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Technical Note Number 5

AERATED LAGOONS FOR SECONDARY TREATMENT

   Historically, effluent data for aerated lagoons treating municipal wastewaters have been expressed in terms of BOD5 and TSS. This is unfortunate. As was discussed in Technical Note 1, the BOD5 parameter is faulty because of its inflation by nitrification taking place in the BOD5 test. Arguing that secondary BOD5 limits were initially established on the basis of values flawed by nitrification, the U.S. EPA has suggested that the CBOD5 limit for secondary treatment be 25 mg/L rather than the 30 mg/L allowed when the limit is stated in terms of BOD5. For this reason, secondary treatment will be defined here as treatment that will consistently meet effluent limits of 25 mg/L for CBOD5 and 30 mg/L for TSS.

    Typically, the general configuration of aerated lagoon systems used in the past to treat domestic wastewaters takes the form of an aerated basin followed by an unaerated polishing pond. Effluent from this type of system rarely meets, on a consistent basis, the secondary limits stated above. Figure 1 illustrates the effluent performance of such a system in Georgia. As was discussed in Technical Note 2, practically all of the effluent TSS and CBOD5 is caused by algae that grows in the lagoon. This being the case, why not design the lagoon system in such a way that algal growth is minimized?

Figure 1.

    Effluent characteristics of an aerated lagoon system in Georgia treating a domestic wastewater. (Courtesy of Bruce Henry)

  Figure 2 illustrates in a conceptual way how algal growth can be minimized through control of the hydraulic retention time (HTR). There is a minimum HTR (point a) required to reduce the influent CBOD5 to an acceptable level. There is also an HRT (point b) beyond which algae become established and grow. The key to the design of a system that will produce an effluent with minimal algae is to design a system where the effective HRT falls between points a and b, preferably close to point b considering the sludge storage function of the system. Also, it must be kept in mind that the effective HRT should be based on a consideration of the initial flow rate as well as the design rate. It is just as important for the system to perform well the day that it goes into operation as it is at the end of its design life. One such system is found in Figure 3.
 

DPMC AERATED LAGOON SYSTEMS

    Figure 3 is a photograph of a dual-power, multicellular (DPMC) aerated lagoon system. The DPMC systems were considered innovative as recent as a decade ago. Now, however, many such systems are operating successfully in the southeastern United States and elsewhere. Design details are found elsewhere (Rich 1999). Essentially, the system consists of four cells in series. For municipal wastewater treatment in the southeastern United States, the system will have, at design flow, a total HRT of 4.5 to 5 days, and a depth of at least 3 m. The first cell (HRT = 1.5 - 2 d) is aerated at 6 W/m3 of volume (30 hp/mgal), a level that will 1) maintain all solids in suspension, and 2) provide oxygen sufficient for the conversion of the influent CBOD to carbon dioxide and biomass. The following three cells, each with a HRT of approximately 1 d, serve the functions of sedimentation, solids stabilization, and sludge storage. Each cell is aerated at 1 W/m3 of volume (5 hp/mgal), a level that permits the settleable solids to settle, but, is sufficient to maintain a thin aerobic layer at the top of the solids deposit. The aerobic layer reduces feed-back of nitrogen and CBOD to the water column, and maintains a stable deposit. Aeration also reduces the dead-space volume of the cells.

    Since the control of algal growth is crucial in the reduction of effluent suspended solids, careful attention is paid to factors influencing such growth. The turbidity created in the first cell by maintaining all settable solids in suspension reduces light in the water column to the extent that very little algal growth occurs in that cell. The focus of concern, therefore, centers on factors in the remaining three cells. Those factors include HRT, multicellular configuration, surface area, and mixing.

Solids stabilization rates are another consideration in the design of DPMC systems. Benthal stabilization occurs as the result of a combination of aerobic and anaerobic mechanisms. The bottom surface area of the cells must be large enough that the solids loading will not exceed that which will result in all biodegradable solids being stabilized over the annual temperature cycle. The frequency at which the sludge must be removed from the system is also a consideration. One DPMC system treating a domestic wastewater for over twelve years at 40 percent of the design hydraulic load has not required sludge removal.

