1. Wastewater characteristics and effluent quality parameters - Chuyên Trang Chia Sẻ Kiến Thức Thời Trang Mới Nhất

In many arid and semi-arid countries water is becoming an increasingly scarce resource and planners are forced to consider any sources of water which might be used economically and effectively to promote further development. At the same time, with population expanding at a high rate, the need for increased food production is apparent. The potential for irrigation to raise both agricultural productivity and the living standards of the rural poor has long been recognized. Irrigated agriculture occupies approximately 17 percent of the world’s total arable land but the production from this land comprises about 34 percent of the world total. This potential is even more pronounced in arid areas, such as the Near East Region, where only 30 percent of the cultivated area is irrigated but it produces about 75 percent of the total agricultural production. In this same region, more than 50 percent of the food requirements are imported and the rate of increase in demand for food exceeds the rate of increase in agricultural production.

Whenever good quality water is scarce, water of marginal quality will have to be considered for use in agriculture. Although there is no universal definition of ‘marginal quality’ water, for all practical purposes it can be defined as water that possesses certain characteristics which have the potential to cause problems when it is used for an intended purpose. For example, brackish water is a marginal quality water for agricultural use because of its high dissolved salt content, and municipal wastewater is a marginal quality water because of the associated health hazards. From the viewpoint of irrigation, use of a ‘marginal’ quality water requires more complex management practices and more stringent monitoring procedures than when good quality water is used. This publication deals with agricultural use of municipal wastewater, which is primarily domestic sewage but possibly contains a proportion of industrial effluents discharged to public sewers.

Expansion of urban populations and increased coverage of domestic water supply and sewerage give rise to greater quantities of municipal wastewater. With the current emphasis on environmental health and water pollution issues, there is an increasing awareness of the need to dispose of these wastewaters safely and beneficially. Use of wastewater in agriculture could be an important consideration when its disposal is being planned in arid and semi-arid regions. However it should be realized that the quantity of wastewater available in most countries will account for only a small fraction of the total irrigation water requirements. Nevertheless, wastewater use will result in the conservation of higher quality water and its use for purposes other than irrigation. As the marginal cost of alternative supplies of good quality water will usually be higher in water-short areas, it makes good sense to incorporate agricultural reuse into water resources and land use planning.

Properly planned use of municipal wastewater alleviates surface water pollution problems and not only conserves valuable water resources but also takes advantage of the nutrients contained in sewage to grow crops. The availability of this additional water near population centres will increase the choice of crops which farmers can grow. The nitrogen and phosphorus content of sewage might reduce or eliminate the requirements for commercial fertilizers. It is advantageous to consider effluent reuse at the same time as wastewater collection, treatment and disposal are planned so that sewerage system design can be optimized in terms of effluent transport and treatment methods. The cost of transmission of effluent from inappropriately sited sewage treatment plants to distant agricultural land is usually prohibitive. Additionally, sewage treatment techniques for effluent discharge to surface waters may not always be appropriate for agricultural use of the effluent.

Many countries have included wastewater reuse as an important dimension of water resources planning. In the more arid areas of Australia and the USA wastewater is used in agriculture, releasing high quality water supplies for potable use. Some countries, for example the Hashemite Kingdom of Jordan and the Kingdom of Saudi Arabia, have a national policy to reuse all treated wastewater effluents and have already made considerable progress towards this end. In China, sewage use in agriculture has developed rapidly since 1958 and now over 1.33 million hectares are irrigated with sewage effluent. It is generally accepted that wastewater use in agriculture is justified on agronomic and economic grounds (see Example 1) but care must be taken to minimize adverse health and environmental impacts. The purpose of this document is to provide countries with guidelines for wastewater use in agriculture which will allow the practice to be adopted with complete health and environmental security.

