Effluents from poorly operated works

Although there has been a great improvement in the treatment of sewage in the last 10-15 years because of investment in new plant and equipment, there are still some sewage works where the wastewater is not adequately treated. This may be because they are old and the equipment is inefficient, or they may be overloaded, i.e. they are receiving more sewage than they were designed to purify. In these situations, the effluent that enters the receiving water still contains many of the substances that were present in the incoming sewage. Some have been only partially broken down by the bacteria so the effluent contains high concentrations of organic matter.

We saw in the description of the operation of sewage works how the presence of oxygen is important for the treatment of sewage. It enables the sewage to be degraded and oxidized by aerobic bacteria (which require free oxygen for living). If the sewage works is overloaded, broken down or operating inefficiently, then there may not be sufficient oxygen for the aerobic bacteria and the organic matter is then broken down by anaerobic bacteria (those that can live in the absence of oxygen). Anaerobic bacteria obtain their oxygen from the breakdown of salts such as phosphates and sulphates. The products of this degradation are often foul smelling and occur as follows:

Organically combined carbon ^ methane

" nitrogen ^ amines and ammonia

" sulphur ^ sulphides and organic sulphur compounds

" phosphorus ^ phosphines and organic phosphorus compounds

The effects of a poor quality effluent on a receiving stream can be very marked and it may take many kilometres for the river to recover from the pollution. This is because the river downstream acts like an extension to the sewage works in that the naturally occurring bacteria and protozoa oxidize the organic matter present in the sewage effluent and, as they do so, they utilize the dissolved oxygen in the river water. However, the amount of oxygen which is dissolved in water is limited, for example at 10°C, water contains only 11.3 mg/l of dissolved oxygen whereas air contains nearly 300 mg/l. The dissolved oxygen may therefore be quickly used up and it takes time for fresh oxygen to dissolve in the water from the air, especially if the water is slow flowing. If the dissolved oxygen is depleted, then the substances formed in anaerobic conditions are present in the water and many of them are poisonous to the river organisms. However, as the river flows over rocks and waterfalls, the oxygen levels are restored and the organic matter is degraded to the extent that aquatic life is returned to normal. This process is called self-purification.

The chemical results for a poor quality effluent and for samples taken upstream and downstream, are shown in Table 4. A poor quality effluent usually contains high concentrations of ammonia as well as organic matter. As it flows downstream, this ammonia is oxidized by bacteria to nitrate. This process is called nitrification and also utilizes dissolved oxygen for the oxidation process. The overall effect on the river chemistry downstream of a poor quality discharge is illustrated in Figure 4.

Table 4. Chemical analysis of a poor quality sewage effluent and the receiving waters

Concentration (in mg/l, except for pH and conductivity)

River water Effluent River water

Chemical determinand

upstream

downstream

Suspended solids

11.0

120.0

46.0

BOD2.81055.4

Permanganate value

9.4

31.6

11.0

Dissolved oxygen

11.7

0.0

6.4

Ammonia nitrogen

0.3

32.0

7.8

Nitrate nitrogen

0.9

1.7

0.5

Detergents

0.1

4.5

0.9

Phosphate phosphorus

0.3

3.7

1.2

Alkalinity

65.0

170.0

115.0

Chloride

4.0

75.0

22.0

pH

7.0

7.7

7.7

Conductivity

143

820

370

Sewage Eutrophication Downstream

Distance downstream

Discharge point

Figure 4. Changes in the chemistry of river water downstream of a poorly treated sewage effluent Source: The Biology of Pollution, Macan, Hodder. Reproduced with permission

Distance downstream

Discharge point

Figure 4. Changes in the chemistry of river water downstream of a poorly treated sewage effluent Source: The Biology of Pollution, Macan, Hodder. Reproduced with permission

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