Plumbing Africa August 2018 - Page 51

HEALTH AND SANITATION 49 << Continued from page 47 1. Sedimentation Sedimentation is recommended as simple treatment of water prior to application of other purification treatments such as filtration and disinfection methods. It is a physical treatment process used to reduce the turbidity of the water. Small particulate suspended matters (sand, silt and clay) and some biological contaminants are removed from water under the influence of gravity. The longer the water is allowed to sediment, the more the suspended solids and pathogens will settle to the bottom of the container. The addition of special chemicals or some natural coagulants, such as indigenous plants, can accelerate sedimentation. Four common chemicals used are aluminium sulphate (alum), polyaluminium chloride (PAC or liquid alum), alum potash and iron salts (ferric sulphate or ferric chloride). Some indigenous plants are also traditionally used in some countries, depending on the local availability, to help with sedimentation. In countries like Malawi, Sudan, Egypt, and Malaysia, the application of Moringa oleifera seeds extract in water coagulation and softening has received a lot of attention. Another natural plant coagulant known to clarify turbid surface water is Strychnos potatorum. This type of coagulant is reported to be used in countries like Southern and central parts of India, Sri Lanka, and Burma. Much of the suspended material can be removed by simply allowing the water to stand and settle for some time. This retention time (from one hour up to two days, the longer the better) is required to settle particles to the bottom. Storing water for at least one day will also promote the natural die-off of some bacteria. Simple sedimentation is often effective in reducing water turbidity, but it is not consistently effective in reducing microbial contamination. However, most viruses, bacteria, and fine clay particles are too small to be settled out by simple gravity se dimentation. However, attachment of these smaller particles (bacteria and viruses) to suspended particles would result in the formation of flocs that can then settle to the bottom of the tank due to their 1 Post-storage treatment of the water is critical for both health of the users and maintenance of the system. The level of treatment will depend on the intended use of the water. Water used for irrigation does not require the same level of treatment as water used for potable indoor purposes. To maximise effectiveness, a multi-barrier approach where more than one method of treatment is used is recommended. approaches comprise: sedimentation, filtration methods, and disinfection methods (to be covered in the next article of the series). Those methods are discussed further in the following sections of this document. 2 increased mass. The addition of coagulants reduces the time required to settle out suspended solids and is very effective in removing fine particles. 2. Filtration methods Filtration is commonly used to reduce turbidity and remove pathogens. It is a physical process that involves passing water through filter media. There are several types of filters; some are designed to grow a biological layer that kills or inactivates pathogens and improves the removal efficiency. Various types of filters are used by households around the world, including bio-sand filters, ceramic pot filters, ceramic candle filters, and membrane filters. • Sand filters Studies have reported that bio-sand filters (BSF) are capable of removing 81–100% bacteria and 99.98– 100% protozoa from harvested rainwater. Reports indicate that treatment of harvested rainwater with BSF can reduce bacteria, viruses, and protozoa by up to 4-log reduction. Further reports show that turbidity can be reduced by 84% with BSF. Other studies have also reported on the removal of microorganisms and turbidity by iron oxide coated sand filters. Results showed that the coated filter medium was able to remove 99% of coliforms and 96% lead. Biosand filters, however, have been reported to have a limited virus removal efficiency. 1. 2. A common filter installed to a storage tank eliminates organic materials entering the system. Organic materials are the most common elements to negotiate in pre-storage harvesting treatment. Continued on page 51 >> August 2018 Volume 24 I Number 6