Test Entry for Holle Polle - Waste Water - an Overview
Created | Updated Jan 28, 2002
Test for Holle Polle to read
Before we look into the various methods of treating waste water, as described by the other entries in this project, we must first ascertain what waste water is? 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?
What is Waste Water and Where Does It Come From?
Waste water must have been used, before it became waste. Generally speaking,l some manipulation or contamination of fresh water has taken place. The resulting waste water is no longer good enough to be used for certain purposes.
For example, cooling water that has performed the task of cooling something has done so by absorbing 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 cycle - waste water.
Another example: Several industrial processes require distilled or de-ionised water. Many cleansing processes use this special water to remove the slightest amounts of dust, dirt, grease, salt or whatever during the production process. In the course of this cleansing, the contamination migrates from those products into the water, which is afterwards no longer de-ionised, 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 into waste
water.
Besides this, sewage, of course, is waste water. Whenever people flush the toilet, wash themselves, use the dishwasher or washing machine, they produce waste water. It runs down pipes and waste water systems 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 streets, pavements and roofs.
To sum it up, waste water can be many different things, depending upon its origin.
What Does Waste Water Consist of That Makes It Different From Fresh Water?
Looking at sewage particularly, waste water consists mainly of water, of course. The water flowing into a sewage plant has a solid content of 1%. These include minerals, micro-organisms and hydrocarbons, which have a certain natural concentration in fresh and waste water. Fresh water, supplied by water works, is usually enriched with chlorine, in order to keep micro-organisms (especially harmful ones) from infecting people.
Apart from natural components, waste water is contaminated with many 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 dissolved. Faeces containing earthlike dirt and micro-organisms are flushed into the system.
Where Does Waste Water Go?
As already mentioned before, waste water usually runs down some kind of pipes or sewer into places where it is collected and
treated. Depending on the local infrastructure, treatment is carried out in different ways (or not at all). The related entries in this project describe the types of treatment employed.
Why Do We Want Fresh Water?
Compared to fresh water the pot-pourri of substances that is waste water is not likely to be used for washing or cooking any more. Besides, there are several other things that water can be used for , which would make the use of it undesirable. To sum it up, water has to be clarified, disinfected. Any unwanted content must be removed.
Other applications for water such as cooling or industrial cleansing processes require their waste water to be treated as well. For re-use as a 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, de-ionised water is often recirculated into cleansing processes after passing through an ion-exchanger. Ion-exchangers usually pass the water through two stages: one for exchanging cations (with a positive electronic charge) and one for exchanging anions (negative electronic charge). Cations are mostly dissolved metals while anions are non-metallic, such as carbonates, sulphates or chlorides.
Whatever the application for which it is intended, fresh water has to meet certain requirements. After use, the water no longer complies with these.
How Can Waste Water Be Converted Back Into Fresh Water?
Well, this is the question that the project as a whole shall cover. There are many ways, indeed. Re-cooling and ion-exchanging are
only two specialised ways. This project will particularly focus on biological treatment, which can be applied to municipal sewage and waste water from several types of industrial plant.
Links to Further Entries Within This Project
This entry is part of the 'Biological Waste Water Treatment' Project.
