Plumbing Africa February 2018 | Page 25

ENVIRONMENT AND ENERGY flushing, resulting in diluted sewage combined with few other wastewater sources. It is important to note that, in many cases, large volumes of wastewaters that are legally discharged to decaying and/or badly operated sewerage networks, both combined and separate, never actually reach a treatment plant. Much is lost en route as a result of broken pipes, or ends up in surface water drains, polluting both groundwater and surface watercourses. There are also many cases of illegal water reuse by communities who deliberately tamper with trunk sewer systems. SANITATION AND THE PRODUCTION OF WASTEWATER IN INFORMAL SETTLEMENTS Wastewater generation is one of the biggest challenges associated to the growth of informal settlements (slum s) in the developing world. In sub-Saharan Africa, 62% of the urban population live in slums. The most alarming statistics can be found in countries emerging from conflict and in West Asia, where the proportion living in slums has increased from 67% to 77% and 21% to 25%, respectively. Slums vary in type, form, and population density. However, most are characterised by a lack of paved roads, durable housing, water and sanitation infrastructure, and drainage. In these situations, high levels of faecal matter and solid waste are disposed of to surface water drainage canals and ditches. Poor solid waste disposal causes blockages in drainage systems, resulting in flooding. Uncollected wastewater and urban run-off flows are often equivalent to sewered wastewater in terms of toxicity and health risks. Although many slums rely on on-site sanitation, faecal matter is not usually contained and wastewater is still produced as residents often use latrines as bathrooms for personal ablutions with so called ‘bucket showers’. Slum dwellers frequently have to rely on un-sewered communal public toilets, use open space or dispose of faeces in polythene bags (i.e. flying toilet). Communal toilets are not widely used, due to lack of water, poor maintenance, and the cost to the user. A study in the slums of Delhi found that the average low-income family of five could spend 37% of its income on communal toilet facilities. Finding a suitable place to go to the toilet is especially problematic for women, causing risks related to personal security, embarrassment, and hygiene. COMPOSITION OF MUNICIPAL AND URBAN WASTEWATER The precise composition of wastewater varies around the world and is governed by a wide range of factors, including domestic water use and the level of commercial/ industrialisation. In developed regions, the BOD:COD* ratio 4 is likely to be lower than in the developing world, due to a higher proportion of industrial wastewater. This will lower the water’s suitability for biological treatment. In some areas, high levels of inorganic substances, sulphates, and alkalinity for example, can affect the wastewater’s suitability for post-treatment use. Sulphates tend to result in hydrogen sulfide production with implications for sewer corrosion. High alkalinities, or water hardness, are likely to cause limescale deposits and will affect the suitability of the water for reuse as process water. WASTEWATER FROM PARTICULARLY HAZARDOUS SOURCES Wastewater from domestic sources are usually relatively free from hazardous substances, but there are growing concerns about commonly used medications that, even at low concentrations, may have long-term impacts. Industries that use red list substances (substances classed with significant hazard) in their production Continued on page 24 >> February 2018 Volume 23 I Number 12 *BOD:COD ratio Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed (i. e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. Chemical oxygen demand (COD) is the standard method for indirect measurement of the amount of pollution (that cannot be oxidised biologically) in a sample of water. The higher the chemical oxygen demand, the higher the amount of pollution (mostly inorganic) in the test sample. If the BOD:COD ratio for untreated wastewater is 0.5 or greater, the waste is considered to be easily treatable by biological means. If the ratio is lower than 0.3 approximately, either the waste may have some toxic components, or acclimated microorganisms may be required for its stabilisation. 23