Facultative waste stabilization ponds, sometimes
referred to as lagoons or ponds, are frequently used
to treat municipal and industrial wastewater in the
United States. The technology associated with
facultative lagoons has been in widespread use in the
United States for at least 90 years, with more than
7,000 facultative lagoons in operation today. These
earthen lagoons are usually 1.2 to 2.4 m (4 to 8 feet)
in depth and are not mechanically mixed or aerated.
The layer of water near the surface contains dissolved
oxygen due to atmospheric reaeration and algal
respiration, a condition that supports aerobic and
facultative organisms. The bottom layer of the lagoon
includes sludge deposits and supports anaerobic
organisms. The intermediate anoxic layer, termed the
facultative zone, ranges from aerobic near the top to
anaerobic at the bottom. These layers may persist for
long periods due to temperature-induced waterdensity
variations. Inversions can occur in the spring
and fall when the surface water layer may have a
higher density than lower layers due to temperature
fluctuations. This higher density water sinks during
these unstable periods, creates turbidity, and
produces objectionable odors.
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.
The oxygen, produced by algae and surface
reaeration, is used by aerobic and facultative bacteria
to stabilize organic material in the upper layer of
water. Anaerobic fermentation is the dominant
activity in the bottom layer in the lagoon. In cold
climates, oxygenation and fermentation reaction rates
are significantly reduced during the winter and early
spring and effluent quality may be reduced to the
equivalent of primary effluent when an ice cover
persists on the water surface. As a result, many states
in the northern United States and Canada prohibit
discharge from facultative lagoons during the winter.
Although the facultative lagoon concept is land
intensive, especially in northern climates, it offers a
reliable and easy-to-operate process that is attractive
to small, rural communities.