Water Use: Recycling and Desalination for Agriculture

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Pereira L.S., Duarte E., and Fragoso R. Water Use: Recycling and Desalination for Agriculture. In: Neal Van Alfen, editor-in-chief. Encyclopedia of Agriculture and Food Systems, Vol. 5, San Diego: Elsevier; 2014. pp. 407-424.

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Author's personal copy

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Water Use: Recycling and Desalination for AgricultureLS Pereira, E Duarte, and R Fragoso, University of Lisbon, Tapada da Ajuda Lisbon, Portugal

r 2014 Elsevier Inc. All rights reserved.

GlossaryBiochemical oxygen demand The amount of dissolvedoxygen needed by aerobic biological organisms in a body ofwater to break down organic material present during 5 daysof incubation at 20 1C (BOD5).Desalination Technological process that allows theremoval of salt and other minerals from saline water toproduce freshwater suitable for human consumption andagricultural or industrial use.Drainage water The water naturally or artificially removedat surface or subsurface from an agricultural area and thatcontains chemicals, mainly salts leached from the soil.Refractory organic chemicals Organic compounds, eithernatural or synthetic, that are resistant to being broken downthrough conventional treatment processes.

cyclopedia of Agriculture and Food Systems, Volume 5 doi:10.1016/B978-0-444

Sodium adsorption ratio (SAR) A measure of the sodicityof soil, hence of the suitability for use in irrigation and isdetermined from the ratio of the concentrations of the Naþ

ion relative to those of the Ca2þ and Mg2þ ions.Water reclamation Treatment or processing of wastewaterto make it reusable.Water recycling The wastewater after appropriatetreatment is redirected back to its original use.Water reuse The use of treated wastewater for beneficialpurposes such as agricultural use.Water scarcity A situation when water availability in acountry or in a region is below 1000 m3 per person per year.Water scarcity refers to various regimes affecting wateravailability: Natural aridity and drought, and man-madewater scarcity and desertification.

Introduction

Global socio demographic and environmental change posesunprecedented challenges to mankind. Drivers of globalchange such as climate change, population growth, urban-ization, industrialization, and rising income, living standard,and energy demand will all characterize the future demand forwater. Population growth and water scarcity also drive theneed to the use/recycling of nonconventional water resources,mainly saline and drainage waters, treated wastewater anddesalinated water for irrigation and other uses in manycountries (Pereira et al., 2009; Wichelns and Drechsel, 2011;Hanjra et al., 2012).

The use of waters with different degrees of salinity, eitherdrainage water or groundwater from low-quality aquifers, is acommon and old practice in water-scarce regions. Differently,the reuse of wastewater or desalinated water is a relativelyrecent practice. All have in common the need to control theimpacts on the environment, to manage crops in agreementwith characteristics of the used water and, in case of waste-water, the need to avoid health consequences for workers andconsumers of the produces.

Appropriately treated wastewater is a drought-proof andrenewable supply of water. Wastewater irrigation can supplyplant food nutrients inexpensively, mitigate water scarcity,save disposal costs, reduce pumping energy cost, and thusminimize carbon emissions. However, the negative health andenvironmental risks of wastewater irrigation need to be ad-dressed, such as pathogens, excess nutrients, salts, toxicelements, and heavy metals (WHO, 2006a,b; Pereira et al.,2009; Hanjra et al., 2012). These negative impacts often resultin negative consumer attitudes toward the use of wastewaterfor irrigation. Nevertheless, awareness of health risks is nothigh among farmers. These facts call attention to the need for

developing educational and awareness programs that maymake easy the adoption of best management practices andacceptability by the consumers.

A better understanding of the positive and negative en-vironmental health impacts of wastewater use in agriculturecan improve management of wastewater use and offer posi-tive-sum solutions for human welfare and the environment.The wastewater demand for irrigation will continue to in-crease, especially by the millions of small farmers who dependon wastewater irrigation (Qadir et al., 2010). Hence, in aimingat health safety of consumers and field workers, the challengeis to satisfy that demand with wastewater treated to the ap-propriate level, which justifies that particular attention is givento wastewater treatment as well as to health-related issues.

Concern about the sustainability of water use for feedingthe future human population is the strong motivation tounderstand the potential of the use/recycling of nonconven-tional water resources and nutrient energy recycling in irrigatedagriculture. These uses call for the development of innovativegovernance strategies to meet the future water demand, toenhance participation of water users in healthy and environ-mentally sound management of water at system and farmlevels. The socioeconomic benefits from treated wastewater usein agriculture (Winpenny et al., 2010) have so far not beenadequately quantified and innovative approaches are required.

Seawater desalination will play an important role in ad-dressing the challenge of global water scarcity. Nevertheless,desalinized water is very expensive and high in energy con-sumption, yet it is available as demineralized. This calls formineralizing it when used for irrigation, so that fertilizers,microelements, and other elements as calcium are added toenhance the balance of nutrients for crops. Differently, usingsaline waters and wastewater requires careful attention to thesalt components to avoid soil degradation by salinity and

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408 Water Use: Recycling and Desalination for Agriculture


plant toxicity by salts or phytotoxic elements and compounds.Attention is therefore given to the impacts on soils and theenvironment.

The sustainable use of nonconventional water resourcesneeds that water uses are mostly beneficial, the water prod-uctivity is high, and the economic results are positive in termsof farming returns. To achieve that sustainable use of water, itis required that the respective pathways be known to clearlyidentify benefits and problems (Figure 1).

When using freshwater or low salinity water, the focus is onwater conservation and saving, thus applying only the waterthat is required for achieving the crop yield, minimizing waterwastages and losses and maximizing the water productivityand the economic return. Differently, when using wastewaterthe priority must be assigned to safe use of water in terms ofpublic health (Figure 1). Maximizing the beneficial uses keepsa high priority for wastewater or saline water uses but an atleast equal priority must be assigned to minimize environ-mental impacts due to pathogens, phytotoxic effect and excessof salts that may degrade the soil, the flora, and the fauna. Thisincludes the preservation of water bodies that function for thedisposal of the excess water and related riparian fauna andflora. Maximizing the water productivity must then be a pri-ority constrained by the control of health and environmentalimpacts and by the economic farm return.

Minimize health impacts

Nonbeneficial

Beneficial

Identify the pathways of nonconventional wa

Waterfor a

Consumedfraction

Minimize environmental impacts

Maximize beneficial uses

Maximize water productivity

Figure 1 Efficient nonconventional water use and identification of related pCoping with Water Scarcity. Addressing the Challenges. Dordrecht: Springer,indicators of water use performance and productivity for sustainable water c

Agricultural Use of Saline Water and Recycling ofDrainage Water

Characteristics and Impacts of Saline Waters

Saline water includes water commonly called brackish, saline,or hypersaline from different sources, including aquifers thatare naturally saline or became saline due to human activities,and drainage effluents from agricultural land. Agriculturaldrainage waters are a resource for irrigation and other uses.However, drainage water may be the source for variouspathogens and appropriate care is required. When treated thiswater may be reused for several processes (Tanji and Kielen,2002; van der Molen et al., 2007). Freshwater is considered tohave a total dissolved solids (TDS) concentration of less than500 mg l�1, that is, electrical conductivity (EC) below0.7 dS m�1. Saline and brackish water will have TDS of500–30 000 mg l�1(0.7–42 dS m�1), whereas seawater hasTDS averaging 35 000 mg l�1(49 dS m�1).

Drainage water is used for irrigation in many parts of theworld, mainly in CA, USA; India; Egypt; Israel; China; NorthAfrica; and Middle East. There are abundant reports on its use(e.g., Minhas et al., 2006; Rhoades et al., 1992; Shahid et al.,2013; Sharma and Minhas, 2005; Singh, 2009). The use ofhighly saline waters may be feasible for halophytes, which

ter use in irrigation

divertedny use

Nonconsumedfraction

Losses

NonreusableReusable

Preservedquality

Degradedquality

Wastagenonbeneficial

athways. Adapted from Pereira, L.S., Cordery, I., Iacovides, I., 2009.p. 382 and Pereira, L.S., Cordery, I., Iacovides, I., 2012. Improvedonservation and saving. Agricultural Water Management 108, 39–51.

MAC_ALT_TEXT Figure 1

Water Use: Recycling and Desalination for Agriculture 409


could be explored for human and animal consumption or forlandscape uses (e.g., Cassaniti et al., 2012; Díaz et al., 2013).

