Like most natural environments, conditions inside facultative lagoons are always changing. Lagoons experience cycles due to variations in the weather, composition of the wastewater and other factors. In general, the wastewater in facultative lagoons naturally settles into three fairly distinct layers or zones. Different conditions exist in each zone, and wastewater treatment takes place in all three.
The top layer in a facultative lagoon is called the aerobic zone, because the majority of oxygen is present there. How deep the aerobic zone is depends on loading, climate, amount of sunlight and wind and how much algae are in the water. The wastewater in this part of the lagoon receives oxygen from air, from algae and from the agitation of the water surface (from wind and rain, for example). This zone also serves as a barrier for the odors from gases produced by the treatment processes occurring in the lower layers.
Names for the middle layer include the facultative, intermediate or aerobic/anaerobic zone. Both aerobic and anaerobic conditions exist in this layer in varying degrees. Depending on the specific conditions in any given part of this zone, different types of bacteria and other organisms are present that contribute to wastewater treatment.
The anaerobic zone is the layer at the very bottom of the lagoon where no oxygen is present. This area includes a layer of sludge, which forms from the solids that settle out of the wastewater. Here, wastewater is treated by anaerobic bacteria, microscopic organisms, such as certain protozoa and sludge worms, all of which thrive in anaerobic conditions.
Facultative lagoons are designed to hold the wastewater long enough for much of the solids in the wastewater to settle and for many disease-causing bacteria, parasites and viruses to either die off or settle out. Time also allows treatment to reduce the overall organic strength of the wastewater or its biochemical oxygen demand (BOD). In addition, some of the wastewater eventually evaporates.
Sunlight is also extremely important to facultative lagoons because it contributes to the growth of green algae on the water surface. Because algae are plants, they require sunlight for photosynthesis. Oxygen is a by-product of photosynthesis, and the presence of green algae contributes significantly to the amount of oxygen in the aerobic zone. The more warmth and light the sun provides, the more green algae and oxygen there is likely to be in the lagoon.
The oxygen in the aerobic zone makes conditions favorable for aerobic bacteria. Both aerobic and anaerobic bacteria are very important to the wastewater treatment process and to each other.
Bacteria treat wastewater by converting it into other substances. Aerobic bacteria convert wastes into carbon dioxide, ammonia and phosphates, which, in turn, are used by the algae as food. Anaerobic bacteria convert substances in wastewater to gases, such as hydrogen sulfide, ammonia and methane. Many of these by-products are then used as food by both the aerobic bacteria and algae in the layers above.
In addition, the sludge layer at the bottom of the lagoon is full of anaerobic bacteria, sludge worms and other organisms, which provide treatment through digestion and prevent the sludge from quickly accumulating to the point where it needs to be removed. How often sludge must be removed from facultative lagoons varies depending on the climate, the individual lagoon design and how well it is maintained. Sludge in all lagoons accumulates more quickly in cold than in warm temperatures.
The information for this blog came from http://water.me.vccs.edu/exam_prep/lagoons.htm.