Pre-treatment Processes
The major pre-treatment processes include the following :

• Aeration
• Settling or Clarification

Aeration

In the process of aeration, air and water are brought into intimate contact with each other to transfer volatile substances to or from the water. The removal of a gas from water is classified as desorption, degasification or stripping. The transfer of a gas to water is called gas adsorption.

Uses of Aeration

Principal uses for aeration in water treatment include :-

• To reduce the concentration of taste – and odor causing substances and , to a limited extent, for oxidation of organic matter
• To remove substances that may in some way interfere with or add to the cost of subsequent water treatment. A prime example is removal of carbon dioxide from water before lime softening and the removal of Carbon Dioxide from water after a Cation Exchanger in a Demineralization Plant.
• To add oxygen to water, primarily for oxidation of iron and manganese so that they may be removed by further treatment
• To remove VOCs considered hazardous to public health

Types Of Aeration Equipment

• Cascade aerators
• Packed columns
• Diffused aeration
• Mechanical surface aerators

Cascade Aerators

With cascade aerators, increases in exposure time and area-volume ratio are obtained by allowing water to flow downward over a series of steps or baffles. The simplest cascade aerator is a concrete step structure that allows water to fall in thin layers from one level to another. The exposure time of air to water can be increased by increasing the number of steps and the area-volume ration can be improved by adding baffles to produce turbulence. In cold climates, these aerators must be housed and adequate provisions must be made for ventilation. As with tray aerators, operating problems include corrosion and slime and algae buildup.

Diffused Aeration

Diffusion or bubble type aerators accomplish gas transfer by discharging bubbles of air into water by means of air-injection devices. Compared with packed columns, diffused aeration provides less interfacial area for mass transfer but greater liquid contact time. On the other hand, packed columns provide a greater effective area but lower liquid volumes.

Diffuser Aerators

The most common type of equipment for diffusion aeration consists of rectangular concrete tanks in which perforated pipes, porous diffuser tubes or plates, or other impingement devices are inserted. Compressed air is injected through the system to produce fine bubbles, which, on rising through the water, produce turbulence resulting in effective water-air mixing. This type of aeration technique is often adapted to existing storage tanks and basins.

Surface Aerators

Mechanical surface aerators are used extensively in wastewater applications for supplying oxygen to water. To a lesser extent, they are used to control taste and odor problems in water treatment plant. They generally consist of an electric motor suspended on float; with a driveshaft operating a propeller located a short distance below the water surface. The water is drawn up by the blade and thrown into the air in tiny droplets so that the water can pick up oxygen.

Settling or Clarification

Clarification in Water Treatment terminology is the process of removing suspended solids from raw water.

Brief Overview of Clarifier Design

Clarifiers are sized based on the surface loading rate, overflow rate and detention time. These are selected based on several parameters such as the type of water being treated, the quality of the treated water required from the unit, the type of the unit, etc.

Brief Overview of Clarifier Components and Operation

Clarifiers will have provisions for feeding raw water, taking water out of the unit and removing the settled sludge from the unit. Some units also have provisions for dosing chemicals. One of the key parameters that has to be determined during start-up of the clarifier and also periodically during its operation is the settling rate of the solids coming into the clarifier. This is normally done with the help of jar tests, which measures the settleability of the solids in the raw water.

Brief Description of Various Types of Clarifiers

Sedimentation Basinss
In water treatment, sedimentation basins are usually rectangular type. Square and Circular basins are used as well. Sedimentation basins are used to reduce the suspended solids load on downstream water treatment units. Sedimentation basins usually have a scraper for settled solids removal.

Tube Settlers
In these types of units, tubes are placed at a 60 Degree angle. The tubes provide a very short settling distance for suspended solids and inclination of the tubes ensures the removal of the settled solids by gravity. Many such parallel shallow tubes are provided within the clarifier resulting in large surface area and a low overflow rate.
Sufficient depth is provided in the tube settler to ensure that there is sufficient room below the tubes for sludge removal equipment and for good distribution of water to the tubes. Settled solids that slide down the tubes collect uniformly below the tubes, and they must be periodically removed. Treated water is collected using pipes with holes or launders placed above the tubes.

Plate Settlers
Reduction in settling distance by providing plates stacked close to each other is the basis of design for Plate Settlers. The plates are inclined to ensure that the solids can be removed and dropped for removal as sludge. The effective settling area is the horizontal projected area of the plate. The distribution of the inlet raw water to each plate and collection of the treated water are critical in obtaining optimum performance from the unit. Settled solids that slide down the plates collect in the basin below the plates and must be removed periodically.

Clariflocculator
This is the name given to a unit wherein flocculation and settling is combined into a single unit. This unit consists of a Flocculation zone and a separate Settling zone. After the water is dosed with chemicals in an upstream unit called Flash Mixer, it comes to the clariflocculators, where it is first led to the Flocculation zone. Paddles are provided in the Flocculation zone, so that the floc mixes together and agglomerate into larger flocs.
A raker arm is provided at the bottom to scrape the sludge into the central portion of the clariflocculator for removal. Clarified water is taken out of the unit by radial or peripheral launders.

Solids Contact Clarifiers
In this unit flocculation and settling is combined into a single unit. Solids which have earlier settled in the clarifier are re-circulated to the mixing zone. These act as initiators for forming floc since there are more chances of this floc for coming into contact with particles in the raw water as also in the growth of new floc. This also results in lower requirement of coagulation chemicals since contact of this floc which has been formed earlier with new floc results in good growth of floc. Hence the surface-loading rate for solids contact clarifiers is higher than that of conventional clariflocculators. Radial Launders are used to collect treated water.
The components in a Solids Contact Clarifier include a central reaction well, recirculation section, influent pipe, and influent baffle. There are two drives in the Solids Contact Clarifier, one drives the sludge disposal rake and the other drives the turbine mixer which re-circulates the sludge. Solids contact clarifiers are also used for hardness / silica reduction along with suitable chemical addition.