The quality of drainage water for use in irrigation may besummarized as follows (van der Molen et al., 2007):

• very good, o1 dS m�1, appropriate for all crops;

• good, 1–2 dS m�1, appropriate for most crops;

• moderate, 2–3 dS m�1, appropriate for tolerant crops;

• poor, 3–6 dS m�1, appropriate for tolerant crops, with ap-propriate leaching;

• very poor, 46 dS m�1, appropriate but not recommen-ded for irrigation. Usable to irrigate halophytes, maintainwater levels in fish ponds, secure water levels in brackishcoastal lakes, leaching salt-affected soils during theinitial stage.

Waterlogging, salinity, and related problems have arisen inmany irrigation areas where freshwater is used for irrigation.Such problems could arise even more quickly and more se-verely when saline water is used. The major potential hazardsassociated with the use of saline water in agriculture may besummarized as follows:

1. Yields decrease, which relate to:

• reduced soil water availability to the crop due to in-creased soil water osmotic potential and reduced soilinfiltration rates;

• soil crusting, affecting infiltration, erosion, and cropemergence;

• toxicity to the crop when the concentration of certainions exceeds that of crop tolerance; and

• imbalance of nutrients available to crops.2. Soil degradation due to:

• salinization when salts accumulate in the root zoneif leaching and drainage are insufficient or inappro-priate;

• sodification when there is high sodium content in re-lation to other cations and the soil complex accumulatesthis excess sodium. Sodification causes soils to lose theirstructure, tilth, become dispersive, and these soils havereduced infiltration and permeability; and

• loss of soil productivity in relation to salinization,sodification, imbalance of nutrients, and reduced wateravailability.

3. Environmental hazards from:

• soil degradation;

• groundwater and surface water salinization;

• damages to the soil environment affecting plant andanimal communities, and soil biodiversity;

• desertification due to degradation of the soil, water, andenvironmental conditions;

• development of aquatic weeds, algal blooms, and eu-trophication in water bodies receiving nutrients anddrainage effluents, particularly nitrates, thus affectingwaterways and canals as well as the wildlife that in-habits ponds, lakes, and reservoirs; and

• presence of particular ions to levels exceeding specifichealth and safety thresholds, which affect either plantssensitive to those ion concentrations, or animals thatdrink those waters, such as the selenium in drainagewaters in California.

4. Public health hazards due to:

• toxic ions such as heavy metals that, despite infrequentingestion, are cancerous when accumulated in humansand

• vectors of disease, such as mosquitoes and snails, whichdevelop better in saline waters, mainly when rich innutrients, which may affect populations living in areasusing saline water or reusing drainage waters.

The use of saline water for irrigation requires:

• integrated management of water of different qualities at thelevels of the farm, the irrigation system, and the basin (e.g.,Rhoades et al., 1992; Wallender and Tanji, 2012);

• adopting irrigation methods with high performance asdiscussed below;

• adopting appropriate irrigation scheduling providing forthe satisfaction of crop water and using minimal leachingfractions to minimize deep percolation and runoff (Allenet al., 1998, 2007);

• monitoring of soil and water quality, providing feedbackthat helps implement the optimal operation and control ofthe irrigation systems (Shahid et al., 2013; Wallender andTanji, 2012);

• promotion of tools, particularly models (Oster et al., 2012),to predict long- and short-term effects of irrigation waterquality on crop yields, soil properties, and quality of theenvironment;

• training of irrigation farmers and agricultural officers; and

• pilot areas to test and assess irrigation and soil and cropmanagement practices for using saline water, includingshallow water tables associated with deficit irrigation.

Criteria and Standards for Assessing the Suitability of Waterfor Irrigation

Consolidated standards have been made available in FAOpublications (Ayers and Westcot, 1985; Rhoades et al., 1992)and further developed in various other publications such asTyagi and Minhas (1998) and Wallender and Tanji (2012).This has made it possible to define the main water qualityparameters and management that need to be known to allowsaline waters to be used safely in irrigation. Standards include:

1. Water quality characteristics to be considered for irrigationto assess the suitability of saline water concerning

• salinity hazards (TDS and EC);

• soil crusting and permeability hazards (sodium ad-sorption ratio (SAR) or adjusted SAR, EC, and pH);

• specific ion toxicity hazard (e.g., Naþ , Cl�, B, and Se,among others); and

• nutrient imbalance hazard (excess NO3�, limited Ca2þ ,

PO43�).

2. Parameters required to evaluate the quality of saline wateron a routine basis including TDS, EC, concentration of ions(mainly Ca2þ , Mg2þ , Naþ , CO3

2�, HCO�3 , Cl�, and

SO42�), SAR, or the adjusted SAR under certain conditions,

trace elements (such as Se, As, B, Mo, Cd, Cr, and Cu), aswell as other potentially toxic substances of agriculturalorigin.

Table 1 Water quality for irrigation and required restrictions in use

Problems Water quality characteristic No restrictions Slight to moderate restrictions Severe restrictions

Salinity effects on water availability Electrical conductivity (dS m�1) o0.7 0.7–3.0 43.0Total dissolved solids (TDS) (mg l�1) o450 450–2000 42000

Salinity effects on soil infiltration Sodium adsorption ratio (SAR)o3 EC40.7 dS m�1 EC: 0.7–0.2 dS m�1 ECo0.2 dS m�1

3–6 41.2 1.2–0.3 o0.36–12 41.9 1.9–0.5 o0.512–20 42.9 2.9–1.3 o1.320–40 45.0 5.0–2.9 o2.9

Toxicity SodiumSurface irrigation: SAR o3 3–9 49Sprinkle/spray (meq l�1) o3 43

ChlorideSurface irrigation (meq l�1) o4 4–10 410Sprinkle/spray (meq l�1) o3 43

Boron (mg l�1) o0.7 0.7–3.0 43.0Trace elements VariableBicarbonate (meq l�1) (sprinkle/spray) o1.5 1.5–8.5 48.5

Plant nutrition pH 6.5–8.5

Source: Adapted from Ayers, R.S., Westcot, D.W., 1985. Water quality for agriculture. FAO Irrigation and Drainage Paper 29 Rev. 1. Rome: FAO.



3. Water quality standards already available should beadjusted for the specific conditions of saline water use,including soils, climate, crops and crop sequences, andirrigation methods.

4. Attention must be paid to possible hazardous effectsof trace elements on people or livestock that consumecrops produced using water having such elements (e.g., Asand Se).

The main water quality characteristics for saline waters aresummarized in Table 1, with indications of the restrictions ontheir use, which mainly concern crop tolerance to salts, theneed for leaching, specific requirements for the irrigationmethods and systems, plus soil management practices andfertilizer practices.

Crop Irrigation Management using Saline Water

Crop responses to salinity vary with species and, to a lesserdegree, with the crop variety. The tolerance of crops to salinityis generally classified into four to six groups from sensitive (ornontolerant), where most horticultural and fruit crops are in-cluded, to the tolerant, which includes barley, cotton, jojoba,sugarbeet, several grass crops, asparagus, and date palm. Fulllists of crop tolerance classes are given by Hoffman andShalhevet (2007). In addition, halophytes may be com-mercially explored using highly saline water (e.g., Cassanitiet al., 2012).

The behavior of various crops under irrigation with waterof different degrees of salinity varies with species, varieties, andthe crop growth stages. As irrigation water salinity increases,germination is delayed. Germination is adversely affected formost field crops when the EC of the irrigation water, or the soilsaturation extract, reaches a threshold of 2.4 dS m�1. Adverseeffects occur at lower values (o1 dS m�1) for nontolerantcrops, and at higher values for tolerant crops (generally notmore than 4 dS m�1). However, responses of a tolerant crop

become affected when the crop is irrigated with saline water insuccessive years.

The germination and seedling stages are the most sensitive.Any adverse effects at such stages will lead to a reduction incrop yields proportional to the degree of plant loss duringgermination and establishment. At this stage, water of goodquality should be used, especially if plants are sensitive. Be-sides germination and crop establishment, most crops aremore sensitive to salinity during the reproductive phase. Othercritical stages vary from crop to crop. Case studies are reportedin the literature (Tyagi and Minhas, 1998; Sharma and Minhas,2005; Wallender and Tanji, 2012).

The effects of the salinity on yields may be estimated asfollows (Ayers and Westcot, 1985; Rhoades et al., 1992; Allenet al., 1998):

Ya

Ym¼ 1� ðECe � ECe thresholdÞ b

100½1�

where Ya and Ym are the actual and potential crops yields (kgha�1), when the crop techniques are appropriate to the localenvironmental conditions and no water stress affects the crop;b is a crop-specific parameter, which describes the rate of yielddecrease per unit of excess salts (% per dS m�1), and ECe andECe threshold are, respectively, the actual EC of the soil satur-ation extract and the crop-specific ECe threshold above which thecrop is affected by salinity. The ECe threshold ranges from1.0 dS m�1 for very sensitive crops, such as carrots and beans,up to more than 7.5 dS m�1 for barley, cotton, and tolerantgrasses. The b rate of decrease in yield per unit increase in ECvaries from more than 15 (% per dS m–1) for the sensitivecrops, down to 5 or less (% per dS m–1) for tolerant crops. Theparameters b and ECe threshold are tabled in Allen et al. (1998)and Hoffman and Shalhevet (2007) for the main crops.

