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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

Biochemical Oxygen Demand (BOD)

In Table 1.1 the river water pollution load is indicated by its biochemical oxygen demand (BOD) concentration. BOD is a measure of the amount of biodegradable organic substances in the water. As naturally occurring bacteria consume these organic substances they take up oxygen from the water for respiration, while converting the substances into energy and materials for growth. On average each person produces about 60 g of BOD in faecal and other materials. This is equivalent to 60,000 mg of BOD. Depending on the volume of water used to convey the faecal materials, the concentration of the BOD in the wastewater varies. For example if the total water usage per person is 200 L per day, then the resulting wastewater will have a BOD concentration of 300 mg/L. Upon discharge to a river the concentration is further diluted by the river water. The BOD of industrial wastewaters can be very high (e.g. abattoir, paper mill).

The river pollution classification (Table 1.1) provides an illustration of the ability of the environment (here the river) to cope with small waste discharges of organic wastes. Small discharges of BOD are diluted by the river water to low levels. If the concentration of BOD in the river water is less than 3 mg/L the river remains 'unpolluted' The oxygen uptake by bacteria, as they consume the organic wastes, is replenished by the continuous transfer of oxygen from the atmosphere to the water. The dissolved oxygen (DO) concentration in the water remains high. This simple process explains the reason why a stream in an undisturbed forest remains clean despite the natural organic wastes produced by animals in the forest. Other physical, chemical and biological processes take place which help in the ability of nature to purify wastes. These are elaborated in Section 2 (2.2).

On the hand the river pollution classification shows that it does not take much for an unpolluted river (class I) to become a grossly polluted river (class IV). When the BOD concentration in the river water is greater than 12 mg/L, the transfer of oxygen from the atmosphere cannot replenish the oxygen demand and the water becomes completely deoxygenated. It is incapable of supporting fish life. The water is dominated by bacteria that thrive on the organic wastes but able to extract oxygen chemically from substances like sulphates in the wastes. Gases such as hydrogen sulphide (rotten egg gas) and methane are generated by these bacteria. Foul odours are the result, and the appearance of the water is grey black with bubbles frothing up.

To prevent degradation of the receiving environment wastewater needs to be treated. This treatment is usually carried out at the point of discharge, also called 'end of pipe' Etreatment (Figure 1.4). Treatment consists of removing solids from the wastewater and reducing its BOD. The degree of treatment that is required is dependent on the capacity of the receiving environment to assimilate the remaining organic wastes.


Figure 1.4: End of pipe treatment of wastewater prior to discharge to the environment

Because the wastewater treatment facility is generally designed for dry-weather flow, its capacity is exceeded in wet weather. Treatment efficiency drops during wet weather, and in high rainfall events a significant volume of combined wastewater and stormwater is not effectively treated. To overcome the problem of wet-weather flow, and recognising that stormwater may not be as contaminated as wastewater, separate collection of wastewater and stormwater have been implemented (Figure 1.5), with stormwater treated only to remove gross solids.


Figure 1.5: Separate collection of wastewater and stormwater

 

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