Control Of Algae And Design Of Settling Basins

Control of algae in wastewater stabilization pond effluents has been a major concern throughout the history of the use of the systems. The use of maturation ponds and polishing ponds following all types of treatment processes has resulted in a need to control algae in the effluent. State design standards have contributed to the problem by requiring long detention times in the final cell in a lagoon system. It has been established that few, if any, of the solids in lagoon effluents are fecal matter or material entering the lagoon system. This fact led to much discussion about the necessity to remove algae from lagoon effluents; however, it was pointed out that the algae die, settle out, and decay, thus inducing an oxygen demand on the receiving stream. This concern about decay and oxygen consumption resulted in investigations into the most effective methods to remove algae and ways to design systems to minimize the growth of algae in settling basins. Methods for removing algae are discussed in Chapter 5, and the design of settling basins is discussed in the following text.

For 18 months, Toms et al. (1975) studied algae growth rates in full-scale polishing lagoons receiving activated sludge effluents. It was concluded that growth rates for the dominant species always were less than 0.48 d1, and, if the hydraulic retention time (HRT) was less than 2 d, algae growth would not become a problem. At a HRT of less than 2.5 d, the effluent TSS decreased, and beyond this HRT the TSS increased. Uhlmann (1971) reported no algae growth in hyper-fertilized ponds when the detention times were less than 2.5 d. Toms et al. (1975) evaluated one-cell and four-cell polishing lagoons and found that, for HRTs beyond 2.5 d, the TSS increased in both lagoons, but significant growth did not occur until after 4 to 5 d in the four-cell lagoon.

Light penetration is reduced as the depth of a lagoon is increased; however, because of the trapezoidal shape of most lagoon cells, little advantage is achieved by increasing the depth beyond 3 or 4 m. Thermal stratification occurs in lagoons without mechanical mixing and provides an excellent environment for algae growth. Disturbing stratification will reduce algae growth. Rich (1999) recommends some degree of aeration for lagoon cells to control algae. The intensity of aeration also has an influence on algae growth by suspending more and more solids as the intensity increases. This results in a reduction in light transmission and consequently fewer algae.

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