The suitability of water for irrigation based on salinity,leaching and drainage requirements, and crop tolerance tosalinity must be related to irrigation management. Water andcrop management must be selected to minimize accumulation



of salts in the active root zone and to eliminate salt stress,especially during the critical growing stages of the plants. Theseinclude (Pereira et al., 2002, 2009; Allen et al., 1998):

• appropriate selection of irrigation methods;

• efficient leaching management, including volumes andfrequency, and respective drainage of the salty water awayfrom the cropped land (Section Leaching Requirementsand Strategies for Controlling Impacts on Soil Salinity);

• proper irrigation scheduling, in agreement with the avail-able irrigation system; and

• crop rotations adapted to the prevailing conditions; con-siderations must include irrigation water quality, soil sal-inity levels, chemical and physical properties of the soils,and climatic conditions.

A major factor when using saline irrigation is schedulingand frequency. Saline water requires more frequent irrigationthan freshwater because salts in the water and the soil increasethe osmotic potential of the soil water, which makes wateruptake by the crop roots more difficult. For irrigation sched-uling purposes, it is possible to consider the total available soilwater smaller than that for nonsaline soils through correctingthe soil water content at the wilting point (θWP, cm

3 cm�3):

θWP salt ¼ θWP þ bECe � ECe threshold

10

� �ðθFC � θWPÞ ½2�

where θFC is the soil water content at field capacity (cm3

cm�3), θWP salt is the soil water content at wilting point (cm3

cm�3) for saline conditions and b, ECe, and ECe threshold are asdefined in eqn [1]. The θWP salt varies from a crop to anotherwith b and ECe threshold, and varies for the soil with ECe.Equation [2] implies the reduction of the depth of water ap-plied at each irrigation and, simultaneously, increasing theirrigation frequency. However, those depths depend on theirrigation method and the off-farm system delivering water tothe fields.

The use of straw mulch to control soil evaporation alsohelps controlling the upward transport of salts to the rootzone, particularly when irrigation and leaching are appropriate(Bezborodov et al., 2010; Pang et al., 2010).

The selection of the irrigation method must consider thequality of the water and the potential for both the water andthe irrigation method to produce negative impacts. A summaryof the more relevant considerations is presented in Table 2.These refer to the capabilities for controlling:

• soil salinity hazards due to salt accumulation in theroot zone;

• toxicity hazards caused by direct contact of the salty waterwith the plant leaves and fruits;

• soil infiltration and permeability hazards caused by themodification of the soil physical properties, mainly due tothe Na ion; and

• yield hazards when the irrigation system does not allowadoption of appropriate irrigation management, that is,frequency and volumes of irrigation.

Irrigation methods are described in various irrigationmanuals and their adaptability for using saline and wastewateris discussed in numerous papers. Surface irrigation methods,

particularly flat basins and borders, are appropriate to applysaline waters and for salt leaching. However, they are not ad-equate to apply small irrigation depths. When water is de-livered to the farms through surface canal systems, the deliveryschedules are generally of the rotation type, and are often rigid,delivering large irrigation volumes at long intervals. Thesesystems are well adapted for leaching, but are less appropriatefor irrigation of less tolerant crops that would require smalland frequent applications, for example, vegetable crops. Dif-ferently, drip irrigation is appropriate for these crops becausesmall and frequent irrigation depths are used. Drip systemsrequire that irrigation be monitored to avoid salts returninginto the wetted bulb. Cracks forming at the soil surface mustbe avoided. Moreover, emitters must be carefully selected, thatis, not having too small orifices, and filtration must be care-fully practised with filter sizes in agreement with sizes of orificeemitters, and the system must be carefully maintained andcleaned to avoid accumulation of salts within the piping,which could induce emitter clogging.

Leaching Requirements and Strategies for ControllingImpacts on Soil Salinity

The leaching requirement is usually computed from (Ayersand Westcot, 1985)

LR ¼ ECiw

5ECe � ECiw½3�

where ECiw is the electrical conductivity of the irrigation waterand ECe is the electrical conductivity of the saturated extract ofthe soil. The ECe should be the average soil salinity toleratedby the crop, not that for achieving maximum yield but toprovide for attaining 70–90% of the potential yield. When aleaching fraction is applied with the irrigation water, the sal-inity built up of the soil is reduced nonlinearly with the size ofthat fraction. This may be expressed as

ECe ¼ 1þ LFLF

� ECiw

5½4�

where LF, the actual leaching fraction, is used in place of theleaching requirement, LR. This equation shows that the soilsalinity ECe increases proportionally to the salinity of the ir-rigation water, ECiw.

The salinity built up cannot be prevented by drasticallyincreasing the leaching fraction because this would dramatic-ally increase the percolation of water, causing the watertable to rise, plus waterlogging and degradation of the groundwater quality. This further contributes to water scarcity.Therefore, depending on the crop, and the salinity of the waterand soil, the minimum LF is recommended. Moreover, theapplication of an LF is only effective when there is high uni-formity of irrigation water distribution. When uniformity islow, water percolates abundantly in some areas of the fieldwhereas other areas are insufficiently wetted. Then farmerstend to apply more water but this produces only more per-colation (e.g., Pereira et al., 2007). Under conditions of waterscarcity, this would be a very poor use of the available water.As reported by Hanson et al. (2007), leaching with drip irri-gation is more effective when application depths are large

Table 2 Evaluation of the irrigation methods for use with saline water

Irrigation method Flat basin irrigation Corrugated basinirrigation

Border irrigation Furrow irrigation Sprinkler irrigation Drip andsubsurface dripirrigation

Microsprinkling andmicrospray

Salt accumulationin the root zone

Not likely to occurexcept for theunderirrigated partsof the field whenuniformity of waterapplication is verypoor. Leachingfraction difficult tocontrol in traditionalsystems

Salts tend toaccumulate onthe tops of theridges. Leachingbefore seeding/planting isrequired forgermination andcropestablishment

As for basin irrigationbut infiltration controlis more difficult, as isthe control of theleaching fraction

Salts tend toaccumulate onthe tops of theridges. Leachingis required beforeseeding/planting

Not likely to occurwhen set systems areused except for theunder irrigated partsof the field due to lowuniformity. Instead,problems occur withequipment designedfor light and frequentirrigation

Not likely to occurexcept for theunder irrigatedparts of the fielddue to lowuniformity,including thatresulting fromnozzle cloggingwhen waterfiltration is poor

Not likely to occurexcept for theunderirrigated partsof the field due to lowuniformity andclogging; leachingcould then be moredifficult

Foliar contact,avoiding toxicity

It is possible only forbottom leaves in lowcrops and foddercrops, and during thefirst stage of growthof annual crops

Unlikely becausecrops are grownon ridges

As for flat basins Unlikely becausecrops are grownon ridges

Severe leaf damage canoccur affectingyields. Damagegreater with morefrequent irrigation

Not likely to occur Leaf damage canoccur, affectingyields of annualcrops. Less damagefor tree crops

Ability to infiltratewater and refillthe root zone

Adequate because largevolumes of water aregenerally applied ateach irrigation andwater remains in thebasin until infiltrationis complete

As for flat basins,above

As water infiltrateswhile flowing on thesoil surface, runofflosses increase wheninfiltration decreases

Salinity-inducedinfiltrationproblems causevery high runofflosses

Salinity-inducedinfiltration problemsincluding soilcrusting may causevery high runofflosses

Problems generallydo not occurexcept whenthere are notenough emittersandunderirrigation ispractised

Problems are similar tothose for setsprinklers, so runofflosses may beimportant

Control of cropstress and yieldreduction

Adequate becausetoxicity is mostlyavoided, salts aremoved down throughthe root zone,infiltration iscompleted, andirrigation can bescheduled to avoidcrop stress

As for flat basinsbut depending onavoiding saltstress at plantemergence andcropestablishment

Crop stress is likely tooccur due to reducedinfiltration, soinducing relativelyhigh yield losses

Crop stress is verylikely to occurdue to reducedinfiltration, soinducing yieldlosses

Crop stress is verylikely to occur due totoxicity by contact ofwater with the leavesand fruits, and due toreduced infiltration,thus crop stress andyield losses mayoccur

These systems areable to providefor crop stressand toxicitycontrol, so yieldlosses areminimized

Toxicity due to directcontact with theleaves. Nonuniformirrigation mayproduce crop stressand runoff. Yieldlosses likely

Source: Adapted from Pereira, L.S., Cordery, I., Iacovides, I., 2009. Coping with Water Scarcity. Addressing the Challenges. Dordrecht: Springer, p. 382.

