text: Objectionable odors are likely when the rate of oxygen transfer from the lagoon surface is less than the rate of oxygen consumption in the lower levels of the lagoon.  A  facultative lagoon might provide 50 pounds of oxygen per day (5 grams of oxygen per square meter per day) for biochemical catabolism.  Biological activity within a facultative lagoon varies directly with temperature.  Warm weather will require large oxygen transfer rates, and waste accumulation during cold weather can cause short-term warm weather oxygen requirements to exceed long-term waste loading rates.  Algae can provide surface oxygen during daylight hours, but algal respiration can require additional oxygen during darkness.  Ice or scum mats can reduce the oxygen transfer surface.  Some facultative lagoons use mechanical surface aerators to increase atmospheric oxygen transfer, but aerator mixing depth should not re-suspend anaerobic sludge from the bottom of the lagoon.  Aerator operation may be limited to periods of heavy waste loads, high temperatures, darkness, low wind velocity, or other conditions threatening to cause anaerobic conditions on the lagoon surface.
  Facultative stabilization ponds stratify with an aerobic surface layer and an anaerobic layer below the surface. The aerobic surface layer limits release of malodorous gas from the anaerobic benthic zone. Algae and cyanobacteria typically grow in the aerobic zone and provide bacteria in the pond with plenty of oxygen during the daytime. However, algal photorespiration may consume oxygen during night time when it is dark. Waste stabilization ponds with large algal populations may show significant diurnal fluctuation in oxygen concentrations with a peak in the late afternoon, and a minimum at dawn.
  Kinds of algae growing in treatment ponds include green, red and brown algae.