Page 345 - Industrial Wastewater Treatment, Recycling and Reuse

Urban Wastewater Treatment 317

In relation to effluent quality, effluents contain valuable resources (water,
organic matter, and nutrients). However, in excessive amounts these can be
detrimental to soils or plant growth. Effluents can also contain chemical con-
taminants, salts, andpathogens that can pose a riskto the wider environment or
public health, or may cause pollution. These risks can be minimized by
applying the criteria and information provided in the Guideline during the site
selection, design, and operation phases of an effluent irrigation system.
Best management practices that optimize the use of the water, nutrients,
and organic matter and reduce the potential for harm from other contam-
inants are also critical. For an effluent irrigation system to be sustainable, the
amount of water, nutrients, and chemicals that will be applied should be
determined to ensure that it is the optimum for the crop or cultivar, the
agronomic system employed, and site-specific factors such as climate, topog-
raphy, and soil.
Adjustments to the amount of effluent applied or the area over which it is
applied can then be made to ensure that irrigated plants and environments
are not stressed by water or by the organic material, nutrients, or chemicals
applied.
Water and nutrient balances are used to calculate the amount of water
and nutrients that should be applied, and at what times, to meet the crop
requirements while ensuring increases in runoff and percolation are mini-
mized. The water balance is calculated to determine the maximum volume
of effluent that can be sustainably used. The elements to be considered in a
water balance are rainfall, evapotranspiration, runoff, and percolation.
For some effluents, the loading rates of nutrients such as nitrogen and
phosphorus can limit the quantity of effluent to be used for irrigation in a
given area. In a nutrient balance the amount of the specific nutrient (e.g.,
nitrogen or phosphorus) assumed to be applied in a year is compared with
the amount taken up by the biological or physical processes of the crop-soil
system. Preirrigation soil nutrient status is also considered.
In some systems the amount of effluent that can be applied is limited by
potential adverse impacts of salinity, heavy metals, and persistent organic
chemicals. The Guideline suggests that key components in managing these
types of limitations include designing the system to avoid any potential
impacts and having in place a management and monitoring system to correct
any emerging problems and to identify when action needs to be taken to
ensure the environmental and agronomic performance of the system.
The National Water Policy 2002 emphasizes a participatory approach in
water resources management. It has been recognized that participation of