© Springer Nature Singapore Pte Ltd. 2019
Rajeev Pratap Singh, Alan S. Kolok and Shannon L. Bartelt-Hunt (eds.)Water Conservation, Recycling and Reuse: Issues and Challengeshttps://doi.org/10.1007/978-981-13-3179-4_13

13. Reuse of Wastewater in Agriculture

M. P. Tripathi1  , Yatnesh Bisen1 and Priti Tiwari1
(1)
Department of Soil and Water Engineering, SVCAET & RS, FAE, IGKV, Raipur, Chhattisgarh, India
 
 
M. P. Tripathi

Abstract

Reuse of water is defined as “water which is used twice or more time before it returns back to the natural water cycle”. Treated wastewater is reused for beneficial purposes which include domestic use as toilet flushing, agricultural and landscape irrigation, industrial processes and replenishing/recharging a groundwater basin. water reuse defined as the dependency on the use of groundwater and surface water sources and can reduce the diversion of water from susceptible ecological systems. Moreover, water reuse may decline the nutrient concentration from wastewater flows into waterways, thereby decreasing and controlling pollution. This chapter built in the different sources and possibilities of reuse wastewater, their advantages, disadvantages and possible risks. The Environmental Protection Agency (EPA), the World Health Organization (WHO) health guidelines for the reuse of wastewater and the Food and Agricultural Organization (FAO) water quality guidelines for irrigation are integrated in this chapter. On the basis of these guidelines, recommendations and policy implementations for safe reuse of wastewater in irrigation and various purposes are suggested in this chapter. Further, the issue of wastewater reclamation is given and discussed properly in this chapter, which can be taken into consideration before implementing the reuse of wastewater for agriculture in India and abroad. The prospective reuse of wastewater depends on the hydraulic and biochemical individuality of wastewater, which determines the systems and extent of treatment required. Irrigation usually requires a lower quality of treatment of wastewater. However, properly designed and adequately implemented wastewater reuse system is an environmental protection measure which is superior to discharging treated wastewater into surface waters. It is the authors’ hope that the content of this chapter will facilitate the consideration of reuse as an integral part of water management strategies in development projects. In this context the aim of the authors is to convey the message that the wastewater irrigation is to maximize the benefits to the poor (who depend on the resource) while minimizing the risks. The integrated guidelines will help task managers and development agency staff to prepare wastewater reuse projects. The information and material about wastewater reuse in agriculture shows that an integrated planning approach, considering economic as well as environmental and health issues, related to water reuse, is essentially a guaranty for the success.

13.1 Introduction

The amount of water is fixed and its form is changing as per atmospheric changes. Water never destroyed, its circulation is a continuous process and only its distribution is changing. Its quantity and quality both should be preserved carefully. It will be the most scarce commodity in the future and has to be dealt carefully. Seventy percent of world water use, including all the water diverted from rivers and pumped from underground, is utilized for irrigation, 20% is used by industry, and 10% goes to residences. The freshwater withdraw in agriculture and the industry worldwide is shown in Fig. 13.1. India utilized its 84% water for agriculture, 4% for drinking purposes and 12% for industry and others. Hence, efficient management in agriculture is very important.
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Fig. 13.1

Freshwater withdraw worldwide by sector in 2000: Source World Resources 2000–2001. People and Ecosystems, Washington, DC 2000

The major objective of wastewater treatment is usually to allocate domestic and industrial effluents to be removed/disposed without any risk to human health or unacceptable harm to the natural environment. In fact, irrigation with wastewater is an effective form of wastewater disposal (as in slow-rate land treatment). It can be completely untreated municipal or industrial wastewater, mechanically purified wastewater or particularly or fully purified wastewater treated biologically (Donta 1997). The amount of treated effluent used in agriculture/horticulture has a great control on the performance and function of the wastewater-soil-plant or aquacultural systems. In the case of irrigation, the required amount of effluent will depend on the different crops, soil conditions and distribution system of effluent to be adopted. Through crop constraint and selection of irrigation methods, which minimize human health, the extent of pre-application wastewater treatment can be decreased.

India represents about 16% of the world’s population. It accounts only about 2.5% of land area and 4% of water resources of the world. The total utilizable water resources of the country are estimated to be about 1123 BCM (690 BCM from surface and 433 BCM from ground), which was found to be 28% of precipitation. About 85% (688 BCM) of water for irrigation is being diverted (Fig. 13.2), and it was estimated to be increased up to 1072 BCM by the year 2050. Major water resource for irrigation is groundwater in India. Annual groundwater recharge in India is about 433 BCM, out of which about 213 BCM is used for irrigation and about 18 BCM for domestic and industrial purposes (CGWB 2011). Water demand for domestic and industrial purposes will be increased up to about 29 BCM till the year 2025. Thus availability of water for irrigation is estimated to reduce up to about 162 BCM.
../images/441018_1_En_13_Chapter/441018_1_En_13_Fig2_HTML.png
Fig. 13.2

Projected water demand by different sectors. (CWC 2010)

The population in India is expected to be more than 1.5 billion by the end of the year 2050 as per the present rate of population growth (1.9% per annum). In India, the average annual freshwater availability per capita has gone down due to increase in population, overall development and change in standard of living. Per capita availability of freshwater per annum in India was reported to be as 5177 m3, 1869 m3 and 1588 m3 for the years 1951, 2001 and 2010, respectively. The effective and efficient water resource management through adopting wastewater treatment process and recycling system is urgently needed to meet the water requirement of the country. The project-wise water required/needed by the various sectors is shown in Fig. 13.2 (CWC 2010). There is an alarming situation of the disposal of wastewater and effluents because of deficit capacity of treatment plants and their increasing rate of sewage production.

The design of wastewater treatment plants is usually based on the need to reduce organic and suspended solid loads to limit the pollution of the environment. Pathogen removal has very rarely been considered an objective, but, for reuse of effluents in agriculture, this must now be of primary concern, and processes should be selected and designed accordingly (Hillman 1988). Treatment to remove wastewater constituents that may be toxic or harmful to crops, aquatic plants (macrophytes) and fish is technically possible but is not normally economically feasible.

The short-period deviations in wastewater disposals noticed at public/municipal wastewater treatment units follow a diurnal pattern. Disposal is usually low during the early-morning hours, when water use is lowest and when the base flow of infiltration-inflow and small amount of sanitary wastewater. A first peak of flow usually occurs in the late morning, when wastewater from the peak morning water consumption reaches the treatment plant, and the second high flow usually occurs in the evening. The relative magnitude of the peaks and the times at which they occur vary from country to country and with the size of the community and the length of the sewers. Small communities with small sewer systems have a much higher ratio of peak flow to average flow than do large communities. Even though the extent of peaks is satisfied as wastewater passes through a treatment plant, the daily differences in flow from a public/municipal treatment unit make it impracticable, in most cases, to irrigate with effluent directly from the treatment unit. The value of reclaimed water in crop irrigation has long been recognized, particularly where fresh water resources are limited (Webster 1954; Mertz 1956; Sepp 1971). Some form of flow equalization or short-term storage of treated wastewater is important to provide a relatively continuous/uniform supply of treated wastewater for efficient irrigation, although extra benefits result from storage.

