A facultative wastewater treatment pond receives domestic waste with no industrial discharge. Over the last few days, odor and pinkish color developing. Explanation MOST likely:

• A. An alkaline condition is developing
• B. An anaerobic condition is developing
• C. An acidic condition is developing
• D. An aerobic condition is developing

Answer: (B)

In a facultative pond, oxygen is supplied mainly at the surface, while the deeper zones depend on what’s happening with the organic matter and its consumption of oxygen. If the input of waste is high and the water isn’t being reoxygenated fast enough, the bottom becomes depleted of oxygen and anaerobic conditions develop. Odor here comes from anaerobic decomposition releasing reduced compounds like hydrogen sulfide, which stinks like rotten eggs, and other gases such as methane. The pinkish color can occur because certain anaerobic, light-loving bacteria (such as purple non-sulfur bacteria) thrive under these conditions and give the water or sludge a pink/purple hue. So the combination of a strong odor and pink tint points to an anaerobic environment forming in the pond, rather than an oxygen-rich (aerobic) condition or a simple pH shift (alkaline or acidic).