Chapter 22

Dairy effluents
Water used in domestic and industrial applications become polluted to a greater or lesser extent. Water is also used as a transport medium to carry away waste products. As awareness of the importance of improved standards of water treatment grows, process requirements become increasingly exacting. The food industry contributes to a great extent to pollution, particularly as the pollutants are of organic origin. Organic pollutants normally consist of 1/3 dissolved, 1/3 colloidal and 1/3 suspended substances, while inorganic materials are usually present mainly in solution.

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Organic pollutants
The normal way to express the concentration of a pollutant is to specify the total quantity per unit volume of sewage. Another and more modern way of analysing the presence and quantities of organic substances in sewage effluent is the use of chromatography, such as High-Performance Liquid Chromatography (HPLC). However, the quantity of organic substances is normally determined in the form of; • biological oxygen demand (BOD) • chemical oxygen demand (COD) • calcining loss • total organic carbon (TOC)

Biological oxygen demand (BOD)
BOD is a measure of the content of biologically degradable substances in sewage. The substances are broken down by micro-organisms in the presence of (and therefore with consumption of) oxygen. Oxygen demand is measured in terms of the quantity of oxygen consumed by micro-organisms over a period of five days (BOD5) or seven days (BOD7), in decomposing the organic pollutants in waste water at a temperature of 20°C. BOD is measured in mg oxygen/l or g oxygen/m3. The following relationship is assumed for municipal sewage: BOD7 = 1.15 x BOD5

BOD is a measure of the content of biologically degradable substances in sewage.

Chemical oxygen demand (COD)
COD indicates the quantity of the pollutants in waste water that can be oxidised by a chemical oxidant. The normal