Wastewater is composed of 99.9% water and 0.1% of other materials like suspended, colloidal and dissolved solids (International reference centres for wastes disposal, 1985). Water resources are so scarce that there is often a major conflict between urban (domestic and industrial) and agricultural demands for water. This conflict can usually only be resolved by the agricultural use of wastewater: the cities must use the fresh water first; urban wastewater after proper treatment must be used for crop irrigation. If such a sequence of water resource utilization is not followed, both urban and agricultural developments may be seriously constrained with consequent adverse effects on national economic development. Moreover, the knowledge of fertilizer value of the effluent is necessary. All of the nitrogen and much of the phosphorus and potassium normally required for agricultural crop production would be supplied by the effluent (Al-Salem 1987). Further, other valuable micronutrients and the organic matter contained in the effluent will provide additional benefits (Abdel-Ghaffar et al. 1988).

13.2 Challenges

Insufficient capacity of wastewater treatment and increasing sewage generation pose a big question of the disposal of wastewater (Bhamoriya 2004). Thus, if the world is facing a water shortage, it is also facing a food shortage. Water deficits, which are already spurring heavy grain imports in numerous smaller countries, may soon do the same in larger countries, such as China and India (Earth Policy Institute 2002). Water supply for agriculture and sanitation will be one of the main future challenges in growing population and industrialization. The growing awareness of water resource scarcity, the competition for water resources and the negative impact of contaminated water on human health and the environment demand the development of adequate strategies in water management. The development of new management strategies to supply fresh water and the issue of treating and recycling wastewater will play an important role in tackling the existing and occurring problems.

It has been reported that application of sewage or sewage mixed with industrial effluents can save 25–50% of N and P fertilizer and produce 15–27% higher crop yield as compared to the normal water (Anonymous 2004). As per an estimate, 73,000 ha of peri-urban agriculture in India is focused to provide irrigation with wastewater (Strauss and Blumenthal 1990). Minhas and Samra (2004) reported that farmers of peri-urban areas generally take up, round the year, rigorous vegetable production system with 300–400% cropping intensity. They also reported that perishable commodity like fodders irrigated with wastewater farmers earn four times more per unit area as compared to normal water. Arar (1988) stated that the most suitable treatment of wastewater to be used prior to effluent application in agriculture is that which will turn out as an effluent meeting the suggested microbiological and chemical quality guidelines both at low cost and at minimum operational and maintenance requirements.

A biological treatment processes alone are not sufficient to meet tightening environmental regulations (Pant and Adholeya 2007). Presently there are no separate regulations/guidelines for safe handling, transport and disposal of wastewater in the country. The existing policies for regulating wastewater management are based on certain environmental laws and certain policies and legal provisions, viz. Constitutional Provisions on sanitation and water pollution; National Environment Policy, 2006; National Sanitation Policy, 2008; Hazardous Waste (Management and Handling) Rules, 1989; Municipalities Act; District Municipalities Act; etc. Water Act 1974 also emphasizes utilization of treated sewage in irrigation, but this issue has been ignored by the State Governments. With the improper design, poor maintenance, frequent electricity breakdowns and lack of technical manpower, the facilities constructed to treat wastewater do not function properly and remain closed most of the time (CPCB 2007). In addition to setting up treatment plants, the Central Government, the State Government and the Board have given financial incentive to industries/investors to encourage them to invest in pollution control (CPCB 2005).

13.3 Irrigation with Wastewater

This section includes (1) conditions for successful irrigation, (2) strategies for managing treated wastewater on the farm, (3) selection of irrigation methods, (4) field management practices in wastewater irrigation and (5) planning for wastewater irrigation.

13.3.1 Conditions for Successful Irrigation

Irrigation is the application of water to plants as per the requirement. It helps to grow agricultural/horticultural crops, maintain landscapes and revegetate disturbed soil in dry areas during the period of insufficient rainfall. In arid and semiarid region of the country, irrigation is important for enhancing the production, where it is usually required on supplementary basis in humid and semi-humid regions.

At the farm level, the following basic conditions should be met to make irrigated farming a success:

  • The required amount of water should be applied.

  • The water should be of acceptable quality.

  • Water application should be properly scheduled.

  • Appropriate irrigation methods should be used.

  • Salt accumulation in the root zone should be prevented by means of leaching.

  • The rise of water table should be controlled by means of appropriate drainage.

  • Plant nutrients should be managed in an optimal way.

The above requirements are equally applicable when the source of irrigation water is treated wastewater. Nutrients in municipal wastewater and treated effluents are a particular advantage of these sources over conventional irrigation water sources, and supplemental fertilizers are sometimes not necessary. However, additional environmental and health requirements must be considered when treated wastewater is the source of irrigation water.

13.3.1.1 Amount of Water to be Applied

It is well known that more than 99% of the water absorbed by plants is lost by transpiration and evaporation from the plant surface. Thus, for all practical purposes, the water requirement of crops is equal to the evapotranspiration requirement, ETc. Crop evapotranspiration is mainly determined by climatic factors and hence can be estimated with reasonable accuracy using meteorological data. An extensive review of this subject and guidelines for estimating ETc, prepared by Doorenbos and Pruitt, are given in Irrigation and Drainage Paper 24 (FAO 1977). A computer program, called CROPWAT, is available in FAO to determine the water requirements of crops from climatic data. Table 13.1 presents the water requirements of some selected crops, reported by Doorenbos and Kassam (FAO 1979). It should be kept in mind that the actual amount of irrigation water to be applied will have to be adjusted for effective rainfall, leaching requirement, application losses and other factors.
Table 13.1

Water requirements, sensitivity to water supply and water utilization efficiency of some selected crops

Crop

Water requirements (mm/growing period)

Sensitivity to water supply (ky)

Water utilization efficiency for harvested yield, Ey, kg/m3 (% moisture)

Alfalfa

800–1600

Low to medium-high (0.7–1.1)

1.5–2.0 hay (10–15%)

Banana

1200–2200

High (1.2–1.35)

Plant crop: 2.5–4

Ratoon: 3.5–6

Fruit (70%)

Bean

300–500

Medium-high (1.15)

Lush: 1.5–2.0 (80–90%)

Dry: 0.3–0.6 (10%)

Cabbage

380–500

Medium-low (0.95)

12–20 head (90–95%)

Citrus

900–1200

Low to medium-high (0.8–1.1)

2–5 fruit (85%, lime: 70%)

Cotton

700–1300

Medium-low (0.85)

0.4–0.6 seed cotton (10%)

Groundnut

500–700

Low (0.7)

