Question

Explain the biological treatment of the major part of the sludge transferred from the large aeration tank into the anaerobic sludge digesters before its final release into the natural water bodies.

Answer 1

A significant portion of the activated sludge that settles in the secondary settling tank is not recycled back to the aeration tank. This excess sludge is transferred into large anaerobic sludge digesters for further biological treatment. The biological treatment in the anaerobic sludge digesters involves the breakdown of the organic matter in the sludge by anaerobic microorganisms under oxygen-free conditions. This process has several important outcomes: Decomposition of complex organics: Anaerobic bacteria, including acidogens and methanogens, work synergistically to decompose the complex organic compounds in the sludge (like proteins, carbohydrates, and lipids) into simpler substances. Acidogens first convert these complex organics into organic acids, hydrogen sulfide, ammonia, and carbon dioxide. Production of biogas: Methanogenic bacteria then utilize these intermediate products, particularly organic acids, hydrogen, and carbon dioxide, to produce biogas, which is primarily a mixture of methane (CH4) and carbon dioxide (CO2). Biogas is a valuable energy source that can be used to generate electricity or heat for the STP. Reduction in sludge volume: Anaerobic digestion significantly reduces the volume and mass of the sludge, making its final disposal more manageable. Stabilization of sludge: The process stabilizes the sludge by reducing the amount of putrescible organic matter, thereby decreasing its odor and the potential for environmental pollution. Production of nutrient-rich digestate: The remaining digested sludge (digestate) is relatively stable and rich in nutrients like nitrogen and phosphorus. After proper treatment, it can be used as a soil conditioner or fertilizer in agriculture. The biologically treated sludge (digestate) is then further processed (e.g., dewatered and dried) before its safe disposal or use, ensuring that the final release into natural water bodies does not cause pollution. The anaerobic digestion of sludge is a crucial step in wastewater treatment as it not only reduces the volume of waste but also recovers energy in the form of biogas and produces a more environmentally benign end product. Correct Answer : In anaerobic sludge digesters, anaerobic bacteria decompose the organic matter in the sludge, producing biogas (mainly methane and CO2), reducing sludge volume, stabilizing it, and yielding nutrient-rich digestate suitable for disposal or use as fertilizer.