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2.6 Water Reuse

Limited freshwater resources is one of the most important obstacles to the economic development of the countries of the West Asia region. This has spurred the search for alternative sources of water to augment the conventional sources, including increased attention to the reuse of wastewater and agricultural drainage water. The intense competition for the limited good-quality water sources has been a cause for this increased attention to the reuse of lower-quality water. This latter water is currently used after treatment and, sometimes, even without treatment.

2.6.1 Reuse of Treated Water

The large expansion of urban settlements in the countries of the West Asia region resulted in a large increase in the quantity of water used for drinking and domestic purposes. These increased water needs include an estimated 2,382 million m³/year for the Arab countries of the Arabian peninsula including Yemen (ACSAD, 1997), 1,280 million m³/year for Iraq, 1,012 million m³/year for Syria, 216 million m³/year for Jordan and 415 million m³/year for Lebanon. This means the total quantity of water used for drinking and domestic purposes in these countries was about 5,305 million m³/year. Most of the countries have resorted to constructing wastewater treatment plants, in order to reduce surface and subsurface water pollution and to conserve the environment. Further, these large quantities of water can be reused to satisfy part of the region’s irrigation water needs, thereby also working to conserve the conventional high-quality water sources. The reuse of this water also facilitates the utilization of the nitrogen and phosphorus available in it, promoting the growth of agricultural crops.

To address this situation, many countries in the region have issued specifications for the water before it can be released into the public sewer network. This is done to avoid putting water in the sewer system that cannot be treated on the basis of the capabilities of the treatment plants. In such situations, pollutants will remain in the water and, if it is reused, it can lead to public health hazards. The specifications of treated water reuse in agriculture have also been developed either from the microbiological aspect or with regard to the water’s content of rare elements and heavy metals. Studies have been conducted on the effects of using treated water (with differing degrees of treatment) for different plants (fruitf trees, grains, weeds, etc.), and also on how the soil retains different materials present in the water.

Technology Description

Protecting the environment and human health is a main goal in planning, constructing and operating wastewater treatment and reuse projects for agricultural and industrial purposes. Careful monitoring of water reuse also is necessary, because of the presence of some chemicals and pathogenic microorganisms that can cause human diseases if transmitted to humans. Although current treatment technologies have advanced to the extent that they remove most of these materials, a number of chemicals are hard to remove completely, the most important being cadmium, zinc, copper, lead and, to a smaller extent, nitrogen. In Saudi Arabia, for example, treated wastewater is reused in Riyadh city for agricultural irrigation. The treatments conducted in the plant include mechanical screening, aeration basins, settling basins, biological filters, secondary settling basins, sludge coagulation basins, sludge decaying reservoirs, and chlorine treatment. This is done in the following stages:

• Pre-treatment – for separating suspended solids and floating oils;
• Primary treatment with chemicals, or physical treatment such as settling and separation;
• Biological treatment -- using aerobic bacteria that subsist on the organic matter in the water;
• Disinfection stage.

The properties of the wastewater obtainable with this treatment sequence are illustrated in Table 20.

Table 20. Properties of treated wastewater in Riyadh City, 1988-1990 (expressed in ppm, unless otherwise noted)

The data in Table 20 indicate that, in regard to its concentration of heavy metals and other components, the water is suitable for irrigation purposes. On the other hand, it also is noted that the wastewater contains several basic nutritional elements necessary for plant growth. These elements have an important economic value for crop production, since they can be used as a replacement for chemical fertilizers (Table 21).

Table 21. Properties of treated wastewater in Riyadh City

As an example of the value of these nutrients, when the water was used to irrigate what (9,000 m³/hectare.season), the quantity of fertilizers available in the water for each hectare included (1) nitrogen - 186 kg, (2) phosphorus - 63 kg, (3) potassium – 135 kg, (4) iron - 2.1 kg, (5) zinc - 1 kg, (6) manganese - 0.36 kg, and (7) copper – 90 grams. These nutrient levels satisfy a large part of the nutrient requirements of the wheat, which are approximately 250-300 kg of nitrogen, 80-100 kg of phosphorus, and 100-120 kg of potassium per hectare per growing season.

