Waste Water Treatment Methods
1. What is Wastewater and where does it come from ?
Wastewater, also known as sewage can be defined in the following way:
Used water that has come into contact with variety of contaminants and concentrations with adversely affected in quality is not fit for human use or environment is called as wastewater.
Waste water majorly comes from:
Wastewater that comes from human and household wastes from kitchen sinks, baths, toilets, showers, laundry etc. is termed as domestic/sanitary wastewater.
Waste water which contains toxic chemicals and other wastes from industries, factories, mills etc. is known as industrial wastewater.
A source of waste water which comes from schools, hospitals, offices, hotels, restaurants, airports and many such called as commercial wastewater.
Wastewater that flows from areas such as roofs, parks, gardens, roads and gutters into drains, after rain is termed as stormwater.
2. Why Wastewater treatment is essential?
Wastewater treatment or sewage treatment is a process to improve the water quality, removing some or all of the contaminants, making it suitable for reuse or discharge back to the environment.
Generally, untreated wastewater contains high levels of organic material, numerous pathogenic microorganisms, as well as nutrients and toxic compounds that can be harmful to human health, environment and waterways, hence effective treatment of wastewater is very much essential.
The major goal of wastewater treatment plants is to eventually produce water that can be reused for various purposes or disposes of in a more ecological and healthy way. Waste water treatment is a major element of water pollution control.
3. Objectives of Wastewater Treatment:
1. To improve quality of wastewater
2. Elimination of pollutants, toxicants and many such
3. Preservation of water quality of natural water resources
4. To make wastewater usable for other purposes
5. Prevention of harmful diseases
4. Methods of Wastewater Treatment:
Wastewater treatment technologies or advanced waste water treatment methods can be broadly classified into three sub divisions and are as follows:
1. Physical treatment method: It involves removal of pollutants/contaminants by physical forces.
2. Chemical treatment method: Removal of impurities or toxic wastes through chemical reactions.
3. Biological treatment method: Ejection of pollutants by biological activities.
5. Industrial Wastewater Treatment Process:
In order to produce a clean effluent that can be safely discharged to water bodies, industrial and municipal wastewater treatment process use four distinct steps of treatment to remove harmful contaminants.
a. Preliminary Treatment of Wastewater:
This is the first step in wastewater treatment and its objective is to remove large debris, coarse solids and heavy inorganic material contained in the wastewater flow. It consists of physical operations such as:
Removes heavy solids in wastewater such as rags, paper, plastics, sticks and metals to prevent damage and clogging of downstream equipment.
It is used for separation of floatable and suspended solid particles from waste water.
Here, grit chambers are used to slow down the flow so that solids such as sand, ash, cinder and eggshells will settle out of the water and it can be removed manually or mechanically.
b. Primary Treatment of Wastewater:
Large debris and grit removed in the preliminary treatment is directed to primary treatment operations and it’s objective is the removal of settleable organic and inorganic solids by sedimentation and the removal of materials that will float (scum) by skimming. It involves physical and /or chemical operations for treatment of wastewater.
- It involves a more sophisticated tank called sedimentation tank or primary clarifier removes most of the suspended solids that will float or settle.
- Sedimentation often uses chemicals like flocculants and coagulants.
- Sludge that settles to the bottom of the clarifier is called as primary sludge and it is collected for further treatment called sludge treatment.
- In this treatment about 50-70% of suspended solids, 35% of BOD will get reduced and it removes very few toxic chemicals.
c. Secondary Treatment of Wastewater:
This treatment involves a biological process and it’s objective is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids.
- Biological treatment process for secondary treatment are classified as aerobic (in the presence of oxygen) and anaerobic (in the absence of oxygen).
- In most of the cases, secondary treatment involves biological treatment processes called Activated sludge Process. Aerated Lagoon, Trickling filters, Oxidation Pond, Rotating biological contactors.
- During this process, primary effluent enters aeration tank where air is mixed with sludge and hence many microorganisms remove biodegradable organic matter.
- A secondary sedimentation tank allows the microorganisms and solid wastes to form clumps and settle.
- After this treatment, It generally removes 80-90% of all the pollutants have been removed and large proportion of toxic chemicals are removed.
d. Tertiary/Advanced Treatment of Wastewater:
It is the final treatment stage of wastewater processing and its main objective is the removal of specific wastewater constituents which cannot be removed in previous stages and thereby increase the quality of the effluent to higher degree.
- It utilizes some form of filtration to remove higher level of suspended solids which was not possible through primary and secondary screening and sedimentation.
- Nutrients, heavy metals, specific toxic chemicals and other pollutants/contaminants are removed during this process.
- It can remove more than 99% of all the impurities from sewage, producing an effluent of almost drinking-water quality.
- It involves Disinfection which can be attained by means of physical disinfectants like UV light and chemical disinfectants like Chlorine.
- During this process significant percentage of pathogenic organisms are killed or controlled.