Environmental Engineering Reference
In-Depth Information
of Na and Cl ions. Wastewater contains high TDS concentrations, which are not removed
during the treatment process. Consequently, prolonged use of wastewater for irrigation
would lead to salinization of the soil. This is one of the most common adverse effects of
wastewater reuse for irrigation. High soil salinity results in high Na concentration in
soils, leading to poor soil structure and dispersion of clay particles. As consequence, the
soil permeability is reduced, leading to water logging, weed growth and erosion. On
drying, the soil forms hard crusts that limit root penetration. Restrictive guidelines with
respect to EC and TDS in wastewater reused for irrigation are presented in Table 10.1,
which has been compiled based on data from Degremont (1991) and Ayers & Westcot
(1985).
The parameter used to evaluate and measure the potential of wastewater to cause
salinity problems is the sodium adsorption ratio (SAR). It is represented by equation
(10.1) and shows the potential of wastewater to supply Na
+
, which would replace Ca
2+
and Mg
2+
in soils and would lead to a change in the soils' structures.
(10.1)
where, the concentrations of the different ions are expressed in meq/l.
A low SAR (2 to 10) indicates little danger, medium (7 to 18) indicates medium
hazards and high hazards are indicated within the range of 11 to 25 (Fetter 1984). High
SAR can destroy the soil structure owing to the dispersion of the clay particles, and may
result in hard setting, reduced penetration and infiltration rates and reduced crop yield
(DWAF 1996b).
3.2
Specific ions
Nutrients in their different forms, present in wastewater used for irrigation are the major
beneficial aspect of this practice, as they provide the quantity necessary for plant uptake
without the need to use artificial fertilizers. Khouri et al. (1994) have performed
comparative studies on the yields of selected crops irrigated with different types of
wastewater and with fresh water combined with the application of a commercial
fertilizer. Results show that stabilization ponds' effluents provide a comparative crop
yield to other forms of wastewater and it is higher than the yield achieved when
commercial fertilizers are applied. It has already been emphasized in previous chapters
that the nutrients loadings should be comparable to the specific plants nutrients uptake
rates to avoid excessive amounts accumulated in soils, leached to ground water or washed
up by runoff. Some adverse effects of nutrients would be associated with the presence of
ammonia, which could cause crop burn. Also, excessive nitrogen in the form of nitrates
and ammonium causes excessive vegetative growth resulting in delayed crop maturity.
High bicarbonates in irrigation water can unsightly deposits on crop leaves that are
sprinkler-irrigated. Table 10.1 presents recommended guidelines with respect to these
elements.
The adverse effect of Na and Cl ions on soils has been discussed, but it should be
noted that these ions, together with boron, could have a toxic effect on some plants as
well. Table 10.1 presents guidelines regarding the recommended concentrations of these
metals in wastewater reused for irrigation purposes.
