The two standards for sewage effluent quality proposed by the Royal Commission were for no more than 30 mg l -1 of suspended solids and 20 mg l-1 for BOD, the so called 30: 20 standard. The Royal Commission envisaged that the effluent of this standard would be diluted 8:1 with clean river water having BOD of 2 mg l -1 or less. This standard was considered to be the normal minimum requirement and was not enforced by statute because the character and use of rivers varied so greatly. Currently, most sewage treatment works are required to meet discharge standards set by the Urban Wastewater Treatment Directive. Sewage treatment now attempts to consistently produce an effluent with a quality superior to its 'Legal Consent' and attempts to achieve an 'Operating Target', frequently half the Legal Consent. In addition considerable importance has been placed upon the concentration of ammonia in the effluent. In the case of a works attempting to nitrify the effluent, the ammonia concentration is frequently limiting. Domestic sewage contains approximately 1000 mg l -1 of impurities of which about two-thirds are organic. Thus sewage is 99.9% water and 0.1% total solids upon evaporation. When present in sewage approximately 50% of this material is dissolved and 50% suspended (see Figure 1). The main components are: nitrogenous compounds - proteins and urea; carbohydrates - sugars, starches and cellulose; fats - soap, cooking oil and greases. Inorganic components include chloride, metallic salts and road grit where combined sewerage is used. Thus sewage is a dilute, heterogeneous medium which tends to be rich in nitrogen.