A brief explanation on the treatment process that takes place at Ng’ethu and Kabete treatment water works is detailed here below.
Abstraction: water is abstracted from the source, which it is usually located at a point that is remote from human activity – which normally brings pollution. From the source, water is carried through a pipe known as the intake pipe which takes the water to the treatment works.
Screening: at the mouth of the intake pipe there are a series of screens whose purpose is to prevent suspended particles from entering the system and finding their way into the pumps. This is the first stage of treatment. The water then enters a tank called a sump from where it is pumped to the next stage of treatment known as clarification.
Clarification: the main reason for clarification is to remove suspended impurities from the water. Most surface waters contain three types of impurities:
- Settle able impurities, which settle out by gravity when the water is allowed to be still
- Suspended impurities which normally remain in suspension and can not be removed settling e.g. clay particles, algae etc.
- Dissolved impurities which comprise various mineral compounds in solution e.g manganese, iron etc
The clarification process which mainly removes second category of particles involves two main processes – Coagulation and Flocculation. A chemical known as a coagulant such as Aluminium Sulphate (or Alum) is mixed in the water. The Alum distorts the properties of the suspended particles that prevent them from settling and causes them to come together (coagulate) to form larger particles called flocs through the flocculation process. As the flocs become bigger and heavier they begin to settle to the bottom of the clarifier from where they are removed. Water leaving the clarification stage is clear but not yet safe for drinking because the addition of alum leaves it slightly acidic and still contains micro – organisms.
The water then moves to the next stage of treatment known as filtration. Filtrar=tion involves causing the water to follow by gravity through graded sand media to remove any suspended particles that may have passed the clarification stage.
Disinfection: This stage of treatment follows filtration and its main purpose is to kill all harmfull micro- organisms that could be in the water. Through disinfection the colour and smell of the water is also improved. The process involves mixing of the water with a saturated solution of a chlorine compound or chlorine gas. The applied chlorine dose ensures that not only are the germs in the water at the time of application are killed, but also those from subsequent contamination of water during distribution and storage will also be killed. This explains why on some occasions water, especially that from kitchen taps which directly connected to supply pipes, has a characteristic smell of chlorine.
After disinfection the water is allowed to settle in a tank known as the Clearwell for a specific period called the retention time during which the chlorine does its intended work. In the Clearwell Soda Ash is added. This the final stage of water treatment.
Water is then pumped to reservoirs which are usually located at a high point from which it is distributed to the public.