412Water

Use:Recyclingand

Desalinationfor

Agriculture




enough to appropriately control the water and salts within thewetted bulbs.

Equation [4] predicts that ECe¼1.5 ECiw, when the salinityof the soil and the irrigation water are in equilibrium. LowerECe can occur if good quality water, including rainwater, isemployed for leaching. Therefore, the conjunctive use for ir-rigation of saline water and good quality water is advocated,and similarly for the case of using wastewater for irrigation. Anupdated discussion on LF practices is given by Hoffman andShalhevet (2007) and by Letey et al. (2011). The latter suggestthat using a smaller LF than usually recommended may be abetter option.

Several strategies are usually adopted to facilitate irrigationwith saline water (e.g., Rhoades et al., 1992; Shahid et al.,2013). The main strategies are listed below:

1. The dual rotation, in which sensitive crops (e.g., lettuce andalfalfa) in the rotation are irrigated with low-salinity riverwater, and salt-tolerant crops (e.g., cotton, sugarbeet, wheat,and barley) are irrigated with saline drainage water. For thetolerant crops, the switch to drainage water is usually madeafter establishment, that is, irrigations at preplanting and atthe initial crop stages are made with low salinity water.Benefits from this strategy include: (1) harmful levels of soilsalinity in the root zone do not occur because saline water isused only for a fraction of the time; (2) substantial allevi-ation of salt buildup in the soil occurs during the time whensalt-sensitive crops are irrigated with freshwater; and (3)proper preplanting irrigation and careful irrigation man-agement during germination and seedling establishmentleach salts out of the seed layer and from shallow soildepths. Difficulties include the complexity of water man-agement for farmers and system managers and the possiblelack of freshwater when it is required.

2. Blending, where water supplies of different salinity levels aremixed in variable proportions. Irrigation water having aquality superior to that of the saline water is obtained.Blending may be more practical on large farms because thefarmer may take into consideration the crops grown andthe respective growth stages. Differently, with large irri-gation systems supplying many small farms it would bedifficult to properly satisfy the requirements of all thecrops, unless a cropping pattern is imposed for all farms.

3. Cyclic application of saline water and freshwater. Salinitymust not be above the acceptable thresholds for the cropsgrown. Cycles of application of freshwater should coincidewith the more sensitive growth stages, particularly forplanting and seedling development, and for the leaching ofthe upper soil layers. This strategy has more potential andflexibility than the blending strategy.

Monitoring and Evaluation

Using brackish and saline irrigation water can be successful onnumerous crops but uncertainty exists concerning the long-term effects of these irrigation practices on the physical andchemical quality of the soil. These effects largely depend onthe chemical and physical characteristics of the soil, on theclimate, and on the possibility of leaching with natural rain orusing high-quality water for leaching. A great concern refers to

the reduction of water infiltration capacity. This is especiallyimportant where reuse is practised on poorly structured soilsand the drainage water has SAR415 (mmol l�1)½. However,the capability to predict changes in soil infiltration and per-meability is still insufficient.

Long term effects on soil salinization are often consideredwith simulation models (Minhas et al., 2006; Oster et al.,2012). However, the existing knowledge is still limited. Soilvariability in space and irrigation variability both in time andspace produce large uncertainty in predictions. Soil salinityunder a cyclic strategy of application will fluctuate more, bothspatially and temporally, than if using a blending strategy.Predicting or anticipating plant response and effects on the soilwould be more difficult when cyclic application is used.However, management strategies must be selected that keepthe average root-zone salinity levels within acceptable limits.

Another cause of uncertainty refers to elements such as theboron and chloride contained in drainage water that mayproduce more long-term detrimental effects than salinity.Furthermore, the reuse of drainage water poses a long-termproblem relative to the potential for accumulation of heavymetals in plants and soils. These metals can be toxic to humanand animal consumers of the crops. Nevertheless, there is onlylimited potential for using prediction models to estimate long-term impacts.

Monitoring soil salinity, leaching, and drainage adequacy istherefore paramount to evaluate the long-term impacts fromusing saline waters, either groundwater or drainage water(Shahid et al., 2013). Monitoring and evaluation should payattention to:

• The dynamics of salts throughout the soil profile for de-tecting temporal changes in salinity levels, particularly toidentify when salt buildup is steadily increasing.

• The functioning and performance of the drainage system,including observations of drainage outflows and saltstransported with the drainage water.

• The irrigation performance, mainly relative to the uni-formity of water distribution and the leaching fractionsactually applied.

• The irrigation schedule with respect to the satisfaction ofirrigation and leaching requirements, and the related con-straints due to delivery, or other restrictions that mayhamper appropriate irrigation management.

• Sampling ECe and ECiw throughout the irrigated area toidentify the occurrence of problem areas requiring specificwater management.

• Sampling for specific ions that may be present in the irri-gation water that could have toxicity effects or create healthrisks, as for heavy metals.

• Follow-up impacts from using saline water such as changesin groundwater quality, plant ecosystems, wetlands, or inriverbed fauna and flora.

New approaches for laboratory and field techniquesare required (e.g., Rhoades et al., 1999; Wallender and Tanji,2012). Environmental impact assessment methodologies forirrigation and drainage projects may also be adapted for sur-veying and monitoring areas where water of inferior quality,such as saline and wastewater, is used for irrigation.



Wastewater Reclamation and Recycling

Wastewater/Effluent Characteristics

General wastewater characteristicsMunicipal wastewater mainly comprises water with relativelysmall concentrations of suspended and dissolved organic andinorganic solids. Organic substances include carbohydrates,lignin, fats, soaps, synthetic detergents, proteins, and theirdecomposition products, as well as various natural and syn-thetic organic chemicals from the process industries. Typicaldomestic wastewater can be classified as strong, medium, orweak, depending on the concentration of the major constitu-ents. Chemical oxygen demand is 1000, 500, and 250 mg l�1,respectively, for strong, medium, and weak wastewater(Sincero and Sincero, 2003). In arid and semiarid countries,domestic water use is often fairly low, so sewage tends to bevery strong.

Municipal wastewater also contains a variety of inorganicsubstances, including heavy metals, which can have phytotoxiceffects and health impacts, thus limiting reuse in agriculture.

Microbiological characteristics – PathogensFrom the point of view of human health, the contaminants ofgreatest concern are the pathogenic micro- and macroorgan-isms. Pathogenic viruses, bacteria, protozoa, and helminthsmay be present in municipal wastewater and will survive in theenvironment for long periods. At one time, coliforms werethought to be exclusively of intestinal origin, but it is nowknown that some are free living in the environment and areassociated with soil and freshwater. Current practice usesEscherichia coli and other fecal indicator organisms, includingintestinal Enterococcus, to ascertain levels of fecal pollution inwater, with their numbers usually being given in the form offecal coliforms (FC) per 100 ml of wastewater. Although theseorganisms may not be pathogenic, there is however well-documented correlation with deleterious health effects when-ever exposed to measureable levels of fecal indicatororganisms.

Table 3 Main pathogenic parameters relative to wastewaters

Pathogens Survival time

Coliforms and fecal coliforms (Citrobacter,Enterobacter, Klebsiella, Escherichia coli)

Up to 60 days in watethe soil

Fecal Streptococci (Streptococci bovis,Streptococci equines, Streptococci faecalis)

–

Clostridium perfringens Survival characteristicviruses or even helm

Salmonella spp. (S. typhi agent for typhoid) If removal of SalmonShigellae and Vibrioprobably also remo

Enteroviruses: Poliomyelitis and Meningitis,and respiratory infections

May attain 120 days

Rotaviruses: Gastrointestinal problems More persistent than

Intestinal Nematodes: Ascaris lumbricoides Several months

Source: Reproduced from Mara, D., Cairncross, S., 1989. Guidelines for the Safe Use of WastM.B., 1992. Wastewater treatment and use in agriculture. FAO Irrigation and Drainage Pape

Pathogenic organisms give rise to the greatest health con-cern in the use of wastewaters. In areas of the world wherehelminthic diseases caused by Ascaris and Trichuris spp. areendemic in the population, and where untreated sewage isused to irrigate salad crops or vegetables eaten uncooked,transmission of these infections is likely to occur through theconsumption of such crops. Further evidence was provided toshow that cholera could be transmitted through the samepathway. Indian studies have shown that farmworkers exposedto sewage wastewater in areas where Ancylostoma (hookworm)and Ascaris (nematode) infections are endemic have signifi-cantly higher levels of infection than other agriculturalworkers. More detailed information on health risks and vector-borne diseases are given by WHO (2006a–c, 2011) and Maraand Sleigh (2010a,b). They can be summarized as follows:

• high risk (high incidence of excess infection) due to hel-minths (Ancylostoma, Ascaris, Trichuris, and Taenia);

• medium risk (low incidence of excess infection) due toenteric bacteria (Vibrio cholera, Salmonella typhosa, andShigella); and

• low risk (low incidence of excess infection) related to en-teric viruses (viral diarrheas and hepatitis A).

