A pond has turned dull grey or colorless and DO & pH have dropped. This condition is most likely caused by?

• A. Excessive recirculation
• B. Excessive aeration
• C. Organic overload
• D. Hydraulic overload

Answer: (C)

A surge in organic matter in a pond triggers a burst of microbial activity that uses oxygen as it decomposes the organics. The rapid respiration drives dissolved oxygen (DO) levels down. As the organics are broken down, carbon dioxide is produced and dissolves in the water, forming carbonic acid and lowering the pH. The combination of depleted DO and lower pH can make the water appear dull gray or colorless, especially as pigments fade and solids settle. This pattern is the clearest sign of organic overload. Excessive aeration would typically raise DO, hydraulic overload affects residence time and overall treatment but doesn’t directly explain the DO and pH drop, and excessive recirculation mainly changes mixing and solids handling rather than causing a rapid DO decline with pH decrease.