Lagoons are relatively inexpensive in terms of equipment, maintenance, operating cost, energy cost and labor. When wastewater enters a lagoon that has a large volumetric capacity, it stays in the lagoon for an extended period of time. This allows bacteria to grow and remove many of the components of the wastewater.
However, solids in the wastewater and bacteria that grow on the contaminants tend to precipitate and may accumulate. Therefore, they need to be dredged or otherwise have solids removed periodically.
Lagoons also require a relatively large area, so there is a substantial capital cost in land acquisition and real estate taxes.
So before you go out and buy property, let’s look at three types of wastewater lagoons to determine which one is right for you.
If the wastewater is not aerated, the lagoon may function in an anaerobic mode in which organic matter is fermented to simple organic acids and eventually converted to methane.
High-strength wastewater, especially from food processing, is often treated in anaerobic lagoons as a first-stage treatment. There is no requirement for aerators but mixers to keep bacteria suspended in the lagoon improve the efficiency.
In some cases, a cover or cap may be present, either by fatty deposits floating and congealing on the surface or by an engineered covering. The cover prevents oxygen from diffusing into the water and reduces the leakage of odors typical of anaerobic digestion but must allow methane and other gases to escape or capture methane for energy use.
Anaerobic lagoons depend on the growth of bacteria to remove contaminants. But because anaerobic bacteria may grow more slowly than aerobic bacteria, and since the bacteria may settle to the bottom if not mechanically mixed, the concentration of bacteria is much lower than in an activated sludge process.
The removal of contaminants measured as chemical oxygen demand (COD) is often in the range of 50-70 percent. Typically, anaerobic lagoons are the first stage in a multistage treatment process.
The problems that frequently occur in anaerobic lagoons are:
Facultative lagoons may be actively aerated or simply have oxygen diffusing from the air into the surface water. Ideally, there are both aerobic and anaerobic bacteria present, active and contributing to the removal of contaminants.
Often, an anaerobic bottom water layer and sediment provide the anaerobic environment for the same processes as the anaerobic lagoon, but aerobic bacteria present in upper water layers can perform additional metabolic processes such as nitrification (oxidation of ammonia to nitrite and nitrate) and more efficient COD removal. Volatile acids are converted to carbon dioxide and water and hydrogen sulfide can be oxidized to elemental sulfur or sulfate compounds.
Nitrite and nitrate formed in the aerobic zone can be reduced to nitrogen gas in the anaerobic zone thus removing some of the dissolved nitrogen. Given enough residence time, wastewater can be cleaned with 90 percent COD removal.
The problems that occur in facultative lagoons include the problems encountered in anaerobic lagoons, plus:
Lagoons are frequently aerated with surface aerators that mix a zone around the aerator as well as increase the dissolved oxygen. Such lagoons are rarely completely mixed or maintain significant dissolved oxygen in 100 percent of the water volume so there are anaerobic processes occurring in the sediment on the bottom and in “dead zones” not thoroughly mixed or aerated.
Aerated lagoons are quite efficient at COD removal and nitrification. Aerobic bacteria produce more biomass than anaerobic bacteria per COD removed so there is more biomass that accumulates if there is a settling zone or if the biomass is removed in a clarifier there is more biomass to be wasted.
If a clarifier is used, bacteria can be recycled to the aerated zone and the lagoon will operate as an activated sludge system with a higher bacteria concentration and thus a smaller required lagoon to achieve the effluent goals.
Problems that frequently occur in aerated lagoons include:
Every type of wastewater lagoon has its pros and cons, which is why there are so many options. But no matter which one you choose, you will need an effective solution for keeping your lagoon clean and running efficiently.
Monera Technologies provides expert technical assistance to diagnose and treat the problems occurring in wastewater treatment. Contact us for bacterial and nutrient products to aid in the correction of treatment deficiencies.