EXAMPLE 1 – AGRONOMIC AND ECONOMIC BENEFITS OF WASTEWATER USE IN IRRIGATION

As an example, a city with a population of 500,000 and water consumption of 200 l/d per person would produce approximately 85,000 m3/d (30 Mm³/year) of wastewater, assuming 85% inflow to the public sewerage system. If treated wastewater effluent is used in carefully controlled irrigation at an application rate of 5000 m3/ha.year, an area of some 6000 ha could be irrigated. In addition to the economic benefit of the water, the fertilizer value of the effluent is of importance. With typical concentrations of nutrients in treated wastewater effluent from conventional sewage treatment processes as follows:

Nitrogen (N) – 50 mg/l
Phosphorus(P) – 10 mg/l
Potassium (K) – 30 mg/l

and assuming an application rate of 5000 m3/ha.year, the fertilizer contribution of the effluent would be:

N – 250 kg/ha. year
P – 50 kg/ha. year
K – 150 kg/ha. year

Thus, all of the nitrogen and much of the phosphorus and potassium normally required for agricultural crop production would be supplied by the effluent. In addition, other valuable micronutrients and the organic matter contained in the effluent will provide additional benefits.

Municipal wastewater is mainly comprised of water (99.9%) together with relatively small concentrations of suspended and dissolved organic and inorganic solids. Among the organic substances present in sewage are carbohydrates, lignin, fats, soaps, synthetic detergents, proteins and their decomposition products, as well as various natural and synthetic organic chemicals from the process industries. Table 1 shows the levels of the major constituents of strong, medium and weak domestic wastewaters. In arid and semi-arid countries, water use is often fairly low and sewage tends to be very strong, as indicated in Table 2 for Amman, Jordan, where water consumption is 90 l/d per person.

Table 1: MAJOR CONSTITUENTS OF TYPICAL DOMESTIC WASTEWATER

Constituent

Concentration, mg/l

Strong

Medium

Weak

Total solids

1200

700

350

Dissolved solids (TDS)1

850

500

250

Suspended solids

350

200

100

Nitrogen (as N)

Phosphorus (as P)

Chloride1

100

Alkalinity (as CaCO3)

200

100

Grease

150

100

BOD52

300

200

100

1 The amounts of TDS and chloride should be increased by the concentrations of these constituents in the carriage water.

2 BOD5 is the biochemical oxygen demand at 20°C over 5 days and is a measure of the biodegradable organic matter in the wastewater.

Source: UN Department of Technical Cooperation for Development (1985)

Municipal wastewater also contains a variety of inorganic substances from domestic and industrial sources (see Table 3), including a number of potentially toxic elements such as arsenic, cadmium, chromium, copper, lead, mercury, zinc, etc. Even if toxic materials are not present in concentrations likely to affect humans, they might well be at phytotoxic levels, which would limit their agricultural use. However, from the point of view of health, a very important consideration in agricultural use of wastewater, the contaminants of greatest concern are the pathogenic micro- and macro-organisms.

Pathogenic viruses, bacteria, protozoa and helminths may be present in raw municipal wastewater at the levels indicated in Table 4 and will survive in the environment for long periods, as summarized in Table 5. Pathogenic bacteria will be present in wastewater at much lower levels than the coliform group of bacteria, which are much easier to identify and enumerate (as total coliforms/100ml). Escherichia coli are the most widely adopted indicator of faecal pollution and they can also be isolated and identified fairly simply, with their numbers usually being given in the form of faecal coliforms (FC)/100 ml of wastewater.

Table 2: AVERAGE COMPOSITION OF WASTEWATER IN AMMAN, JORDAN

Constituent

Concentration mg/l

Dissolved solids (TDS)

1170

Suspended solids

900

Nitrogen (as N)

150

Phosphorus (as P)

Alkalinity (as CaCO3)

850

Sulphate (as SO4)

BOD5

770

COD1

1830

TOC1

220

1 COD is chemical oxygen demand
2 TOC is total organic carbon

Source: Al-Salem (1987)