The main microbiological parameters that are particularlyimportant from the health point of view when characterizingwastewaters (Pereira et al., 2009) are summarized in Table 3.

Table 3 clearly shows that there is a wide variety ofpathogens present in wastewater, depending upon the regionconcerned. These pathogens are the cause for a variety of dis-eases; some of them are of high risk for human health careworkers and consumers of produces. Moreover, their survivaltime in water may be very large. These conditions imply ap-propriate care when treating and using wastewater.

XenobioticsXenobiotics are either naturally occurring or syntheticcompounds that interfere with the functioning of endo-crine systems causing unnatural responses. For that reasonthey are receiving growing attention. Xenobiotics include

Presence in wastewater

r and 70 days in E. coli count is a main indicator

Occur both in man and in other animals

s similar tointh eggs

Useful in wastewater quality reuse studies

ellae is achievedcholera are

ved

Typical in a tropical urban sewage

in water Especially under tropical conditions

enteroviruses Removal in parallel with that of SS, virus aresolids-associated

Infections can be spread by effluent reusepractices

ewater and Excreta in Agriculture and Aquaculture. WHO and UNEP, Geneva and Pescod,r 47. Rome: FAO.



pharmaceuticals (e.g., antibiotics as trimethoprim and ery-tromycine; analgesics and antiinflammatory drugs as codein,ibuprofen, and diclofenac), plus personal care products (e.g.,fragrances, sunscreen agents), pesticides, and disinfectants.Some of these compounds, for example, pentabromobiphe-nylether, 4-nonylphenol, C10–C13 chloroalkanes, and the di(2-ethylhexyl)phthalate (DEHP), have been listed as priorityhazardous substances in the field of water policy by theEuropean Directive on Environmental Quality Standards(Directive 2008/105/EC). WHO (2011) also identified someof the above-mentioned xenobiotics through the hazardidentification process.

In wastewater treatment plants (WTP), hydrophobic com-pounds are adsorbed into the sludge, whereas persistenthydrophilic compounds remain in the treated effluents.Whenever these compounds, their metabolites, or transfor-mation products are not removed during wastewater treatmentprocess, they may enter the aquatic environment. As a result,increased attention is being given to the use of reclaimed waterto avoid the introduction of xenobiotic compounds into theenvironment, which is highly relevant in areas where re-claimed water is used for irrigation.

GraywaterGraywater (GW) is defined as the urban wastewater that in-cludes water from baths, showers, hand wash basins, washingmachines, dishwashers, and kitchen sinks, but excludesstreams from toilets (Santos et al., 2012). GW is an importantalternative source of water as it makes up the largest volume ofthe waste flow from households, and it has a nutrient contentthat can be beneficially used by crops, and also has a lowpathogen content (WHO, 2006c). Owing to the low levels ofpathogens and nitrogen, reuse and recycling of GW is receivingmore and more attention (Li et al., 2008) as it can be used forany purpose provided it is treated to the desired qualitystandard. The treatment system varies based on the site con-ditions and GW characteristics.

The design of a graywater reuse system primarily de-pends on the quantity to be treated and reuse applications(WHO, 2006c). The reclaimed graywater should fulfill thefour criteria (hygienic safety, esthetics, environmental toler-ance, and economical feasibility) for reuse. One shouldalso keep in mind that different reuse applications requiredifferent water quality specifications and thus demand differ-ent treatments varying from simple processes to more ad-vanced ones.

Various studies show the usefulness and limitations of GWfor irrigation. Rodda et al. (2011) showed that irrigation withGW increased plant growth and yield relative to crops irrigatedwith tap water. However, the soil irrigated with GW showedincreased EC and increased concentrations of metals over time,coupled with an increase in sodium and metal concentrationsin crops. Accumulation of surfactants and sodium in soil mayalso occur due to GW reuse, then affecting adversely agri-cultural productivity and environmental sustainability. Traviset al. (2010) verified that GW could change soil propertiesimpacting the movement of water within the soil and thetransport of contaminants in the vadose zone. As for thedrainage reuse referred before, appropriate monitoring is

required to support further expansion of GW reuse in irri-gation and a better definition of treatment requirements.

Wastewater Treatment

General aspectsAn appropriate degree of treatment needs to be provided toraw municipal wastewater before it can be used for agriculturaland landscape irrigation, or other uses. The required quality ofeffluent will depend on the proposed water uses, crops to beirrigated, soil conditions, and the irrigation system.

The best available wastewater treatment for agriculturaluses is that which allows meeting the recommended micro-biological and chemical quality guidelines at low cost, min-imizes operational and maintenance requirements, and keepsnutrients in the reclaimed water. Simplicity and robustness ofthe treatment process is even more important in developingcountries. Pathogen removal is a primary concern (Asano et al.,2006; WHO, 2006c).

Irrigation with effluent directly from the treatment plant isgenerally not feasible and some form of storage of the treatedeffluent is necessary. Additional benefits can result from stor-age in reservoirs but their management is particularly de-manding because of the need to manage both water quantityand quality (Kfir et al., 2012).

Conventional Wastewater Treatment

Conventional wastewater treatment consists of a combinationof physical, chemical, and biological processes and operationsto remove solids, organic matter, and, sometimes, nutrientsfrom wastewater (Figure 2).

Different degrees of treatment can be considered:

1. Preliminary treatment: The objective is the removal ofcoarse solids and other large materials from the raw was-tewater. Treatment includes coarse screening, grit removal,and trituration.

2. Primary treatment: The objective is the removal ofsettable organic and inorganic solids. Large fractions of thetotal suspended solids, oils, and grease are removed butonly a fraction of the biochemical oxygen demand (BOD5,when the test is carried out during 5 days) is removed. Ifcoagulants are used, particulate and colloidal matter areremoved together with heavy metals, polychlorinated bi-phenyls (PCBs), and polycyclic aromatic hydrocarbons(PAHs). This may be considered a sufficient treatment if thewastewater is to be used to irrigate crops that are notconsumed by humans or to irrigate orchards, vineyards,and some processed food crops as discussed in the SectionManagement Strategies for Minimizing Health Hazards inWastewater Use in Irrigation.

3. Secondary treatment: The objective here is to remove theresidual organics (soluble BOD5) and suspended solids. Itoften involves the removal of biodegradable dissolved andcolloidal organic matter using aerobic biological treatmentprocesses. When coupled with a disinfection step such aschlorination, these processes can provide substantial re-moval of bacteria and viruses. The use of membrane bior-eactors (MBR), coupling of a membrane to a bioreactor,

Wastewater

Primary sedimentation fororganic and inorganic solids,skimming of oils and greases

Secondary sedimentation,removal of suspended solids

Biological aerobic treatment,removal of residual organics,

soluble BOD5 and suspended solids

Clarification and removal ofmicroorganisms

Chemical coagulation, sedimentation,and filtration for removal of

suspended and colloidal solids

Disinfection

Reverse osmosis or distillationand remineralization

Advanced oxidation processes

Wastewater for unrestricted uses

Wastewater for street and floor washingfor irrigation of gardens, golf courses, and crops,

which are not eaten without cooking

Wastewater for irrigation of crops,which are not eaten without cooking

Wastewater for irrigation of nonfood crops,full health restrictions must apply

Separation of solids,coarse screening

Primarytreatment

Pretreatment

Secondarytreatment

Tertiarytreatment

Advancedtreatment

Figure 2 Conventional wastewater treatment and related conditions for water reuse. Modified from Pereira, L.S., Cordery, I., Iacovides, I.,2009. Coping with Water Scarcity. Addressing the Challenges. Dordrecht: Springer, p. 382. Boxes with stippled borders refer to supplementaltreatments.



allows achieving an effluent that can be reused withoutrestrictions in irrigation (Fatone et al., 2011).

4. Tertiary treatment is employed when specific undesirablewastewater constituents cannot be removed by secondarytreatment, for example, additional suspended solids andrefractory organic chemicals (resistant to be broken downthrough conventional treatment processes).

