What is the Role of Activated Sludge in Sewage Treatment Plant?
After primary settling, wastewater still contains dissolved organic matter, which needs decomposition before environmentally safe discharge. Biological treatment using 'activated sludge' is a secondary process where microbes break down soluble compounds into more stable end products. Activated sludge is indispensable in removing organic pollutants in sewage treatment plants.
What is Activated Sludge?
Activated sludge is a suspension of metabolically active microorganisms, including bacteria, protozoa and fungi, that rapidly mineralise organic matter. It forms flocs that bio-oxidize carbon sources and convert nutrients such as nitrogen, phosphates and other chemicals in wastewater. These diverse groups of microbes grow and settle within aeration basins. Essentially, activated sludge fosters desirable micro-environments that transform contaminants into simpler byproducts.
How Activated Sludge Systems Work?
Wastewater in secondary tanks is mixed and aerated with previously enriched sludge from aerobic or anaerobic digesters. This provides a tailored cocktail of thriving cultures.
As air pumps lead to oxygen transfer through diffusers, they stimulate microbial growth and volatile solids biodegradation. Carbonaceous BOD and ammonia levels decrease.
Flocs slowly develop where bacteria associate with protozoa and fungi embedded in a mucilaginous matrix. These flocs aggregate suspended solids that flocculate.
After a specific retention period, a proportion of nutrient-rich activated sludge is recirculated to continue supporting upcoming batches. The remainder undergoes further sludge management as effluent gets discharged/disinfected.
Influencing Factors
Careful monitoring and control of variables like pH, temperature, nutrients, and dissolved oxygen is vital for optimal activity:
1- Maintain aerobic conditions above 0.5-1.0 mg/L to facilitate breakdown of organics
2- Adjust alkalinity, micronutrient availability and BOD loading rates
3- Prevent growth by limiting toxicity from chemicals or heavy metals
4- Provide enough mixing for contact between microbes and the food supply
Key Benefits of Activated Sludge
Leveraging activated sludge offers multiple advantages in sewage processing:
1- Effectively reduces soluble BOD by 90% through catabolic action
2- Removes nutrient nitrogen/phosphorus, preventing algae growth and eutrophication
3- Allows handling fluctuating flows and higher loading rates
4- Well-developed flocs settle faster, enabling effluent clarity
5- Valuable biosolids are created that can amend agricultural land or generate energy
6- Without adequately optimised activated sludge, discharge limits would frequently be violated, making the process indispensable.
Conclusion
In sewage treatment's secondary phase, activated sludge is pivotal for metabolising dissolved organics after primary sedimentation removes solid particulates. Maintaining the right mix of bacteria and protozoa breaks down pollutants through aerobic digestion and bioflocculation. Activated sludge equipment ranging from diffusers and basins to clarifiers enables convenient sludge age control critical for reducing effluent BOD/nutrients before final polishing and disinfection. Understanding the composition and function of activated sludge provides deeper insight into biological wastewater remediation.
The activated sludge process revolutionised sewage treatment over a century ago and is an indispensable technology today. As wastewater quality regulations grow more stringent, optimising sludge age and SRT and developing newer modifications will be key to maximising treatment capacity. The more we understand balanced microbial ecosystems, the better we can leverage Nature's helping hand in cleaning contaminated water for beneficial reuse in human habitats.
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