12 WATER ENGINEERING
Desalination by reverse osmosis
In reverse osmosis (RO) plants, the input water or feedwater is pumped at high pressure through special permeable membranes to separate the salt and impurities (including dissolved solids, organic matter and toxic metals) from the water molecules. RO plants can filter out around 95–99% of impurities to provide water that is clean and safe to drink. The resulting water quality, however, depends on the salinity level of the feedwater, the pump pressure, and the quality and maintenance of the membranes.
Stages of reverse osmosis
1 Pre-treatment of feedwater All types of feedwater require pre-treatment to remove suspended solids and impurities that would otherwise hinder the desalting process. Pre-treatment is the most important stage in the case of RO plants, as untreated water would clog the membranes and affect their performance. Common pre-treatment includes filtration, disinfection, and the addition of anti-scalant chemicals. Pre-treatment filters need to be cleaned by backwashing every few days to remove the build-up of solids and matter.
2 Reverse osmosis After pre-treatment, pressure is increased to force the saline feedwater through a series of membrane modules in order to separate the salt and residual impurities and to allow the clean water to pass through. The percentage of feedwater recovered varies: around 40–60% of seawater and 50–90% of brackish water processed through RO can be recovered as product water.
RO membranes are tubes of polymer composite layers with tiny perforations that range in size from millionths to billionths of a metre. Since they are expensive to replace, they need to be maintained to the highest standard. While the membranes resist most impurities, they are still prone to fouling over time and require regular cleaning. Microorganisms not filtered out in pre- treatment can build up on membrane surfaces and adjoining pipes. Without timely cleaning, these deposits can damage membranes and affect the success of the process. Heavy fouling makes the cleaning process difficult by preventing the chemicals from soaking deep into the foulants and flushing them away. Examples of pollutants include calcium scale, metal oxides, silica, organic material and biological matter, such as algae and mold.
3 Wastewater discharge Desalination generates liquid waste containing high salt concentrations, chemicals from pre- treatment and cleaning, and toxic metals. This concentrate, known as brine, is disposed of as waste. Currently, methods include discharge directly into the sea, into a sewage treatment plant, into deep saline aquifers, evaporation ponds, or, in the case of inland facilities, the brine is dried out and disposed of in landfill.
The disposal of brine waste remains an obvious environmental challenge. However, the impact of liquid waste handling varies depending on factors such as the site of a desalination plant and the means of disposal.
4 Post-treatment Desalinated water is highly acidic, corrosive and unpalatable because it is stripped of its natural minerals as well as impurities. To overcome this problem, the water has to be remineralized before it can be used, otherwise it will be unhealthy and will corrode piping systems. Remineralization is a process of adding minerals, such as limestone, to stabilize the acid/alkaline balance. Product water normally contains traces of bacteria and viruses remaining after desalination, so post-treatment also typically includes a disinfection stage to make the water potable.
Osmosis is a process that occurs in nature in living organisms where a low solute (high water content) migrates across a semi-permeable membrane toward a high solute (low water) on the other side, gradually balancing the composition of each solution. Osmosis is the chief means by which water passes into and out of cells.
Reverse osmosis, by definition, is the opposite of this natural process.
Osmosis Semi-permeable membrane
High solute
Low solute
97
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134
