SURFACE WATER TREATMENT
Water companies get their surface water mainly from reservoirs which are topped up by rivers. South Staffordshire Water PLC has two large reservoirs from which it can get surface water supplies, namely Blithfield Reservoir in Staffordshire, which is fed by the River Blithe, and Chelmarsh Reservoir near Brigdnorth, which is fed by the River Severn.
Surface Water is much dirtier than ground water and so needs a much more involved process for cleaning it if it to become wholesome, potable, drinking water.
Essentially there are seven stages to this cleaning process.
1: SCREENING
As reservoirs are open to the elements they are also open to large pieces of natural and man-made rubbish such as branches, bottle, cans, carrier bags and all sorts of other litter. The water leaves the reservoir through large pipes called OUTLET MAINS, which carry it all the way to the WATER TREATMENT WORKS. It is essential that these pipes do not get clogged up by the large pieces of rubbish as, if this were to happen, no water would get to the water treatment works. To prevent the pipes from getting clogged up a large metal net is fixed over the open end of the pipe. This metal net is known as a SCREEN and is designed to prevent the large bits of rubbish from entering the pipe. This first part of the surface water treatment process is known as SCREENING because it is achieved using a SCREEN.
Over time some debris may build up on the screen and cause it to become blocked. Hence, to prevent this from happening, periodically divers are asked to inspect the screen and clean off any debris. Some of the debris such as smaller branches and leaves will inevitably rot away and break up quite naturally and so will not cause too much of a problem.
Once it has passed through the screen the water starts its journey through the OUTLET MAINS until it reaches its destination at the WATER TREATMENT WORKS.
2: CLARIFICATION
When it arrives at the water treatment works the water could appear anything from slightly cloudy to very dirty, depending on how the reservoir has been affected by recent weather conditions such as storms and floods or prolonged hot spells where algal blooms have occurred. In any case the next task is to make the water look CLEARER, i.e. transparent, so we can see through it clearly. To do this we move onto stage two of the WATER TREATMENT PROCESS. Stage two is known as CLARIFICATION, because the aim of it is to CLARIFY the water, i.e. to make it clearer.
CLARIFICATION can be done in one of two ways, either traditionally or by using the more modern DAF Plant method. The two methods are essentially the same apart from one major difference. Traditional clarification relies on the dirt sinking to the bottom of the clarification tank, whereas DAF Plant clarification relies on the dirt rising to the top of the tank. To explain why there needs to be a difference you must think about the plant, algae, that is responsible for bad tastes, smells, (odours), and colours in the water. Algae is a plant and as such it relies on the sun for its energy. Therefore it likes to float around at the top of the reservoir where it can get the most sunlight energy. As traditional clarification relies on things sinking to the bottom it is not very good at removing algae from the water. DAF Plant Clarification on the other hand relies on things floating, which is exactly what the algae likes to do. Hence, it is much better at removing algae than traditional clarification.
So how does Clarification work?
