Q.What is the key difference between primary and secondary sewage treatment?
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Start your 14-day free trial to unlock the full solution →Primary sewage treatment physically removes large and small solid particles, while secondary treatment biologically degrades organic matter using microbes.
Untreated sewage, or wastewater, poses significant health and environmental hazards. It contains a vast amount of organic matter, along with numerous pathogenic microbes that can cause serious diseases if released directly into natural water bodies. To mitigate these risks, sewage undergoes a multi-stage treatment process in Sewage Treatment Plants (STPs) before it is discharged. This treatment aims to reduce its polluting potential, making it safer for the environment. The initial two crucial stages in this process are primary and secondary treatment, each targeting different components of the sewage.
Primary Treatment: The Physical Stage
Primary treatment is fundamentally a physical process designed to remove large and small solid particles from the sewage. It acts as the first line of defense, essentially cleaning the wastewater of its most obvious contaminants.
The process typically involves two main steps:
- Sequential Filtration: The raw sewage is first passed through a series of screens. These screens physically block and remove floating debris such as plastic, rags, sticks, and other large suspended solids. This step prevents larger items from damaging subsequent treatment machinery.
- Sedimentation: After filtration, the wastewater flows into large, open settling tanks. Here, the velocity of the water is significantly reduced, allowing heavier particles like grit (soil and small pebbles) to settle down to the bottom due to gravity.
The solids that settle at the bottom of these tanks form what is known as primary sludge. The liquid portion that remains above the sludge, now largely free of large suspended solids and grit, is called the primary effluent. This effluent still contains a substantial amount of dissolved organic matter and microbes, making it unsuitable for direct discharge.
Primary treatment does not significantly reduce the Biochemical Oxygen Demand (BOD) of the sewage, as it primarily removes physical solids, not dissolved organic pollutants.
Secondary Treatment: The Biological Stage
Secondary treatment, also known as biological treatment, is the stage where the primary effluent is subjected to microbial action to reduce its organic content. This is the most critical step for purifying the water biologically.
The primary effluent is transferred into large aeration tanks. Here, two key conditions are maintained:
- Constant Agitation: The effluent is continuously stirred mechanically.
- Air Pumping: Air is pumped into the tanks.
These conditions promote the vigorous growth of useful aerobic microbes. These microbes form masses called flocs, which are essentially networks of bacteria associated with fungal filaments, creating a mesh-like structure. The microbes within these flocs consume the major part of the organic matter present in the effluent. As they consume organic matter, they significantly reduce the Biochemical Oxygen Demand (BOD) of the effluent. BOD is a measure of the amount of oxygen that would be consumed by microbes if all the organic matter in one litre of water were oxidised. A high BOD indicates a high level of organic pollution. …
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