Lecture # 11 Water treatment:

Atypical water treatment plant is diagrammed in Fig. below Atypical water treatment plant is diagrammed in Fig. below. It is designed to remove odors, color, and turbidity as well as bacteria and other contaminants 1- Rapid mixing 2- Flocculation 3- Stilling 4- Filtration 5- Disinfection 6- Clear well storage 7- Pumping to distribution system

3- Settling When the flocs have been formed they must be separated from the water. This is invariably done in gravity settling tanks that allow the heavier-than-water particles to settle to the bottom. Settling tanks are designed to approximate uniform flow and to minimize turbulence. the two critical elements of a settling tank are the entrance and exit configurations.

Figure below shows one type of entrance and exit configuration used for distributing the flow entering and leaving the water treatment settling tank.

The sludge in water treatment plants is composed of aluminum hydroxides, calcium carbonates, and clays, so it is not highly biodegradable and will not decompose at the bottom of the tank. After some time, usually several weeks, the accumulation of alum sludge at the bottom of the tank is such that it must be removed.

Typically, the sludge exits through a mud valve at the bottom and is wasted either into a sewer or to a sludge holding and drying pond. The water leaving a settling tank is essentially clear. Polishing is performed with a rapid sand filter.

4- Filtration Many of the contaminants in water can be removed by movement of water through the soil. this natural process to water treatment systems and developed what is now known as the rapid sand filter. The operation of a rapid sand filter involves two phases: filtration and backwashing.

Illustrating the process of rapid sand filter Water from the settling basins enters the filter and seeps through the sand and gravel bed, through a false floor, and out into a clear well that stores the finished water. During filtration, valves A and C are open. Sometimes anthracite, a type of carbon, is also used in the filter bed; it can remove dissolved organic materials. The suspended solids that escape the flocculation and settling steps are caught on the filter sand particles and eventually the rapid sand filter becomes clogged, resulting in greater head loss through the filter, so it must be cleaned. This cleaning is performed hydraulically by a process called backwashing.

The operator first shuts off the flow of water to the filter (closing valves A and C) and then opens valves D and B, which allow wash water (clean water stored in an elevated tank or pumped from the clear well) to enter below the filter bed. This rush of water forces the sand and gravel bed to expand (fluidize) and lead to individual sand particles into motion, rubbing them against their neighbors. The suspended solids trapped within the filter are released and escape with the wash water. After at least 15 min, the wash water is shut off and filtration is resumed. Treatment plants want to minimize the frequency of backwashing because it uses energy and a significant amount of water, the product of the treatment plant. In addition, this water may require treatment prior to discharge.

5- Disinfection Disinfection kills the remaining microorganisms in the water, some of which may be pathogenic. When chlorine comes in contact with organic matter, including microorganisms, it oxidizes this material and is in turn itself reduced.