Technical Note Number 2
AERATED LAGOON EFFLUENTS
There are two great myths in aerated lagoon technology. The first myth is that effluent BOD5 measures the biodegradable carbonaceous material in the effluent. The fallacy of this myth was discussed in Technical Note Number 1. Practically all effluent BOD5 values are inflated by nitrification that occurs in the 5-day BOD5 test itself. Such inflation is avoided by using the CBOD5 test in which nitrification is suppressed. The second myth is that the effluent BOD5, or CBOD5, is the residual of the BOD5 in the influent to the lagoon. In fact, most of the effluent CBOD5 is the result of algae that grows in the lagoon. By discarding these two myths, one is in a much better position to understand the performance of aerated lagoon. For domestic wastewaters, the BOD5, or CBOD5, in the influent to a lagoon system consists of two fractions – a particulate fraction consisting of 70 to 80 percent of the total, and a soluble fraction, making up the remainder. In an aerated lagoon cell in which all settleable solids are maintained in suspension by aeration, the removal of the particulate fraction is very rapid, probably no more than 4 to 5 hours. Removal is the result of the physical capture and adsorption by the suspended floc. The time required for the removal of the soluble fraction is somewhat longer, but still quite rapid. The mechanism involved here, is the assimilation of the organic materials for growth.
Attention is directed to Fig. 1. The data points shown there illustrate the soluble BOD5 remaining in the effluent of a full-scale aerated lagoon treating a domestic wastewater at different hydraulic retention times for a range of temperatures varying from 16° to 20° C (Fleckseder and Malina 1970). The curve in Fig. 1 is a plot of an equation that predicts the effluent BOD5 (Rich 1991) using coefficients determined by Jorden et al. (1971) for domestic wastewaters at 20° C. The equation, and, hence the prediction, was developed on the assumption that the BOD5 values truly represented the carbonaceous demand. As was discussed in Technical Note Number 1, the BOD5 values in the plot were most likely to have been inflated by nitrification in the 5-day test. Therefore, the curve is seen to provide a conservative estimate of the soluble BOD5, let alone the soluble CBOD5. From the figure, it is obvious that the residual of the CBOD5 in the influent that is found in the effluent will be quite small.
Figure 1. Effluent soluble BOD5 as a function of the hydraulic retention time. (Taken from Rich (1993))
Municipal wastewaters have an abundance of nitrogen and phosphorus, and, thus,
when treated in lagoon systems with excessive hydraulic retention times, provide
an optimal environment for the growth of algae. The concentration of algae in
the effluent is reflected in the magnitude of the total suspended solids (TSS).
In the absence of algae, the TSS of the effluent of a lagoon system with a
terminal settling cell will normally be less than 10 mg/L. Not only will algae
increase effluent TSS, they will also increase the CBOD5. Such increase is the
result of the respiration of algae during the 5-day test. On the average (Toms
et al. 1975),
CBOD5 = 0.5 TSS (1)
In summary, most of the TSS and CBOD5 in the effluents of lagoons is caused by algae growing in the lagoon. Very little, if any, TSS and CBOD5 in the effluents are residuals of the TSS and CBOD5 that enter the lagoon. Figure 2 illustrates the impact that nitrification in the BOD5 test and algae have on the effluent BOD5 of an aerated lagoon system located in South Carolina. The values with the legend "ABOD5" were the effluent CBOD5 values whereas those with the legend "NBOD5" were derived from the differences in the BOD5 and the CBOD5 values.
Figure 2. Effluent BOD5 and its components - BOD5 caused by algal respiration
(ABOD5) and BOD5 caused by nitrification in the BOD5 (NBOD5).
As discussed in Technical Note Number 1, effluent BOD5 is worthless as a performance parameter. Consequently, most historical records in terms of this parameter are of little value in determining performance, especially for aerated lagoons. However, approximate performance records in terms of CBOD5 can be reconstructed using effluent TSS data.
The effluent CBOD5 can be estimated, using
CBOD5 = SCBOD5 + 0.5 TSS (2)
where SCBOD5 is the soluble CBOD5. It is seen in Fig. 2 of Technical Note Number
1 that the SCBOD5 component of CBOD5 is generally less than 10 mg/L. Thus, if an
aerated lagoon had an effluent TSS of 50 mg/L, the effluent CBOD5 can be
estimated roughly as being
CBOD5 = 10 + 0.5(50)
Fleckseder, H. R. and Malina, J. F. (1970). "Performance of the aerated lagoon process." Technical Report CRWR-71, Center for Research in Water Resources, University of Texas, Austin, TX.
Jorden, W. L. et al. (1971). "Evaluating treatability of elected industrial wastes." Proc. 26th Annual Purdue Indust. Wastes Conf., Purdue University, Lafayette, IN.
Rich, L. G. (1993). "Technical Note No. 1, Aerated Lagoons". Office of Continuing Engineering Education, Clemson University, Clemson, SC.
Toms, I. P. et al. (1975). "Observations on the performance of polishing ponds." Water Pollution Control, 74, 383-401