4.10.1.2 - Potable water and water analysis

Potable water and chemically pure water

Water quality describes whether a sample is suitable for a particular use, rather than simply whether it looks clear.

Potable water is water that is safe to drink. It must contain sufficiently low levels of dissolved salts and harmful microbes. Microbes are microscopic organisms; some cause disease. A sample can be clear while containing dissolved substances or harmful microbes that cannot be seen. A pH close to 7 does not prove that the sample is safe either.

Potable water is usually not chemically pure. It contains small amounts of dissolved substances at levels suitable for drinking. Groundwater can dissolve mineral compounds as it moves through rocks. Those dissolved ions do not make it automatically unsafe; their identity and concentration matter. Conversely, removing visible particles does not establish that dissolved salts or microbes have been removed.

The method used to make potable water depends on the starting supply and local conditions. A community with accessible fresh water usually needs different treatment from one relying on seawater. Energy supply, equipment, contamination and available quantities affect the choice. Treatment must address the actual impurities: filtering sand particles and removing dissolved salt are different separation problems.

Producing potable water from fresh water

Fresh water has relatively low levels of dissolved salts, so its treatment usually focuses on particles and microbes.

In the United Kingdom, rain supplies fresh water that collects in the ground and in lakes and rivers. The first step is to choose an appropriate source: a supply with manageable contamination and sufficient quantity. Groundwater has often passed through soil and rock, which can remove suspended particles, but it can still contain dissolved substances and microbes. River water may contain suspended soil or contamination from upstream activities. Neither should be assumed safe without treatment and checks.

Water is passed through filter beds, such as layers of sand and gravel, to remove suspended insoluble particles. These particles are trapped while water passes through. Dissolved ions are much smaller and remain in the water; ordinary filtration does not desalinate it. Removing solids also helps later treatment reach microbes effectively.

The filtered water is sterilised to destroy harmful microbes. Chlorine and ozone are chemical sterilising agents; ultraviolet light can also be used. Filtration and sterilisation therefore have distinct roles: filtration removes particles, while sterilisation reduces the microbial danger. A statement that water is “cleaned” does not explain which problem the step solves.

Groundwater with low dissolved-salt levels commonly needs these steps rather than an energy-intensive salt-removal process. An unusually contaminated source can require further specialist treatment. The choice is based on the sample, not a rule that all groundwater is automatically potable.

Desalination by distillation and membranes

Salty water needs removal of dissolved salts before it can provide a suitable drinking-water supply.

Desalination means removing salts from salty water or seawater. It may be necessary where fresh-water supplies are limited. A filter bed cannot perform this job because it mainly retains suspended particles rather than dissolved ions.

In distillation, water is heated to form water vapour. Dissolved salts remain behind because they do not evaporate with the water under these conditions. The vapour is cooled and condenses to liquid water, which is collected separately. Heating and boiling large quantities of water requires substantial energy. The process separates a volatile solvent from non-volatile dissolved substances; it does not simply collect the salty water after warming it.

Reverse osmosis uses a membrane that allows water through while preventing most dissolved salts from passing. High pressure forces water from the salty side through the membrane, leaving a more concentrated salt solution behind. Pumps providing that pressure use energy. Membranes need suitable pretreatment and maintenance so particles or deposits do not block them.

Both methods require large amounts of energy, although their exact costs depend on the design and local supplies. Distillation mainly needs heating; membrane processes need pressure. Desalination produces a concentrated salty waste stream that must be managed. Compare this with groundwater that already has low salt levels: filtering and sterilising that source generally avoids the additional salt-separation demand. A treatment route must still deliver appropriate quality after any separation.

Singapore's Tuas South Desalination Plant opened in 2013. PUB describes its use of ultrafiltration and reverse osmosis. The two operations solve different problems: pretreatment removes particles that could block the membranes, while reverse osmosis separates water from dissolved salts. A large plant therefore combines methods rather than assuming that an ordinary particle filter can desalinate seawater.