Which of the following methods would least effectively maintain dissolved oxygen levels in a treatment process?

Maintaining adequate dissolved oxygen (DO) levels in wastewater treatment processes is crucial for the effectiveness of biological treatment methods. Passive venting involves allowing air to circulate naturally without the use of mechanical devices or energy to increase oxygen transfer. This method relies on passive airflow, which may not provide sufficient oxygenation in systems that require higher DO levels for optimal microbial activity.

In comparison, surface aeration introduces air directly into the water, effectively increasing DO by agitating the surface and allowing air exchange. Mixing tanks promote homogenous mixing and can enhance the interaction between water and air, also supporting oxygen transfer. Mechanical aerators actively agitate the water and facilitate oxygen transfer through various mechanisms like splashing or introducing bubbles, which is very effective in elevating DO levels. Therefore, while passive venting might maintain some level of dissolved oxygen, it is the least effective among the methods listed for ensuring necessary oxygenation in treatment processes.