Table 3: CHEMICAL COMPOSITION OF WASTEWATERS IN ALEXANDRIA AND GIZA, EGYPT

Constituent

Alexandria

Giza

Unit

Concentration

Unit

Concentration

dS/m

3.10

dS/m

1.7

7.80

7.1

SAR

9.30

2.8

Na2+

me/l

24.60

mg/l

205

Ca2+

me/I

1.50

mg/l

128

me/I

3.20

mg/l

K+

me/I

1.80

mg/l

Cl-

me/I

62.00

mg/l

320

SO42-

me/I

35.00

mg/l

138

CO3

me/I

1.10

HCO3-

me/I

6.60

NH4+

mg/l

2.50

NO3

mg/l

10.10

mg/l

8.50

mg/l

0.20

mg/l

0.7

mg/l

1.10

mg/l

0.4

mg/l

0.80

mg/l

1.4

Source: Abdel-Ghaffar et al. (1988)

Table 4: POSSIBLE LEVELS OF PATHOGENS IN WASTEWATER

Type of pathogen

Possible concentration per litre in municipal wastewater1

Viruses:

Enteroviruses2

5000

Bacteria:

Pathogenic E. coli3

Salmonella spp.

7000

Shigella spp.

7000

Vibrio cholerae

1000

Protozoa:

Entamoeba histolytica

4500

Helminths:

Ascaris Lumbricoides

600

Hookworms4

Schistosoma mansoni

Taenia saginata

Trichuris trichiura

120

?Uncertain
1Based on 100 lpcd of municipal sewage and 90% inactivation of excreted pathogens
2Includes polio-, echo- and coxsackieviruses
3Includes enterotoxigenic, enteroinvasive and enteropathogenic E. coli
4Anglostoma duedenale and Necator americanus

Source: Feachem et al. (1983)

Table 5: SURVIVAL OF EXCRETED PATHOGENS (at 20-30°C)

Type of pathogen

Survival times in days

In faeces, nightsoil and sludge

In fresh water and sewage

In the soil

On crops

Viruses

Enteroviruses

<100 (<20)

<120 (<50)

<100 (<20)

<60 (<15)*

Bacteria

Faecal Coliforms

<90 (<50)

<60 (<30)

<70 (<20)

<30 (<15)

Salmonella spp.

<60 (<30)

<70 (<20)

<30 (<15)

Shigella spp.

<30 (<10)

–

<10 (<5)

Vibrio cholerae

<30 (<5)

<30 (<10)

<20 (<10)

< 5 (<2)

Protozoa

<30 (<15)

<20 (<10)

<10 (< 2)

Entamoeba histolytica cysts

<30 (<15)

<20 (<10)

<10 (< 2)

Helminths

Many

<60 (<30)

Ascaris lunbricoides eggs

Months

* Figures in brackets show the usual survival time.

Source: Feachem et al. (1983)

Organic chemicals usually exist in municipal wastewaters at very low concentrations and ingestion over prolonged periods would be necessary to produce detrimental effects on human health. This is not likely to occur with agricultural/aquacultural use of wastewater, unless cross-connections with potable supplies occur or agricultural workers are not properly instructed, and can normally be ignored. The principal health hazards associated with the chemical constituents of wastewaters, therefore, arise from the contamination of crops or groundwaters. Hillman (1988) has drawn attention to the particular concern attached to the cumulative poisons, principally heavy metals, and carcinogens, mainly organic chemicals. World Health Organization guidelines for drinking water quality (WHO 1984) include limit values for the organic and toxic substances given in Table 6, based on acceptable daily intakes (ADI). These can be adopted directly for groundwater protection purposes but, in view of the possible accumulation of certain toxic elements in plants (for example, cadmium and selenium) the intake of toxic materials through eating the crops irrigated with contaminated wastewater must be carefully assessed.

Table 6: ORGANIC AND INORGANIC CONSTITUENTS OF DRINKING WATER OF HEALTH SIGNIFICANCE

Organic

Inorganic

Aldrin and dieldrin

Arsenic

Benzene

Cadmium

Benzo-a-pyrene

Chromium

Carbon tetrachloride

Cyanide

Chlordane

Fluoride

Chloroform

Lead

2,4 D

Mercury

DDT

Nitrate

1,2 Dichloroethane

Selenium

1,1 Dichlorethylene

Heptachlor and heptachlor epoxide

Hexachlorobenzene

Lindane

Methoxychlor

Pentachlorophenol

Tetrachlorethylene

2, 4, 6 Trichloroethylene

Trichlorophenol

Source: WHO (1984)