Control of the HRT in DPMC systems is critical to good performance. Where initial flow rates are significantly lower than design flow rates, three options are available to the designer. One option is to divide the system into parallel trains, additional trains to be brought into operation as the wastewater flows increase. The second is to design the system to operate at multiple depths. Thanks to the shallow slopes of typical lagoon basins, a volume increase of almost 50 percent can be attained by increasing the operating depth from 3 m to 4 m. The third option is to install effluent weirs in all three settling cells, and discharge from the cell giving the best effluent.
 

PERFORMANCE

   Since the DPMC system is in the public domain and no warranties apply to ensure proper design, many engineers inject their own biases and irrational reasoning when they design these systems. One of the biggest mistakes is to add additional HRT as a safety factor. Another is to omit aeration in the last settling cell. Furthermore, even if all settling cells are provided with aeration, operators, to save on power costs, will often operate the aerators intermittently. Of course these mistakes result in more algae, and hence, higher effluent TSS and CBOD5 values. For these reasons, the performance data presented here are for systems in which the author is confident that they have been designed and operated correctly.

   Figure 4 illustrates the performance record of a DPMC system located in Berkeley Co., SC. The record was taken from the monthly discharge monitoring reports submitted to the state regulatory agency. Typically, BOD5 was determined rather than CBOD5. In the 9.5 year record presented in the figure, the TSS and BOD5 values never reached 30 mg/L. Keeping in mind that the CBOD5 in the effluent would be less than the corresponding value of the TSS, one can see that the system can clearly meet the 25/30 limits on a consistent basis. The average of the parameter values, both BOD5 and TSS, measured during this period of time was about 12 mg/L.

     Figure 5 illustrates the performance of a DPMC lagoon (2 mgal/d) located at Hampton, SC. The lagoon is followed by an intermittent sand filter for nitrification. Consequently, effluent data is normally collected for the entire system, rather than just for the lagoon. However, a short study was conducted to evaluate the performance of just the lagoon. The first point to be made about Fig. 5 is that the lagoon effluent TSS values were amazingly low, this in spite of the fact that the system was operating at a flow rate of 25 percent of the design rate. The second point is that the BOD5 values are all grossly inflated by nitrification occurring in the BOD5 test. The corresponding CBOD5 values, had they been determined, would have been less than the TSS values.
 

     Figure 6 is a sketch of another DPMC lagoon - intermittent sand filter system. This system, with a design capacity of 3.4 mgal/d, is located at North Myrtle Beach, SC, a resort community where the flows during the summer months are about 3 to 4 times the flow in the winter months. For this reason, the system was designed with two DPMC lagoons in parallel, both discharging to one of nine intermittent sand filters. The system has been in operation for about 12 years, and, for this period of time, only the effluent from the entire system has routinely been evaluated. However, in October 1997, the U. S. EPA, Region 4, Enforcement and Investigations Branch conducted an intensive three day, on-site study of the plant followed by a six-month post evaluation to confirm its performance and to study its costs. Table 1 tabulates the results in terms of the means of two, 24 hour composite samples. The sampling points are indicated in Fig. 6. Flow through the plant during the studies was approximately 50 percent of the design flow.
 

a Standard 5-day, 20°C biochemical oxygen demand
b Carbonaceous BOD5
c Soluble carbonaceous BOD5
d Total suspended solids
e Alkalinity as CaCO3
f Ammonia nitrogen
g Nitrate + nitrite nitrogen
h Total Kjeldahl nitrogen
i Total phosphorus
j Chlorophyll a

 

REFERENCE

Rich, L. G. (1999). High Performance Aerated Lagoon Systems. American Academy of Environmental Engineers, Annapolis, MD