The treatment of water/wastewater, in general, means the partial reduction or complete removal of excessive impurities present in water/wastewater. Physical, Chemical or Biological means are applied for the required degree of treatment, and the treatment units are designed to carry out specific functions on the principles of either one or combinations of means employed.
The type of combination used from the available unit operations and process for treatment of particular water/wastewater is defined as the water/wastewater treatment system. The treatment systems include:
- Preliminary Treatment System
- Primary Treatment System
- Secondary Treatment System
- Tertiary or Advanced Treatment System
The preliminary treatment system includes:
- Screen Chamber
- Grit Chamber
- Skimming Tanks
Primary treatment system addresses all the units of preliminary treatment system and primary sedimentation tank (PST), also known as primary clarifier. When only these units are provided for treatment, it is called primary treatment system of wastewater.
In the primary treatment system, the removal of most of the large floating materials take place in the screen chamber, and most of the heavy suspended solids are separated in the grit chamber.
The Primary Clarifier (PST) then reduces about 60-70 % of the fine settleable suspended solids, which include about 20-30 % of organic suspended solids. It should be noted that colloidal and soluble (dissolved) organic content of wastewater is not removed in this system.
After primary treatment, if wastewater is further treated for the removal of colloidal and soluble organic matter present in the wastewater, then it is called a secondary treatment system of wastewater.
Normally, biological processes are employed to remove the remaining soluble and colloidal organic content. The treatment system provided usually consists of Activated Sludge Process (ASP), or Trickling Filters and a Secondary Settling Tank (SST) also known as the secondary clarifier in a conventional treatment plant.
Other biological treatment units usually provided for secondary treatment to cater specific needs include:
- Waste Stabilization Ponds / Oxidation Ponds (WSP)
- Oxidation Lagoons / Aerated Lagoons (AL)
- Oxidation Ditches / Extended Aeration System (OD)
- Rotating Biological Contractors (RBC)
- Membrane Bio-Reactor (MBR)
Note: When both primary and secondary treatment systems are provided, then it is generally known as complete treatment of wastewater, because the quality of final effluent of wastewater obtained normally satisfies the prescribed limits set by the authorities for disposal into the receiving water bodies.
Effluent from the secondary treatment system is further treated to reduce or remove the concentrations of residuals impurities; it is called a tertiary or advanced treatment of wastewater. The system is normally provided when it is found that:
- The quality of conventionally treated wastewater (secondary effluent) is unsuitable for final disposal requirements into the water body or onto the land.
- The concentration (s) of residual organic materials or suspended solids require further reduction or complete removal for specific reuse or recycling of wastewater after the secondary treatment.
Different techniques are available for the tertiary treatment:
- Granular Media Filtration, Ultra Filtration
- Biological Nitrification/De-nitrification
- Biological and Chemical Process
- Ion Exchange, Reverse Osmosis, Electro Dialysis, Chemical Precipitation
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