Pathogenic organisms give rise to the greatest health concern in agricultural use of wastewaters, yet few epidemological studies have established definitive adverse health impacts attributable to the practice. Shuval et al. (1985) reported on one of the earliest evidences connecting agricultural wastewater reuse with the occurrence of disease (Figure 1). It would appear that in areas of the world where helminthic diseases caused by Ascaris and Trichuris spp. are endemic in the population and where raw untreated sewage is used to irrigate salad crops and/or vegetables eaten uncooked, transmission of these infections is likely to occur through the consumption of such crops. A study in West Germany (reported by Shuval et al. 1986) provides additional evidence (Figure 2) to support this hypothesis and further evidence was also provided by Shuval et al. (1985; 1986) to show that cholera can be tranmitted through the same channel.

Figure 1: Prevalence of Ascaris-positive stool samples in West Jerusalem population during various periods, with and without supply of vegetables and salad crops irrigated with raw wastewater (Gunnerson, Shuval and Arlosoroff 1984)

There is only limited evidence indicating that beef tapeworm (Taenia saginata) can be transmitted to the population consuming the meat of cattle grazing on wastewater irrigated fields or fed crops from such fields. However, there is strong evidence from Melbourne, Australia and from Denmark (reported by Shuval et al. 1985) that cattle grazing on fields freshly irrigated with raw wastewater, or drinking from raw wastewater canals or ponds, can become heavily infected with the disease (cysticerosis).

Indian studies, reported by Shuval et al. (1986), have shown that sewage farm workers exposed to raw wastewater in areas where Ancylostoma (hookworm) and Ascaris (nematode) infections are endemic have significantly excess levels of infection with these two parasites compared with other agricultural workers in similar occupations. Furthermore, the studies indicated that the intensity of the Ascaris infections (the number of worms infesting the intestinal tract of an individual) in the sample of sewage farm workers was very much greater than in the control sample. In the case of the hookworm infections, the severity of the health effects was a function of the worm load of individuals, which was found to be related to the degree of exposure and the length of time of exposure to the hookworm larvae. Sewage farm workers are also liable to become infected with cholera if practising irrigation with raw wastewater derived from an urban area in which a cholera epidemic is in progress (Shuval et al. 1985). Morbidity and serological studies on wastewater irrigation workers or wastewater treatment plant workers occupationally exposed to wastewater directly and to wastewater aerosols have not been able to demonstrate excess prevalence of viral diseases.

Figure 2: Wastewater irrigation of vegetables and Ascaris prevalence in Darmstadt and Berlin, compared with other cities in Germany not practising wastewater irrigation (Gunnerson, Shuval and Arlosoroff 1984)

No strong evidence has been adduced to suggest that population groups residing near wastewater treatment plants or wastewater irrigation sites are at greater risk from pathogens in aerosolized wastewater resulting from aeration processes or sprinkler irrigation. Shuval et al. (1986) suggest that the high levels of inmunity against most viruses endemic in the community essentially block environmental transmission by wastewater irrigation.

Finally, in respect of the health impact of use of wastewater in agriculture, Shuval et al. (1986) rank pathogenic agents in the order of priority shown in Example 2. They pointed out that negative health effects were only detected in association with the use of raw or poorly-settled wastewater, while inconclusive evidence suggested that appropriate wastewater treatment could provide a high level of health protection.

EXAMPLE 2 – RELATIVE HEALTH IMPACT OF PATHOGENIC AGENTS

High Risk
(high incidence of excess infection)

Helminths
(Ancylostoma, Ascaris, Trichuris and Taenia)

Medium Risk
(low incidence of excess infection)

Enteric Bacteria
(Cholera vibrio, Salmonella typhosa, Shigella and possibly others)

Low Risk
(low incidence of excess infection)

