructed for the long term and based on local needs.
Keywords: Agriculture, Effects, Management practices, Non-intentional reuse, Wastewater.
1. Introduction
Irrigation is a key factor in securing food supplies in many developing countries. Of the world’s total arable land, 17 percent is irrigated and produces 34 percent of the crops (Pescod 1992). Three-quarters of the irrigated area (192 million hectares) is located in developing countries (United Nations 2003), and as a consequence there is a high dependence on water for food production (figure 1). Frequently in these countries, wastewater is used to irrigate land because of high demand for water (70 percent of total use), the availability of wastewater, the productivity boost that the added nutrients and organic matter provide, and the possibility to sow all year round. Wastewater irrigation can be very important locally.
Wastewater is used to irrigate in many forms. It can be used as treated (reclaimed water) or nontreated (raw wastewater) and it can be applied directly to crops or indirectly after discharge and dilution with water from rivers or reservoirs. Sometimes reuse is part of a planned project, but most of the time—and particularly in developing countries—it just happens. In industrialized countries water reuse is part of a strategy to protect water bodies and to reduce wastewater treatment costs. It is usually performed only after high ecological standards of wastewater treatment have been achieved, and as a consequence reclaimed water has a low organic matter and nutrient content. In contrast, in developing countries reuse is frequently a spontaneous response to a shortage of water and job opportunities. It is generally practiced with “poor quality” water (even raw wastewater), which farmers like for its
a. Treatment and Reuse Group, Instituto de Ingeniería, Universidad Nacional Autónoma de México (UNAM), México.
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fertilizing properties but mostly because it is the only way to earn a living (Jiménez and Garduño 2001; IWMI 2003).

Figure 1. Freshwater withdrawals for agricultural use in 2000
Source: World Resources Institute 2000.
Wastewater can even be used for agricultural irrigation in cities. This “urban agriculture” is practiced in urban and peri-urban areas of arid or wet countries, depending on wastewater availability, local demand for fresh food products, and people living on the verge of poverty who have no job opportunities. Wastewater flowing in open channels is used to irrigate very small plots of land where trees, fodder, or any other product that can be introduced to the market in small quantities (flowers and vegetables) or be used as part of the family diet are grown (Cockram and Feldman 1996; Ensink et al. 2004b).
Like any activity, the use of wastewater to irrigate has both advantages and drawbacks. This paper discusses these aspects, and based on scientific work and practical experiences it proposes ways to obtain maximum benefits while reducing the risks.
1.1. Advantages of using wastewater for agricultural irrigation
x It permits higher crop yields, year-round production, and enlarges the range of crops that can be irrigated, particularly in (but not limited to) arid and semi-arid areas.
x Recycles organic matter and other nutrients to soils. x It therefore reduces the cost of fertilizers (or simply makes them more accessible to poor farmers). x Reduces the use of synthetic fertilizer.
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x Acts as a low-cost wastewater disposal method that can also be hygienic (under controlled conditions).
x Avoids discharging pollutants to surface water bodies (which have a considerably lower treatment capability than soils).
x Increases the economic efficiency of investments in wastewater disposal and irrigation. x Conserves freshwater sources and reduces negative impacts on surface water bodies. x Can recharge aquifers through infiltration. x Improves soil properties (soil fertility and texture). x The cost of pumping wastewater from nearby channels is lower than the cost of pumping
groundwater. x It offers additional benefits such as greater income generation from cultivation and marketing of
high-value crops, which contribute to improved nutrition and better education opportunities for children.
1.2. Risks and drawbacks of using wastewater for agricultural irrigation
x To maximize the benefits and minimize drawbacks, wastewater reuse must be carefully planned. x Because the impact of pollution is generally less and takes longer in soils (and aquifers) than in
surface water, some governments may delay the construction of necessary wastewater treatment facilities. x Water salinity and metal content in soils is increased in the long term. x Storage capacity is needed to adapt/reconcile continuous wastewater production with crops’ water demand and water supplied by precipitation. x Under non-controlled conditions (a) pathogens contained in wastewater can cause health problems for humans and cattle; (b) some substances that may be present in wastewater can be toxic to plants, cattle, or humans consuming crops; (c) some substances that may be present in wastewater can reduce soil productivity; and (d) infiltration of wastewater to aquifers may cause aquifer pollution with pathogens and organic matter.

