Lagoons

A relatively inexpensive design for biological waste treatment is called an oxidation pond or lagoon.  Sunlight promotes
photosynthesis so these units are rich in oxygen near the top but a deep lagoon may be anaerobic at its bottom.  While agitators are sometimes used, many lagoons have no added mixing and must rely on flow, diffusion, wind, and the buoyancy or sinking of microorganisms for mixing.  Short circuiting of flow can make some regions of the lagoon relatively stagnant and ineffective.  Deflectors or baffles may distribute the flow, but a raceway is a more reliable method of encouraging plug flow.  Our program LABYR.BAS is obsolete because of the following Java applet (typical plan view of raceways):

The idea is to have the flow go back and forth through the unit with no dead spots.  Generally, earth moving equipment scrapes out a lagoon and the soil that is removed forms the walls and spacers.  Clay or a plastic liner may cover the bottom to prevent leaks, but it is common practice to depend on microorganisms to plug the pores even though this may take many months.  During this time wastewater has easy access to groundwater.

Because of the low capital and operating costs, lagoons are very popular for industrial waste treatment in the southern United States.  They are less common in the North because they may freeze in cold weather.  Nevertheless, every state, even Alaska, has some lagoons for industrial wastes that are warm enough for good microbial activity despite very severe weather.

The rest of the chapter is covered by various web pages about biological reactors.