Geoscience Reference
In-Depth Information
Water is supplemented with water from purer river
water pumped from further afield.
Eutrophication of water can occur at what appear
to be very low nutrient levels. As an example the
drinking water standard for nitrate-nitrogen is
around 12 mg/l (depending on country) but concen-
trations as low as 2-3 mg/l can cause eutrophication
problems in water bodies.
Table 7.5 shows some of the indicators used in
a quantitative example of defined trophic levels
developed for the Organisation for Economic Co-
operation and Development (OECD). The chloro-
phyll is an indicator of algal growth in the water,
while phosphorus and dissolved oxygen are more
traditional water-quality measures. The dissolved
oxygen is taken from the bottom of the lake because
this is where the vegetative decomposition is tak-
ing place. The dissolved oxygen level near to the
surface will vary more because of the proximity to
the water/air interface and the oxygen produced
in photosynthesis by aquatic plants. It is worth
noting that heavily eutrophied water samples will
sometimes have a dissolved oxygen greater than 100
per cent. This is due to the oxygen being produced
by algae which can supersaturate the water.
used for industrial waste water is dependent on
the type of waste being produced. In this section the
processes described are those generally found in sew-
age treatment rather than in specialised industrial
waste water treatment.
There are two major objectives for successful
sewage treatment: to control the spread of disease
from waste products and to break down the organic
waste products into relatively harmless metabolites
(i.e. by-products of metabolism by bacteria, etc.).
The first objective is achieved by isolating the waste
away from animal hosts so that viruses and other
pathogens die. The second objective is particularly
important for the protection of where the treated
effluent ends up - frequently a river environment.
In Britain the first attempt to give guidelines
for standards of sewage effluent discharge were
provided by the Royal Commission on Sewage
Disposal which sat between 1898 and 1915. The
guidelines are based on two water-quality para-
meters described earlier in this chapter: suspended
solids and biochemical oxygen demand (BOD).
The Royal Commission set the so-called 30:20
standard which is still applicable today (i.e. 30 mg/l
of suspended solids and 20 mg/l of BOD). The
standard was based on a dilution ratio of 8:1 with
river water. Where river flow is greater than eight
times the amount of sewage effluent discharge
the effluent should have a TSS of less than 30 mg/l
and a BOD of less than 20 mg/l. There was also the
recommendation that if the river is used for
drinking water extraction further downstream the
standard should be tightened to 10:10. This was
CONTROLLING WATER QUALITY
Waste water treatment
The treatment of waste water is a relatively simple
process that mimics natural processes in a controlled,
unnatural environment. The treatment processes
Table 7.5
OECD classification of lakes and reservoirs for temperate climates
Trophic level
Average total P (mg/l)
Dissolved oxygen
Max. chlorophyll
(% saturation)
(mg/l) (at depth)
Ultra-oligotrophic
0.004
>90
0.0025
Oligotrophic
0.01
>80
0.008
Mesotrophic
0.01-0.035
40-89
0.008-0.025
Eutrophic
0.035-0.1
0-40
0.025-0.075
Hypertrophic
>0.1
0-10
>0.075
Source
: Adapted from Meybeck
et al
. (1989)
