WATER PROCESSING CYCLE
NZOWASCO employs full conventional water treatment processes
- Water sources include all rivers along Nzoia which includes Rivers; Kuywa, Kapolet, Kibisi
- Intake structures are all built along rivers for abstraction of raw water
- Raw water is pumped or gravitated to the treatment works through raw water mains
- Water is mixed with Alum
- Water is allowed to flow into flocculation basins
- Water flows into the segmentation basin
- Water flows through filtration units (Sand filters) to trap suspended materials
- Filtered water is allowed to flow to clear water reservoirs
- Chlorine is dossed into water at entry point for disinfection
- Water is now pumped to the storage tank ready for release to distribution lines
Water supply is composed of the following treatment process as illustrated on the diagram below:
Water is abstracted from river with the intake at falls that act as a natural weir. Intake is essential to divert some water from rivers or lakes in to transmission conduits of the water works system. The intake is located upstream at a higher ground so as to utilize the gravitational flow that reduces the cost of operating the treatment plant.
It is important to consider the location of the intake where highest quality of water can be abstracted and where minimal human activities of the surrounding land can interfere with water quality. The intake source however has some challenges where pollution of water due to agricultural activities occurs especially during the rainy seasons.
The intake works consist of an intake chamber fitted with a bar screen and with 3no valve chamber. Water is conveyed through the intake where the treatment of water process begins.
Screening process is done to remove large debris such as sticks, leaves, trash, and other large particles, with may interfere with subsequent filtration process. The intake is installed with a course and a medium screen.
Chemical mixing and dosing building
Inside the chemical mixing and dosing building there are a number of facilities that are used to make the first chemical treatment process to raw water after screening. They include three Alum mixing tanks, two soda-ash mixing tanks each equipped with a motor for stirring purpose, also from each tank there is a dozer that is used for dosing Alum and Soda Ash at the rate determined from jar and pH tests done daily at 8:00am and 6:00pm. At the back of the mixing tanks, there is a store to keep bags of Alum and Soda Ash.
Raw water was dozed with Soda Ash solution if its pH level was less than 6.5 to raise it to around 8.0 hence making it alkaline that ensures coagulation and flocculation processes work effectively. Then it’s dosed with Alum to allow to allow the formation of flocs. To ensure there is maximum mixing of chemicals with raw water a wedge was created along the channel after the dosing pipes that changes the lamina flow into turbulent flow. Water then flows into flocculation basin.
Flocculation is the process which clarifies the water, clarifying means removing any turbidity or colour so that the water is clear and colorless. Clarification is done by causing a precipitate to form in the water which can be removed using simple physical method. This is achieved by passing water mixer into flocculation chamber that has a series of baffles that utilizes the principle of up and down flow from the first chamber to the third one, to ensure thorough and homogeneous mixer. In the process the precipitate forms as very small particles. Small particles that where originally present in the raw water absorb on to the surface of these small precipitate particles and so get incorporate into the large particles that coagulation produces. In this way, the coagulated precipitate takes most of the suspended matter out of the water. The flocs formed can now be allowed to settle down by directing it to the sedimentation basins.
Water exiting the flocculation basin enters the sedimentation basin, also called a clarifier. It was located closer to the flocculation basin in such a way that the transit between does not permit settlement or flocs break up. The basins are three in number and are rectangular in shape such that water moved from one end to the other. The basins are deep enough to ensure that they retain water for a period of 8 hour, to reduce the horizontal velocity of water to give flocs time to settle.as particles settle to the bottom of the basin, a layer of sludge is formed on the floor of the tank. This layer of sludge is weekly removed and from each tank, through the process called disladging before it was rechanneled to the river. Sedimentation basin outflow was typically over a weir so that only a thin top layer that furthest from the sediments exist.
Filtration is the process of passing the water through sand medium. After separation most of the flocs water is filtered as the final step to remove remaining suspended particle and unsettled flocs. A rapid type of filtration is used. The effective size of sand used is 0.45mm to 0.7mm. The sand is placed in three layers, the top layer is the fine sand, the medium sand in the middle and course sand at the bottom. This type of filtration removes turbidity but not bacteria, colour, odour and tasteless water characteristics. As water pass through the top most layer most of the fine particles that escape from the sedimentation basin is trapped here. For cleaning of rapid sand filters back washing is used.
Disinfection is the process of killing of pathogenic bacteria. Chlorine is added to the filtered water to disinfect the water free of viruses, bacteria (Escherichia coli, campylobacter and shigella), and protozoa. The filter water is directed to the chlorine dosing chamber which has an open channel where dosing pipes from the chlorine mixing room is dozed at the rate of 100ml/second. Soda ash is dozed when the pH is below 6.5 to boost the to about 7.5, this is to avoid the acidic effect that result in corrosion to supplying pipes and health complications to consumers suffering from stomach problems. Residual chlorine of 0.2mg/liter is allowed to safeguard against contamination of water during distribution.
Clear water tank
Filtered chlorinated water enters in to the clear water tank of capacity 1500m3 tank for storage before it is distributed to the consumers. The tank has been design in such a way that water drops through some height before it falls to the ground of the tank. Bacteria, colour, odour are removed. The water has tasteless required characteristics after the process of mixing the chemicals with water. The tank has several partitioning walls inside for maximum mixing of chlorine and soda ash and a retention contact period of 30-45minute before it is pumped to a distribution tank.
Distribution of treated water
The system of water distribution is combined gravity and pumping system. Water is pumped from the clear water tank located in treatment works to the distribution tank 3350m3 and supplied by gravity to customers in its environs. The system of supply used is intermittent system. This is because water is supplied during the day and closed during the night where there is less usage of water. The type of distribution system is both combined Ring/Circular and Dead End Systems. Ring/circular system exist in the town Area while Dead End system exist in the extended areas away from the town.