The facultative lagoon in the pond sequence functions like the primary clarifier of a conventional sewage treatment system.  Heavy solids will settle to the bottom of the lagoon, and lighter solids will float.  This facultative lagoon lacks the sludge removal capability of a primary clarifier, so a population of anaerobic organisms will colonize accumulated sludge on the bottom of the lagoon.  The surface area of the lagoon should be large enough to provide an atmospheric oxygen transfer rate adequate to prevent anaerobic conditions on the lagoon surface.  Intermediate depths of the lagoon support facultative micro-organisms capable of oxidizing both the dissolved and suspended organics from the original wastewater and the products of anaerobic catabolism on the bottom of the lagoon.
Areas with a consistently cool, but frost-free, climate may sustain facultative conditions in the first stabilization pond when treating lightly polluted water at low temperatures favorable to high concentrations of dissolved oxygen with low metabolic rates. Facultative pond stratification becomes unstable during cold weather increasing release of malodorous gas when water temperatures drop below 4 degrees Celsius (39 degrees Fahrenheit); and formation of ice on the pond surface will effectively prevent transfer of atmospheric oxygen to the pond biome. Stabilization ponds in climates with significant seasonal temperature variation may release malodorous gas during the season of rising temperatures as the pond biome consumes wastes accumulated during cold weather with increasing metabolic rates exceeding the atmospheric oxygen transfer rate at the pond surface.