Figure 14.5 Comparison of simplified model of Chapter 5 to observed CMAL performance. The data shown were taken by L. G. Rich from H. R. Fleckseder and J. F. Malina, Performance of the Aerated Lagoon Process, Technical Report CRWR-71, Center for Research in Water Resources, University of Texas, Austin, TX, 1970. (From L. G. Rich, "Technical Note No. 1, Aerated Lagoons.'' Office of Continuing Engineering Education, Clemson University, Clemson, SC, 1993. Reprinted with permission.)
The kinetics of algal growth in lagoon systems are not as well characterized as the kinetics of bacterial growth. However, data from Toms et al.'" suggests an effective, average algal maximum specific growth rate of 0.48 day ', which corresponds to a minimum SRT of about 2 days. This is consistent with the observation that algal growth can typically be minimized in a single lagoon by maintaining an HRT of less than 2 days. Thus, an SRT (HRT) of 2 days is a reasonable "benchmark" of whether algae can grow.
Although the SRT is equal to the HRT for CMALs, the bioreactor configuration is not as well characterized or controlled for other types of lagoons, making calculation of the SRT less straightforward. ' " Anaerobic lagoons could be sized bv assuming complete mixing with no biomass recycle or retention, but this would give a very conservative design because experience indicates that significant biomass retention occurs due to the settling of particulate matter, which provides surfaces for growth of anaerobic bacteria. This makes the SRT greater than the HRT. The mixing pattern in facultative and facultative/aerated lagoons is quite complex, and changes on a diurnal, daily, and seasonal basis. Consequently, it is not possible to calculate the SRT for such systems. However, since long HRTs are used, the SRTs are also quite long.
Just as for anaerobic processes, the volumetric organic loading (VOL) is the mass of biodegradable organic matter applied to the lagoon system per unit time per unit volume. It is denoted symbolically by I\ s and is expressed in units of kg COD or BODJ(m'-day). It is calculated with Eq. 13.1 in terms of the flow rate and reactor volume or with Eq. 13.2 in terms of the HRT. As illustrated by Eq. 13.4, the VOL for a suspended growth process is related to the SRT by the concentration of active biomass in the reactor and the biomass net process yield. For processes such as anaerobic and facultative lagoons, it is not possible to determine the active biomass concentration. However, the VOL can be used to characterize process performance just as it was used for the various anaerobic processes described in Chapter 13 for which biomass concentrations could not be determined. Thus, it is useful as a design parameter for these more poorly defined systems. Volumetric organic loadings for anaerobic lagoons generally range from 0.2 to 1 kg BOD,/(m -day),2'4144 although somewhat higher loadings can be used in some instances. For facultative and facultative/aerated lagoons they generally range from 0.1 to 0.3 kg BOD,/(m ■ day).'14144 Since the SRT is well defined for CMAL processes, VOL need not be used to design them. However, the VOL is related to the volumetric oxygen requirement, and can be used to estimate whether adequate oxygen transfer can be achieved.
Lagoon performance will deteriorate slightly as the VOL is increased within the applicable loading range, but will deteriorate rapidly as the VOL is increased beyond the applicable range. This is consistent with the typical relationship observed between process performance and SRT. Process performance is relatively independent of the SRT for values significantly in excess of the minimum SRT for the process, but becomes quite sensitive to SRT as it approaches the minimum value. Limiting VOLs corresponds to ensuring adequate SRTs.
