What is the ideal range of dissolved oxygen levels in a conventional activated sludge system for producing good effluent?

In a conventional activated sludge system, maintaining the ideal range of dissolved oxygen (DO) levels is crucial for effective biological treatment. The correct answer indicates that maintaining dissolved oxygen levels within the range of 2 to 4 mg/l is optimal for producing good effluent.

This level of dissolved oxygen is sufficient for aerobic microorganisms to effectively metabolize organic matter present in the wastewater, promoting the breakdown of pollutants. When DO levels are within this range, the microorganisms can thrive, leading to efficient processes such as nitrification, which is essential for converting ammonia to nitrate and thus improving the quality of the effluent.

Higher DO levels can lead to increased energy costs for aeration without significantly improving treatment efficiency, while extremely low levels of DO (like those stated in the other choices) can inhibit microbial activity, resulting in poor treatment performance and potentially leading to the release of unprocessed or partially processed pollutants in the discharge. Therefore, the 2 to 4 mg/l range is considered to balance the needs of the aerobic bacteria and the operational efficiency of the treatment system.