0.6–0.8 unshelled dry nut (15%)

Maize

500–800

High (1.25)

0.8–1.6 grain (10–13%)

Potato

500–700

Medium-high (1.1)

4–7 fresh tuber (70–75%)

Rice

350–700

High

0.7–1.1 paddy (15–20%)

Safflower

600–1200

Low (0.8)

0.2–0.5 seed (8–10%)

Sorghum

450–650

Medium-low (0.9)

0.6–1.0 grain (12–15%)

Wheat

450–650

Medium high (spring: 1.15; winter: 1.0)

0.8–1.0 grain (12–15%)

Source: FAO (1979)

13.3.1.2 Quality of Water to be Applied

Irrigation water quality requirements from the point of view of crop production will have to be adjusted depending on the local climate, soil conditions and other factors. In addition, farm practices, such as the type of crop to be grown, irrigation method and agronomic practices, will determine to a great extent the quality suitability of irrigation water. Some of the important farm practices aimed at optimizing crop production when treated sewage effluent is used as irrigation water. The suitability of water for irrigation will greatly depend on the climatic conditions, the physical and chemical properties of the soil, the salt tolerance of the crop grown and the management practices. Thus, classification of water for irrigation will always be general in nature and applicable under average use conditions. Ayers and Westcot (FAO 1985) classified irrigation water into three groups based on salinity, sodicity, toxicity and miscellaneous hazards, as shown in Table 13.2. These general water quality classification guidelines help to identify potential crop production problems associated with the use of conventional water sources. The guidelines are equally applicable to evaluate wastewaters for irrigation purposes in terms of their chemical constituents, such as dissolved salts, relative sodium content and toxic ions.
Table 13.2

Guidelines for interpretation of water quality for irrigation

Potential irrigation problem

Units

Degree of restriction on use

None

Slight to moderate

Severe

Salinity

Ecw a

dS/m

< 0.7

0.7–3.0

> 3.0

or

TDS

mg/l

< 450

450–2000

> 2000

Infiltration

SARb = 0–3 and ECw

 

> 0.7

0.7–0.2

< 0.2

 3–6

 

> 1.2

1.2–0.3

< 0.3

 6–12

 

> 1.9

1.9–0.5

< 0.5

 12–20

 

> 2.9

2.9–1.3

< 1.3

 20–40

 

> 5.0

5.0–2.9

< 2.9

Specific ion toxicity

Sodium (Na)

 Surface irrigation

SAR

< 3

3–9

> 9

 Sprinkler irrigation

me/I

< 3

> 3

 

Chloride (Cl)

 Surface irrigation

me/I

< 4

4–10

> 10

 Sprinkler irrigation

m3/l

< 3

> 3

 

Boron (B)

mg/l

< 0.7

0.7–3.0

> 3.0

Trace elements

Miscellaneous effects

Nitrogen (NO3-N)c

mg/l

< 5

5–30

> 30

Bicarbonate (HCO3)

me/I

< 1.5

1.5–8.5

> 8.5

pH

Normal range 6.5–8

Source: FAO (1985)

aECw means electrical conductivity in deciSiemens per metre at 25 °C

bSAR means sodium adsorption ratio

cNO3-N means nitrate nitrogen reported in terms of elemental nitrogen

Municipal wastewater effluents may contain a number of toxic elements, including heavy metals, because under practical conditions wastes from many small and informal industrial sites are directly discharged into the common sewer system. These toxic elements are normally present in small amounts, and hence, they are called trace elements. Some of them may be removed during the treatment process, but others will persist and could present phytotoxic problems. Thus, municipal wastewater effluents should be checked for trace element toxicity hazards, particularly when trace element contamination is suspected. Table 13.3 presents phytotoxic threshold levels of some selected trace elements.
Table 13.3

Threshold levels of trace elements for crop production

 

Element

Recommended maximum concentration (mg/l)

Remarks

Al

(Aluminium)

5.0

Can cause nonproductivity in acid soils (pH < 5.5), but more alkaline soils at pH > 7.0 will precipitate the ion and eliminate any toxicity

As

(Arsenic)

0.10

Toxicity to plants varies widely, ranging from 12 mg/l for Sudan grass to less than 0.05 mg/l for rice

Be

(Beryllium)

0.10

Toxicity to plants varies widely, ranging from 5 mg/l for kale to 0.5 mg/l for bush beans

Cd

(Cadmium)

0.01

Toxic to beans, beets and turnips at concentrations as low as 0.1 mg/l in nutrient solutions. Conservative limits recommended due to its potential for accumulation in plants and soils to concentrations that may be harmful to humans

Co

(Cobalt)

0.05

Toxic to tomato plants at 0.1 mg/l in nutrient solution. Tends to be inactivated by neutral and alkaline soils

Cr

(Chromium)

0.10

Not generally recognized as an essential growth element. Conservative limits recommended due to the lack of knowledge on its toxicity to plants

Cu

(Copper)

0.20

Toxic to a number of plants at 0.1–1.0 mg/l in nutrient solutions

F

(Fluoride)

1.0

Inactivated by neutral and alkaline soils

Fe

(Iron)

5.0

Not toxic to plants in aerated soils but can contribute to soil acidification and loss of availability of essential phosphorus and molybdenum. Overhead sprinkling may result in unsightly deposits on plants, equipment and buildings

Li

(Lithium)

2.5

Tolerated by most crops up to 5 mg/l; mobile in soil. Toxic to citrus at low concentrations (<0.075 mg/l). Acts similarly to boron

Mn

(Manganese)

0.20

Toxic to a number of crops at a few tenths to a few mg/l but usually only in acid soils

Mo

(Molybdenum)

0.01

Not toxic to plants at normal concentrations in soil and water. Can be toxic to livestock if forage is grown in soils with high concentrations of available molybdenum

Ni

(Nickel)

0.20

Toxic to a number of plants at 0.5 mg/l to 1.0 mg/l; reduced toxicity at neutral or alkaline pH

Pd

(Lead)

5.0

Can inhibit plant cell growth at very high concentrations

Se

(Selenium)

0.02

Toxic to plants at concentrations as low as 0.025 mg/l and toxic to livestock if forage is grown in soils with relatively high levels of added selenium. As essential element to animals but in very low concentrations

Sn

(Tin)

  

Ti

(Titanium)

–

Effectively excluded by plants; specific tolerance unknown

W

(Tungsten)

  

C

(Vanadium)

0.10

Toxic to many plants at relatively low concentrations

Zn

(Zinc)

2.0

Toxic to many plants at widely varying concentrations; reduced toxicity at pH > 6.0 and in fine-textured or organic soils

Source: Adapted from National Academy of Sciences (1972) and Pratt (1972)

Note: The maximum concentration is based on a water application rate which is consistent with good irrigation practices (10,000 m3 per hectare per year). If the water application rate greatly exceeds this, the maximum concentrations should be adjusted downwards accordingly. No adjustment should be made for application rates less than 10,000 m3 per hectare per year. The values given are for water used on a continuous basis at one site