2. Extent of wastewater use

There is no complete global inventory on the extent to which wastewater is used to irrigate land, mostly due to a lack of heterogeneous data and the fear that countries have about disclosing information; economic penalties can be imposed if produce is found to have been irrigated with low-quality water.1 Nonetheless, the global figure commonly cited is at least 20 million hectares in 50 countries (around 10 percent of irrigated land) are irrigated with raw or partially treated wastewater (United Nations 2003).
It is also estimated that one-tenth or more of the world’s population consumes crops irrigated with wastewater (Smit and Nasr 1992). Of course, wastewater use varies considerably from one region to another. In Hanoi, Vietnam, for instance, up to 80 percent of vegetables produced are irrigated with wastewater (Ensink et al. 2004a). The regional situation certainly depends on the level of wastewater
1. For instance, Jordan’s export market was seriously impacted in 1991 when countries from the Arabian Peninsula and the Persian Gulf restricted imports of fruit and vegetables irrigated with inadequately treated wastewater (McCornick et al. 2004).

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treatment (35 percent on average in Asia, 14 percent in Latin America and the Caribbean, and levels approaching 0 percent in Africa [WHO and UNICEF 2000]). And because the cost of improving sanitation is considerable compared to other needs, it is estimated that, for the foreseeable future, untreated wastewater will continue to be used for irrigation. Figures 2 and 3 are a collection of nonhomogeneous data from different countries that gives an idea of the number of hectares irrigated with treated and non-treated wastewater.

0
Mexico Egypt Israel Cyprus
Argentina Chile
United States Australia Jordan Kuwait Turkey Tunisia
Saudi Arabia Germany
South Africa France Bahrain
Sweden Japan* Oman*

10,000

20,000

30,000

40,000

50,000

Hectares 60,000 70,000

Figure 2. The number of hectares irrigated with reclaimed and treated wastewater
Note: Information may vary from source to source. Some countries report agricultural wastewater use without mentioning the amount of hectares involved.
*No data available.

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Hectares 0
China
Mexico India
Colombia Pakistan
South Africa Ghana
Vietnam Peru
Morocco Tunisia Italy
Argentina Sudan Kenya Egypt
Greece* Iran*
Lebanon* Nepal*
Palestine* Senegal*
Syria* Yemen* Zimbabwe*

20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 180,000 200,000 1,330,000 ha

Figure 3. The number of hectares in selected countries irrigated directly and indirectly with wastewater
Note: Information may vary from source to source. Some countries report agricultural wastewater use without mentioning the amount of hectares involved.
*No data available.

3. Effects on human health
Surprisingly, the health effects of irrigating with wastewater can be both positive and negative. The positive effects have not been fully studied, but they have begun to be recognized in literature and are related to food security in poor areas. Thanks to wastewater, it is possible (and commonly the only way) to produce food and increase income in poor areas, thus also increasing nutrition and the quality of life. Malnutrition plays a significant role in the death of 50 percent of all children in developing countries (10.4 million children under the age of five die annually from it, according to Rice et al. [2000]). A study in Tanzania showed that a village where a rice irrigation scheme had been developed with wastewater had more malaria vectors than a nearby savannah village but a lower level of malaria
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transmission. The village with the irrigation scheme had more resources to buy food, children had a better nutritional status, and the villagers were more likely to buy and use mosquito nets (Ijumba 1997).
Negative effects are due to the presence in wastewater of pathogens and toxic chemical compounds. Four groups are at risk: (1) agricultural workers and their families; (2) crop handlers; (3) consumers of crops, meat, and milk; and (4) those living near the areas irrigated with wastewater, particularly children and the elderly. Wastewater contains a variety of excreted organisms, and the types and concentrations vary depending upon the background levels of disease in the population. Many pathogens can survive for long enough periods of time in soil or on crop surfaces and thus be transmitted to humans or animals. The most environmentally resistant pathogens are helminth (parasitic worm) eggs, and they are recognized as the main health risk in the use of wastewater for irrigation because of their resistance and persistence (WHO 1989), particularly for developing countries where levels found in wastewater are seven to 80 times greater than those found in developed countries’ wastewater (Jiménez 2003).
Helminthiases (infestation with parasitic worms) are common diseases with an uneven distribution around the world. In developing countries, the affected population is 25–33 percent, whereas in developed ones it is less than 1.5 percent. The problem is more severe in regions where poverty and poor sanitary conditions prevail; under these conditions helminthiasis reaches 90 percent of the population (Bratton and Nesse 1993). There are several kinds of helminthiasis; ascariasis is the most common and is endemic in Africa, Latin America, and the Far East. There are 1.3 billion infections globally. Furthermore, even though it is a disease with a low mortality rate, most of the people affected are children under 15 years with problems of faltering growth and/or impaired fitness. Approximately 1.5 million of these children will probably never catch up, even if treated (Silva et al. 1997).
Besides helminthiasis, other diseases related to the use of wastewater are as follows: cholera, typhoid, shigellosis, gastric ulcers caused by Helicobacter pylori, giardiasis, amebiasis, and spoon-shaped nails (Blumenthal and Peasey 2002). There are gender implications of using wastewater, because crops such as vegetables need a high labor input, which is often supplemented by female households. Transfer of pathogens to other family members could occur if basic standards of hygiene are not maintained when women return to household activities and do evening cooking chores (Van der Hoek et al. 2002).
Regarding chemical compounds in wastewater, the major health concern is due to metals. Many of them are biologically beneficial in small quantities but become harmful at high levels of exposure. For some, no human toxicological threshold has yet been established for wastewater intended for irrigation (i.e., cobalt and copper) or the thresholds are rather high (i.e., boron, fluorine, and zinc). Cobalt, copper, and zinc are not considered here because plants are not likely to absorb them in sufficient quantities to prove harmful to consumers and are toxic to plants far before reaching a content that is toxic to humans (Chang et al. 2002). There is a limit for hexavalent chromium, however, because it is rapidly reduced to trivalent chromium, which forms a less soluble solid phase in wastewater or soils. Cadmium is the metal that causes the largest risk. Its uptake can increase with time, depending on soil concentration, and is toxic to humans and animals in doses much lower than those that visibly affect plants. Absorbed cadmium is stored in the kidney and liver, but meat and milk products are unaffected (Pescod 1992).

