Water Treatment Process

The surface water contains wide variety of contaminants and thus unfit for use. The contaminants are broadly categorized as Physical water quality parameters, Chemical water quality parameters and Bacteriological water quality parameter. Physical water quality parameters include suspended solids, turbidity, color, taste and odor and temperature. Chemical water quality parameter include total dissolved solids, alkalinity, hardness, metals, fluoride and organic, etc. Bacteriological water quality parameters include pathogens. The aim of water treatment is to produce and maintain water that is hygienically safe, aesthetically attractive and palatable, in an economical manner. The amount and type of treatment process will depend on the quality of raw water and the standards of quality to be required after treatment. The raw water from the river or an other source is pumped to the water treatment plant through rising main. The unit operation in water treatment process constitute aeration, flocculation, and clarification, filtration, disinfection etc.

Aeration

Aeration is the process treating the water to promote exchange of gases between the water and atmosphere to remove taste and odor and the dissolved harmful gases from water.
The purpose of aeration is

  • To add Oxygen to water for imparting freshness.
  • Expulsion of carbon dioxide, hydrogen sulfide and other volatile substances causing taste and odor.
  • To precipitate impurities like iron and manganese in certain forms

Coagulation and flocculation

The raw water usually contains a large variety of suspended and colloidal particles which can not be remove by gravitational force only. In order to remove very fine suspended and colloidal particles coagulation and flocculation is necessary.

The terms coagulation and flocculation are often used indiscriminately to describe to describe the process of removal of turbidity caused by fine suspensions, colloids and organic odor.

Coagulation is the process of destabilization of the charged suspended/colloidal particle’s. Operationally this is achieved by the addition of appropriate coagulant like alum and rapid intense mixing for obtaining uniform dispersion of the chemical.

Flocculation is second stage of the formation of settleable particles or flocs from destabilized colloidal particles. Flocculation is achieved by gentle and prolonged mixing of raw water.

It is a common practice to provide an initial flash mixing for dispersal of the coagulant or other chemicals into the water followed by slow mixing where growth of floc takes place.

Coagulation and flocculation are greatly influenced by the by physical and chemical forces such as electrical charges on particles, exchange capacity, particle size and concentration, pH, water temperature, electrolyte concentrations and mixing.

Sedimentation

Sedimentation is the separation of the suspended particles that are heavier than water from water by gravitational force only. Sedimentation is used to remove readily settling sediments such as sand and silt, coagulated impurities, and precipitated impurities such as hardness and iron. When suspended solids are separated from water by the mere action of the gravitational force only, it is called plain sedimentation. Finely divided solids and colloidal particles which can not be removed by plain sedimentation within commonly used detention period of few hours, are converted to settleable flocs by coagulation and flocculation and subsequently settled in sedimentation tank. The settled solids at the bottom of tank is termed as sludge which is removed at certain time interval.

Filtration

Filtration is a process for separating suspended and colloidal impurities from water by passage through a porous medium or porous media i.e. through the bed of sand and gravel. Filtration process is used in water treatment process to effectively remove turbidity, color, microorganism, precipitated hardness from chemically softened water and precipitated iron and manganese from aerated water.

Removal of turbidity is essential not only from the requirement of aesthetic acceptability but also for the efficient disinfection which is difficult in the presence of suspended and colloidal impurities that serve as hideout for the microorganism.

In this process water is passed through the bed of sand and gravel and collected through the under drainage system. After certain period the filter gets clogged causing reduced rate of flow and increase loss of head. At that time it necessary to cleanup the filter bed. It is done by backwashing. Back washing is done by passing fresh water through the bottom of the filter bed and comes out through the trough over the filter bed. The residual after backwash is again recycled back to the system.

Disinfection

The treatment process like coagulation, flocculation, sedimentation, filtration, aeration etc produce water that are aesthetically acceptable and economical to use. But for utmost safety of water for drinking purpose, disinfection is extremely essential treatment process. Disinfection is the process of killing the disease producing organisms. Bacteria, viruses and amoebic cysts constitute the three main types of human enteric pathogens and effective disinfection is aimed at destruction or inactivation of these and other pathogens such as helminthes responsible for water borne diseases. The need for disinfection in ensuring protection against transmission of water borne diseases can not overemphasize.

Modern disinfection process includes use of :

  • Physical method such as thermal treatment and ultrasonic waves.
  • Chemicals including oxidizing chemicals such as Chlorine and its compound, Bromine. Iodine, Potassium permanganate, Ozone and metal like silver.
  • Radiation.

In municipal water treatment process chlorine and its compound is mostly used. The dose of chlorine should be such that there will be residual chlorine in water at consumer end to prevent further contamination.

Storage

The treated water after disinfection is stored in a reservoir where sufficient contact time is provided for effective disinfection. The water from the storage reservoir is pumped to the distribution system or elevated storage reservoir for further distribution.