Geoscience Reference
In-Depth Information
In some cases, particularly when dealing with
waste water, the oxygen demand will be higher than
total saturation. In this case the sample needs to
be diluted with distilled water. The maximum dis-
solved oxygen content at 20°C is 9.1 mg/l, so any
water sample with a BOD
5
value higher than 9 will
require dilution. After the diluted test a calculation
needs to be performed to find the actual oxygen
demand. If you have diluted the sample by half
then you need to double your measured BOD
5
value, and so on.
A normal unpolluted stream should have a BOD
5
value of less than 5 mg/l. Untreated sewage is some-
where between 220 and 500 mg/l; while milk has a
BOD
5
value of 140,000 mg/l. From these values it
is possible to see why a spillage of milk into a stream
can have such detrimental effects on the aquatic
fauna. The milk is not toxic in its own right, but
bacteria consuming the milk will strip the water
of any dissolved oxygen and therefore deprive fish of
the opportunity to breathe.
There are three reasons why BOD
5
means that there are plenty of data to compare
readings against.
It also important to realise that BOD is not a
direct measure of pollution; rather, it measures the
effects of pollution. It also should be borne in mind
that there may be other substances present in your
water sample that inhibit the natural bacteria (e.g.
toxins). In this case the BOD
5
reading may be low
despite a high organic load.
Trace organics
Over six hundred organic compounds have been
detected in river water, mostly from human activity
(Tebbutt, 1993). Examples include benzene, chloro-
phenols, pesticides, trihalomethanes and poly-
nuclear aromatic hydrocarbons (PAH). These would
normally be found in extremely low concentrations
but do present significant health risks over the long
term. The data for pesticide concentrations (see
Table 7.4) in European water resources show that it
is a significant problem. This indicates that all water
extracted from surface water supplies in Belgium
(supplies approximately 30 per cent of the Belgian
population) will require pesticide removal before
reticulation to customers (Eureau, 2001). Although
Germany appears to have no pesticide problem,
10 per cent of its surface water resources occasionally
have pesticide levels greater than 0.1
g/l and 90
per cent have pesticides in concentrations less than
0.1
g/l (but still present) (Eureau, 2001).
is such a
crucial test for water quality:
•
Dissolved oxygen is critical to aquatic fauna and
the ability to lose dissolved oxygen through
organic matter decay is an important measure of
stream health.
•
It is an indirect measure of the amount of organic
matter in the water sample.
•
It is the most frequently measured water quality
test and has become a standard measure; this
Table 7.4
Percentage of water resources with pesticide
concentrations regularly greater than 0.1 µg/l (European
Union drinking water standard) for selected European
countries
Country
Surface water (%)
Groundwater (%)
Belgium
100
5.2
Denmark
n/a
8.9
Germany
0.0
0.0
Netherlands
50.0
5.0
UK
77.0
6.0
Source
: Data from Eureau (2001)