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Cadmium is a particular concern when industrial wastewater alone or mixed with sewage is used to irrigate.
Wastewater contains a wide variety of organic compounds, some of them toxic or having cancer or embryo/fetal effects. The specific effect depends on the type of compound, its concentration, and the route and duration of exposure. Normally, the effects are long term. Of particular concern is a specific kind of organic compound, named endocrine disrupters,2 that has been recently identified in municipal wastewater. Endocrine disruptors derive from many sources, including pesticides, persistent organic pollutants, nonionic detergents, and human pharmaceutical residues. Many of these substances are resistant to conventional wastewater treatment and may persist in the environment for some time. Human health effects potentially linked to exposure to these chemicals include breast, prostate, and testicular cancer; diminished semen quantity and quality; and impaired behavioral/mental, immune, and thyroid function in children. Although direct evidence of adverse health effects in humans is lacking, reproductive abnormalities, altered immune function, and population disruption potentially linked to exposure to these substances have been observed in amphibians, birds, fish, invertebrates, mammals, and reptiles (WHO 1999).
Organic compounds (including endocrine disrupters) have not been studied to a large extent, but in general it is known that even if recalcitrant in water they are reduced by several mechanisms in soils (British Geological Survey et al. 1998). And if wastewater is treated, they are at least partially removed. Nevertheless, these health risks associated with chemicals found in wastewater need to be given more attention, particularly in developing countries where the pace of industrialization is accelerating without proper treatment and disposal. In these countries, municipal and industrial wastewater are often not segregated—creating a potentially dangerous mixture of toxic substances that must be handled cautiously. And, in particular, care must be taken with phtalates isolated from aquifers that have formed with the infiltration of wastewater used to irrigate land (Jiménez 2004; British Geological Survey et al. 1998).

4. Effects on soils

Soil is a very complex mixture of mineral and organic substances in concentrations that vary widely in different regions and climates. For this reason, it is very difficult to say whether wastewater compounds and in what concentrations cause problems or provide benefits. Nonetheless, it is currently known that the most visible effect of using wastewater for irrigation is a productivity increase due to its content of nutrients and organic matter (Mara 2003; U.S. EPA 1992). Nutrients make wastewater an effective fertilizer, while organic matter improves soil texture.
Nitrogen is present in several chemical forms (nitrate, ammoniacal nitrogen, organic nitrogen, and nitrites). Most crops only absorb nitrates, but the other forms are transformed into them in soils (National Research Council et al. 1996). Nitrates are very soluble in water, and as a consequence they are washed out of soil by irrigation and polluting aquifers or surface water bodies. It is therefore

2. Chemicals that mimic hormones or have anti-hormone activity and interfere with the functioning of endocrine systems in various species.