13.3.1.3 Scheduling of Irrigation

To obtain the maximum yields, water should be applied to crops before the soil moisture potential reaches a level at which the evapotranspiration rate is likely to be reduced below its potential. The relationship of actual and maximum yields to actual and potential evapotranspiration is illustrated in the following equation:
$$ \left(1-\frac{{\mathrm{Y}}_{\mathrm{a}}}{{\mathrm{Y}}_{\mathrm{m}}}\right)\kern0.5em =\kern0.5em \mathrm{ky}\left(1-\frac{{\mathrm{ET}}_{\mathrm{a}}}{{\mathrm{ET}}_{\mathrm{m}}}\right) $$
(13.1)
where

  • Ya = actual harvested yield

  • Ym = maximum harvested yield

  • ky = yield response factor

  • ETa = actual evapotranspiration

  • ETm = maximum evapotranspiration

Several methods are available to determine optimum irrigation scheduling. The factors that determine irrigation scheduling are available water-holding capacity of the soils, depth of root zone, evapotranspiration rate and amount of water to be applied per irrigation, irrigation method and drainage conditions.

13.3.1.4 Irrigation Methods

Many different methods are used by farmers to irrigate crops. They range from watering individual plants from a can of water to highly automated irrigation by a centre pivot system. However, from the point of wetting the soil, these methods can be grouped under five headings, namely:
  1. (i)

    Flood irrigation – water is applied over the entire field to infiltrate into the soil (e.g. wild flooding, contour flooding, borders, basins, etc.).

     
  2. (ii)

    Furrow irrigation – water is applied between ridges (e.g. level and graded furrows, contour furrows, corrugations, etc.). Water reaches the ridge, where the plant roots are concentrated, by capillary action.

     
  3. (iii)

    Sprinkler irrigation – water is applied in the form of a spray and reaches the soil very much like rain (e.g. portable, semiportable and solid set sprinklers, travelling sprinklers, spray guns, centre pivot systems, etc.). The rate of application is adjusted so that it does not create ponding of water on the surface.

     
  4. (iv)

    Subirrigation – water is applied beneath the root zone in such a manner that it wets the root zone by capillary rise (e.g. sub-surface irrigation canals, buried pipes, etc.). Deep surface canals or buried pipes are used for this purpose.

     
  5. (v)

    Localized irrigation – water is applied around each plant or a group of plants so as to wet locally and the root zone only (e.g. drip irrigation, bubblers, micro-sprinklers, etc.). The application rate is adjusted to meet evapotranspiration needs so that percolation losses are minimized. Table 13.4 presents some basic features of selected irrigation systems as reported by Doneen and Westcot (FAO 1988).

     
Table 13.4

Basic features of some selected irrigation systems

Irrigation method

Topography

Crops

Remarks

Widely spaced borders

Land slopes capable of being graded to less than 1% slope and preferably 0.2%

Alfalfa and other deep rooted close-growing crops and orchards

The most desirable surface method for irrigating close-growing crops where topographical conditions are favourable. Even grade in the direction of irrigation is required on flat land and is desirable but not essential on slopes of more than 0.5%. Grade changes should be slight and reverse grades must be avoided. Cross slops is permissible when confined to differences in elevation between border strips of 6–9 cm. Water application efficiency 45–60%

Graded contour furrows

Variable land slopes of 2–25% but preferable less

Row crops and fruit

Especially adapted to row crops on steep land, though hazardous due to possible erosion from heavy rainfall. Unsuitable for rodent-infested fields or soils that crack excessively. Actual grade in the direction of irrigation 0.5–1.5%. No grading required beyond filling gullies and removal of abrupt ridges. Water application efficiency 50–65%

Rectangular checks (levees)

Land slopes capable of being graded so single or multiple tree basins will be levelled within 6 cm

Orchard

Especially adapted to soils that have either a relatively high or low water intake rate. May require considerable grading. Water application efficiency 40–60%

Subirrigation

Smooth flat

Shallow rooted crops such as potatoes or grass

Requires a water table, very permeable subsoil conditions and precise levelling. Very few areas adapted to this method. Water application efficiency 50–70%

Sprinkler

Undulating 1–> 35% slope

All crops

High operation and maintenance costs. Good for rough or very sandy lands in areas of high production and good markets. Good method where power costs are low. May be the only practical method in areas of steep or rough topography. Good for high rainfall areas where only a small supplementary water supply is needed. Water application efficiency 60–70%

Localized (drip, trickle, etc.)

Any topographic condition suitable for row crop farming

Row crops or fruit

Perforated pipe on the soil surface drips water at base of individual vegetable plants or around fruit trees. Has been successfully used in Israel with saline irrigation water. Still in the development stage. Water application efficiency 75–85%

Source: FAO (1988)

13.3.1.5 Leaching

Under irrigated agriculture, a certain amount of excess irrigation water is required to percolate through the root zone so as to remove the salts which have accumulated as a result of evapotranspiration from the original irrigation water. This process of displacing the salts from the root zone is called leaching, and that portion of the irrigation water which mobilizes the excess of salts is called the leaching fraction, LF.
$$ \mathrm{Leaching}\kern0.5em \mathrm{Fraction}\kern0.5em \left(\mathrm{LF}\right)=\frac{\mathrm{depth}\kern0.5em \mathrm{of}\kern0.5em \mathrm{water}\kern0.5em \mathrm{leached}\kern0.5em \mathrm{below}\kern0.5em \mathrm{the}\kern0.17em \mathrm{root}\kern0.17em \mathrm{zone}}{\mathrm{depth}\kern0.5em \mathrm{of}\kern0.5em \mathrm{water}\kern0.5em \mathrm{applied}\;\mathrm{at}\kern0.5em \mathrm{the}\kern0.17em \mathrm{surface}} $$
(13.2)

Salinity control by effective leaching of the root zone becomes more important as irrigation water becomes more saline.

13.3.1.6 Drainage

Drainage is defined as the removal of excess water from the soil surface and below so as to permit optimum growth of plants. Removal of excess surface water is termed surface drainage, while the removal of excess water from beneath the soil surface is termed sub-surface drainage. The importance of drainage for successful irrigated agriculture has been well demonstrated. It is particularly important in semiarid and arid areas to prevent secondary salinization. In these areas, the water table will rise with irrigation when the natural internal drainage of the soil is not adequate. When the water table is within a few metres of the soil surface, capillary rise of saline groundwater will transport salts to the soil surface. At the surface, water evaporates, leaving the salts behind. If this process is not arrested, salt accumulation will continue, resulting in salinization of the soil. In such cases, sub-surface drainage can control the rise of the water table and hence prevent salinization.

