Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 370

may not be appropriate to discharge saline drainage water into a river of lake, when
that surface water body is being used for domestic or agricultural water supplies.
Disposal of runoff and drainage waters into natural depressions has been practiced
for centuries. The impounded waters are dissipated by evaporation, seepage, and
transpiration losses. The use of constructed disposal basins for saline agricultural
drainage waters are also common worldwide where there are constraints on dis-
charging into natural salt sinks such as the oceans and inland closed basins. In the
Murray-Darling basin in Australia, some of the constructed evaporation basins are
intended to hold saline water only temporarily. The stored waters are then released
during high river flows.
9.9.2 Treatment of Drainage Water
Treatment approaches of drainage water can be divided into three general cate-
gories:
- physical
- chemical
- biological
Many processes exhibit both physical and chemical aspects and so are sometimes
called physical/chemical or physiochemical treatment.
The first step in the selection of any treatment process for improving drainage
water quality is to thoroughly define the problem and to determine what the treat-
ment process has to achieve. In most cases, either regulatory requirements or the
desire to re-use the water will be the driving force in defining the treatment issues to
be selected for a particular drainage water. A thorough knowledge and understand-
ing of these water quality criteria is required prior to select any particular treatment
process.
An introductory description is given below for the most common treatment meth-
ods, which have application in treating agricultural drainage water. More detailed
description of common treatment processes and design procedures of treatment
plant (Waste water treatment) can be found in “Wastewater treatment” texts/books.
9.9.2.1 Physical/Chemical Treatment
Particle Removal
Several physical processes aim to remove suspended particulate matter. Subsurface
drainage water itself is usually low in suspended particles. These processes might
be used in an overall treatment process for the removal of particulates formed in
another stages of the treatment, such as removal of bacteria from a biological sys-
tem or removal of precipitates formed in a chemical treatment process. Particle
removal unit processes include sedimentation, flotation, centrifugation, and filtra-
tion. Filtration further includes granular media beds, vacuum filters, belt presses,
and filter presses.