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important to adjust the amount of nitrogen added with the wastewater. This amount depends on the soil’s original nitrogen content (0.05–2 percent) and crop demand (from 50 to 350 kilograms of nitrogen per hectare [kg/ha] [Girovich 1996]), values that are equivalent to irrigation rates of 125–875 millimeters for domestic wastewater with a medium nitrogen content and that indicate, for most crops, that wastewater has a greater nitrogen content than needed. With phosphorus, it is the opposite. Phosphorus is very scarce in soils and must almost always be added. Wastewater normally contains lower amounts of phosphorus than required by crops (6–12 milligrams of phosphorus per liter [mg/L]), and does not negatively impact the environment, even if applied for long periods through effluents, because it is stable and can be accumulated in soils (Girovich 1996). The third macronutrient, potassium, exists in high concentrations in soils (3 percent) but is not bio-available to plants. Approximately 185 kg/ha of potassium are required and sewage can supply part of this demand (Mikkelsen and Camberato 1995).
Besides adding nutrients, irrigating with wastewater enriches the humic content by supplying organic matter, which increases soil humidity, retains metals (through cationic exchange and the formation of organo-metallic compounds), and enhances microbial activity (Ortega-Larrocea et al. 2002). If organic matter content in wastewater is less than 350–500 mg/L, all these effects enhance soil productivity by avoiding soil clogging. Recycling nitrogen, phosphorus, potassium, and organic matter to soil is important because it closes their ecological cycles instead of interrupting them, as is traditionally done when these compounds are removed from wastewater, trapped into sludge, and dumped with it in confinement sites or landfills. But in the case of phosphorus, recycling is even more important because its reserves are limited and dwindling; recycling it is even being promoted by the phosphate industry (CEEP 2001).
Irrigating with wastewater also has negative effects on soils. The most common one reported is an increase in metal content that, depending on the level, may or may not be harmful. The use of domestic wastewater (treated or not) to irrigate results in the accumulation of metals in upper layers of soil with no negative effects on crops, even when applied over long periods of time (several decades). However, wastewaters containing industrial effluents with high metal contents not only accumulate metals but also cause damage to crops and eventually to consumers. Regardless of the wastewater metal content, for metal uptake by crops a certain level has to be reached in soils but also be present in the mobile fraction. Metals are fixed to soils with a pH of 6.5–8.5 and/or with high organic matter content. Fortunately, sewage pH is always slightly alkaline (7.2–7.6). This value, combined with an important soil and wastewater alkalinity maintains original soil pH. The elements of major concern are cadmium, copper, molybdenum, nickel, and zinc. In some cases, the presence or absence of other divalent metals in the soil can influence the uptake of heavy metals.
Wastewater containing solids may clog soils, depending on its concentration (100–350 mg/L), soil porosity, and chemical composition (mineral ones that are not biodegraded are the worst). This will require regular soil drying and periodic removal of soil by raking or scraping for infiltration recovery.
Long term, the main problem that water reuse causes is soil and groundwater salinization. This occurs even with freshwater if appropriate soil washing and land drainage are not furnished, and in that sense

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wastewater reuse will accelerate the processes due to a higher salt content. Salinity effects are of concern particularly in arid and semi-arid regions where accumulated salts are not flushed from the soil profile by natural precipitation and where wastewater reuse is a necessity. The salinization build-up rate also depends on the water quality, soil transmissivity, organic matter content, land drainage, irrigation rate, and depth to the groundwater level. Depending on the type of soils and the washing and drainage conditions, salinity problems can occur with conductivities greater than 3 deciSiemens per meter (dS/m) in dissolved solids greater than 500 mg/L (being severe if greater than 2,000 mg/L), chlorine less than 140 mg/L, and a sodium absorption ratio (SAR) greater than 3–9. Other problems related to salinity are toxic effects caused by sodium, bicarbonates, and boron.
Israel, as an example, uses 70 percent of its municipal effluents for agricultural irrigation and has experience with soil salinization. Because removing salts from wastewater is much more expensive than preventing their entry into it, an extensive salt control program has been adopted. This program includes saline discharge control to sewerage and regulation of the quantity of salts (sodium, boron, chlorides, and fluorides) used for ion exchanger regeneration and in detergents. As a result of this measure, chlorides in sewage have dropped from 120 mg/L in 1992 to 60 mg/L in 2002, and boron has dropped from 0.6 mg/L in 1999 to 0.3 mg/L in 2002 (expected to reach 0.2 mg/L by 2008) (Weber and Juanicó 2004). Certainly, countries reusing wastewater to irrigate will follow this example.

