Controlled Discharge

Controlled discharge is defined as limiting the discharge from a lagoon system to those periods when the effluent quality will satisfy existing discharge requirements. The usual practice is to prevent discharge from the lagoon during the winter period and during the spring and fall overturn periods and algal bloom periods. Many countries currently do not permit lagoon discharges during winter months.

Pierce (1974) reported on the quality of lagoon effluent obtained from 49 lagoon installations in Michigan that practice controlled discharge. Of these 49 lagoon systems, 27 have two cells, 19 have three cells, 2 have four cells, and 1 has five cells. Discharge from these systems is generally limited to late spring and early fall; however, several of the systems discharged at various times throughout the year. The period of discharge varied from fewer than 5 d to more than 31 d. The lagoons were emptied to a minimum depth of approximately 0.46 m (18 in.) during each controlled discharge to provide storage capacity for the non-discharge periods.

(X) PROBABILITY OF NOT EXCEEDING

FIGURE 5.7 Comparison of the effluent quality of two-cell systems vs. lagoon systems with three or more cells with long storage periods before discharge (Michigan). (Pierce, D.M., in Upgrading Wastewater Stabilization Ponds To Meet New Discharge Standards, PRWG151, Utah Water Research Laboratory, Utah State University, Logan, 1974.)

(X) PROBABILITY OF NOT EXCEEDING

FIGURE 5.7 Comparison of the effluent quality of two-cell systems vs. lagoon systems with three or more cells with long storage periods before discharge (Michigan). (Pierce, D.M., in Upgrading Wastewater Stabilization Ponds To Meet New Discharge Standards, PRWG151, Utah Water Research Laboratory, Utah State University, Logan, 1974.)

During the discharge period, the lagoon effluent was monitored for BOD5, TSS, and FC. The effluent BOD5 and TSS concentrations measured during the study are illustrated in terms of probability of occurrence in Figure 5.7. All values are arranged in order of magnitude and plotted on normal probability paper with concentration (mg/L) plotted against the probability that the value would not be exceeded under similar conditions. The plot compares the performance of two-cell lagoon systems vs. lagoon systems with three or more cells. The results shown in Figure 5.7 are summarized in Table 5.16.

The results of the study indicated that the most probable effluent BOD5 concentration for controlled discharge systems was 17 mg/L for the two-cell lagoon systems and 14 mg/L for lagoon systems with three or more cells. There was a 90% probability that the effluent BOD5 concentration from two-cell systems and lagoon systems with three or more cells would not exceed 27 mg/L. This value was slightly less than the 30-mg/L BOD5 U.S. Federal Secondary Treatment Standard. The most probable effluent TSS concentration was found to be 30 mg/L for two-cell lagoon systems and 27 mg/L for lagoon systems with three or more cells. The 90% probability levels for effluent TSS concentrations were 46 mg/L for two-cell lagoon systems and 47 mg/L for lagoon systems with three or more cells. The results of the study also indicate that the FC levels were generally less than 200/100 mL, although this standard was exceeded on several occasions when chlorination was not employed.

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