All surface waters have the potential to carry pathogenic (disease-causing) microorganisms and must be disinfected prior to human consumption. Since the adequacy of disinfection cannot be assured in the presence of turbidity, it is first necessary to remove the suspended solids causing the water to be turbid. This is accomplished by a sequence of treatment processes that typically includes coagulation, flocculation, sedimentation, and filtration. Coagulation is accomplished by adding chemical coagulants, usually aluminum or iron salts, to neutralize the negative charge on the surfaces of the particles (suspended solids) present in the water, thereby eliminating the repulsive forces between the particles and enabling them to aggregate. Coagulants are usually dispersed in the water by rapid mixing. Water purification is the removal of contaminants from raw water to produce drinking water that is pure enough for human consumption or for industrial use. There are a number of other processes that may be employed to treat water, depending on the quality of the source water and the desired quality of the treated water. Processes that may be used to treat either surface water or groundwater include:
• Lime softening, which involves the addition of lime during rapid mixing to precipitate calcium and magnesium ions;
• Stabilization, to prevent corrosion and scale formation, usually by adjusting the pH or alkalinity of the water or by adding scale inhibitors;
• Activated carbon adsorption, to remove taste- and odor-causing chemicals or synthetic organic contaminants; and
• Fluoridation, to increase the concentration of fluoride to the optimum level for the prevention of dental cavities.
Municipal waters, for example, consist of surface water and ground water, and their treatment is to be distinguished from that of industrial water supplies. Municipal water supplies are treated by public or private water utilities to make the water potable (safe to drink) and palatable (aesthetically pleasing) and to insure an adequate supply of water to meet the needs of the community at a reasonable cost.
Processes
Raw water
Raw (untreated) water is withdrawn from either a surface water supply, in the case of Alice water treatment works the source of raw water is Bin field dam. The water is pumped to a central treatment facility. Large municipalities may utilize more than one source and may have more than one treatment facility. There is a chemical storage unit; this is where the chemical coagulant is stored. The coagulants used vary according to the water quality of that particular place. In the chemical storage unit there is a regulation of how much of the water dosage is needed. This is achieved by doing a drop down test; which has a time frame of 60 seconds.
Phase 1
The treatment process is done in various phases. The first phase is that of mixing the raw water with the coagulant. Other chemicals may be added at the same time, including powdered activated carbon (to absorb taste- and odor causing chemicals or to remove synthetic chemicals); chemical oxidants such as chlorine, ozone, chlorine dioxide, or potassium permanganate (to initiate disinfection, to oxidize organic contaminants, to control taste and odor, or to oxidize inorganic At the first stage there are 2 pipes, a large one from which the water flows from the storage tank and a smaller pipe in which the coagulant is transported. The plant is designed so as to allow proper mixing. This is achieved by the channels in which it is designed.
Phase 2
The next phase is the settle tanks. The more settle tanks there are, the more the volume of the water to be treated. This is where sedimentation takes place. Firstly flocculation occurs then the density of the flocs is increased by bringing the flocs together. As soon as the density of the flocs is more than that of water they begin to coagulate the sediment. Not all the flocs coagulate but the remaining flocs are filtered in the last level of the settle tanks so that they are not passed through to the next stage of filtration. Everything settled in the tanks is found in the sludge lagoons. The term activated sludge refers to suspended aerobic sludge consisting of flocs of active bacteria, which consume and remove aerobically biodegradable organic substances from screened or screened and pre-settled water. It is important that every day the lagoon is emptied.
Phase 3
Following next are the filters. This is a further treatment process as to ensure that there is a very limited number of flocs in the water. A process of backwashing is involved in this stage. Backwashing employs the aid of sand to wash of any possible impurities in the water. If the water is tested or even appears to be turbid, then backwashing is employed. The water is emptied from the tanks into the channels and is mixed with sand. This is done by rapid mixed, so as ensure proper washing off. The water flows into the ponds then flows back into the storage tanks and start over the entire treatment process from mixing with the coagulant. If backwashing is not necessary the water transfers into the disinfection stage in the reservoir.
Phase 4
The final process involved is disinfection. Once the water has been filtered, it can be satisfactorily disinfected. Disinfection is the elimination of pathogenic microorganisms from the water. It does not render the water completely sterile but does make it safe to drink from a microbial standpoint. The most used disinfectant is chlorine, preferably in gaseous form. In instances where there is no chlorine on stock, alternatively hth tablets are used. When in granules it will be more costly because the granules run out quickly.
Maintenance
Highly trained staff is required for maintenance and trouble-shooting. The mechanical equipment (mixers, aerators and pumps) must be constantly maintained. A continuous supply of oxygen and sludge is essential Control of concentrations of sludge and oxygen levels in the aeration tanks is required and technical appliances (e.g. pH-meter, temperature, oxygen content etc.) need to be maintained carefully. As well, the influent and effluent must be constantly monitored and the control parameters adjusted.