To clarify the water a chemical known as a COAGULANT is used, an example of which is PAC, (Poly Aluminium Chloride). A carefully calculated amount of the coagulant is added to the dirty water in a large tank known as a CLARIFICATION TANK. This is mixed with the water quite quickly at first to help it to get into contact with all the small bits of dirt that are floating around in the water. After this fast mixing, known as FLASH MIXING, the mixing is slowed down. The coagulant has the effect of making the little bits of dirt that are floating around in the water to stick one another, i.e. to COAGULATE. As the bits of dirt stick together they form larger clumps of dirt known as FLOC. This is known as FLOCCULATION. The flocs get bigger and heavier and eventually get too big and heavy to float and so start to sink to the bottom of the clarification tank. In traditional clarification the flocs are allowed to settle and are removed from near the bottom of the clarification tank, with the clarified water being taken away at the top. In the DAF Plant, water which has been held under pressure and so has been allowed to absorb lots of air, is released into the dirty water in which the flocs are forming. As the pressurised water enters the dirty water the pressure difference causes the air to leave the water in the form of millions of very tiny air bubbles. (A similar effect can be seen when opening a bottle of fizzy pop where the carbon dioxide can be seen to rush upwards in the bottle). The air bubbles attach themselves to the flocs and lift them to the surface using the force of UP THRUST, i.e. they increase the buoyancy of the flocs. Hence, in the DAF Plant the dirt is lifted to the surface where it forms a thin layer about 25mm thick of scum, which is known as MOUSSE. At about 15 - 30 minute intervals a large scraping arm scrapes the mousse from the surface and the resultant sludge is transported along channels to the SLUDGE TREATMENT part of the water treatment works. The air bubbles and the rising flocs trap the algae and lift them to the surface as well. Hence, the DAF Plant is much better at removing algae than the traditional clarification method. The name DAF Plant comes from the fact that the process relies on air being dissolved in the water to float the dirt to the surface. Hence, DAF stands for DISSOLVED AIR FLOTATION.
3: FILTRATION
Once the water has been CLARIFIED it will look reasonably clear but will still have some bits of dirt and flocs floating around in it. Hence, these must be removed. If this water had been affected by algae in the reservoir then at this stage it could still have a poor taste, smell, and colour and so these problems must also be removed.
As with clarification there is a traditional method and a more modern method of FILTERING the water. In order to carry out FILTRATION the water must be passed through a device known as a FILTER. Traditionally the filters were made out of sand and gravel and were very good at removing bits of dirt from the water. They work because of the clever way they are designed and built. The gravel is laid in the bottom of the filter and has the simple job of providing a GRAVEL BED onto which the sand can sit without falling out the bottom of the filter. Hence, the filter is made of a layer of gravel at the bottom with a thicker layer of sand sitting on top of it. It is the sand which does the hard work of removing the bits of dirt from the water. The clarified water enters the filter at the top. When the filter is made the grains of sand used are all about the same size, but they are not exactly the same shape. This means that when they are piled up together in the filter they do not fit together exactly. hence, there are little gaps between the grains of sand. It is these gaps which the water, being a liquid, can find its way through, pulled through by the force of gravity. Hence, as the water enters the top of the filter it gradually starts to work itself through the gaps, passes through the sand, through the gaps between the grains of gravel and out through the bottom of the filter. The bits of dirt on the other hand are too big to get through these gaps, especially as they have stuck together in the clarification part of the process, and so they get stuck in the sand. Hence, if the dirt gets stuck but the water passes through unhindered it stands to reason that the water coming out of the bottom of the filter will look very clear. In fact it will not have any dirt left in it at all.
Something which is not removed by the sand and gravel filters is the waste that has been released into the water by the algae giving it a poor taste, smell and colour. These waste materials are dissolved in the water and so the sand and gravel filters are unable to remove them. They pass through the gaps with the water. In order to remove these problems a different type of filter is used and this is now the modern way that filtration is undertaken. Instead of having sand in the filter an element called CARBON is used. This is the element from which COAL and CHARCOAL is made. The carbon is specially made so that is in the shape of small grains just like the sand. hence, it is just as good as sand at removing the dirt. However, the carbon is specially treated by heating it up to about 900 degrees celsius which makes it 'activated'. The GRANULAR ACTIVATED CARBON as it is known is able to remove the waste which is dissolved in the water and so removes the tastes, smells, and colours from the water. It is such a useful substance that it has found a variety of uses in the home, including being in shoe inserts to help reduce the smell of sweaty feet!
In both traditional filtration using sand, and the more modern method using carbon, the force of gravity is used to allow the water to pass through the filters. Hence, the filters are commonly known as GRAVITY FILTERS.
Over time the filters will become clogged with all the dirt that they have removed from the water. Hence, periodically the filters have to washed to remove the dirt. This is done by blasting compressed air backwards through them to break up the filter beds, then forcing water backwards through the filter to wash away the dirt that has been dislodged by the air. The filters are then left to settle and are rinsed to flush away any stray dirt before being reused to filter the water.