Enteric viruses

The following microbiological parameters are particularly important from the health point of view:

i. Indicator Organisms

a. Coliforms and Faecal Coliforms. The Coliform group of bacteria comprises mainly species of the genera Citrobacter, Enterobacter, Escherichia and Klebsiella and includes Faecal Coliforms, of which Escherichia coli is the predominant species. Several of the Coliforms are able to grow outside of the intestine, especially in hot climates, hence their enumeration is unsuitable as a parameter for monitoring wastewater reuse systems. The Faecal Coliform test may also include some non-faecal organisms which can grow at 44°C, so the E. coli count is the most satisfactory indicator parameter for wastewater use in agriculture.

b. Faecal Streptococci. This group of organisms includes species mainly associated with animals (Streptococcus bovis and S. equinus), other species with a wider distribution (e.g. S. faecalis and S. faecium, which occur both in man and in other animals) as well as two biotypes (S. faecalis var liquefaciens and an a typical S. faecalis that hydrolyzes starch) which appear to be ubiquitous, occurring in both polluted and non-polluted environments. The enumeration of Faecal Streptococci in effluents is a simple routine procedure but has the following limitations: the possible presence of the non-faecal biotypes as part of the natural microflora on crops may detract from their utility in assessing the bacterial quality of wastewater irrigated crops; and the poorer survival of Faecal Streptococci at high than at low temperatures. Further studies are still warranted on the use of Faecal Streptococci as an indicator in tropical conditions and especially to compare survival with that of Salmonellae.

c. Clostridium perfringens. This bacterium is an exclusively faecal spore-forming anaerobe normally used to detect intermittent or previous pollution of water, due to the prolonged survival of its spores. Although this extended survival is usually considered to be a disadvantage for normal purposes, it may prove to be very useful in wastewater reuse studies, as Clostridium perfringens may be found to have survival characteristics similar to those of viruses or even helminth eggs.

ii. Pathogens

The following pathogenic parameters can only be considered if suitable laboratory facilities and suitably trained staff are available:

a. Salmonella spp. Several species of Salmonellae may be present in raw sewage from an urban community in a tropical developing country, including S. typhi (causative agent for typhoid) and many others. It is estimated (Doran et al. 1977) that a count of 7000 Salmonellae/litre is typical in a tropical urban sewage with similar numbers of Shigellae, and perhaps 1000 Vibrio cholera/litre. Both Shigella spp and V. cholera are more rapidly killed in the environment, so if removal of Salmonellae can be achieved, then the majority of other bacterial pathogens will also have been removed.

b. Enteroviruses. May give rise to severe diseases, such as Poliomyelitis and Meningitis, or to a range of minor illnesses such as respiratory infections. Although there is no strong epidemiological evidence for the spread of these diseases via sewage irrigation systems, there is some risk and it is desirable to know to what extent viruses are removed by existing and new treatment processes, especially under tropical conditions. Virus counts can only be undertaken in a dedicated laboratory, as the cell culture techniques required are very susceptible to bacterial and fungal contamination.

c. Rotaviruses. These viruses are known to cause gastro-intestinal problems and, though usually present in lower numbers than enteroviruses in sewage, they are known to be more persistent, so it is necessary to establish their survival characteristics relative to enteroviruses and relative to the indicator organisms in wastewaters. It has been claimed that the removal of viruses in wastewater treatment occurs in parallel with the removal of suspended solids, as most virus particles are solids-associated. Hence, the measurement of suspended solids in treated effluents should be carried out as a matter of routine.

d. Intestinal Nematodes. It is known that nematode infections, in particular from the roundworm Ascaris lumbricoides, can be spread by effluent reuse practices. The eggs of A. lumbricoides are fairly large (45-70 m m x 35-50 m m) and several techniques for enumeration of nematodes have been developed (WHO 1989).

The quality of irrigation water is of particular importance in arid zones where extremes of temperature and low relative humidity result in high rates of evaporation, with consequent deposition of salt which tends to accumulate in the soil profile. The physical and mechanical properties of the soil, such as dispersion of particles, stability of aggregates, soil structure and permeability, are very sensitive to the type of exchangeable ions present in irrigation water. Thus, when effluent use is being planned, several factors related to soil properties must be taken into consideration. A thorough treatise on the subject prepared by Ayers and Westcot is contained in the FAO Irrigation and Drainage Paper No 29 Rev. 1 (FAO 1985).