5. Advanced treatment aims at unrestricted water use,namely in irrigation. It is required because secondary ef-fluents commonly still contain low levels of pathogens,nutrients, and dissolved solids. It includes membranetechnology, ultra-/nanofiltration, or reverse osmosis (RO)as well as water remineralization RO membrane filtration.This has been accepted as a proven technology to achievethe quality required for unlimited irrigation reuse (Oronet al., 2008). However, its application is limited due to therequirement for extensive pretreatment along with the as-sociated high operational and maintenance costs. Mean-while, low-pressure RO is emerging as an alternative to theconventional method. Advanced oxidation processes areconsidered a highly competitive treatment for removal ofrefractory organic chemicals. These processes includephotochemical degradation, photocatalysis, and chemicaloxidation processes. Disinfection normally uses a chlorine

solution but ozone and ultraviolet irradiation can alsobe used.

Natural/biological treatment systems and present advancesNatural/biological treatment systems consist of the following:

1. Stabilization ponds, which according to the metabolic re-gime present, can be classified as anaerobic, facultative,aerobic or maturation, and aerated ponds. Dissolved orsuspended organic matter is metabolized by heterotrophicbacteria with uptake of oxygen, whereas dissolved oxygen isreplaced through photosynthetic oxygen produced bymicroalgae.

2. Overland treatment of wastewater, where the effluent isdistributed over gently sloping grassland on fairly im-permeable soils, and it moves evenly down the slope tocollecting ditches. Suspended and colloidal organic ma-terials are then removed by sedimentation and filtrationthrough the surface grass and organic layers. Pathogens arealso removed at levels comparable with conventional sec-ondary treatment systems, without chlorination. The im-pact on groundwater should be considered in the case ofhighly permeable soils.


Lowest risk

To the consumer, butfor the field worker

protection is needed

• Crops not for humanconsumption

• Crops processed by heator drying before humanconsumption

• Vegetables and fruitsgrown exclusively forcanning or processing

• Fodder crops/animalfeed crops

• Pasture, green foddercrops

• Crops that do not comeinto direct contact withwastewater

• Crops eaten only aftercooking

• Crops the peel of whichis not eaten

• Crops under sprinkling

Increased risk

To consumer, fieldworker, and handler

High risk

To consumer, fieldworker, and handler

• Crops eaten uncooked grown in close contact with wastewater

• Landscape irrigation with public access

Public health risk from using wastewater in irrigation

Figure 3 Levels of health risk for consumers and workers due to wastewater use in irrigation. Adapted from Westcot, D.W., 1997. Quality controlof wastewater for irrigated agriculture. FAO Water Report 10. Rome: FAO and Pereira, L.S., Cordery, I., Iacovides, I., 2009. Coping with WaterScarcity. Addressing the Challenges. Dordrecht: Springer, p. 382.



3. Wetland treatment systems, where treatment occurs inmaturation ponds that incorporate floating, submerged, oremergent aquatic species (macrophytes). Floating macro-phytes, desirably having large root systems and very effi-cient nutrient extraction, include the Eichornia crassipes andthe Ceratophyllum demersum. In wetlands having emergentmacrophytes, for example, Phragmites communis and Scirpuslacustris, aerobic treatment takes place in the rhizosphere asoxygen passes to it from the atmosphere through leaves,stems, and roots, whereas anoxic and anaerobic treatmenttakes place in the surrounding soil. Nutrients and heavymetals can be removed by plant uptake.

4. Constructed wetlands (CW) have the appearance of a nat-ural wetland habitat and employ many of the biologicalprocesses found in natural wetland ecosystems. CW canproduce a biologically treated effluent with sufficiently lowpathogen content to be safely used for irrigation. As thesesystems can be constructed from local materials and aresimple to operate and maintain, they have the potential tobe applied in a variety of countries (Tanaka et al., 2011).Removal processes depend on CW type/configuration andinflow loading (Vymazal and Kröpfelová, 2008). Operationand maintenance activities are essential to guarantee anappropriate performance of CW, including water-levelcontrol, weeds control, and plant inspection (Galvão et al.,2009).

Management Strategies for Minimizing Health Hazards inWastewater Use in Irrigation

Pathogens brought with the wastewater can survive for manydays in the soil or on the crops. Thus, a potential for diseasetransmission exists when wastewater is used for irrigation.Factors influencing transmission of disease include the degree

of wastewater treatment, the crops grown, the irrigationmethod used to apply the wastewater, and the crop and har-vesting practices adopted (Westcot, 1997). These factors definethe degree of public health risk as depicted in Figure 3. Ex-amples of crops representing low risk include olive trees, datepalms, and energy crops.

The risk of infection of field workers relates to their directcontact with the crop or soil in the area where wastewater isused. This risk directly relates to the level of protection neededfor field workers. Workers safety requires adoption of pre-ventive measures against infection. The following risk situ-ations for field workers are identified (Westcot, 1997):

1. Low risk of infection:

• mechanized cropping practices;

• mechanized harvesting practices;

• irrigation ceasing long before harvesting; and

• long dry periods between irrigations.2. High risk of infection:

• high wind and dust areas;

• hand cultivation and hand harvesting;

• moving of sprinkler or other irrigation equipment; and

• direct contact with irrigation water.

Preventive measures to minimize health hazards for fieldworkers include wearing protective clothing, for example,impermeable boots that prevent any direct skin contact withthe wastewater, maintenance of high levels of hygiene, andimmunization against infections that are likely to occur.

International guidelines or standards for the micro-biological quality of wastewater for irrigation when used for aparticular crop do not exist. The standards and guidelines forthe quality of wastewater used for irrigation are focused oneffluent standards at the wastewater treatment plant, ratherthan at the point of use. These standards are often used for


Table 4 Health-based targets for wastewater use in agriculture

Exposurescenario

Health-based target (DALYa perperson per year)b

Log10 pathogenreduction neededb

Number of Helmintheggs per litrec,d

Escherichia coli (number per 100 ml)

Unrestrictedirrigation

r10�6 o103

Lettuce 6 r1 Relaxed to o104 for high-growing leafcrops or drip irrigationOnion 7 r1

Restrictedirrigation

r10�6 o105

Highlymechanized

3 r1 Relaxed to o106

Labor intensive 4 r1 When exposure is limited or regrowth islikelyLocalized (drip)

irrigationr10�6

High-growingcrops

2 No recommendatione

Low-growingcrops

4 r1d

aDisability adjusted life years.bThe health-based target can be achieved, for unrestricted and localized irrigation, by a 6−7 log unit pathogen reduction (obtained by a combination of wastewater treatmentand other health protection measures). For restricted irrigation, it is achieved by a 2−3 log unit pathogen reduction.cWhen children under 15 years are exposed, additional health protection measures should be used (e.g., treatment to r0.1 egg l�1).dThe mean value of r1 egg l�1 should be obtained for at least 90% of samples.eWhen no crops to be picked up from the soil.Source: Adapted from WHO, 2006b. WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater, vol. II. Wastewater Use in Agriculture. Geneva: World Health Organizationand WHO, 2006c. WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater, vol. IV. Excreta and Greywater Use in Agriculture. Geneva: World Health Organization.



process control at wastewater treatment plants. On the basis ofan epidemiological review, WHO adopted water qualityguidelines for wastewater use in agriculture as shown inTable 4. These guidelines are for the microbiological quality oftreated effluent from a wastewater plant when that water isintended for irrigation.

It is advisable to utilize the WHO (2006a–c) guidelines forcontrolling the quality of water used in irrigation (Table 4).Using them as irrigation standards would help to:

• identify the areas currently being contaminated;

• reduce the disease infection risk until suitable wastewatertreatment is adopted;

• improve the basic health level in rural areas; and

• provide indication and data of unsatisfactory areas to assistin planning for wastewater management.

Using the helminth standard throughout all cropping sys-tems aims at increasing the level of protection for agriculturalworkers, who are at high risk of intestinal nematode infection(WHO, 2006b). It is assumed that if the recommended hel-minth egg limit could be reached, then equally high removalsof all protozoa would be achieved. No bacterial guidelineswere considered by WHO for protection of agricultural work-ers because there was little evidence of such a risk to workers,and some degree of reduction in bacterial concentration wouldbe achieved with efforts to meet the helminth levels. Guide-lines shown in Table 4 should be adapted according to localrequirements and specifications.

As urban populations grow enormously, the degree of riverand irrigation water supply contamination in developingcountries will likely increase. Pressure will also increase to usepartially treated wastewater for irrigation until adequate

treatment facilities can be constructed. In addition, predictinglong-term effects of wastewater use is yet not supported bysufficient research (Xu et al., 2010). Thus, there is an imme-diate need to control wastewater use in high-risk croppingsystems such as vegetable crop production. The guidelinesproposed by WHO (2006a–c) could be applied in areas wherewastewater is utilized directly for irrigation or where use isindirect, for example, where contaminated river water, in-cluding when a treatment plant exists, is diverted for irrigation.

