Production process of potable water employed at NAWASA
The National Water and Sewerage Authority (NAWASA) obtains water from the following sources to treat and produce potable water to its customers:
- Underground borehole wells
- Protected springs
- Surface sources
95% of NAWASA’s water production comes from surface sources while borehole constitutes 5%.
Before water is consumed, it must be treated in order to make it safe for use. Depending on the source, treatment is done at varying levels. In Grenada the type of treatment most employed is the slow sand filtration process for water from surface sources. Water from boreholes and springs requires only disinfection before distributed to the public.
Surface waters are treated to eliminate turbidity (brownish colour imparted to water by dissolved and suspended soil constituents) and bacterial contaminations originating primarily from the local environment. The Slow Sand Filtration Treatment process involves collecting and transporting the raw water from the dams in the rivers to the treatment plant at which the following treatment process are undertaken:
Raw water from the river contains many impurities including very fine clay materials (turbidity) that do not settle out of water easily and therefore requires the addition of chemicals to the water in order to bring the particles together and increase their mass (coagulation) to allow them to settle out of the water.
Coagulation involves the addition of a nontoxic chemical (Coagulant), example Aluminium Sulphate, and poly Aluminium Chloride to the raw water through a rapid mixing process. This process introduces a charge to the water that results in the formation of flocks (bringing together of particles to form clumps).
The clumping of the particles which is facilitated by a gentle agitation is called flocculation.
Due to the weight of these clumps they are able to settle under the influence of gravity, this is the process of sedimentation. The three processes so far take place within one basin circular or rectangular, concrete or metallic.
The previous three processes would have addressed the issue mainly related to turbidity and to some extent the removal of some of the bacteria in the water.
The next stage is filtration, that is, the passing of the water from the sedimentation basin through a bed of sand with the sand grains having a specific size. This process removes up to 95% of bacteria in the water. Filtration is much more than a physical process of straining, but also entails biological and chemical processes. The surface of the filter bed (sand bed) is very important, since it is within the first few inches of the bed that the main processes take place.
After passing through the filter bed, the water is now clear, having turbidity less than 5 and with little bacteria. Note, turbidity is how clear the water is.
The final stage is the addition of chlorine to the filtered water which is called disinfection. This process inactivates or kills the remaining bacteria, thus rendering the water safe for human consumption. From this point the treated water is transmitted and then distributed to customers.