Desalination and Other Forms of Water Treatment
Despite covering about 71 per cent of the planet’s surface, seawater is not suitable for consumption by land-based animals, and only specialised marine plants can survive in it. Unfortunately, most freshwater is locked in glaciers and icebergs, so little is readily accessible for human use. In practice, of the 2,5 per cent that is drinkable, a mere 0,3 per cent is both in liquid form and present above ground. When combined with a global population now approaching 8 billion, these statistics underline the importance of desalination and other forms of water treatment.
While people have found simple ways to remove the salt from seawater for many centuries, only during recent years have scientists developed the technology to turn this simple concept into a viable commercial proposition. Before the breakthrough, the focus was almost exclusively on recycling wastewater using a range of purification techniques. Of these, filtration was the simplest. The modern methods now in use developed from lo-tech methods used in the distant past. These involved passing the liquid through linen, porous clay pots and sand. Sand filtration remains a standard and effective water treatment, despite continued advances in desalination technology.
The simple act of boiling water is sufficient to free it of bacteria. However, boiling will not improve its clarity or rid the liquid of any dissolved solids, such as heavy metal salts, that may be present. While there are also more effective filtration techniques to eradicate microorganisms, it will generally be necessary to remove any particulate matter before using such methods. Typically, an effective purification process can require several steps. For example, before commencing the desalination process, just as with other water treatment routines, the first step must be to remove any undissolved solids.
In a municipal plant, that process begins with screening to remove large items, like twigs, leaves and litter, before transfer to sedimentation tanks. Here, most of the visible particles sink to form a layer of sediment on the bottom. The clarification process is only complete after passing the upper layer of liquid through a bed of sand and gravel, which also removes most of the microorganisms. In some plants, a process known as reverse osmosis (RO) is included before chlorination. Both the “polishing” stage used in conventional water treatment plants and the desalination process leverage RO technology.
Until relatively recently, seawater processing plants employed distillation to separate salt. However, boiling vast volumes of seawater is expensive. Consequently, the end-product proved too costly to compete with conventional purification methods. Today, reverse osmosis is proving to be a gamechanger. It appears destined to become the method of choice in desalination plants and water treatment, wherever exceptional purity levels are critical.