13.3.2 Strategies for Managing Treated Wastewater on the Farm

Success in using treated wastewater for crop production will largely depend on adopting appropriate strategies aimed at optimizing crop yields and quality, maintaining soil productivity and safeguarding the environment. Several alternatives are available, and a combination of these alternatives will offer an optimum solution for a given set of conditions. The user should have prior information on effluent supply and its quality, as indicated in Table 13.3, to ensure the formulation and adoption of an appropriate on-farm management strategy.

Basically, the components of an on-farm strategy in using treated wastewater will consist of a combination of:

  • Crop selection

  • Selection of irrigation method

  • Adoption of appropriate management practices

Furthermore, when the farmer has additional sources of water supply, such as a limited amount of normal irrigation water, he will then have an option to use both the effluent and the conventional source of water in two ways, namely:

  • By blending conventional water with treated effluent

  • By using the two sources in rotation

These are discussed briefly in the following sections (Table 13.5).
Table 13.5

Information required on effluent supply and quality

Information

Decision on irrigation management

Effluent supply

The total amount of effluent that would be made available during the crop growing season

Total area that could be irrigated

Effluent available throughout the year

Storage facility during non-crop growing period either at the farm or near wastewater treatment plant and possible use for aquaculture

The rate of delivery of effluent either as m3 per day or litres per second

Area that could be irrigated at any given time, layout of fields and facilities and system of irrigation

Type of delivery: continuous or intermittent or on demand

Layout of fields and facilities, irrigation system and irrigation scheduling

Mode of supply: supply at farm gate or effluent available in a storage reservoir to be pumped by the farmer

The need to install pumps and pipes to transport effluent and irrigation system

Effluent quality

Total salt concentration and/or electrical conductivity of the effluent

Selection of crops, irrigation method, leaching and other management practices

Concentrations of cations, such as Ca++, Mg++ and Na+

To assess sodium hazard and undertake appropriate measures

Concentration of toxic ions, such as heavy metals, Boron and Cl−

To assess toxicities that are likely to be caused by these elements and take appropriate measures

Concentration of trace elements (particularly those which are suspected of being phytotoxic)

To assess trace toxicities and take appropriate measures

Concentration of nutrients, particularly nitrate-N

To adjust fertilizer levels, avoid over-fertilization and select crop

Level of suspended sediments

To select appropriate irrigation system and measures to prevent clogging problems

Levels of intestinal nematodes and faecal coliforms

To select appropriate crops and irrigation systems

13.3.3 Selection of Irrigation Methods

Under normal conditions, the type of irrigation method selected will depend on water supply conditions, climate, soil, crops to be grown, cost of irrigation method and the ability of the farmer to manage the system. However, when using wastewater as the source of irrigation, other factors, such as contamination of plants and harvested product, farm workers and the environment and salinity and toxicity hazards, will need to be considered. There is considerable scope for reducing the undesirable effects of wastewater use in irrigation through selection of appropriate irrigation methods.

The choice of irrigation method in using wastewater is governed by the following technical factors:

  • The choice of crops

  • The wetting of foliage, fruits and aerial parts

  • The distribution of water, salts and contaminants in the soil

  • The ease with which high soil water potential could be maintained

  • The efficiency of application

  • The potential to contaminate farm workers and the environment

Table 13.6 presents an analysis of these factors in relation to four widely practised irrigation methods, namely, border, furrow, sprinkler and drip irrigation.
Table 13.6

Evaluation of common irrigation methods in relation to the use of treated wastewater

Parameters of evaluation

Furrow irrigation

Border irrigation

Sprinkler irrigation

Drip irrigation

1. Foliar wetting and consequent leaf damage resulting in poor yield

No foliar injury as the crop is planted on the ridge

Some bottom leaves may be affected, but the damage is not so serious as to reduce yield

Severe leaf damage can occur resulting in significant yield loss

No foliar injury occurs under this method of irrigation

2. Salt accumulation in the root zone with repeated applications

Salts tend to accumulate in the ridge which could harm the crop

Salts move vertically downwards and are not likely to accumulate in the root zone

Salt movement is downwards, and root zone is not likely to accumulate salts

Salt movement is radial along the direction of water movement. A salt wedge is formed between drip points

3. Ability to maintain high soil water potential

Plants may be subject to stress between irrigations

Plants may be subject to water stress between irrigations

Not possible to maintain high soil water potential throughout the growing season

Possible to maintain high soil water potential throughout the growing season and minimize the effect of salinity

4. Suitability to handle brackish wastewater without significant yield loss

Fair to medium. With good management and drainage acceptable yields are possible

Fair to medium. Good irrigation and drainage practices can produce acceptable levels of yield

Poor to fair. Most crops suffer from leaf damage and yield is low

Excellent to good. Almost all crops can be grown with very little reduction in yield

Source: Kandiah (1990b)

A border (and basin or any flood irrigation) system involves complete coverage of the soil surface with treated effluent and is normally not an efficient method of irrigation. This system will also contaminate vegetable crops growing near the ground and root crops and will expose farm workers to the effluent more than any other method. Thus, from both the health and water conservation points of view, border irrigation with wastewater is not satisfactory.

Furrow irrigation, on the other hand, does not wet the entire soil surface. This method can reduce crop contamination, since plants are grown on the ridges, but complete health protection cannot be guaranteed. Contamination of farm workers is potentially medium to high, depending on automation. If the effluent is transported through pipes and delivered into individual furrows by means of gated pipes, risk to irrigation workers will be minimum.

The efficiency of surface irrigation methods, in general, borders, basins and furrows, is not greatly affected by water quality, although the health risk inherent in these systems is most certainly of concern. Some problems might arise if the effluent contains large quantities of suspended solids, and these settle out and restrict flow in transporting channels, gates, pipes and appurtenances. The use of primary treated sewage will overcome many of such problems. To avoid surface ponding of stagnant effluent, land levelling should be carried out carefully, and appropriate land gradients should be provided.

Sprinkler, or spray, irrigation methods are generally more efficient in terms of water use, since greater uniformity of application can be achieved. However, these overhead irrigation methods may contaminate ground crops, fruit trees and farm workers. In addition, pathogens contained in aerosolized effluent may be transported downwind and create a health risk to nearby residents. Generally, mechanized or automated systems have relatively high capital costs and low labour costs compared with manually moved sprinkler systems. Rough land levelling is necessary for sprinkler systems, to prevent excessive head losses and achieve uniformity of wetting. Sprinkler systems are more affected by water quality than surface irrigation systems, primarily as a result of the clogging of orifices in sprinkler heads, potential leaf burns and phytotoxicity when water is saline and contains excessive toxic elements and sediment accumulation in pipes, valves and distribution systems. Secondary wastewater treatment has generally been found to produce an effluent suitable for distribution through sprinklers, provided that the effluent is not too saline. Further precautionary measures, such as treatment with granular filters or micro-strainers and enlargement of nozzle orifice diameters to not less than 5 mm, are often adopted.

