The purposes of pollution control endeavours should
be (1) to protect the assimilative capacity of surface
waters; (2) to protect shellfish, finfish and wildlife;
(3) to preserve or restore the aesthetic and recreational
value of surface waters; (4) to protect humans
from adverse water quality conditions.
The selection and design of treatment facilities is
based on a study of
• the physical, chemical and biological characteristics
of the wastewater
• the quality that must be maintained in the
environment to which the wastewater is to be
discharged or for the reuse of the wastewater
• the applicable environmental standards or discharge
requirements that must be met
The main chemical characteristics of wastewater are
divided into two classes, inorganic and organic. Because
of their special importance, priority pollutants
and volatile organic compounds (VOCs) are usually
considered separately.
Physical characteristics
The principal physical characteristics of wastewater
are its solids content, colour, odour and temperature.
The total solids in a wastewater consist of the
insoluble or suspended solids and the soluble compounds
dissolved in water. The suspended solids
content is found by drying and weighing the residue
removed by the filtering of the sample. When
this residue is ignited the volatile solids are burned
off. Volatile solids are presumed to be organic
matter, although some organic matter will not burn
and some inorganic salts break down at high temperatures.
The organic matter consists mainly of
proteins, carbohydrates and fats.
Between 40 and 65 % of the solids in an average
wastewater are suspended. Settleable solids, expressed
as millilitres per litre, are those that can be
removed by sedimentation. Usually about 60 % of
the suspended solids in a municipal wastewater are
settleable. Solids may be classified in another way
as well: those that are volatilised at a high temperature
(600 °C) and those that are not. The former are
known as volatile solids, the latter as fixed solids.
Usually, volatile solids are organic.
Colour is a qualitative characteristic that can be
used to assess the general condition of wastewater.
Wastewater that is light brown in colour is less than
6 h old, while a light-to- medium grey colour is
characteristic of wastewaters that have undergone
some degree of decomposition or that have been
in the collection system for some time. Lastly, if
the colour is dark grey or black, the wastewater is
typically septic, having undergone extensive bacterial
decomposition under anaerobic conditions. The
blackening of wastewater is often due to the formation
of various sulphides, particularly, ferrous sulphide.
This results when hydrogen sulphide produced
under anaerobic conditions combines with
divalent metal, such as iron, which may be present.
Colour is measured by comparison with standards.
The determination of odour has become increasingly
important, as the general public has become
more concerned with the proper operation
of wastewater treatment facilities. The odour of
fresh wastewater is usually not offensive, but a
variety of odorous compounds are released when
wastewater is decomposed biologically under
anaerobic conditions. The principal odorous compound
is hydrogen sulphide (the smell of rotten
eggs). Other compounds, such as indol, skatol,
cadaverin and mercaptan, formed under anaerobic
conditions or present in the effluents of pulp
and paper mills (hydrogen sulphide, mercaptan,
dimethylsulphide etc.), may also cause a rather
offensive odour. Odour is measured by successive
dilutions of the sample with odour-free water until
the odour is no longer detectable.
The temperature of wastewater is commonly
higher than that of the water supply because warm
municipal water has been added. The measurement
of temperature is important because most
wastewater treatment schemes include biological
processes that are temperature dependent. The
temperature of wastewater will vary from season
to season and also with geographic location. In
cold regions the temperature will vary from about 7
to 18 °C, while in warmer regions the temperatures
vary from 13 to 24 °C.
Chemical characteristics
Inorganic chemicals
The principal chemical tests include free ammonia,
organic nitrogen, nitrites, nitrates, organic phosphorus
and inorganic phosphorus. Nitrogen and phosphorus
are important because these two nutrients are
responsible for the growth of aquatic plants.
Other tests, such as chloride, sulphate, pH and
alkalinity, are performed to assess the suitability of
reusing treated wastewater and in controlling the
various treatment processes.
Trace elements, which include some heavy metals,
are not determined routinely, but trace elements
may be a factor in the biological treatment of
wastewater. All living organisms require varying
amounts of some trace elements, such as iron, copper,
zinc and cobalt, for proper growth. Heavy metals can
also produce toxic effects; therefore, determination of
the amounts of heavy metals is especially important
where the further use of treated effluent or sludge is
to be evaluated. Many of the metals are also classified
as priority pollutants (see below).
Measurements of gases, such as hydrogen sulphide,
oxygen, methane and carbon dioxide, are made
to help the system to operate. The presence of hydrogen
sulphide needs to be determined not only
because it is an odorous gas but also because it can
affect the maintenance of long sewers on flat slopes,
since it can cause corrosion. Measurements of dissolved
oxygen are made in order to monitor and control
aerobic biological treatment processes. Methane
and carbon dioxide measurements are used in connection
with the operation of anaerobic digesters
Volatile Organic Carbons (VOC)
Volatile organic compounds (VOC) such as benzene,
toluene, xylenes, dichloromethane, trichloroethane
and trichloroethylene, are common soil pollutants in
industrialised and commercialised areas. One of the
more common sources of these contaminants is leaking
underground storage tanks. Improperly discarded
solvents and landfills, built before the introduction
of current stringent regulations, are also significant
sources of soil VOCs.
In Table 18.1 a list of typical inorganic and organic
substances present in industrial effluents is presented
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