Lagoons are one of the oldest wastewater treatment systems created by mankind. They consist of lined in-ground earthen basins in which the wastewater is detained for a specified time (detention time) and then discharged. The size and depths can vary, as well as the degree of treatment. Although these lagoons, or ponds as they are sometimes called, are very simple in design, there are complex chemical, biological and physical processes occurring. There are five main types of lagoons: facultative lagoon (stabilization pond), aerobic lagoon, anaerobic lagoon, partial mixed aerated lagoon, and completely mixed aerated lagoon.
A completely mixed aerated lagoon in wastewater treatment is a relatively shallow basin (with a depth between 5 and 15 feet, or between 152 cm and 457 cm) with a large surface area (of several acres), which operates on a flow-through mode, in many ways similar to CSTRs. They are designed and operated to exclude algae by completely mixing the solids and therefore blocking all light. The earthen basin is generally lined with impervious material such as asphalt or plastic. The complete mixing is achieved through the use of mechanical surface aerators of either the fixed or floated type. The mixing energy must be sufficient enough so that all solids are suspended and light is reduced in the wastewater to the extent that very little algal growth occurs (because the control of algal growth is crucial in the reduction of effluent suspended solids). The holding time in a CMAL is typically 7 to 10 days. The degree of treatment is a function of the mass of organic matters in suspension and holding time.
The microbial population in a CMAL is predominately heterotrophic, and aerobic respiration is the path of metabolism of the microbial population in excess of oxygen. The bacteria in a CMAL utilize the organic matters, and other higher-ups, such as protozoans, rotifers, daphnia, and insect larvae in the food chain are likely to chew on these bacteria and their predators. The ecology of a CMAL is rather complex and may also involve growth of algae; however, algae growth should be suppressed. This issue and other possible interactions within the flora of a CMAL should be an important factor during the treatability studies of wastewater streams. A photograph of a typical CMAL installation is shown in Fig. 4.5.
In a typical CMAL treatment system, a series of lagoons is placed to treat expected ranges of food and agricultural wastewater. The number of lagoons can vary; so can the sizes. When CMALs are used for wastewater treatment before land applications, the first successive lagoons provide biological treatment of the wastewater, and the last lagoon provides storage. CMALs are usually employed to achieve one of two objectives: degrading soluble solids into insoluble biomass (microbial cells) and stabilizing the organic solids. CMALs are used as a pretreatment for industrial wastewaters or as total wastewater treatment systems for a small community. The main advantage to using lagoon systems such as CMALs is their simplicity of treatment. Lagoons treat wastewater over an extended period of time and can be designed to completely degrade sludge. They are not as susceptible to "shock" loading. For example, should a toxic chemical or high pH load be introduced into the lagoon system, the constituent is diluted and can be isolated. The regular wastewater can be bypassed to the next lagoon for continued treatment. Lagoon systems also help equalize peak inflows. Lagoon systems are simple, low-capital investment systems and also have low operation and maintenance costs since fewer staff and less mechanical equipment are needed to operate this type of system. Minimal sludge is produced, which reduces capital and operational costs associated with sludge stabilization, conditioning, dewatering, and disposal. The main disadvantages of lagoon systems are that they require more land than mechanical wastewater treatment facilities and thus are not a viable option for communities with high population intensity, and that there is no operational control over the rates of biochemical reactions; the degree of treatment is affected by the temperature as temperature effects the kinetics of biochemical reactions. Seasonal changes tend to have impact on the degree of the treatment with CMALs.
Completely mixed aerated lagoons offer a reasonable treatment alternative to more costly mechanical biological wastewater treatment such as trickling filters and activated sludge systems for readily biodegradable wastewater—those streams from food processing operations. However, the decision of whether to use CMALs should be based on the total cost analysis, treatability study, and local environmental regulations.
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