Indicative expected removal levels of pathogens are shownin Table 5 for various treatment processes. Results depend notonly upon the treatment, but also on the time of detention(Westcot, 1997; Pereira et al., 2009). Further disinfection maybe required for products that are eaten fresh (Bichai et al.,2012).

Crop Restrictions and Irrigation Practices

To minimize the health risk from using wastewater in irri-gation, the prime approach is to treat the wastewater to thelevel recommended above. However, untreated or insuffi-ciently treated wastewaters are still used for irrigation whenuncontrolled flows are used. Then, the application of croprestrictions is a required measure to protect the consumer andto protect public health. Crop restrictions focus on salad orvegetable crops that are normally eaten raw as indicated be-fore. Unfortunately, in countries where institutional arrange-ments and enforcement of standards are usually not strongenough, crop restrictions are not effectively enforced and donot ensure that unsafe food products are not presented to theconsuming public.

Table 5 Indicative log unit reductions or inactivation of pathogens achieved by selected wastewater treatment processes

Treatment process Log unit pathogens removalsa

Viruses Bacteria Cysts Helminth eggs

Low-rate biological processesWaste stabilization ponds 1–4 1–6 1–4 1–3Wastewater storages and treatment reservoirs 1–4 1–6 1–4 1–3Constructed wetlands 1–2 0.5–3 0.5–2 1–3

High-rate processesPrimary treatmentPrimary sedimentation 0–1 0–1 0–1 0 to o1Chemically enhanced primary treatment 1–2 1–2 1–2 1–3Secondary treatmentActivated sludgeþ secondary sedimentation 0–2 1–2 0–1 1 to o2Trickling filtersþ secondary sedimentation 0–2 1–2 0–1 1–2Aerated lagoonþ settling pond 1–2 1–2 0–1 1–3Tertiary treatmentCoagulation/flocculation 1–3 0–1 1–3 2High-rate granular or slow-rate sand filtration 1–3 0–3 0–3 1–3Dual-media filtration 1–3 0–1 1–3 2–3Membranes 2.5 to 46 3.5 to 46 46 43DisinfectionChlorination 1–3 2–6 0–1.5 0 to o1Ozonation 3–6 2–6 1–2 0–2Ultraviolet radiation 1 to 43 2 to 44 43 0

aThe log unit reductions are log10 unit reductions. A 1 log unit reduction¼90% reduction; a 2 log unit reduction¼99% reduction; a 3 log unit reduction¼99.9% reduction;and so on.Source: Adapted from WHO, 2006b. WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater, vol. II. Wastewater Use in Agriculture. Geneva: World HealthOrganization.



Crop restrictions need a strong institutional framework andthe capacity to monitor and control compliance with theregulations (Pereira et al., 2009). Regulations and governanceon the use of wastewater, namely for irrigation, are required(Hanjra et al., 2012). Planning, management, and operation ofreservoirs for wastewater reuse are essential in terms of was-tewater quality and related reuse performance (Kfir et al.,2012).

The following factors favor the adoption of croprestrictions:

• A law-abiding society or strong law enforcement.

• Allocation of wastewater is controlled by a public body thathas legal authority to enforce crop restrictions.

• The irrigation water conveyance and distribution system iscontrolled by strong central management.

• There is high demand and price advantage for the un-restricted crops.

• There is little market pressure in favor of the excludedcrops.

• Wastewater is used by a small number of large farms.

Very large, dispersed irrigation schemes and those havingpoor or weak management make it difficult to enforce croprestrictions. Difficulties also occur when producers are mainlysmall farmers and the market prices do not favor adoptinglower risk crops. In many developing countries, when waste-water, including untreated effluent, is discharged directly tosurface waters and these are diverted downstream for irrigationpurposes there is a widespread distribution of the wastewaterand crop restriction becomes extremely difficult.

Irrigation practices should be designed according to thequality of used wastewater. Salinity hazards associated withwastewater are considered in the Section Crop IrrigationManagement Using Saline Water (Table 2). Aspects referring tothe control of health hazards were discussed in the precedentsections. However, the selection of methods of irrigation withwastewater to comply with guidelines for controlling healthrisks requires particular attention. Irrigation methods differ onseveral aspects as summarized in Table 6 They mainly concern:

• The probability of direct contact of workers with the irri-gation water, thus referring to the need for adopting morestringent preventive measures.

• The direct contact of the water with the harvestable yield,thus implying the need for a careful implementation ofconsumer protection measures, including crop restrictions.

• The foliar contact with the water that may cause phytotoxicproblems.

• The capability for avoiding salt concentration in the croproot zone, which would require measures to avoid soildegradation (Muyen et al., 2011; Xu et al., 2010) as referredabove in relation to the use of saline water.

Drip irrigation is often preferred. Then filtering is essentialfor appropriate functioning of the system. Media filters may berecommended due to their ability to remove turbidity and theability to recover dissolved oxygen (Elbana et al., 2012).

Aspects relative to health risk reduction have been dis-cussed in the sections above and may be found in severalreferences, such as WHO (2006a), Pescod (1992), and Westcot(1997). Toxicity hazards to the crops (Ayers and Westcot,

Table 6 Evaluation of the irrigation methods for irrigation with wastewater

Irrigation methods Basin irrigation andborder irrigation

Corrugated basinirrigation

Furrow irrigation Sprinkler irrigation Micro irrigation: drip andsubsurface irrigation

Microirrigation:microsprinkling andmicrospray

Human contact (healthhazard)

Likely to occur, mainlywhen water iscontrolled manually.Preventive measuresincluding clothingrequirements

Likely to occur whenwater is controlledmanually, less withautomation. Preventivemeasures includingclothing are required

Likely to occur, whenwater is controlledmanually, less whenautomation is adopted

Workers may havecontact with wettedequipment. Smalldroplets will inevitablybe spread by wind, soworkers will always beat risk

Not likely to occur exceptcontact with wettedirrigation equipment

Generally workers are notin the field whenirrigating but they mayhave contact withwetted equipments.Small drops willinevitably be spread bywind, so workers willalways be at risk

Contact with fruits andharvestable yield(contamination hazard)

Not occurring for treecrops and vines, mosthorticultural and fieldcrops. May occur forlow vegetable cropssuch as lettuce andmelon

Not likely to occurbecause crops aregrown on ridges

Not likely to occurbecause crops aregrown on ridges

Fruits andharvestable yield arecontaminated

Not likely to occur Fruits and harvestableyield of vegetable cropsmay be contaminated.Less likely for under-tree irrigation with nowind

Salt accumulation in theroot zone (salinityhazard)

Not likely to occur exceptfor the underirrigatedparts of the field whenuniformity of waterapplication is very poor

Salts tend to accumulateon the top of the ridge.Leaching beforeseeding or planting isrequired for assuringgermination and plantestablishment

Salts tend to accumulateon the top of the ridge.Leaching is requiredbefore seeding/planting

Not likely to occur exceptfor the underirrigatedparts of the fieldresulting from lowuniformity of waterapplication



Foliar contact (toxicityhazard)

Possible for bottomleaves in low crops(e.g., lettuce andmelon) and foddercrops during first stageof annual crops

Exceptionally becausecrops are grown onridges and water flowsin furrows betweenthem

Exceptionally becausecrops are grown onridges

Severe leaf damage canoccur affecting yields.Edible leaves would becontaminated

Not likely to occur Severe leaf damage canoccur definitelyaffecting yields ofannual crops, but notfor irrigation of treesand vines if drops arelarge enough and jetsare oriented to the soil.Hazardous for edibleleaves

Source: Reproduced from Pereira, L.S., Cordery, I., Iacovides, I., 2009. Coping with Water Scarcity. Addressing the Challenges. Dordrecht: Springer, p. 382.

420Water

Use:Recyclingand

Desalinationfor

Agriculture




1985) mainly require avoiding direct contact between thewater charged with toxic ions and the crop leaves and othersensitive parts of the crop. Dilution of ion concentrations bymixing charged waters with freshwater is generally more costlythan to select an irrigation method where such foliar contact isminimized.

Salts in wastewater used for irrigation are a source of soilpollution (Kalavrouziotis and Koukoulakis, 2012). The saltaccumulation in the root zone is generally controlled byleaching of the salts from the root zone naturally when rainfallis abundant, or by applying a leaching fraction with the irri-gation water. Leaching is discussed in the Section LeachingRequirements and Strategies for Controlling Impacts on SoilSalinity. Appropriate application of a leaching fraction de-pends on the irrigation method and the performance of theirrigation system. Practising overirrigation to be sure that saltsare leached down from the root zone is common but leads toexcessive percolation to the groundwater and may causewaterlogging. The groundwater quality can be degraded.Drainage has to be considered. However, problems may becontrolled easily if the irrigation system is designed carefullyallowing for control of volumes applied and for a uniformdistribution of water over the field.

