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REVIEW OF LAGOON OPERATION
Technical Note 1
Lagoons are frequently used to treat sewage generated by a town or industry. They are an enhancement of natural treatment of waste products and are considered to be low-tech. The objective of a lagoon or other type of wastewater treatment system is to stabilize the organic material and other matter that makes up the waste stream so that it no longer exerts an oxygen demand on our streams and lakes. Lagoons can meet this objective if properly maintained and managed.
Lagoons come in many forms and shapes, but they all have some common characteristics depending upon the type of treatment that is desired.
Most lagoons begin operation as facultative lagoons. That is, they have a layer at the top of the water that is aerated naturally by wind and wave action. This layer is described as aerobic because the micro-organisms that dwell there require oxygen to function properly. The bottom layer contains micro-organisms that function without oxygen and are called anaerobes. In between these two layers is a section that contains micro-organisms that can function with or without oxygen. Facultative lagoons are generally 4-8 feet deep and have long detention times.
Anaerobic lagoons operate almost entirely without oxygen. They work like a large septic tank. Frequently they are used to treat high oxygen-demanding domestic or industrial waste. Anaerobic lagoons are usually 15-20 feet deep. They are very effective at reducing the oxygen demand of such wastes and are less costly to operate. Although aerators are not needed in anaerobic lagoons, slow speed mixers are frequently used to keep the waste homogenous throughout the lagoon. Complete treatment using the anaerobic process produces very little sludge.
Aerobic lagoons introduce oxygen into the water through the use of aerators. Aerators may be of the floating type or submerged. In either case, both aeration and mixing are important in the lagoon. Aeration and mixing can minimize the growth of algae and increase the treating capacity of a lagoon from 3 to 10 times above an aerobic lagoon that depends upon the exchange of oxygen at the surface by wind and wave action. Depths of 5-6 feet are common.
There are several significant advantages to lagoon treatment of wastewater. Some of these have been listed below:
. Cost effective to design and construct
. Simple to operate
. Can handle shock and intermittent loadings
. Very effective at removing pathogens
. Effluent can be used for irrigation
. Less energy than most other treatment methods
. Less manpower needed
. Lower chemical cost
. Minimal site maintenance
Lagoons also have some disadvantages as noted below:
. Require more land
. Frequent odor problems
. May provide breeding for insects
. Must control algae for satisfactory treatment
It is generally accepted that two to four lagoons are better than one. However, one large lagoon can become a 2, 3, or 4-celled lagoon through the use of dividing curtains or baffles.
This document was written by Dr. Donald G. Adams of Environmental Services Company, LLC. Ruston, LA. It is not to be duplicated in any form without the express written consent of the owner. It may not be used for any purpose other than its intended purpose.
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