Lagoon Systems

 

Lagoons are one of the oldest wastewater treatment systems created by humans. They consist of in-ground earthen basins in which the waste 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 lagoon, or ponds as sometimes called, are very simple in design, there are complex chemical, biological and physical processes taking place. Due to the mechanical simplicity and low maintenance requirement, lagoons are well suited for the developing world. Lagoons are typically classified by their degree of mixing and amount of oxygen dissolved in the water. For the most part, the more air and mixing supplied, the better the effluent quality of the lagoon.

Types

Anaerobic Lagoon - These lagoons are typically 6 to 30 feet deep. They have no aerobic zones, which may cause the production of odors. The detention time can vary from 20 - 50 days. These systems have typically been used to treat high strength wastewater. The low-rate anaerobic process stabilizes organic matter, converting it to carbon dioxide and methane. If sulfur is present, hydrogen sulfide, an odorus compond, may be produced. Particulate matter that does not get degraded will settle to the bottom.

 

Facultative Lagoon - These lagoons have both an aerobic zone and an anaerobic zone. They are typically 4 to 8 feet deep, with a retention time around 25 days. Algae thrive in the top layer of the lagoon and supply oxygen via photosynthesis. Aeration in the top layer can also come from suface mixing due to wind. As depth increases the lagoon gradually becomes anaerobic, with a sludge layer at the bottom. Biodegradable organic matter is stabilized by both the anaerobic an aerobic processes, releasing methane an carbon dioxide. Since the top layer contains oxygen, reduced compounds (e.g. H2S) can be oxidized before leaving, preventing the release of odors. These lagoons are also known as stabiliztion ponds.

 

Aerobic Lagoon - These lagoons are designed to be completely aerobic, stabilizing organic matter at a higher rate than the other lagoons. The mechanism for providing oxygen can very greatly. If algal photosythesis is the only source of oxygen, the lagoon should allow light penetration through the entire water column. The depth should not exceed 5 feet, and the detention time should be around 10 to 40 days.

Partial-mix Aerated Lagoon - These lagoons are supplied with oxygen by floating surface aerators or submerged diffusers. The depth of these lagoons can be from 6 to 20 feet, with a detention time of 5 to 20 days. The combination of a greater depth and shoter detention time makes the land requirement much less than the other lagoons. However, the introduction of aeration devices increases the operation and maintenance costs.

Completely Mixed Aerated Lagoons - These lagoons are used throughout developed world, and can produce excellent effluent quality. They are designed and operated to exclude algae by completely mixing the solids, and therefore blocking all light. Algae can cause the effluent to exceed the total suspended solids (TSS) limit.

Links

Tecnical Notes - This site may be the most comprehensive online analysis on lagoons. Dr. L Rich, from Clemson University, has written 8 technical notes that provide great information on lagoon performance and trouble shooting.

Lagoons Online - Although this is a website from Maine, USA, this site has excellent descriptions of the aerated lagoon process and examples of aeration equipment that can be used for tropical lagoon design..

Tropical Public Health Engineering - From the University of Leeds, UK. This site has nice photos and explantions of lagoons in the the developing world. Photos from Brasil and Colombia.

FAO (click on Natural Biological Treatment Systems)- This site discusses wastewater treatment in the developing world. A World Bank Report describes "Stabilization ponds are the preferred wastewater treatment process in developing countries, where land is often available at reasonable opportunity cost and skilled labor is in short supply."
Equations for designing lagoons are also provided.

LAS International - Wastewater Treatment facilities for developing Countries. This site has good examples and pictures of Aerated Lagoons.

Innovative and Low-Cost Wastewater Treatment Technology
- This site discusses the application of chemically enhanced primary treatment (CEPT) to achieve better effluent quality. This can be applied to lagoons.

Duckweed in Lagoons. Application of Duckweed to lagoons has become common in many places. This site explains the many benefits. Also see wetland section.

Good Texts for Design

Crites, R., Tchobanoglous, George, Small and Decentralized Wastewater Management Systems, McGraw-Hill, 1998

Grady, Leslie C.P., Daigger, Glenn T., Biological Wastewater Treatment, Marcel Dekker, New York, 1999

Metcalf & Eddy, Inc., Wastewater Engineering: Treatment, Disposal, and Reuse, McGraw-Hill, 1991

EPA Design Manual: Municipal Wastewater Stabilization Ponds

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