Environmental Engineering Reference
In-Depth Information
tains less than 3 mg/L of soluble P, which rapidly
becomes adsorbed to soil particles after irrigation
(Kelly et al. 2006 ). When plant demand is low, P
accumulation and immobilisation in the soil is
more likely than leaching or over-fertilisation;
the exception is in sandy soils where there is
some risk of leaching (Kelly et al. 2006 ). The
main concerns associated with P are its potential
toxicity to Australian natives which has evolved
on our low P soils and run-off or accidental
discharges to water bodies leading to eutrophica-
tion (NRMMCEP and HCAHMC 2006 ).
Cadmium presents the highest health risk of
all the heavy metal(loid)s in reclaimed water; it is
loosely bound to soil and will cause phytotoxic-
ity at relatively low levels; it is a particular threat
to humans and animals because toxicity occurs at
a lower threshold than for plants. Consequently,
there are national and international schemes to
monitor the Cd concentration in foods (Unkovich
et al. 2006 ).
3.3
Organic Contaminants
Three main groups of organic contaminants are
found in reclaimed wastewater (GuangGuo 2006 ;
Müller et al. 2007 ) that include (1) natural organic
matter (NOM) which consists of refractory mole-
cules like fulvic and humic acids; (2) disinfection
by-products which are formed during chlorination
and (3) synthetic organic compounds including
pesticides, organohalothanes, phthalates, aro-
matic hydrocarbons, surfactants, endocrine dis-
rupting chemicals (EDCs), pharmaceuticals and
personal care products.
Although NOM can induce putrefaction in
stored reclaimed water by depleting oxygen, there
is little concern with discharging NOM onto
agricultural land because it should eventually be
broken down by the natural microbial populations
(GuangGuo 2006 ). Disinfection by-products
(DBP) can be formed by the reaction of chlorine
with NOM (Singer 1999 ). Trihalomethanes are the
most well known of the DBPs which are consid-
ered carcinogenic, mutagenic and clastogenic
(Kim and Clevenger 2007 ). Synthetic organic
compounds represent a wide range of chemicals.
Some are susceptible to wastewater treatment
processes whilst others fall into the group of stable
organics which may remain in very small amounts
in reclaimed water. Many have been implicated as
EDCs which interfere with normal hormone com-
munication systems; they impact adversely on
growth, reproduction and development. There is
limited data on their presence in wastewater, and
due to their potential to cause adverse environ-
mental and human health impacts, further moni-
toring and research are warranted (Holmes et al.
2010 ).
3.2
Heavy Metal(loid)s
Most heavy metal(loid)s are very effectively
removed from wastewater in the treatment pro-
cess so that their levels are very low in reclaimed
water. Boron and Cd are the only two heavy
metal(loid)s included in the list of key environ-
mental hazards in the current Australian
Guidelines for Water Recycling - they are not
as readily separated from reclaimed water dur-
ing standard treatment (NRMMCEP and
HCAHMC 2006 ).
Metal(loid)s partition to the biosolids formed
during sedimentation processes because their
ionic nature causes them to sorb strongly to
charged organic matter and clays (Unkovich et al.
2006 ). At low levels some heavy metal(loid)s are
considered as micronutrients, but above plant
requirements foliar application can produce phy-
totoxicity (Bolan et al. 2011 ). By virtue of their
persistent nature, they can also accumulate in the
soil, thereby resulting in soil biota toxicity, phy-
totoxicity through root uptake and entry into the
food chain leading to negative impacts on food
quality and human health.
Boron is not retained in biosolids because it
exists as an uncharged species within the normal
pH range of wastewater and thus remains in the
reclaimed water. Boron is a micronutrient at very
low levels; it has a narrow safety margin and, if
leaching fractions are insuffi cient, it can accumu-
late in the soil profi le and cause a reduction in
yield and also phytotoxicity in sensitive species
(Unkovich et al. 2006 ).
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