Environmental Engineering Reference
In-Depth Information
The AGWR does away with the class-based
system and advocates a risk assessment-based
approach. Each scheme is individually
assessed; water quality targets, treatment pro-
cesses and additional preventative measures are
tailored to produce a safety level consistent with
the proposed end use of the reclaimed water. The
emphasis is no longer on end of line testing but on
developing a multi-barrier approach to reduce risk
to an acceptable level known as the “tolerable
risk”. The risk assessment is carried out largely in
relation to hazards to human health, and in this
regard microbial pathogens are the greatest threat
(NRMMCEP and HCAHMC
2006
). The AGWR
national guidelines are not mandatory, and several
states have elected not to adopt the new approach
at this point in time (Power
2010
).
ment so the concentration of these major nutri-
ents still remains higher in treated sewage than in
irrigation water from other sources (Kelly et al.
2006
).
Whilst the use of treated wastewater can ben-
efi t crop nutrient management, application in
excess can be detrimental to both crops and the
local environment. The nutrient load supplied to
a crop is determined by the nutrient concentra-
tion of the reclaimed water and the irrigation
depth (Kelly et al.
2006
). Table
2
outlines the
macronutrient uptake of a range of vegetables
and the nutrient load supplied by an irrigation
depth of 1,000 mm from wastewater which is
treated to a tertiary level; the data demonstrates
that at this level of irrigation, some nutrients
would be supplied in excess of requirements,
thereby likely to result in the loss of nutrients
through leaching and surface run-off.
Nitrate is the most mobile form in soil and can
be subject to leaching if nitrate and water are
applied in excess of the plants' needs, this is a
particular risk in colder, wetter seasons where
plant growth is slow (Kelly et al.
2006
). Nitrate
can reach surface waters through run-off, con-
taminate groundwater and impact on public
health if the water is used as a potable resource
and
3
Key Environmental Hazards
The contaminants in reclaimed water that present
the greatest risk to the receiving environment
include boron (B), cadmium (Cd), chlorine (Cl)
and disinfection residuals, N, P, Sodium (Na) and
Chloride.
potentially
cause
eutrophication
of
3.1
Nutrients
groundwater- dependent ecosystems.
Australia has some of the oldest and least fer-
tile soils in the world; therefore, the P in waste
water is generally of great benefi t to crops in
Australia. Reclaimed wastewater typically con-
Human and domestic wastes contribute large
amounts of N and P to sewage. Only 50 % of the
N and 60 % of the P are removed during treat-
Table 2
Crop macronutrient uptake and supply in reclaimed water (kg/ha)
Nutrient contents (kg/ha)
N
Crop
Typical yield (t/ha)
P
K
Ca
Mg
Cabbage
50
147
24
147
36
13
Capsicum
20
41
4
69
52
7
Carrot
44
210
19
270
175
10
Caulifl ower
50
181
28
225
127
18
Celery
190
308
97
700
290
38
Cucumber
18
66
12
120
34
8
Lettuce
50
100
18
180
10
3
Potato
40
264
23
310
66
21
Tomato
194
572
133
856
348
87
Reclaimed water
a
1,000 mm
82
11.5
468
399
308
Source
: Kelly et al. (
2006
)
a
Nutrients applied in 1,000 mm from the Virginia Pipeline Scheme, South Australia
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