This water was used to irrigate wheat, corn and alfalfa. The experimental results indicated that the average productivity of wheat reached 6.74 ton/hectare using this water, compared to 5.95 ton/hectare using groundwater. Further, the concentration of heavy poisonous metals in the water (copper, lead, nickel) did not exceed allowable limits.

The use of the wastewater reuse technology is limited to irrigate green fields, or agriculture in a limited form for specific crops. The idea for wastewater reuse, however, for potable water and domestic uses is still unacceptable for psychological, sociological, and religious considerations, as well as lack of practical experience in the field of advanced purification. Strict and continuous monitoring of the treated water also is required. Overall, therefore, it is considered not acceptable to undergo the risk of exposing residents to potential health hazards and disease. As a result, the wastewater reuse technology is concentrated only in the irrigation and industrial (water recycling) sectors.

Some countries have set special standards for wastewater use in irrigation, and for the relation between cultivated crops and the degree of treatment. The initial treatment is suggested to be sufficient to irrigate green fields, or some crops not directly consumed by humans. For crops directly used by humans, a second wastewater treatment is required, in addition to filtering and disinfection. Table 22 illustrates the types of plants and selected crops that can be irrigated with treated wastewater.

Table 22. Plants and crops that can be irrigated with treated wastewater

It is absolutely prohibited to use treated wastewater to irrigate crops with fruit this is eaten fresh. This is because the treated wastewater for irrigation typically contains organic materials and basic nutrients necessary for plant growth, as well as some poisonous materials, that are added to the soil during the irrigation process. The plants will absorb part of the elements. The remaining elements will accumulate in the soil, and their concentrations increase over time. Table 23 shows the maximum permissible limits (kg/hectare) for the element concentrations in agricultural lands.
The maximum permissible limits for different rare and poisonous elements in the water, as determined by international organizations, are illustrated in Tables 24 and 25.

Table 23. Maximum permissible limits for accumulation of rare elements in agricultural soils

Table 24. Maximum permissible limits of rare elements in treated wastewater

Table 25. Recommended microbiological quality guidelines for wastewater use in agriculture

Category and reuse conditions	Exposed group	Intestinal nematodes2 (arithmetic mean of eggs/L)3	Fecal coliform (geometric mean/100 ml)3	Wastewater treatment expected to achieve microbiological quiality
A. Irrigation of crops likely to be eaten uncooked, sport fields, public parks	Workers, consumers, public	1	100	A series of stabilization ponds designed to achieve the indicated microbiological quality or equivalent treatment
B. Irrigation of cereal crops, industrial crops, fodder crops, pastures and trees5	Workers	1	Not Applicable	Retention in stabilization ponds for 8 to 10 days, or equivalent helminth and fecal coliform removal
C. Localized irrigation of crops in category (B), if exposure of workers and the public does not occu.r	None	Not applicable	Not applicable	Pre-treatment as required by the irrigation technology, but no less than primary sedimentation

N.B.	(1)	In special cases, local epidemiological, socio-cultural, and environmental factors should be taken into account, and the guidelines modified accordingly.
	(2)	Ascaris as Trichuris and hookworms.
	(3)	During the irrigation period.
	(4)	A more stringent guideline ( 200 fecal coliforms /100 ml) is applied for public lawns, with which the public may have direct contact.
	(5)	In the case of fruit trees, irrigation should cease two weeks before the fruit is picked, and no fruit should be picked off the ground. Sprinkler irrigation should not be used.

Some Arab countries (e.g., Saudi Arabia) have set specific standards for the use of wastewater in agriculture. Table 26 identifies the standard specifications recommended by the Ministry of Agriculture and Water for the unrestricted use of wastewater in irrigation. It should be noted that meeting these standards requires tertiary treatment of the wastewater, including secondary treatment, chemical treatment, filtering of the water through sand filters, and then disinfection (Al-Sa’aty, 1995).

 

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