Wastewater Treatment: 3 Types of Lagoon Designs

 

Lagoons are one of the most common wastewater treatment options because they are cost-effective and easy to maintain. But how do you know which design is right for your wastewater treatment plant? In this blog, we look at three types of wastewater lagoon designs.

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.

Anaerobic Lagoons

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:  

• Acid lock: the fermentation reactions produce simple acids that lower pH. Methanogens that normally metabolize the acids and neutralize pH may not be growing adequately and low pH inhibits the growth of methanogens and other types of bacteria.
• Low COD removal: the absence of essential nutrients (often trace elements) or inhibition by chemicals in the wastewater may slow or stop bacterial growth. Rarely, bacteria may sediment out too rapidly or remain in suspension and be washed out with the effluent leaving too low a concentration in the water column.
• Low methane production: similar to low COD removal but may be due to nutrient limitation of elements specifically needed by methanogens or inhibitors specific to them.
• High odor generation: anaerobic processes such as sulfate reduction generate hydrogen sulfide. The simple acids that are produced are volatile and include acetic, propionic and butyric acids- all of which are odorous. Ammonia is also released and depending on pH and concentration can contribute to significant odor.

Facultative Lagoons

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:

• Foaming: may be due to surfactants present in the wastewater or filamentous bacteria that grow due to specific conditions such as low dissolved oxygen, unbalanced nutrients, high fat, oil and grease (FOG), etc.
• Dispersed biomass: high levels of suspended solids in the effluent may be due to high salt concentration, toxic compounds or rapid bacterial growth when slugs of high organic matter concentration enter the lagoon.
• Inversion of water layers: wind and/or temperature changes may result in the mixing of the surface and bottom layers of the lagoon. Sediment and odors from the bottom mix with aerated water from the top to disrupt both aerobic and anaerobic processes.
• Loss of nitrification: nitrifying bacteria are sensitive to toxins, low pH, and mineral element deficiency. Because they grow slowly they can be washed out of the system if flow increases or their growth is slowed by the above conditions.

Aerated Lagoons

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:

• Low COD removal: the absence of essential nutrients (often trace elements) or inhibition by chemicals in the wastewater may slow or stop bacterial growth. Rarely, bacteria may sediment out too rapidly or remain in suspension and be washed out with the effluent leaving too low a concentration in the water column.
• Foaming: foaming may be due to surfactants present in the wastewater or filamentous bacteria that grow due to specific conditions such as low dissolved oxygen, unbalanced nutrients, high fat, oil and grease (FOG), etc.
• Dispersed biomass: high levels of suspended solids in the effluent may be due to high salt concentration, toxic compounds or rapid bacterial growth when slugs of high organic matter concentration enter the lagoon.
• Pass-through of specific compounds: a type of bacteria necessary for the degradation of a chemical compound may be absent or at too low a concentration. This may be due to the sporadic presence of the compound in the wastewater (or widely varying concentrations) or the compound may be difficult to biodegrade.

Conclusion

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.