What is waste water? What does it consist of? Where does it come from? Where does it go? What makes it different from fresh water? Why do we want fresh water - and not waste water? And finally: How can waste water be converted into fresh water again?
These are the general questions posed and answered by this entry.
What is Waste Water and Where Does it Come From?
Waste water must have been used before it became waste. In general, some manipulation or contamination of fresh water has taken place. Waste water is no longer good enough for being used for a given purpose.
For example, cooling water that has performed the task of cooling something has done so by receiving heat from whatever it was supposed to keep or make cool. In other words: it got warmer. Warm water is not good for cooling any more. It is - in terms of the cooling task - waste water.
Another example: In several industrial processes distilled or deionised water is needed. Many cleaning processes use it in order to remove slightest amounts of dust, dirt, grease, salt or whatever from the desired products. By cleaning, the contamination migrates from those products into the water, which is afterwards no longer deionised, but contaminated. Even if the amount of certain ions may be lower than that in natural or fresh water, it may no longer be pure enough for the cleaning operation - it has turned to be waste water.
Besides, waste water is sewage, of course. Whenever people are flushing the toilets, wash themselves, use the dishwasher or washing machine, they produce waste water. It runs down pipes and canals into places where it might be collected and treated, depending on local infrastructure. In addition to this, rain washes dirt, oil, dust and garbage off from streets, pavements and roofs.
To sum it up, waste water can be many different things depending on its origin.
What's the Difference Between Waste and Fresh Water?
Looking at sewage particularly, waste water consist mainly of water, of course. Furthermore there are minerals, micro-organisms and hydrocarbons, which have a certain natural concentration in fresh and waste water. Fresh water, supplied by water works usually is enriched with chlorine, in order to keep micro-organisms (especially harmful ones) from infecting people.
Apart from natural components, waste water is contaminated with different things, such as dirt and dust and pieces of garbage. Oils, fat and grease cover surfaces or are emulsified by surfactants. Acids or bases, eventually forming and reforming salts, are soluted. Faecal matter is flushed, containing earth-like dirt and micro-organisms.
Where Does Waste Water Go?
As already mentioned before, waste water usually runs down any kind of pipes or canals into places where it is collected and treated. Depending on local infrastructure, treatment is performed in different ways (or not at all). How this treatment can be done, is the subject of related articles.
Why Do We Want Fresh Water?
Compared to fresh water, the potpourri provided with sewage is not likely to be used for washing or cooking any more. Besides, there are several other things that water can be used for . To sum it up, water has to be clarified, disinfected. Any unwanted content must be removed.
Other applications of water such as cooling or industrial cleaning processes require their waste water to be treated as well. For the reuse as cooling medium warm water has to be cooled down again. This is often done in those huge cooling towers that are commonly located near power plants.
Contaminated deionised water is often recirculated into cleaning processes after passing an ion-exchanger. Ion-exchangers usually consist of two stages: one for exchanging cations with a positive electronic load and one for exchanging anions, carrying negative electronic loads. Cations are mostly soluted metals while anions are non-metallic, such as carbonate, sulphate or chloride.
Whatever application you look at, fresh water has to meet certain requirements, which are usually no longer met after application.
There are many ways of converting waste water back into fresh water; indeed. Recooling and ion-exchanging are only two methods. This project particularly focuses on biological treatment.