Localized irrigation, particularly when the soil surface is covered with plastic sheeting or other mulch, uses effluent more efficiently, can often produce higher crop yields and certainly provides the greatest degree of health protection for farm workers and consumers. Trickle and drip irrigation systems are expensive, however, and require a high quality of effluent to prevent clogging of the emitters through which water is slowly released into the soil. Table 13.7 presents water quality requirements to prevent clogging in localized irrigation systems. Solids in the effluent or biological growth at the emitters will create problems, but gravel filtration of secondary treated effluent and regular flushing of lines have been found to be effective in preventing such problems in Cyprus (Papadopoulos and Stylianou 1988). Bubbler irrigation, a technique developed for the localized irrigation of tree crops, avoids the need for small emitter orifices, but careful setting is required for its successful application (Hillel 1987).
Table 13.7

Water quality and clogging potential in drip irrigation systems

Potential problem

Units

Degree of restriction on use

None

Slight to moderate

Severe

Physical

Suspended solids

mg/l

< 50

50–100

> 100

Chemical

pH

 

< 7.0

7.0–8.0

> 8.0

Dissolved solids

mg/l

< 500

500–2000

> 2000

Manganese

mg/l

< 0.1

0.1–1.5

> 1.5

Iron

mg/l

< 0.1

0.1–1.5

> 1.5

Hydrogen sulphide

mg/l

< 0.5

0.5–2.0

> 2.0

Biological

Maximum

   

Bacterial populations

Number/ml

< 10,000

10,000–50,000

> 50,000

Source: Adapted from Nakayama (1982)

When compared with other systems, the main advantages of trickle irrigation seem to be:
  1. (i)

    Increased crop growth and yield achieved by optimizing the water, nutrients and air regimes in the root zone.

     
  2. (ii)

    High irrigation efficiency – no canopy interception, wind drift or conveyance losses and minimal drainage losses.

     
  3. (iii)

    Minimal contact between farm workers and effluent.

     
  4. (iv)

    Low-energy requirements – the trickle system requires a water pressure of only 100–300 k pa (1–3 bar).

     
  5. (v)

    Low labour requirements – the trickle system can easily be automated, even to allow combined irrigation and fertilization (sometimes terms fertigation).

     

Apart from the high capital costs of trickle irrigation systems, another limiting factor in their use is that they are only suited to the irrigation of row crops. Relocation of sub-surface systems can be prohibitively expensive.

Clearly, the decision on irrigation system selection will be mainly a financial one, but it is to be hoped that the health risks associated with the different methods will be taken into account. Each measure will interact with others, and thus a decision on irrigation system selection will have an influence on wastewater treatment requirements, human exposure control and crop selection (e.g. row crops are dictated by trickle irrigation). At the same time, the irrigation techniques feasible will depend on crop selection, and the choice of irrigation system might be limited if wastewater treatment has already been decided before effluent use is considered.

13.3.4 Field Management Practices in Wastewater Irrigation

Management of water, soil, crop and operational procedures, including precautions to protect farm workers, plays an important role in the successful use of sewage effluent for irrigation.

13.3.4.1 Water Management

Most treated wastewaters are not very saline, salinity levels usually ranging between 500 and 200 mg/l (ECw = 0.7–3.0 dS/m). However, there may be instances where the salinity concentration exceeds the 2000 mg/l level. In any case, appropriate water management practices will have to be followed to prevent salinization, irrespective of whether the salt content in the wastewater is high or low. It is interesting to note that even the application of a nonsaline wastewater, such as one containing 200–500 mg/l, when applied at a rate of 20,000 m3 per hectare, a fairly typical irrigation rate, will add between 2 and 5 tonnes of salt annually to the soil. If this is not flushed out of the root zone by leaching and removed from the soil by effective drainage, salinity problems can build up rapidly. Leaching and drainage are thus two important water management practices to avoid salinization of soils.

13.3.4.2 Leaching

The concept of leaching has already been discussed. The question that arises is how much water should be used for leaching, i.e. what is the leaching requirement? To estimate the leaching requirement, both the salinity of the irrigation water (ECw) and the crop tolerance to soil salinity (ECe) must be known. The necessary leaching requirement (LR) can be estimated from Fig. 13.3 for general crop rotations reported by Ayers and Westcot (FAO 1985). A more exact estimate of the leaching requirement for a particular crop can be obtained using the following equation:
$$ \mathrm{LR}=\frac{{\mathrm{EC}}_{\mathrm{W}}}{5\left({\mathrm{EC}}_{\mathrm{e}}-{\mathrm{EC}}_{\mathrm{W}}\right)} $$
(13.3)
where LR is the minimum leaching requirement needed to control salts within the tolerance (ECe) of the crop with ordinary surface methods of irrigation, ECw salinity of the applied irrigation water in dS/m and ECe average soil salinity tolerated by the crop as measured on a soil saturation extract. It is recommended that the ECe value that can be expected to result in at least a 90% or greater yield can be used in the calculation.
../images/441018_1_En_13_Chapter/441018_1_En_13_Fig3_HTML.png
Fig. 13.3

Relationship between applied water salinity and soil water salinity at different leaching fractions (FAO 1985)

Figure 13.3 was developed using ECe values for the 90% yield potential. For water in the moderate to high salinity range (>1.5 dS/m), it might be better to use the ECe value for maximum yield potential (100%) since salinity control is critical in obtaining good yields. Further information on this is contained in Irrigation and Drainage Paper 29, Rev. 1 (FAO 1985).

Where water is scarce and expensive, leaching practices should be designed to maximize crop production per unit volume of water applied, to meet both the consumptive use and leaching requirements. Depending on the salinity status, leaching can be carried out at each irrigation, each alternative irrigation or less frequently, such as seasonally or at even longer intervals, as necessary to keep the salinity in the soil below the threshold above which yield might be affected to an unacceptable level. With good-quality irrigation water, the irrigation application level will almost always apply sufficient extra water to accomplish leaching. With high salinity irrigation water, meeting the leaching requirement is difficult and requires large amounts of water. Rainfall must be considered in estimating the leaching requirement and in choosing the leaching method.

The following practices are suggested for increasing the efficiency of leaching and reducing the amount of water needed:
  1. (i)

    Leach during cool seasons instead of during warm periods, to increase the efficiency and ease of leaching, since the total annual crop water demand (ET, mm/year) losses are lower.

     
  2. (ii)

    Use more salt-tolerant crops which require a lower leaching requirement (LR) and thus have a lower water demand.

     
  3. (iii)

    Use tillage to slow overland water flow and reduce the number of surface cracks which bypass flow through large pores and decrease leaching efficiency.

     
  4. (iv)

    Use sprinkler irrigation at an application rate below the soil infiltration rate as this favours unsaturated flow, which is significantly more efficient for leaching than saturated flow. More irrigation time but less water is required than for continuous ponding.