Another aspect of agricultural concern is the effect of dissolved solids (TDS) in the irrigation water on the growth of plants. Dissolved salts increase the osmotic potential of soil water and an increase in osmotic pressure of the soil solution increases the amount of energy which plants must expend to take up water from the soil. As a result, respiration is increased and the growth and yield of most plants decline progressively as osmotic pressure increases. Although most plants respond to salinity as a function of the total osmotic potential of soil water, some plants are susceptible to specific ion toxicity.

Many of the ions which are harmless or even beneficial at relatively low concentrations may become toxic to plants at high concentration, either through direct interference with metabolic processes or through indirect effects on other nutrients, which might be rendered inaccessible. Morishita (1985) has reported that irrigation with nitrogen-enriched polluted water can supply a considerable excess of nutrient nitrogen to growing rice plants and can result in a significant yield loss of rice through lodging, failure to ripen and increased susceptibility to pests and diseases as a result of over-luxuriant growth. He further reported that non-polluted soil, having around 0.4 and 0.5 ppm cadmium, may produce about 0.08 ppm Cd in brown rice, while only a little increase up to 0.82, 1.25 or 2.1 ppm of soil Cd has the potential to produce heavily polluted brown rice with 1.0 ppm Cd.

Important agricultural water quality parameters include a number of specific properties of water that are relevant in relation to the yield and quality crops, maintenance of soil productivity and protection of the environment. These parameters mainly consist of certain physical and chemical characteristics of the water. Table 7 presents a list of some of the important physical and chemical characteristics that are used in the evaluation of agricultural water quality. The primary wastewater quality parameters of importance from an agricultural viewpoint are:

Table 7: PARAMETERS USED IN THE EVALUATION OF AGRICULTURAL WATER QUALITY

Parameters

Symbol

Unit

Physical

Total dissolved solids

TDS

mg/l

Electrical conductivity

Ecw

dS/m1

Temperature

°C

Colour/Turbidity

NTU/JTU2

Hardness

mg equiv. CaCO3/l

Sediments

g/l

Chemical

Acidity/Basicity

Type and concentration of anions and cations:

Calcium

Ca++

me/l3

Magnesium

Mg++

me/l

Sodium

Na+

me/l

Carbonate

CO3–

me/l

Bicarbonate

HCO3-

me/l

Chloride

Cl-

me/l

Sulphate

SO4–

me/l

Sodium adsorption ratio

SAR

Boron

mg/l4

Trace metals

mg/l

Heavy metals

mg/l

Nitrate-Nitrogen

NO3-N

mg/l

Phosphate Phosphorus

PO4-P

mg/l

Potassium

mg/l

1 dS/m = deciSiemen/metre in SI Units (equivalent to 1 mmho/cm)
2 NTU/JTU = Nephelometric Turbidity Units/Jackson Turbidity Units
3 me/l = milliequivalent per litre
4 mg/l == milligrams per litre = parts per million (ppm); also,
mg/l ~ 640 x EC in dS/m

Source: Kandiah (1990a)

i. Total Salt Concentration

Total salt concentration (for all practical purposes, the total dissolved solids) is one of the most important agricultural water quality parameters. This is because the salinity of the soil water is related to, and often determined by, the salinity of the irrigation water. Accordingly, plant growth, crop yield and quality of produce are affected by the total dissolved salts in the irrigation water. Equally, the rate of accumulation of salts in the soil, or soil salinization, is also directly affected by the salinity of the irrigation water. Total salt concentration is expressed in milligrams per litre (mg/l) or parts per million (ppm).

ii. Electrical Conductivity

Electrical conductivity is widely used to indicate the total ionized constituents of water. It is directly related to the sum of the cations (or anions), as determined chemically and is closely correlated, in general, with the total salt concentration. Electrical conductivity is a rapid and reasonably precise determination and values are always expressed at a standard temperature of 25°C to enable comparison of readings taken under varying climatic conditions. It should be noted that the electrical conductivity of solutions increases approximately 2 percent per °C increase in temperature. In this publication, the symbol ECw, is used to represent the electrical conductivity of irrigation water and the symbol ECe is used to designate the electrical conductivity of the soil saturation extract. The unit of electrical conductivity is deciSiemen per metre (dS/m).

