Sludge lagoon

Figure 22. Sludge lagoon used by a treatment plant in Russia.

As the precipitate settles slowly in a secondary basin, it adsorbs any remaining suspended CaCX)., bacteria, and other particles, such as dirt and algae. Precipitates from the primary and secondary basins are combined in a sludge lagoon for disposal. The water is then passed through a sand filter to remove any remaining suspended particles. 

The limitations for evaporation lagoons include local health ordinances potential for odors and health hazard when not properly designed land area requirements dependence on meteorological and climatological conditions. Sludge lagoons may require provisions to add makeup water to maintain a miiumum depth during dry, hot seasons. Finally, public access restrictions are necessary. 

The US EPA published information on capital cost of constructing sludge lagoons. Using an ENR CC Index equal to 6390.21 (January 2002 Cost), typical excavation and liner costs associated with a two-bedroom residence are as follows ... 

Adapted from Krofta and Wang (30,31)- Fresh raw alum sludge (non-dewatered) was discharged directly into the sludge lagoon 1 gpm = 1 gallon per minute = 3.785 liters per minute. 

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