The areal organic loading (AOL) is the mass of biodegradable organic matter applied per unit time per unit lagoon surface area, A,. It is denoted symbolically as FAS, and is typically expressed in units of kg COD or BODs/(ha-day). It is calculated as:
The logic behind this loading parameter is made obvious when it is recognized that it is used for lagoons in which the growth of algae is necessary. It represents the balance between the loading of biodegradable organic matter and the production of oxygen by algal growth, which is dependent on the penetration of light across the lagoon surface area. Consequently, it is typically used to characterize the performance of facultative and facultative/aerated lagoons.21 "44
As with the other performance parameters, facultative lagoon performance is relatively insensitive to AOL over a wide range of loadings, but deteriorates rapidly as the normal range is exceeded. Acceptable values of AOL vary widely from one geographic location to another. For example, values on the order of 50 to 70 kg BOD,/(ha ■ day) can be used quite successfully in the Southeastern portion of the United States, while loadings of no more than 20 to 40 kg BODt/(ha ■ day) can be used in the more Northern regions. This range reflects variations in ambient temperature and solar radiation over the annual cycle. Local experience and practice should be consulted to select the appropriate AOL for a particular application.
The AOL can also be used to characterize loadings when particulate biodegradable organic matter is being stabilized anaerobically in benthal deposits. Again, the process oxygen demands are being balanced with the supply of oxygen to the process. Typically, the AOL should not exceed 80 g biodegradable VSS/(m -day) [about 115 g biodegradable COD/(nr • day)] for AEL processes with benthal stabi lization.'Mechanical aeration is used to maintain an oxygen concentration in the overlying liquid of 2 mg/L or more. At this loading and residual oxygen concentration. the reduced products (such as organic acids, hydrogen sulfide, etc.) diffusing from the anaerobic benthal deposits are oxidized by a thin aerobic layer at the top of the deposits. Higher loadings or lower oxygen concentrations result in anaerobic conditions throughout the benthal deposit and the release of odoriferous compounds to the overlying water column and, subsequently, to the atmosphere. Likewise, facultative solids storage lagoons, which provide long-term storage of anaerobically digested solids and maintain an aerobic "cap" by algal growth in the overlying clear fluid, are typically sized based on an AOL of 22.5 kg VS/(ha • year)."
The degree of mixing provided in lagoons differs dramatically from type to type. These differences, coupled with variations in lagoon configuration, result in differences in flow patterns.-1 '
Mechanical mixing is generally not provided in anaerobic lagoons, although some mixing is contributed by gas evolution from the digesting material. The absence of intentional mixing results in settling of solids within the lagoon and the retention of biomass. However, the absence of controlled mixing also causes complex llow patterns and the potential for short-circuiting. Inlet and outlet locations and configurations can be selected to minimize short-circuiting. 1 ""'
Mixing is provided in facultative lagoons by several mechanisms, including wind action, gas evolution, and thermal gradients caused by diurnal heating and cooling of the lagoon surface. As with anaerobic lagoons, the uncontrolled nature of the mixing results in poorly defined How patterns and the potential for short-circuiting. Facultative lagoons tend to be well mixed, and must be configured as tanks in series to achieve some degree of plug flow. As with anaerobic lagoons, short-circuiting is controlled by proper selection of the inlet and outlet location and configuration. The reader is referred elsewhere for a more complete discussion of facultative lagoon configuration. I vu
Mixing is provided in F/ALs by an oxygen transfer device. The power input to that device generally determines the degree of mixing, but different volumetric power inputs are required to achieve complete mixing of soluble versus particulate constituents. A volumetric power input of about 1 kW/1000 trf is adequate to achieve a uniform concentration of dissolved species such as oxygen within the zone of influence of the aerator, " although this power input may not produce a residence time distribution reflective of complete mixing. In contrast, a volumetric power input on the order of 6 to 10 kW/1000 m" may be required to achieve uniform concentrations of settleable solids, with the input depending on the concentration.'1 Another consideration is the volumetric power input required to suspend settleable solids. At power densities below 2 kW/1000 m' all settleable solids will be removed from suspension, leaving only nonsettleable solids. v 11 ' The impacts of this are illustrated in Figure 14.7. There it can be seen that algal concentrations in aerated lagoons (as indicated by the chlorophyll a concentration) decrease significantly at power densities above 2 kW/1000 m\ This is because light penetration is reduced by the solids in suspension. A volumetric power input of at least 6 kW 11)00 m' is
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