Nutrients
Wastewaters commonly contain plant nutrients, particularly nitrogen and
phosphorus, that are often applied as artificial fertilizers to irrigated fodders and
crops. Use of wastewater can reduce or even replace the need for other
fertilisers.
Other nutrients common in wastewater include potassium, sulphur, calcium and
magnesium.
The wastewater may supply a surplus of nutrients, ie., more is applied in the
irrigation water than is removed from the area in plant and animal products or
fixed in the soil (strongly bound to clay particles). If this occurs, then the surplus
may become a risk to the environment through leaching into ground or surface
waters.
Irrigation with wastewater should be limited to either the water or nutrient needs
of the crop. An agronomist will be able to determine whether the application at a
particular site will be limited by the capacity of the crop and soil to use the water
or the nutrients.
Salinity
Wastewaters commonly contain higher levels of salinity than the original water
sources, due to additions from various sources and concentration by evaporation.
The risks related to use of saline wastewater are no different to those associated
with other saline water sources, and include:
The salt can cause burning of the leaves of sensitive plant species.
Salt can accumulate in the soil (salinisation) and eventually make the soil
so saline that plant production is reduced or threatened.
Sodium is a common constituent of salts dissolved in irrigation water, and it can
cause the soil to disperse and thereby inhibit drainage and create waterlogging.
Organic components
Wastewaters generally contain suspended solids, including organic matter.
Breakdown of these components requires oxygen. If the levels of organic
compounds are too high, soil oxygen levels can be depleted to such an extent
that the soil becomes anaerobic (devoid of oxygen) and unpleasant odours can
be produced.
Application rates of sewage wastewater are generally low enough that soils do
not become anaerobic, and the organic materials can improve the condition of the
soil.
Living organisms
Wastewaters may contain viruses, bacteria and other organisms. While these are
significantly reduced by the water treatment activities, undesirable organisms will
still be present in the wastewater.
Therefore, it is important to exercise care in the use of wastewater.
Normal hygiene practices, such as taking off muddy boots before going inside
and washing hands before eating, are an important part of reuse practice.
Farmers should be careful that appropriate signage exists to warn others about
the use of recycled water. The standard “Trespassers Prosecuted” and
appropriate signage on taps stating “recycled water – not for drinking” is required
by the Health Department. The use of purple colored pipe or taping may also be
appropriate where supply lines approach the vicinity of potable water lines.