     
  5. (v)

    Use alternate ponding and drying instead of continuous ponding as this is more efficient for leaching and uses less water, although the time required to leach is greater. This may have drawbacks in areas having a high water table, which allows secondary salinization between poundings.

     
  6. (vi)

    Where possible, schedule leaching at periods of low crop water use or postpone leaching until after the cropping season.

     
  7. (vii)

    Avoid fallow periods, particularly during hot summers, when rapid secondary soil salinization from high water tables can occur.

     
  8. (viii)

    If infiltration rates are low, consider pre-planting irrigations or off-season leaching to avoid excessive water applications during the crop season.

     
  9. (ix)

    Use one irrigation before the start of the rainy season if total rainfall is normally expected to be insufficient for a complete leaching. Rainfall is often the most efficient leaching method because it provides high-quality water at relatively low rates of application.

     

Drainage

Salinity problems in many irrigation projects in arid and semiarid areas are associated with the presence of a shallow water table. The role of drainage in this context is to lower the water table to a desirable level, at which it does not contribute to the transport of salts to the root zone and the soil surface by capillarity. What is important is to maintain a downward movement of water through soils. Van Schilfgaarde (1984) reported that drainage criteria are frequently expressed in terms of critical water table depths; although this is a useful concept, prevention of salinization depends on the establishment, averaged over a period of time, of a downward flux of water. Another important element of the total drainage system is its ability to transport the desired amount of drained water out of the irrigation scheme and dispose it safely. Such disposal can pose a serious problem, particularly when the source of irrigation water is treated wastewater, depending on the composition of the drainage effluent.

Timing of Irrigation

The timing of irrigation, including irrigation frequency, pre-planting irrigation and irrigation prior to a winter rainy season, can reduce the salinity hazard and avoid water stress between irrigations. Some of these practices are readily applicable to wastewater irrigation.

In terms of meeting the water needs of crops, increasing the frequency of irrigation will be desirable as it eliminates water stress between irrigations. However, from the point of view of overall water management, this may not always produce the desired results. For example, with border, basin and other flood irrigation methods, frequent irrigations may result in an unacceptable increase in the quantity of water applied, decrease in water use efficiency and larger amounts of water to be drained. However, with sprinklers and localized irrigation methods, frequent applications with smaller amounts may not result in decrease in water use efficiency and, indeed, could help to overcome the salinity problem associated with saline irrigation water.

Pre-planting irrigation is practised in many irrigation schemes for two reasons, namely, (i) to leach salts from the soil surface which may have accumulated during the previous cropping period and to provide a salt-free environment to germinating seeds (it should be noted that for most crops, the seed germination and seedling stages are most sensitive to salinity) and (ii) to provide adequate moisture to germinating seeds and young seedlings. A common practice among growers of lettuce, tomatoes and other vegetable crops is to pre-irrigate the field before planting, since irrigation soon after planting could create local water stagnation and wet spots that are not desirable. Treated wastewater is a good source for pre-irrigation as it is normally not saline and the health hazards are practically nil.

Blending of Wastewater with Other Water Supplies

One of the options that may be available to farmers is the blending of treated sewage with conventional sources of water, canal water or ground water, if multiple sources are available. It is possible that a farmer may have saline ground water and, if he has nonsaline treated wastewater, could blend the two sources to obtain a blended water of acceptable salinity level. Further, by blending, the microbial quality of the resulting mixture could be superior to that of the unblended wastewater.

Alternating Treated Wastewater with Other Water Sources

Another strategy is to use the treated wastewater alternately with the canal water or groundwater, instead of blending. From the point of view of salinity control, alternate applications of the two sources will be superior to blending. However, an alternating application strategy will require duel conveyance systems and availability of the effluent dictated by the alternate schedule of application.

13.3.5 Planning for Wastewater Irrigation

13.3.5.1 Central Planning

Government policy on effluent use in agriculture will have a deciding effect on what control measures can be achieved through careful selection of site and crops to be irrigated with treated effluent. A decision to make treated effluent available to farmers for unrestricted irrigation or to irrigate public parks and urban green areas with effluent will remove the possibility of taking advantage of careful selection of sites, irrigation techniques and crops in limiting the health risks and minimizing environmental impacts. However, if a government decides that effluent irrigation will only be applied in specific controlled areas, even if crop selection is not limited (i.e. unrestricted irrigation is allowed within these areas), public access to the irrigated areas will be prevented, and the control measures can be applied. Without doubt, the greatest security against health risk and adverse environmental impact will be achieved by limiting the effluent use to restricted irrigation on controlled areas to which the public has no access, but even imposing restrictions on effluent irrigation by farmers, if properly enforced, can achieve a degree of control.

A number of key issues or tasks were likely to have a significant effect on the ultimate success of effluent irrigation as follows:
  1. (i)

    Organizational and managerial provisions made to administer the resource, to select the effluent use plan and to implement it

     
  2. (ii)

    The importance attached to public health considerations and the levels of risk taken

     
  3. (iii)

    The choice of single-use or multiple-use strategies

     
  4. (iv)

    The criteria adopted in evaluating alternative reuse proposals

     
  5. (v)

    The level of appreciation of the scope for establishing a forest resource

     

13.3.5.2 Desirable Site Characteristics

The features which are critical in deciding the viability of a land disposal project are the location of available land and public attitudes. Land which is far distant from the sewage treatment plant will incur high costs for transporting treated effluent to site and will generally not be suitable. Hence, the availability of land for effluent irrigation should be considered when sewerage is being planned, and sewage treatment plants should be strategically located in relation to suitable agricultural sites. Ideally, these sites should not be closed to residential areas, but even remote land might not be acceptable to the public if the social, cultural or religious attitudes are opposed to the practice of wastewater irrigation. The potential health hazards associated with effluent irrigation can make this a very sensitive issue, and public concern will only be mollified by the application of strict control measures. In arid areas, the importance of agricultural use of treated effluent makes it advisable to be as systematic as possible in planning, developing and managing effluent irrigation projects, and the public must be kept informed at all stages.

The ideal objective in site selection is to find a suitable area where long-term application of treated effluent will be feasible without adverse environmental or public health impacts. It might be possible in a particular instance to identify several potential sites within reasonable distance of the sewered community, and the problem will be to select the most suitable area or areas, taking all relevant factors into account. The following basic information on an area under consideration will be of value, if available:

  • A topographic map

  • Agricultural soil surveys

  • Aerial photographs

  • Geological maps and reports

  • Groundwater reports and well logs

  • Boring logs and soil test results

  • Other soil and piezometric data

At this preliminary stage of investigation, it should be possible to assess the potential impact of treated effluent application on any usable aquifer in the area(s) concerned. The first ranking of sites should consider other factors, such as the cost and location of the land, its present use and availability and social factors, in addition to soil and groundwater conditions.

