Sewage and its oxygen demand

Fertilisers, pesticides and chemicals from homes, farms and industries pollute air, water and soil. Using living organisms to clean up pollution is bioremediation. This chapter looks at the main kinds of waste and how biotechnology helps deal with them.

Sewage is the waste water from homes, communities and industries. It is about 99% water and only 1% organic and inorganic matter, either:

• suspended (lignocellulose, proteins, fats), or
• dissolved (sugars, fatty acids, amino acids, salts).

Industries add detergents, antibiotics, paints and biocides.

Microbes in water feed on the organic matter and use up oxygen as they do. The biochemical oxygen demand (BOD) is the amount of oxygen microbes need to oxidise the decomposable matter in a sample. It measures the organic load: the more organic matter, the higher the BOD. Strong, high-BOD sewage poured into a river or lake uses up its oxygen, and fish die.

Measuring BOD. The sample is diluted with water saturated with oxygen, and its oxygen content measured (D1). It is then kept sealed at 20 °C for five days, beside a control, and the oxygen measured again (D2). The oxygen used, scaled up for the dilution, gives the BOD:

BOD = (D1 − D2) × dilution factor

For example, 100 mL of sample made up with 900 mL of water has a dilution factor of 10.

Sewage also carries pathogens: E. coli; Salmonella typhi (typhoid); Vibrio cholerae (cholera); the hepatitis A virus; Entamoeba histolytica (amoebic dysentery); Giardia; and worms such as Ascaris.