To understand the aims of sewage treatment it is helpful to understand what is in sewage and what needs to be removed by the treatment process. What is sewage? Sewage typically contains over 99% water Impurities in sewage - Micro organisms Often includes disease causing organisms (pathogens), including bacteria, viruses, protozoa and parasitic worms. Usually measured by the presence of E.coli (a common type of bacteria found in the intestines). 100 mL of sewage contains around 10 million E. coli bacteria. Phosphorus compounds These are present in human wastes (faeces) and in detergents. Typical levels are 10 to 15 mg per litre of sewage. Nitrogen compounds These are mostly present as ammonia or urea. Typical levels are up to 55 mg per litre of sewage. Suspended solids Includes inert material such as sand and organic solids such as food scraps. Typical levels in sewage are 250 mg per litre. Organic matter Usually measured by BOD (Biological Oxygen Demand) or COD (Chemical Oxygen Demand). This can include faeces, fats and oils. Typical BOD levels in sewage are 250 mg per litre. Additionally, sewage will contain rags, plastics, sanitary products and other large solids. Trace amounts of other compounds, including heavy metals and various organic compounds (e.g. pesticides and weedicides) will also be present, even in purely domestic sewage and in higher levels if incorrectly treated trade wastes have entered the sewerage system