Chemistry, Third edition

418 22 · ENVIRONMENTAL CHEMISTRY

Food waste

Food wastes, although they are considered ‘natural’ can have a very damaging effect on the

environment if they are simply discharged into rivers and lakes. This is because they have a

high biochemical oxygen demand.

Suppose that the effluent from a food manufacturing industry has a glucose (C 6 H 12 O 6 )

content of 600 mg dm^3.

(i)Write the chemical equation for the complete oxidation of glucose.

(ii)What is the oxygen requirement (in mg dm–3) for the complete oxidation of 1.0  105 dm^3

of this waste?

Exercise 22D

pH

The pH of drinking water should be between 5.5 and 9.5. A decrease in the pH of the

water increases the solubility of metal ions.

Other metals

The maximum recommended levels of impurities for drinking water, set by the EU,

are as follows

Impurity Concentration/ppm or mg dm^3

Zn 5

Fe 0.2

Mn 0.05

Cu 3

Cd 0.005

Al 0.2

Sulfate

Sulfate is harmless at moderate levels, but excessive sulfate (500 ppm) is thought

to have a laxative effect.

Nitrate

Excess nitrate in drinking water can lead to methemoglobinemia (‘blue-baby’ syn-

drome). It also may be linked to stomach cancer, although this link has not been

proven. The EU has set a maximum limit of 50 ppm in drinking water.

Quality of river water

TheEnvironmental Agency– formerly the National River Authority (NRA) is

responsible for the conservation of water resources in England and Wales. Their

duties include monitoring and improving river quality. Rivers are classified accord-

ing to their quality as shown in Table 22.1.