59. Household water treatment 2
This Technical Brief is the second of two which examine the treatment of water at household level. The first (No.58) introduced the topic and covered treatment by straining, storage, settlement, solar disinfection, chemical disinfection and boiling. This Brief considers treat- ment by coagulation, flocculation, filtration and solar distillation and covers aspects of the reduction of some chemical concentrations.
Coagulation and Straining, and settlement (both of fine sand. For the filter to described in TB 58) are treatment perform well there should be no sudden flocculation methods which usefully precede changes in the flow rate and the water If raw water contains a large amount of filtration to reduce the amount of should not be very turbid (cloudy with fine suspended solids, coagulation and suspended solids which enter the suspended solids), or the filter will flocculation can be used to remove filtration stage. This increases the period quickly become blocked. Good slow much of this material. In coagulation, a for which a filter can operate before it sand filters can produce good quality substance (usually in a liquid form), is needs cleaning. Coagulation and drinking-water. added to the water to change the flocculation are also useful treatments to behaviour of the suspended particles. It precede settlement and improve still There are a number of processes which causes the particles, which previously further the removal of solids before improve the water quality as it passes tended to repel each other, to be filtration. If an effective system of through the filter, but pathogens are mainly attracted towards each other, or removing the flocculated particles from removed in the very top layer of the filter towards the added material. the filter is feasible, then coagulation bed where a biological film (called the and flocculation can also be used ‘schmutzdecke’) builds up. In a well- Coagulation takes place during a rapid before coarse sand filtration. Larger designed and well-operated filter this film mixing/stirring process which pathogens (e.g. parasitic worm eggs) strains out bacteria. Deeper in the sand immediately follows the addition of the are more readily removed by filtration bed bacteria that pass through the coagulant. than smaller pathogens (e.g. viruses). schmutzdecke are killed by other micro-organisms, or they become The flocculation process, which follows attached to particles of sand until they die. coagulation, usually consists of slow, Sand filtration The schmutzdecke takes time to gentle stirring. During flocculation, as Slow sand filtration become effective, so water needs to the particles come into contact with In slow sand filtration the water passes flow though a new filter for at least a each other they cling together to form slowly (e.g. flow velocity of 0.1 to 0.2m/h – week before the filter will work efficiently. larger particles which can be removed i.e. m3/m2/h) downwards, through a bed afterwards by settlement (see TB 58) or The raw water should contain a fair filtration. A chemical which is often used amount of oxygen to promote the is alum (aluminium sulphate). Natural useful biological activity both in the coagulants include some types of clay Level of Water should be schmutzdecke and further down into the (e.g. bentonite) and powdered seeds of discharge regularly topped point to be filter bed. the Moringa olifeira tree. The best type up by slowly pouring above sand above stone of coagulant and the required dose will After a period of use, the material filtered depend on the physical properties out of the water blocks the surface of the Flat stone (particularly the alkalinity/acidity) of the sand and reduces the flow rate to an raw water and the amount and type of unacceptable level. When this happens the filter is drained to expose the sand, suspended solids. and the top 15 to 20mm of the bed is