Potable Water Production
Without water we cannot survive. Worldwide, tremendous pressures are being placed on water resource due to increasing volumes of domestic, industrial and agricultural wastes as well as other ecological substances. The result is pollution of water sources, which in turn may contribute to a rise in water-related diseases. In fact, World Health Organization (WHO) rates poor water quality together with inadequate sanitation, as a leading cause of death in poorer communities. The quality of water used for domestic purposes and the protection of water sources are everyone’s concern.
Water Sources of Lesotho
Water moves continuously through natural cycle of transpiration, evaporation, condensation, precipitation as rain and snow, infiltration into the soil, emergence in the form of springs and run-off (rivers) and storage dams.
The source of supply for potable water is the point in the cycle where water is extracted for use. At this point in time WASCO relies on pumping which is very expensive regardless of the advantage of the country’s topography where service storage could be built and rely on gravity and less pollution.
WASCO uses different types of water sources. The main source of water for Maseru is the Metolong dam, which also serves TY, Roma, Mazenod and Morija.
The Metolong treatment plant has the Capacity of producing 75ML per day on average and maximum production of 94ML per day.
The Metolong water treatment plant uses the latest technology in water treatment as a result, it produces high quality water than the conventional plants used in the past. The presence of organic matter, algae, dissolved hydrogen Sulphide and contamination due to industrial wastes containing phenol, excessive chlorine, etc. and dissolved iron and manganese salts impart color, odor and taste to the water. But the plant is equipped with PAC dosing pump for odor control, potassium permanganate and activated carbon for algae control.
Objectives of Water Treatment
Lesotho as a developing country gradually came to realize that its water supplies are contaminated and that some treatment should be applied in order to improve their quality and to meet the demand, this is why WASCO was established.
Unlike in the past, mountain streams were always cited as examples of pure wholesome water in which trout lived under ideal conditions, since trout are very sensitive to pollution, the fact that they re found in a stream is indeed an indication that seriously pollution does not exist there. However, the quality of water in many streams and rivers in the country is not as high as it as before the development of the industrial areas. Urban run-off and industrial effluents may contain many chemical substances which could affect the health of humans and animals.
There are several types of treatment processes:
- Conventional treatment processes
- Supplementary and special treatment processes
- Flotation treatment processes
- Aeration processes
The commonly used in Lesotho Water Treatment Plants is conventional treatment process whereby water pumped from source to chemicals dosing chambers for addition of coagulants such as Aluminium Sulphate and Polyelectrolytes.
These are mixed together with water to form flocs which settle in the sedimentation tanks, clean water overflowing from the top while sludge being withdrawn from the bottom to waste, then the water flows to clarification stage where the purpose is still to settle out flocs. The following stage is filtration whereby finer particles are removed and then disinfection takes place by addition of Chlorine to kill bacteria and then pumped again to service reservoirs for distribution.
A number of tests are carried out on potable water in order to establish its suitability for drinking. Some of the tests are also conducted to all stages at treatment plant as a way of routine process control.
Water Treatment Process
Pre-settlers with the three compartments are operated to remove excess silt with retention time of two hours. This detention reduces the turbidity by 40% due to natural settlement.
Coagulation takes place in the dosing chambers after the addition of polymers . Since 6th of December 1999 Organic Polymer Coagulants are used to replace aluminium sulphate which was used in the past.
Flocculation takes place in inner zones of the primary and secondary sedimentation tanks. At this stage the particles (flocs) coalesce due to gentle stirring or agitation.
Sedimentation takes place in the outer zone of both primary and secondary sedimentation tanks when particles (flocs) conglomerate and increase in mass, then settling takes place at a faster rate. For a maximum flow rate, retention time in the primary sedimentation tank two hours. The target turbidity for primary effluent is a maximum of 100 NTU, while in the secondary sedimentation tanks; the effluent is expected to be a maximum of 4 NTU. The maximum raw water turbidity from Mohokare is 6000 NTU in rainy season and below 100 NTU in dry season. The raw water from Maqalika storage has turbidity in the range of 10-150 NTU which implies that some sedimentation has already taken place in the reservoir.
This process is carried out at the beginning and at the end of the water treatment process. At the beginning pre-chlorination is applied to sterilize the treatment plant in order to destroy and algae and to retard its growth. As raw waters are mostly contaminated to a certain extent by bacteria and viruses which may cause diseases such as hepatitis, typhoid, gastro-enteritis and cholera, post-chlorination is introduced at any of the three stages – that is after filtration,before clean water storage, and after clean water storage as water is pumped to service reservoirs. The last stage of chlorination is for the destruction of pathogenic and other organisms before water is distributed and reticulated for general use by the public. As treated water leaves the plant to service reservoir, residual chlorine must be at a range of 1.0 – 1.5 mg/l, and must be at minimum of 0.2mg/l at consumer’s tap. The resident time for chlorine effectiveness is from 6 to 8 hours.
As indicated above, WASCO treatment plants use a multiple-barrier concept whereby coagulation, flocculation, sedimentation, filtration and chlorination perform complementary roles that remove and neutralise disease-causing pathogens and other unwanted chemicals in water. Continuous analysis is conducted at these plants to ensure treatment efficacy.
WASCO routinely monitors the quality of water at consumer taps at all its centres, through regular sampling and analysis, in order to give complete assurance in the safety of drinking water. A comprehensive list of parameters is analysed, in line with World Health Organisation Guidelines for Drinking Water Quality.