The treatment of wastewater or industrial effluent occurs in multiple stages or steps. As the wastewater passes through these different steps, it is treated by increasingly advanced methods of treatment that filter and break down the dissolved impurities. In this article I explain what these steps in treatment are.
The process of treating effluent or wastewater can be grouped together under 4 categories or steps.
Step 1: Preliminary Treatment – The raw wastewater contains several impurities like suspended particles and solids besides oil and grease. An undesirable feature at this point is odor. All these particles have to be filtered or removed from wastewater to prevent them from interfering in the downstream treatment processes. The primary treatment methods implemented in this stage are – application of grates and screens to filter large solids, grinders or comminutors to pulverize coarse solids, grease traps to filter the grease and minimal aeration of wastewater to prevent odor.
Step 2: Primary Treatment – During this stage in the treatment process, the readily settleable solids are allowed to settle to the bottom forming a sediment layer. Hence this step is also known as primary clarification. This stage will be succeeded by biological treatment. The main method of clarification involves the use of sedimentation tanks or settling basins. Other auxiliary steps in treatment of wastewater can also be used along with this clarification process. These involve the use of fine screens for filtration apart from flocculation and flotation.
Step 3: Secondary Treatment – The aim at this stage is to purify the wastewater. This is achieved by adopting methods that encourage the microorganisms in the wastewater to decompose and breakdown the organic matter in the wastewater. Such organic matter may be in a dissolved or suspended state in wastewater. There are many ways in which this biological treatment method can be implemented. But all these methods can be categorized as anaerobic treatment method, pond or lagoon based treatment method, land treatment method, activated sludge method, or biological filtration. One of the ways in which biological filtration is carried out involves the use of biological filters in the shape of rotating discs.
Step 4: Auxiliary Treatment – This step is also known as Tertiary Treatment or Secondary Clarification. During this stage in the treatment process a wide variety of physical and chemical processes are used to treat the wastewater. The auxiliary steps in treatment of wastewater can be implemented even prior to or after the the secondary treatment. These physical and chemical processes are often implemented to achieve specific treatment targets or standards.
While designing an effluent treatment plant, several procedures for the treatment of wastewater may be considered. Which procedure is finally chosen to be implemented depends largely on the efficiency of the procedure in filtering a particular group of contaminants.