Water purification processes

Water Treatment

The main objective of a water treatment plant is to produce water that meets drinking water quality standards at a reasonable cost to the consumer. In a treatment plant, various processes are used to produce water of the desired quality. These processes are generally divided into two general categories: operational units and process units.

Operational water treatment unit

In the operational unit, the removal of contaminants is carried out by physical forces.

Process water treatment unit

In the process unit, the treatment of water is carried out by chemical and biological reactions.

Most of the terms operational units and process units are used interchangeably. This is because many processes are completed by a combination of operations. For example, the removal of turbidity in the coagulation process is carried out by a combination of chemical addition, mixing and dispersion, flocculation, and sedimentation.

The general arrangement of the various process and operational units is called the process chain, flow chart, general diagram, or general scheme of the treatment. The main considerations for developing an overall treatment pattern depend on the characteristics and seasonal variations of the raw water quality, legal restrictions, site characteristics, unit economics, and many other factors. The goal is to remove undesirable components from the water and to process the waste in a way that can be easily and properly disposed of or reused. The following is an overview of the various operational units and processes used for waste treatment and processing.

Raw Water Intake, Pumping, Conveying and Metering

Water treatment plants may use systems to ingest, pump, convey and meter raw water. Although these units do not perform any treatment, they are essential as part of the overall water treatment process.

Raw Water Intake

Raw water intakes are constructed to extract water from a river, lake or reservoir from a specified height. The structure of this unit can vary from a simple submerged intake pipe to complex tower-like units that house valves, strainers, control valves, pumps and chemical feeders. Intakes may be submerged, floating or tower-mounted.

Tower-mounted intakes are used in large projects and may be part of a dam or a separate structure at another location. Submerged or floating intakes are used for small water supply projects.

Raw Water Pumping

Raw water pumping stations are usually located in the intake building. The purpose of pumping is to raise water from the source to a height sufficient to allow it to flow by gravity. The pumping height of raw water is the sum of the static head, the pressure drop in the main pipeline, and any partial losses. Centrifugal pumps are most commonly used in water pumping. These pumps may be suspended, submerged, or dry sump.

Raw water conveyance

A raw water conveyance system is designed for the controlled movement of water from the intake to the treatment plant. Typically, the treatment plant is built near a city. Therefore, long lines are required to convey water from the source to the treatment plant. Topography, available elevation, construction materials, economics, and water quality are the primary considerations in selecting a suitable route and conduit. The conduit may be a canal, stream, tunnel, or pressurized pipeline, or a combination of these.

Flow measurement

Measurement of the flow rate of raw water and treated water is essential for operation, process control, billing, and data recording of the treatment plant. Flow measuring devices may be installed on the raw water transmission line, on the main distribution lines, or at other locations within a facility. Often, more than one measuring device is required in different facilities. In general, water flow in pressurized pipes is measured by mechanical height-measuring devices such as venturi meters, nozzles, or orifices. Flow in an open channel is measured by a weir or venturi-shaped flume such as a partial flume.