Geoscience Reference
In-Depth Information
Table 7.6
Changes in suspended solids and biochemical oxygen demand
through sewage treatment. These are typical values which will vary
considerably between treatment works
Stage of treatment
Suspended solids (mg/l)
BOD (mg/l)
Raw sewage
400
300
After primary treatment
150
200
After biological treatment
300
20
Effluent discharged to river
30
20
reaction with the dissolved phosphorus so that an
insoluble form of phosphate settles out. This is
particularly useful where the receiving water for
the final effluent has problems with eutrophication.
The average phosphorus concentration in raw
sewage is 5-20 mg/l, of which only 1-2 mg/l is
removed in biological treatment.
In some cases, particularly in the USA, chlori-
nation of the discharging effluent can take place.
Chlorine is used as a disinfectant to kill any patho-
gens left after sewage treatment. This is a noble
aim but creates its own difficulties. The chlorine
can attach to organic matter left in the effluent and
create far worse substances such as polychlorinated
biphenyl (PCB) compounds. Another safer form
of disinfection is to use ultraviolet light, although
this can be expensive to install and maintain.
Source control
The best way of controlling any pollution is to
try and prevent it happening in the first place. In
order to achieve this differentiation has to be made
between point source and diffuse pollutants (see
p. 129). When control over the source of pollutants
is achieved dramatic improvements in river-water
quality can be achieved. An example of this is
shown in the Case Study of the Nashua River in
Massachusetts, USA.
Case study
CONTROLLING WATER QUALITY OF THE NASHUA RIVER
The Nashua river is an aquatic ecosystem that has
undergone remarkable change in the last one
hundred years. It drains an area of approximately
1,400 km
2
in the state of Massachusetts, USA, and
is a tributary of the much larger Merrimack river
which eventually flows into the sea in Boston
Harbor (see Figure 7.7). The land use of the
Nashua catchment is predominantly forest and
agricultural, with a series of towns along the river.
It is the industry associated with these towns that
has brought about the changes in the Nashua,
predominantly through the twentieth century.
The latter-day changes are well illustrated by the
two photographs at the same stretch of the
Nashua, in 1965 and 1995 (see Plates 9 and 10).
Prior to European colonisation of North
America the Nashua valley was home to the
Nashaway tribe, and the Nashua river could be
considered to be in a pristine condition. With the
arrival of European settlers to New England the
area was used for agriculture and the saw milling
of the extensive forests. The Industrial Revolution
of the nineteenth century brought manufacturing
to the area and mills sprang up along the river. By
