The presence of algae in the aerobic and facultative
zones is essential to the successful performance of
facultative ponds. In sunlight, the algal cells utilize
CO2
from the water and release O2 produced from
photosynthesis. On warm, sunny days, the oxygen
concentration in the surface water can exceed
saturation levels. Conversely, oxygen levels are
decreased at night. In addition, the pH of the near
surface water can exceed 10 due to the intense use of
CO2
by algae, creating conditions favorable for
ammonia removal via volatilization. This
photosynthetic activity occurs on a diurnal basis,
causing both oxygen and pH levels to shift from a
maximum in daylight hours to a minimum at night.