The characteristics of the soil profile underlying a particular site are very important in deciding on its suitability for effluent irrigation and the methods of application to be employed. Among the soil properties important from the point of view of wastewater application and agricultural production are physical parameters (such as texture, grading, liquid and plastic limits, etc.), permeability, water-holding capacity, pH, salinity and chemical composition. Preliminary observation of sites, which could include shallow hand auger borings and identification of vegetation, will often allow the elimination of clearly unsatisfactory sites. After elimination of marginal sites, each site under serious consideration must be investigated by on-site borings to ascertain the soil profile, soil characteristics and location of the water table. Piezometers should be located in each borehole, and these can be used for subsequent groundwater sampling. A procedure for such site assessment has been described by Hall and Thompson (1981) and, if applied, should not only allow the most suitable site among several possible to be selected but permit the impact of effluent irrigation at the chosen site to be modelled. When a site is developed, a long-term groundwater monitoring programme should be an essential feature of its management.

13.3.5.3 Economic, Institutional and Policy Issues

While the overall benefits of wastewater use in agriculture are obvious and the technology and expertise exist to allow it to be achieved without detriment to public health or the environment, governments must be prepared to control the process within a broader framework of a national effluent use policy forming part of the national plan for water resources. Lines of responsibility and cost allocation formulae have to be worked out between the various sectors involved: local authorities responsible for wastewater treatment and disposal, farmers who will benefit from any effluent use scheme and the state which is concerned with the provision of adequate water supplies, the protection of the environment and the promotion of public health. Sufficient attention must be given to the social, institutional and organizational aspects of effluent use in agriculture and aquaculture to ensure long-term sustainability.

13.3.5.4 Economic and Financial Implications

Although the responsibility for collecting, treating and disposing of urban wastewater will normally lie with a local water or sewerage authority or municipality, farmers wishing to take advantage of the effluent are often able and willing to pay for what they use but are not prepared to subsidize general disposal costs. They will base their decision on whether or not they will be better off paying for the effluent rather than doing without it, considering the quantity, timing, quality and cost of the treated effluent. The local sewerage authority should acknowledge their financial responsibility for the basic system to achieve environmental protection objectives and only charge farmers for any incremental costs associated with additional treatment or distribution required specifically for effluent use in agriculture or aquaculture. In practice, if the effluent use scheme is considered at the time the sewerage project is being planned, treatment costs might well be reduced over those normally required for environmental protection.

Payments by farmers might take the form of direct effluent use tariffs paid to the authority or contributions to the capital and/or operating costs of the wastewater treatment plant and effluent conveyance system. Cost sharing can be by cash payments or in-kind contributions, such as land for siting treatment or storage facilities and labour for operation and maintenance. Bartone (1986) has indicated that benefit-cost studies made in Peru showed that the irrigation components in effluent irrigation schemes were economically viable even if land costs and operation and maintenance for wastewater treatment were charged to farmers but not if the full cost of investment in treatment facilities was charged against the agricultural component. In the latter case, feasibility depended on the alternative minimum cost of treatment required for disposal without reuse.

Since wastewater treatment is a major cost in effluent use systems, accepting that local authorities are fully responsible for wastewater collection, it is essential that treatment process selection is made in conjunction with decisions on crop and irrigation system selection. Only in this way can a minimal investment in treatment be achieved without compromising the health risks of an effluent use scheme. Once a decision on effluent quality has been taken, the required standard must be achieved consistently, and the effluent treatment and conveyance system must be operated with complete reliability. Fluctuating production and demand for effluent created by seasonal and diurnal patters of water use, cropping and crop water needs must be accommodated at all times, even if the price of the effluent is varied, to be higher in the hot season.

13.3.5.5 Institutional Organization

The scope and success of any effluent use scheme will depend to a large extent on the administrative skills applied. Wastewater collection and treatment and effluent use in agriculture and aquaculture span a wide range of both urban-based and rural-based interests at both local and regional levels, and institutional responsibilities must be clearly defined. Decisions will have to be taken on:

  • Allocation of effluent among competing uses

  • Maintenance of quality standards and system reliability

  • Investment in supporting resources, especially managerial and technical staff, required to administer each component of an effluent use scheme

Policy decisions should normally be taken by a national or regional body, with executive responsibilities in the hands of a regional organization. Such a regional organization would be responsible for project implementation and operation and would provide the criteria, framework and administrative mechanisms necessary for effective effluent utilization. However, they would also be responsible for effective monitoring and control of the crops irrigated, the quality of effluent and associated health and environmental impacts.

One of the most important features of a successful effluent use scheme is the supervision provided at all stages of the system. Strict control must be applied from the wastewater treatment plant, through the conveyance and irrigation systems, to the quality of the resulting products, whether they are of commercial or environmental value. The management, monitoring and public relations procedures are as important as the technological hardware involved in the system, and managers of regional organizations set up to administer effluent use projects must be firm if the schemes are to realize their full potential. Managerial and technical staff must be properly qualified and suitably trained to carry out their functions effectively. Treated effluent use in agriculture is a major resource development activity and requires an appropriate institutional structure, provided with adequate resources, to be successful.

13.3.5.6 Policy Issues

The legislative framework for effluent use in agriculture can have a significant influence on project feasibility. Bartone (1986) has indicated that the authorities in Mexico are able to impose effective crop restriction measures in irrigation districts because they are empowered to withhold effluent from farmers not observing the regulations, whereas in Chile the sanitary authorities have little leverage. Chilean water law vests water rights in the farmers (landowners), and the authorities have never been successful in imposing crop restrictions, even though lettuce and other vegetables being irrigated with raw sewage have been implicated in annual typhoid epidemics in Santiago.

A coherent national policy for wastewater uses in agriculture is essential. This must define the division of responsibilities among involved ministries and authorities and provide for their collaboration. Institutional mechanisms for implementation of the national policy must be established and legal backing provided for enforcement of regulations. Realistic standards must be adopted to safeguard public health and protect against adverse environmental impacts. Environmental issues associated with wastewater use are the main subject of a UNEP (1991) document. Provisions should be made to adequately staff and resource organizations charged with the responsibility for assessing, implementing, operating and monitoring effluent use schemes and enforcing compliance with regulations. A distinction between the upgrading of existing wastewater use schemes and the development of new schemes is drawn in Mara and Cairncross (1989). In addressing the former, it is stressed attention should be paid not only to the technical improvements required or feasible but also to the need for better management of existing schemes and to their improved operation and maintenance.

A national and/or regional consultative committee will often be of value in developing policy guidelines. Serving on this committee should be a representative of all the main interest groups, including water resources planning, public health, public works (municipalities), agriculture and forestry, environmental protection, trade and commercial interests (including farmers’ representatives). Policies emanating from such a committee should be free of local or partisan influences but, nevertheless, should be pragmatic. In particular, enforcement legislation must be unequivocal, unambiguous and addressed to the main problem areas. The committee should also be charged with assessing the epidemiological and agricultural impacts of effluent use schemes.