Although trees, particularly eucalypts, are
often recommended as an appropriate means of processing wastewater
(by utilising effluent nutrients) there are some significant
limitations to their practical effectiveness:
Due to concerns about unwanted deep drainage of nutrients,
wastewater can only be pumped onto trees when soil moisture
levels are low.
Once a plantation of eucalypts closes its canopy, all
its nutrient requirements are met by internal nutrient cycling,
including retranslocation within the tree. After this time,
any additional nutrients added to the site will either be
bound to the soil (if not saturated already) or leached off-site.
To allow further nutrients to be added, nutrients within the
plants must then be harvested and removed offsite. This may
be highly impractical. Wood is relatively low in nutrients,
the bulk being stored in the bark, leaves and small branches.
Commercial markets for these products are limited.
Continual addition of effluent high in salts may result
in a decline in soil structure resulting in reduced permeability
and subsequent plant growth and health.
Most agricultural point sources, such as dairy sheds,
are on high value land. Removing flat or gently sloping land
with soils of good structure from agricultural production,
as required for an effluent plantation, is unlikely to be
acceptable to farmers.
Notwithstanding these concerns and limitations, there is a
great deal of interest in the potential for wastewater from
regional cities to be used to support commercial forestry
in inland areas where evaporation rates are high. CSIRO research
at Wagga Wagga has led to the publication of detailed guidelines
for the development of such plantations.
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