From rain to reservoir the story of water is the ultimate story of recycling. A detailed visit plan is available for this topic. The day can include an introductory presentation on water by Museum staff and up to seven water-based activities. A dedicated corner of the exhibition contains models and display boards showing Water story (see below) and giving a number of fascinating facts about our dependence on and use of this precious resource.
The heat from the sun evaporates water from the sea and forms clouds in the sky.
When these clouds come inland and pass over high ground rain can fall. The water on the ground drains into ditches and then into rivers from where it can be pumped to and stored in reservoirs.
The water can then be pumped from the reservoir to a water treatment plant where it is rendered safe and palatable to drink. It is then pumped into the distribution network.
Sewage works are usually sited near to a stream or river so that the effluent from the treated sewage can easily be piped from the sewage works to the river.
As the effluent mixes with the river water and flows downstream further treatment occurs naturally as oxygen from the air is absorbed into the water.
Also some remaining pollutants in the water are taken up into the plant life in and along the edges of the river.
After an appropriate distance down stream, (a few miles) the water is of sufficient quality that it may be extracted and pumped back to a reservoir.
Clearly this is an excellent example of recycling and is vital in a densely populated country as there would not be enough water from natural rainfall to provide all of the fresh water that is required.
As the effluent mixes with the river water and flows down stream, further treatment occurs naturally as further oxygen from the air is absorbed into the water. Also some remaining pollutants in the effluent are taken up into the plant life in and along the edges of the river.
The treatment of sewage can take many forms. The processes displayed here are just some of the methods used to render the safe disposal of sewage into waterways. Sewage is first passed into settling tanks where it remains stationary so that the sludge settles to the bottom and grease and foam float to the top.
The grease and foam are swept off, and the liquid is pumped to the aerobic treatment tanks.
The sludge is pumped to the sludge processing tank.
In the aerobic treatment tanks the liquid is sprayed over a bed of coke from a rotating boom while air is blown up through the bed.
The porous nature of the coke provides a large surface area for the interaction of the oxygen in the air and the liquid passing down through the bed and this kills off many pathogens (germs harmful to human health).
The treated fluid then passes on to the Anaerobic treatment tanks.
The fluid from the aerobic tanks then pass to the anaerobic treatment tanks.
The liquid remains in these tanks for a period of time where it remains starved of oxygen and the absence of oxygen kills other pathogens.
Sometimes other treatments are used to render the liquid completely safe so that it can be released into rivers and thence to the sea.
The treatment of sewage is so advanced these days that in many instances, when the treated liquid is released into rivers, and after it has flowed some miles down stream, it may be pumped back again into reservoirs and reused.
In some large treatment works the wet sludge from the settling tanks is pumped from the settling tanks to a sludge processing chamber where it is heat treated to dry it, this also produces methane gas.
The gas is then stored in a methane gas tank and may be used in a gas fired power station which helps to run the various pumps etc. in the plant, and surplus electricity can be exported to the national grid.
The dried sludge may then be stored in a treated storage hopper and later transported away for disposal. It can also be used as a fertiliser for some agricultural purposes.
In actual fact water is not purified, but is made fit and safe to drink by a series of processes.
The various treatments used in a particular waterworks depends on the source of the water and can vary during the course of the year, because the nature of the water changes. It is probably true that no two waterworks are the same.
The first treatment involves removing solid materials from the water and one way in which this is done is with a drum filter.
The second treatment involves removing fine particles from the water which is known as clarifying the water.
The third treatment involves various chemical treatments of the water to adjust the acidity and or hardness of the water, if necessary, and finally to disinfect the water by killing bacteria using chlorine in one form or another.
Water supplied by water companies is not pure water. Raw water, which may be direct from rivers, or reservoirs fed from rivers, or from bore holes, is treated in a series of processes to render it clear, of acceptable taste, and above all for it to be safe to drink.
Nearly all water treatment plants are different because raw water supplies come from all sorts of different sources and can also vary with the time of year.
There are many different ways in which the above processes may be performed some of which are illustrated on the diagrams below and described in the accompanying text.
Debris can be removed from the raw water using a drum filter. This consists of a fine mesh filter supported on a drum shaped frame which rotates slowly.
Raw water is fed through the mesh at the bottom of the drum where debris is trapped on the inside of the mesh. At the top of the drum clean water is fed through the mesh from the outside so as to remove any debris trapped on the inside of the mesh which can then be carried away to a sludge lagoon.
The principle of operation of this type of filter is that a heavier than water chemical, such as ferrous (iron) sulphate, is fed into the incoming turbid water at the inlet at the bottom of an inverted pyramid shaped chamber. As the water rises in the chamber it gradually slows down and at a particular level the downward force of the chemical equals the upward force of the rising water and a "sludge" blanket layer is formed. This tends to trap particles in the water as it rises through the "sludge" blanket leaving the water much clearer. This is collected at the top of the chamber where it is run off to the next process. A drain at the "sludge" blanket level is gradually run off taking the trapped particles with it.
This type of filter works by passing the water down through a bed of sand. Water is fed in through the inlet channel at a controlled flow rate so as to maintain a fixed height of water over the sand bed which forces the water down through it. As the sand bed picks up particles from the water the flow rate slows down and at a particular point the water flow is stopped. The remaining water gradually filters down through until it is just above the sand level.
Air is then blasted up through the bed to dislodge the particles and the water is passed back up through through the bed to clean it. The backwash water passes out and is stored in a lagoon where the particles gradually settle out over time and the water can be used again.
The last process in water treatment is to render the water safe to drink and all bacteria in the water must be killed. To do this chlorine, in one form or another, is used.
The water to be disinfected is passed through a long channel where it flows under and over a series of weirs to ensure it is thoroughly mixed with the chlorine gas that is introduced as the water enters the channel.
18 parts per million by injection of sulphur dioxide gas. The water is then pumped out into the distribution network.
Domestic water use includes water for:
The average use per person per day for items 1 to 5 above is about 160 litres per day. So a household with 2 adults and 2 children will use around 600 litres per day.
Agricultural usage depends upon the type of farm. For example dairy farms will use about 200 litres per day per cow. This includes water for the animal to drink, water for washing down equipment and water for milk cooling purposes. So the use for a herd of 50 cows will be around 10,000 litres per day. Some crops have to be irrigated which can use a considerable amount of water, but in some cases water can be extracted direct from rivers under license. Some farms have their own reservoirs for irrigation purposes.
Beer is about 90% water. A beer barrel holds about 30 gallons of beer, which is 240 pints which is around 140 litres. One brewery in East Anglia produces around 230 barrels per day which requires over 32,000 litres per day. On top of this a considerable quantity of water is required for cleaning and sterilising and also for cooling purposes.
Use depends upon the type of industry. Some examples are. Steel making requires around 300,000 litres of water per tonne of steel produced. But with good recycling practice, fresh water requirements may reduce to 90,000 litres per tonne of steel. Paper making uses considerable quantities of water, anywhere between 5 and 15 cubic metres (5000 and 15000 litres) may be used to produce 1 tonne (1000kg) of paper depending upon the type of paper. Industrial production of food also requires considerable quantities of water.
Water is required in commercial and office premises for sanitary purposes for the staff working in the premises and also for food preparation where such facilities are provided. Other large users of water are hospitals, laundries, hotels, and restaurants.
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