Percent Probability of Occurrence
Effluent BOD5 Concentration (mg/L)
Three or Two Cells More Cells
Effluent SS Concentration (mg/L)
Three or More Cells
50% probability (most probable) 17
90% probability (will not be 27
exceeded 9 out of 10 samples)
Source: Pierce, D.M., in Upgrading Wastewater Stabilization Ponds To Meet New Discharge Standards, PRWG151, Utah Water Research Laboratory, Utah State University, Logan, 1974.
A similar study of controlled discharge lagoon systems was conducted in Minnesota (Pierce, 1974). The discharge practices of the 39 installations studied were similar to those employed in Michigan. The results of that study from the fall discharge period indicated that the effluent BOD5 concentrations for 36 of 39 installations sampled were less than 25 mg/L, and the effluent TSS concentrations were less than 30 mg/L. In addition, effluent FC concentrations were measured at 17 of the lagoon installations studied. All the installations reported effluent FC concentrations of less than 200/100 mL.
During the spring discharge period, 49 municipal lagoon installations were monitored. Effluent BOD5 concentrations exceeded 30 mg/L at only three installations, while the maximum effluent BOD5 concentration reported was only 39 mg/L. Effluent TSS concentrations ranged from 7 to 128 mg/L, with 16 of the 49 installations reporting effluent TSS concentrations greater than 30 mg/L. Only 3 of the 45 installations monitored for effluent FC concentrations exceeded 200/100 mL.
The controlled discharge of lagoon effluent is a simple, economical, and practical method of achieving a high degree of treatment. Experience indicates that routine monitoring of the lagoon effluent is necessary to determine the proper discharge period; however, these discharge periods may extend throughout the major portion of the year. It will be necessary to increase the storage capacity of certain lagoon systems that employ controlled discharge. Many lagoon systems already have additional freeboard and storage capacity that could be utilized without significant modification.
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