In a 10 MGD conventional water treatment plant, continuous raw water pH monitoring shows pH rising during daylight hours and dropping during the night. What condition is indicated by this pattern?

• A. Chlorine demand rise
• B. Alkalinity increase
• C. Thermal stratification
• D. Impending taste and odor problem

Answer: (D)

Diurnal pH fluctuations caused by photosynthetic activity indicate algae in the source water. During daylight, algae and cyanobacteria consume CO2 for photosynthesis, which reduces carbonic acid in the water and raises the pH. At night, photosynthesis stops and respiration releases CO2, lowering the pH. This day–night pattern points to active algal populations, which are often the source of taste and odor problems because algae can produce compounds like geosmin and MIB that affect flavor and odor in finished water. The other options don’t align with the pattern: chlorine demand changes are not driven by a daily pH cycle; an alkalinity increase would buffer pH and not produce such pronounced diurnal swings; thermal stratification affects temperature layers, not the characteristic pH diurnal pattern tied to photosynthesis and algal activity.