In summary, the safe use of wastewater in irrigation re-quires not only compliance with guidelines for the control ofhealth risks, but also well-designed and efficiently managedirrigation systems. Several case study examples are provided inthe literature (e.g., Oron et al., 1999).

Monitoring and Control for Safe Wastewater use in Irrigation

Developing a program to promote safe crop production areasshould occur alongside and as an alternative to crop re-strictions. This can be achieved with a phased process as listedbelow:

1. Development and implementation of a sound program forwater quality monitoring to evaluate the existing levels ofcontamination in the water and its use, including

• appropriate recognition of observation areas and sites;

• selection of water quality and contamination indicators;

• selection of analytical methods and participatinglaboratories;

• identification of sampling techniques; and

• selection of field sites, considering
J the water sources,J crop patterns, andJ cropping and irrigation practices.
2. Conducting the field water quality monitoring program,evaluating the water quality data, and developing pro-cedures to assess the levels of contamination. In addition, itis advisable to follow-up soil salinity aiming at protectingthe soil resource.

3. Developing a database aimed at defining safe productionareas and to control or regulate contaminated water use invegetable or other high risk production areas.

4. Through the combination of field monitoring and ex-ploring the database, to install certification programs thatassure consumers on the quality and safety of productionmethods applied in a given area. The last phase is

developing mechanisms to regulate the use of potentiallycontaminated water on high-risks crops, thus also sup-porting certification programs.

A follow-up of farmers’ attitudes and behavior relative tothe use of wastewater, particularly when it does not complywith the required guidelines, is also important because chan-ges in water use aimed at health safety of consumers andworkers can only be effective with the involvement of farmersand their institutions (e.g., Qadir et al., 2010).

Desalination as an Alternative Source of Water forAgriculture

General Aspects and Treatment Processes

The main desalination processes (Fritzmann et al., 2007; Liet al., 2008) are listed below:

1. Thermal distillation (TD): When a saline solution is boiled,the vapor that comes off is pure water and when this iscooled (condensed), the resulting water contains no salt.Three types of thermal distillation units are used com-mercially, namely, multistage flash, multiple effect distil-lation, and vapor compression.

2. Electrodialysis (ED): If a current is passed through a salinesolution, the different ions (cations and anions) in thesolution will carry the current from one electrode tothe other by drifting in opposite directions. If an ion-selective membrane is placed in this flow, say an anion-permeable membrane, only the anions will manage topass through. ED is only an economical process when usedon brackish water, and tends to be most economical atTDS levels of up to 4000–5000 mg l�1.

3. RO: This is a membrane separation process in which thepressure of the water is raised above the osmotic pressure ofthe membrane. No heating or phase change is necessary forthis separation, and the major energy requirement is forpressurizing the feed water. Seawater reverse osmosis(SWRO) has emerged as the leading technology for futureseawater desalination facilities because of its relatively lowenergy consumption and produced water cost comparedwith thermal desalination technologies (Greenlee et al.,2009). However, the perceived costs and energy require-ments of seawater desalination continue to be a barrier toits implementation, especially seawater desalination foragricultural use. The stringent boron and chloride standardsmake the process more energy intensive than desalinationfor potable use. The total energy consumption by a state-of-the art SWRO facility producing agricultural irrigation wateris in the range of 3–7 kWh m�3 of produced water (WRA,2011). A combined seawater desalination process usingemerging forward (FO) technology coupled with RO couldpotentially reduce the energy consumption of the seawaterdesalination process, and thus lower barriers to its imple-mentation. The FO process is strictly direct osmosis acrossan RO-type membrane.

4. Desalination can use photovoltaic technology to provideelectrical energy to operate standard desalting processes likeRO or electrodialysis. Solar desalination has been receiving



considerable interest as it allows producing potable waterin a sustainable way, which is particularly important inarid areas.

Use in Agriculture, Impacts, and Constraints

In the past, the high cost of desalinating and the energy re-quired have been the major constraints on large-scale pro-duction of fresh water from brackish waters and seawater.However, desalinated water is becoming more competitivebecause desalinating costs are declining and the costs of sur-face water and groundwater are increasing. Nevertheless, thecosts of desalinating water do not allow its full use in irrigatedagriculture, with the exception of intensive horticulture forhigh-value cash crops, such as vegetables and flowers, mainlygrown in greenhouses (Martínez-Beltrán and Koo-Oshima,2006).

However, besides cost, there are other important drawbacksthat prevent the wide use of desalinated water in agriculture,because when essential nutrients, including Ca, Mg, and S, re-moved during RO, are not reintroduced as observed byBen-Gal et al. (2009) serious deficiencies in crops occur. Thedeficiency of calcium may harm the development of terminalbuds and of root apical tips (Yermiyahu et al., 2007), whereasmagnesium is vital to the photosynthesis and protein synthesisoperations of plants. Irrigation with low-magnesium-contentwater (10 mg l�1) resulted in a decrease in leaf magnesium andchlorophyll contents, and the fruit produced was lacking in thebasic nutrients like magnesium, calcium, and sulfate. This im-plies the need for additional fertilization (Birnhack and Lahav,2007). Moreover, the very low buffering capacity of desalinatedwater can cause sudden changes in pH during fertilizer additionand can have a great impact on nutrient availability and ulti-mately on agricultural productivity (Yermiyahu et al., 2007). Inaddition, the high levels of boron (B) have been found to causeboron toxicity in a number of B-sensitive crops (Yermiyahuet al., 2007).

To overcome these negative impacts there are two possiblestrategies, either previously mineralizing the desalinated water,or blending desalinated and natural water in order to achievethe required quality. Ben-Gal et al. (2009) evaluated both thestrategies for supplying these nutrients: Blending 30% salinewater with 70% desalinated water brought Ca, Mg, and Sminerals to satisfactory levels, whereas producing water withsalinity of EC¼1.35 dS m�1 which led to satisfactory yields.These researchers noted that the environmental cost of theincrease in irrigation water salinity through blending wassubstantial. Summarizing, fertilizer management must becarefully defined to avoid problems because of the absence ofCa, Mg, and other elements as well as to avoid problems ofinhibition of use of nutrients due to unbalanced compositionof the soil water and soil extract.

The main environmental impacts of water desalinationrelate to the production of a flow of brine containing the saltsremoved from the raw water. This needs to be disposed. En-vironmentally safe disposal depends mainly on the site of thetreatment plant. If situated near the sea or close to brackishenvironments, such as estuaries, brine disposal is compara-tively easier than that from inland desalinating facilities.

In addition, it is worth mentioning that the energy con-sumption associated with the desalination process has an in-direct environmental impact on greenhouse gas emissions.

Concluding Remarks

The use of low quality waters in agriculture, either drainagewater or groundwater from low-quality aquifers, as well as thereuse of wastewater or desalinated water, need appropriatepractices for controlling the impacts on the environment, on thecrops, and particularly on human health. Crops, soil, and irri-gation must be managed in agreement with characteristics ofthe used water in relation to the sensitivity of the crops, thefragility of the soils and environment, and, in case of waste-water, to avoid health consequences for workers and con-sumers. When appropriate technologies and management areapplied, large benefits result from recycling and desalination foragricultural use helping to cope with water scarcity and drought.As described in this article, it is possible to develop strategiesbased on appropriate resource management and the adoptionof the best available technologies, which support human healthcontrol, minimizing environmental impacts, and achievingpositive farm returns, thus leading to sustainable development.Nevertheless, it is definitely required to develop educationaland awareness programs that may make easy the adoption ofbest management practices and acceptability by the consumers.

See also: Climate Change: Agricultural Mitigation. Climate Change,Society, and Agriculture: An Economic and Policy Perspective.Computer Modeling: Applications to Environment and FoodSecurity. Computer Modeling: Policy Analysis and Simulation.Critical Tracking Events Approach to Food Traceability. Economicsof Natural Resources and Environment in Agriculture. Edaphic SoilScience, Introduction to. Farm Management. Food Safety: EmergingPathogens. Global Food Supply Chains. Land Use: CatchmentManagement. Land Use, Land Cover, and Food-Energy-EnvironmentTrade-Off: Key Issues and Insights for Millennium DevelopmentGoals. Land Use: Restoration and Rehabilitation. Plant AbioticStress: Salt. Plant Abiotic Stress: Water. Precision Agriculture:Irrigation. Root and Tuber Crops. Slum Livestock Agriculture. Soil:Conservation Practices. Soil Fertility and Plant Nutrition. Soil:Nutrient Cycling. Tree Fruits and Nuts. Water: Advanced IrrigationTechnologies. Water: Water Quality and Challenges fromAgriculture. World Water Supply and Use: Challenges for the Future

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