1. Lecture (8): Purification of groundwater, surface water,

2. Lecture (8): Purification of groundwater, surface water,
Lecture (8): Lecture objectives:
By the end of this lecture students should be able to:
To know the water treatment processes.
To know the water disinfection.

3. Lecture (8): Purification of groundwater, surface water,
The methods used for purifying water depend on the volume and quality of the water to be treated. Where underground supplies of good quality are involved, simple treatment. Surface supplies usually require more extensive treatment, For some waters, additional processes such as softening, activated carbon filtration are employed. Where the local health authority has requested it.

4. Treatment processes: (1) Storage:
Storage of polluted water in reservoirs has been a recognized treatment process for many years. Because of low nutrient levels and natural predation, during long periods of storage pathogenic bacteria decay to relatively low numbers. Suspended matter also settles out-. A significant reduction in nitrate levels may occur by the action of bottom muds (the benthic layer), and hardness values can change. Mixing devices are installed in some reservoirs to aerate the water and overcome this problem. Aeration devices can be fitted on the bottom of reservoirs, or pumps can be fitted with jetting nozzles to induce turbulence.

5. (2) Screening:
Virtually all intakes to water supply systems are screened by filters of varying sizes. The devices used vary in size from simple bar types to complex microstrainers. The nature of the screening is at least partly determined by the quality of the raw water entering the treatment works.

6. (3) Slow Sand Filtration:
This process, which was one of the earliest and best has largely been superseded by chemical coagulation and rapid sand filters. Slow sand filters consist of large open filter beds constructed with impermeable walls and floors and containing about 1 m of fine sand laid on 0.3 m of shingle with suitable drainage tiles at the bottom. Filters work under a head of about 1 m of water, percolating through at a rate of 100–300 l/m2 per hour.

7. Coagulation, Flocculation And Sedimentation
These are the processes used for the removal of impurities in most water treatment works. Coagulation:Is the process of aggregation of the colloidal-sized negatively charged particles in the raw water. It is brought about by the addition of coagulants, usually either aluminium or ferric sulphate or polyelectrolytes, which have positively charged ions. The effectiveness of the process is also dependent on the pH over a range of values of alkalinity. The end result of this process is a relatively dense precipitate known as floc. These larger particles will settle out more readily or be more effectively removed by filtration than the original particulate and organic contents of the water. Sedimentation is achieved in tanks, of which there are many designs. Up-flow rates vary from 1 to 3 m/h, but higher rates can be used with some designs.

8. (5) Rapid Sand Filtration:
Sedimentation is usually followed by rapid sand filtration to ensure removal of any floc carried through with the decanted water. Rapid gravity filters are up to 40 times as fast as slow sand filters and normally employ filtration rates up to 20 m/h with peak flows as high as 50 m/h. They do not however employ any biological activity and merely act as physical filters. The filter normally consists of a large open tank containing a bed of sand 1–1.2 m deep supported on layers of coarse sand and gravel.

9. (6) Disinfection 6:1: Chlorination
Chlorine is used as a disinfectant almost universally in the water industry to ensure that water is free from harmful microorganisms. It is added either as a gas or as a solution of sodium hypochlorite. When the source is very pure, for example borehole water, the dose rarely exceeds 0.5 mg/l and the process is known as marginal chlorination. It is therefore usual to add just enough chlorine to overcome the chlorine demand and to maintain a small free-chlorine residual. This amount of free chlorine is 0.1–0.2 mg/l. This process is known as breakpoint chlorination. Although it needs to be sufficient to ensure effective bactericidal activity, the amount of free residual should be low enough to leave no taste in the water.

10. 6:2: Ultraviolet (UV)
6:2: Ultraviolet Irradaition Shortwave ultraviolet (UV) radiation at around 254 nm is effective against most microorganisms and may be used to disinfect water supplies. This UV spectrum is harmful to the eyes and skin and manufacturers’ safety precautions must be adhered to. UV treatment has no smell associated with the treated water but there is no residual effect, so it is usually used for smaller supplies with short pipe runs. It is therefore popular for private water supplies.

11. 6:3: Ozonization Ozone
The triatomic form of oxygen (O3), water treatment plants throughout Europe have used this process for protozoa and virus elimination, as well as the removal of pesticides and other organic chemicals. Ozonization is relatively expensive when compared with chlorination and has no residual effect, but it has some advantages:
ozone is better at removing viruses and protozoan parasites such as Cryptosporidium and Giardia than chlorine; it can be very effective in removing colour from moorland water; organic chemical, tastes and odours are removed; there is no smell associated with the treated water.

12. (7) Removal of Iron, Manganese and Aluminum
Depending on the source of the water and the resulting level of contaminants, other treatment may be necessary. High manganese, aluminium and iron levels for instance, occur in upland surface waters that are low in pH. Balancing the pH with the addition of potassium permanganate or aeration will change the metal ions so that they precipitate out. This is followed by sedimentation or filtration, if necessary in two stages.

13. (8) Removal of tastes and odours:
These may be reduced by the use of granular activated carbon, where the aromatic materials adhere to the charcoal or by ozonation.

14. (1) Fluoridation
Sodium fluoride is added to the public supply at the request of the health authority in order to reduce the incidence of dental caries.