5. Effects on crops

There are two types of effects on crops: (1) those that affect yields and (2) those that modify crop quality (appearance, flavor, or pollutant presence). As already mentioned, yield is in general increased by the fertilizing compounds present in wastewater, but it can also be diminished if toxic compounds are present. For instance, nitrogen applied to plants when it is not needed may induce more vegetative than fruit growth and also delay ripening. This has been observed for beets, cane, and rice (Pescod 1992; Morishita 1988). Concerning phosphorus, high contents (above those commonly present in municipal wastewater) reduce copper, iron, and zinc availability in alkaline soils. Boron is toxic to several crops. Salinity, besides reducing soil productivity, increases salt content in crops. This can be a problem for some crops such as vineyards for wine production. And crops’ appearance is affected by chlorides (less than 140 mg/L in sensitive ones or greater than 350 mg/L in resistant ones) and carbonates (greater than 500 mg/L of calcium carbonate). Both compounds burn leaves when sprinklers are used to irrigate (Pescod 1992).
Concerning pollution, crops can be contaminated with microbes, heavy metals, and organic toxic compounds (in that order of frequency and importance). Contamination can happen by direct contact of irrigation water with edible parts or, in the case of metals, through absorption from soils, depending on environmental conditions and the type of plants. Crop pollution depends not only on water quality but also on agricultural practice (quantity of water applied and irrigation method). Oron et al. (1992) and Najafi et al. (2003) found that microbial pollution is reduced if irrigation is performed by subsurface dripping rather than through sprinklers or furrows. Pollution depends also on the type of crops. For example, Armon et al. (2002) found that zucchini spray-irrigated with poor-quality wastewater

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accumulated higher levels of cryptosporidium oocysts (160–20,000 oocysts/kg) on the surface than other types of crops. Zucchini has hairy, sticky surfaces and grows close to the ground and therefore may concentrate certain types of pathogens on its surface. Trees are less likely to produce polluted fruits because they are located far from the irrigating sites and polluted soils. Crop contamination occurs not only as a result of wastewater irrigation but also during washing, packing, transportation, and marketing, which are frequently not addressed by water reuse criteria, giving the impression that irrigation is the only problem.
Generally speaking, toxic organic compounds have a large size and high molecular weight that do not allow them to be absorbed by plants (Pahren et al. 1979), but some toxic organic compounds present in wastewater can remain in fruits and leaves by direct contact. Pesticides are a great concern, but the main polluting pathway is their direct application to fields rather than their introduction through wastewater. Endocrine disruptors might also pose some concerns. Mansell et al. (2004) have demonstrated that hormones like 7-estradiol, estriol, and testosterone have very low sensitivity to photodegradation through ultraviolet light exposure over a 24-hour period (less than 10 percent).

6. Effects on cattle

Cattle can suffer health or growth problems if they consume forage polluted with wastewater. Nevertheless, in some areas of the developing word where water is scarce, cattle are not only fed with forage grown with wastewater but also allowed to drink it (Ensink et al. 2004a). Some protozoan can infect animals if they survive in irrigated crops, although this is not the main transmission pathway. There is only limited evidence indicating that beef tapeworm (Taenia saginata) can be transmitted to the population consuming the meat of cattle grazing on wastewater irrigated fields or fed crops from such fields. There is strong evidence, however, that cattle grazing on fields freshly irrigated with raw wastewater or drinking from raw wastewater canals or ponds can become heavily infected by Taenia, causing cysticerosis (Shuval et al. 1985).
Although no problems have been reported in relation to cattle that consume fodder irrigated with wastewater, fodder irrigated with high nitrogen content water can cause grass tetany, a disease related to an imbalance of nitrogen, potassium, and magnesium in pasture grasses. Cadmium in much lower doses that visibly affects plants may be harmful to animals. Absorbed cadmium is stored in the kidney and liver, leaving meat and milk products unaffected. Something similar happens with copper, which may be harmful to ruminants (cows and sheep but not to mono-gastric animals) at concentrations too low to visibly affect plants. Molybdenum causes adverse effects in animals consuming forage with 10–20 parts per million and low copper content. The consumption of crops with more than five milligrams of molybdenum per kilogram of feed is toxic to cattle, particularly ruminants. Toxicity is related to the ingestion of copper and sulphates.

7. Effects on water

Irrigating with wastewater modifies not only the quality of the wastewater itself but also affects surface and water bodies.

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WastewaterCropsCountriesSoilsWater