iii. Sodium Adsorption Ratio

Sodium is an unique cation because of its effect on soil. When present in the soil in exchangeable form, it causes adverse physico-chemical changes in the soil, particularly to soil structure. It has the ability to disperse soil, when present above a certain threshold value, relative to the concentration of total dissolved salts. Dispersion of soils results in reduced infiltration rates of water and air into the soil. When dried, dispersed soil forms crusts which are hard to till and interfere with germination and seedling emergence. Irrigation water could be a source of excess sodium in the soil solution and hence it should be evaluated for this hazard.

The most reliable index of the sodium hazard of irrigation water is the sodium adsorption ration, SAR. The sodium adsorption ratio is defined by the formula:

(1)

where the ionic concentrations are expressed in me/l.

A nomogram for determining the SAR value of irrigation water is presented in Figure 3 (US Salinity Laboratory 1954). An exchangeable sodium percentage (ESP) scale is included in the nomogram to estimate the ESP value of the soil that is at equilibrium with the irrigation water. Using the nomogram, it is possible to estimate the ESP value of a soil that is at equilibrium with irrigation water of a known SAR value. Under field conditions, the actual ESP may be slightly higher than the estimated equilibrium value because the total salt concentration of the soil solution is increased by evaporation and plant trans-piration, which results in a higher SAR and a corres-pondingly higher ESP value.

It should also be noted that the SAR from Eq 1 does not take into account changes in calcium ion concentration in the soil water due to changes in solubility of calcium resulting from precipitation or dissolution during or following an irrigation. However, the SAR calculated according to Eq 1 is considered an acceptable evaluation procedure for most of the irrigation waters encountered in agriculture. If significant precipitation or dissolution of calcium due to the effect of carbon dioxide (CO2), bicarbonate (HCO3-) and total salinity (ECw) is suspected, an alternative procedure for calculating an Adjusted Sodium Adsorption Ratio, SARadj. can be used. The details of this procedure are reported by Ayers and Westcot (FAO (1985).

iv. Toxic Ions

Irrigation water that contains certain ions at concentrations above threshold values can cause plant toxicity problems. Toxicity normally results in impaired growth, reduced yield, changes in the morphology of the plant and even its death. The degree of damage depends on the crop, its stage of growth, the concentration of the toxic ion, climate and soil conditions.

The most common phytotoxic ions that may be present in municipal sewage and treated effluents in concentrations such as to cause toxicity are: boron (B), chloride (Cl) and sodium (Na). Hence, the concentration of these ions will have to be determined to assess the suitability of waste-water quality for use in agriculture.

Figure 3: A nomogram for determining sodium adsorption ratio (US Salinity Laboratory 1954)

v. Trace Elements and Heavy Metals

A number of elements are normally present in relatively low concentrations, usually less than a few mg/l, in conventional irrigation waters and are called trace elements. They are not normally included in routine analysis of regular irrigation water, but attention should be paid to them when using sewage effluents, particularly if contamination with industrial wastewater discharges is suspected. These include Aluminium (A1), Beryllium (Be), Cobalt (Co), Fluoride (F), Iron (Fe), Lithium (Li), Manganese (Mn), Molybdenum (Mo), Selenium (Se), Tin (Sn), Titanium (Ti), Tungsten (W) and Vanadium (V). Heavy metals are a special group of trace elements which have been shown to create definite health hazards when taken up by plants. Under this group are included, Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Lead (Pb), Mercury (Hg) and Zinc (Zn). These are called heavy metals because in their metallic form, their densities are greater than 4g/cc.

vi. pH

pH is an indicator of the acidity or basicity of water but is seldom a problem by itself. The normal pH range for irrigation water is from 6.5 to 8.4; pH values outside this range are a good warning that the water is abnormal in quality. Normally, pH is a routine measurement in irrigation water quality assessment.