A question here is, "Can you drink the water after filtration?"
The answer is NO. Although the dirt has now been removed, there is something in the water that has not yet been dealt with in the surface water treatment process, namely GERMS. Germs, or bad bacteria as they are sometimes known, are so small that we cannot see them. They are so small that they can easily pass through the gaps between the grains of sand or carbon. Filtration will not normally remove the germs and so if the water were to be drunk at this point in the process then it is possible that the person who has drunk it would get an unpleasant stomach upset the next day. Hence, this means that the process is not yet finished.
4: DISINFECTION
In order to remove the risk of people who drink water getting a stomach upset, the GERMS in the water must be killed. This is known as DISINFECTION and involves the use of a chemical called CHLORINE. It is the same chemical which is put in the water in many swimming pools to kill the germs in the pool water.
In disinfecting the water using Chlorine there are two main things to think about:
a: How much Chlorine should be used?
b: How long does the Chlorine need to do its job?
Only a very small amount of Chlorine is actually needed, much less than in the swimming pool. At the WATER TREATMENT WORKS the chlorine is added to the water in a CONTACT TANK and it takes least 30 minutes for the water and chlorine to pass through this tank. This is enough time for the chlorine to do its job of killing the germs.
After the water has been disinfected a small amount of chlorine is left in the water, known as the RESIDUAL CHLORINE level. This is actually very useful, as if any pipes get broken the Chlorine in the water will help to kill any germs that might find their way into the broken pipe.
After all the dirt has been removed and the germs have been killed this is the first point in the surface water treatment process where it would be safe to drink the water.
5: FLUORIDATION
One more chemical is added before the water is sent to your homes. This chemical is called FLUORIDE and is added at the request of local health authorities because it has been shown that it helps to reduce tooth decay, particularly in young people. Hence, in response to this request the water has a very small amount of fluoride added. The process of adding FLUORIDE is known as FLUORIDATION.
6: DISTRIBUTION
Once the water has been SCREENED, CLARIFIED, FILTERED, DISINFECTED AND FLUORIDATED it is ready to drink. Hence, all that remains is to use big pumps to pump the water along a very complex network made up of many kilometres of pipe work, until it eventually arrives at the homes of customers. And so the SURFACE WATER TREATMENT PROCESS enables dirty water from a reservoir to be turned into clean, wholesome, potable, drinking water which is safe and enjoyable to drink.
7: SLUDGE TREATMENT
There is one part of the process that must not be forgotten as it raises an important question, "what happens to the waste from the process?"
Waste comes from two main sources in the SURFACE WATER TREATMENT PROCESS.
They are:
a: Flocs from traditional clarifiers and the DAF Plant in the form of a sludge
b: Dirty water that comes from the filters as they are washed
To deal with this waste it enters the SLUDGE TREATMENT part of the WATER TREATMENT WORKS where a POLYMER is added to it. The polymer has the effect of thickening the sludge making it easier to extract the water from it. As in any good water conservation scheme the water which has been extracted from the sludge is returned to the surface water treatment process. The thickened sludge is stirred to ensure an even consistency at which point it looks a bit like molten chocolate in a saucepan. It is then pumped into a CENTRIFUGE which spins the sludge at high speed, (just like the spin cycle in an automatic washing machine). This has the effect of extracting more water from the sludge which is also returned to the surface water treatment process. The sludge which comes out of the centrifuge has a consistency similar to chocolate cake mix and so is known as CAKE .
The cake is deposited into a number of trailers and removed from site. Traditionally this waste would have gone to landfill sites. However, innovative thinking has brought about the realisation that the waste is actually a very good soil conditioner and so nowadays it is passed to farmers who plough it into their fields, where it conditions the soils and helps their crops to grow.
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© South Staffordshire Water 2006