Water Treatments System

Objectives of Water Treatment

 The Water Treatments are to remove the different impurities present in the raw water, to render the water safe and clean and to ensure the water treatment process and treated water quality meets the drinking water standards. The type of water treatment required depends on the characteristics of the raw water. The characteristics of the raw water is assessed by taking sample of water from the source during different seasons of the year and analyzing for physical, chemical and bacteriological quality parameters.

Unit Operation and Process

Water treatment involves removal of undesirable constituents from water and then disposal of them in easiest and safest manner. To achieve these goals, a variety of water treatment operation and processes are utilized, which exploit various physical and chemical phenomena to remove or reduce the undesirable constituents from the water. Those operations used in the treatment of water in which change is brought about by means of or through application of physical forces are known as Water Treatment Process unit operations . Those Process used for the Water Treatment system in which is brought about by means of chemical reaction are known Unit Process.

It might be noted that several unit operations and processes discussed in various chapters of this book require techniques, understanding, and input from a wide range of disciplines including engineering, chemistry and microbiology. Each unit operation / process plays an important role at the various stages of the treatment train. The predominant role and responsibility rest with the design engineer for selecting and designing the appropriate treatment operation/ process in the overall treatment train. The chemical processes commonly used in water and wastewater treatment system.

Chemical precipitation :

Enhancement of removal of suspended solids by chemical addition.

• Coagulation in Water Treatment Process:  Coagulation is the addition and rapid mixing of coagulant resulting in destabilization of the colloidal particles and formation of micro flocs.
• Ion exchange in Water Treatment Process : The cations and anions in water are selectively removed when water is percolated through beds containing cation and anion exchange resins.
• Aeration or Gas transfer in Water Treatment Process : Addition or removal of gases from liquid phase.
• Disinfection in Water Treatment Process : Selective destruction of disease-causing organisms present in water.

Screening

Screening is a unit operation that removes floating and suspended larger material from water.

Screens may be classified as trash racks, coarse screen;. “fine screen or micro strainer depending on the size of material removed. Screen may be located at the intakes, raw water pumping stations and water treatment plant (WT P).

Trash racks

Trash racks are provided to intake ports in the intake structure to prevent large objects from entering the conveyance system. The screens consist of vertical bars with clear opening of 50 to 80 mm. The velocity of flow through the coarse screen is generally limited to less than 8.0 cm/ s.

Coarse Screens

Coarse screens in water treatment fields are used to remove smaller objects that may damage the pumps. They may be located either in the intake structure or in the raw water pump house. In the case of gravity conveyance system, the coarse screen may be provided at the Water Treatment Process. These screens consist of heavy wire mesh with 5 mm square openings or of circular passive screens with similar widths. The velocity through the coarse screen is maintained in the range of 0.4 to 0.8m/ s.

Micro strainers: 

A micro strainer consists of a rotating cylindrical frame covered with fine wire-mesh fabric. Water enters the cylinder and moves radially out. Deposited solids on the screen are removed by a jet of water from the top and discharged into a trough.

Raw water containing a heavy population of algae and plankton is difficult to coagulate. Algae and plankton usually float, because their specific gravity is less than unity. If raw water is pre chlorinated, certain species of algae produce intense taste and odor problems forming substitution products. Removal of algae prior to pre-chlorination has beneficial effects on taste and odor control. Hence, the removal of plankton and algae from the water from an impounding reservoir is essential. Therefore, micro strainer, if designed and installed before chemical coagulation, will often improve the performance of the Water Treatment Process.

Aeration

Aeration is the unit water treatment process for the exchange of gases between water and atmosphere. Aeration or gas transfer involves either bringing air or other gases in contact with water or to transfer volatile substances from the liquid to the gaseous phase. The purposes of aeration are:

• To reduce the concentration of taste and odor producing substances; such as hydrogen sulphide and various organic compounds from the water drawn from deep layers of an impounding reservoir,
• To dissolve oxygen to water for importing freshness when groundwater source is devoid of oxygen,
• To oxidize and precipitate impurities like iron and manganese in certain forms in the water from groundwater sources.

Coagulation and Flocculation

Coagulation and flocculation are one of the water treatment system processes to convert the stable colloidal particles into settle able flocs by destabilizing the charge on the colloids so as to remove turbidity from the water. Water with little or no turbidity will be clear. In addition to removing turbidity from the

Sedimentation

Sedimentation is a unit operation to settle out the suspended particles in water by gravitational force. This is achieved by lowering the flow velocity of the water below the suspension velocity in a basin to settle out suspended particles by gravity. The process is also known as settling or clarification.

Filtration

Filtration is the unit operation in Water Treatment. Filtration using a filter media to separate the suspended particles mechanically from water to render water free from turbidity.

Disinfection

Disinfection is the unit process employed to inactivate the disease producing bacteria (pathogens) present in water by addition of certain chemicals in order to render the water safe for consumption. Common disinfectant is chlorine and the process of addition of chlorine is known as chlorination.

Demineralization

Demineralization is the unit process of removing dissolved minerals and mineral salts, present in the form of mineral ions in water. This process strips out all chemical impurities present in water.

RESIDUALS MANAGEMENT

Water treatment residuals are materials that are removed in various treatment operations and processes along with large quantities of water for transport of the materials. These residuals include turbidity and color causing materials, organic and inorganic solids, algae, bacteria, viruses and precipitated chemicals. These are the by-products of chemical coagulation, iron and manganese precipitation, filter back wash, softening and regeneration brine. The total liquid content of the residuals varies from 3 to 5 percent of the volume of raw water processed. The solids concentrations in the liquid residual may vary from 0.1 to 4 percent depending on the process and on the raw water quality. These residuals cannot be discharged as such into the stream, river, or lake from which they originated because they cause disturbance to the aquatic ecosystem.

In general the residuals-management processes include settling, thickening, conditioning, dewatering, drying, recovery, and disposal. Selection of any of these processes depends greatly on such factors as space available at the site, type and quantity of sludge, local weather conditions, cost of raw chemicals, and type of ultimate disposal available.

SELECTION OF WATER TREATMENT PROCESS

The following considerations influence the selection of a treatment operation/ process train:

PRE CHLORINATION —>AERATION —> RAPID MIXING —>CLARI-FLOCCULATION —>RAPID SAND FILTRATION —> POST CHLORINATION —> PLAIN SEDIMENTATION —>SLOW SAND FILTRATION

Process flow diagram of typical treatment of surface water and ground water sources