The bulk water treatment processes described in Section 4B produce water that is good enough to drink, and which is vastly better than that drunk by millions of people in the developing world. Nevertheless, it is not good enough for many people, and greater degrees of purification are accordingly necessary to satisfy these needs. This […]
Water desalination by the reverse electrodialysis process is less commonly employed than reverse osmosis. It typically uses two types of ion exchange membrane, one allowing only the passage of negatively charged ions (anions), and the other only allows the passage of positive ions (cations). Electrodialysis was (and is) a laboratory process, used to remove salts […]
The key to the desalination of water by reverse osmosis lies in the proper selection of the membrane through which the separation of salt and water occurs. The original RO membranes were made from cellulose derivatives, but are now much more likely to be made from synthetic polymers. Two membrane formats are most commonly used […]
If a tank is set up divided into two compartments by a vertical barrier that is permeable to water only, and one compartment is partly filled with pure water and the other is filled to the same level with a salt solution, then the chemistry of the system is such that water will flow through the […]
Various methods from evaporation to distillation and chemical treatment may be employed for producing fresh water from sea water and other salty water sources. The different types of technology employed include multi-stage flash distillation, thermo-compression distillation, reverse osmosis and reverse electrodialysis. A multi-stage flash distillation plant typically consists of a series of twenty or more […]
The ubiquitous hyberbolic cooling tower is one of the most evocative sights when the theme of ‘industry’ is discussed. Its plume of steam – assumed to be a more toxic effluent – is a symbol to many of the environmental impact feared from industrial activity. The plume is, of course, a sign of heat going […]
Much thought is currently being given to the immediate collection of rainwater and its use for low grade fresh water applications, such as toilet flushing, garden irrigation and so on. An automatic system that collects rain and wastewater from roofs and pavements in and around a building, stores it in a tank and then filters […]
The major development of the deep-bed filter has been to allow the bed material to move continuously down through the filter vessel, and then to be carried back up to the top of the bed, through a cleansing zone. Manufacturers have concentrated on the development of the moving bed filter, which provides better cleansing of […]
The alternative approach to match flow direction with pore size is to undertake the filtration in upwards flow. With an upflow sand filter, flow is from the bottom through to the top of the bed. The result is that the entire bed depth is utilized to trap solids, with the fine top layer acting as […]
Conventional downflow sand filters are effective for liquid-solid separation at flow rates up to about 15m3/h.m2 of filter area, although higher rate downflow filters are available. With proper selection of filter media, gelatinous as well as granular suspended matter can be filtered out, without a rapid differential pressure build-up. The bed is cleaned by a […]
