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Figure 14.2 Facultative lagoon (vertical dimension exaggerated).

ment, biodegradable organic matter is stabilized by aerobic metabolism in the upper zone. The aerobic zone also provides an aerobic "cap" that oxidizes reduced compounds produced in the underlying anaerobic zone, minimizing odor release and oxidizing soluble oxygen-demanding compounds. Effluent is withdrawn from the aerobic zone.

A synergistic relationship exists between the bacteria and the algae in a facultative lagoon. Bacteria stabilize organic matter via anaerobic and aerobic metabolism, resulting in new biomass. During daylight hours, algae produce oxygen in the upper portion of the lagoon, and the bacteria there use it as their electron acceptor. Carbon dioxide produced by the bacteria serves as the carbon source for algal growth, while sunlight provides the necessary energy. However, when light is not available, algae use molecular oxygen to oxidize biodegradable organic matter and obtain energy by heterotrophic metabolism. Although the presence of algae produces most of the oxygen needed by the bacteria for aerobic metabolism, it also results in a decrease in waste stabilization because a portion of the carbon dioxide produced by the bacteria is converted back into particulate organic matter in the form of algal cells. Experience indicates that many algae do not settle well and pass into the lagoon effluent where they contribute biodegradable organic matter and total suspended solids."s

Algal growth is promoted by constructing facultative lagoons as shallow basins, generally 1 to 2 m deep, thereby allowing maximum exposure of the lagoon contents to sunlight; minimizing mixing, so that light can penetrate the upper layers of the lagoon; and balancing the organic loading with the production of oxygen by the algae. Because oxygen production is generally limited by the light available to the algae, and the light available is determined by the lagoon surface area, the organic loading is generally expressed as the mass of biodegradable organic matter applied per day per unit of surface area. The allowable organic loading rates generally result in HRTs of 25 days or more.

Diurnal variations in incident light cause significant changes in the environmental conditions within facultative lagoons."41 During the day, when light is available and algae produce oxygen, the size of the aerobic zone is significant. During the night, however, when light is not available, the size of the aerobic layer is reduced, perhaps to zero. In addition, the diurnal variation in algal activity causes the carbon dioxide concentration to vary, which produces pH variations. During the day, the pH in the aerobic zone can reach values as high as 10 as carbon dioxide is consumed by the algae. During the night, on the other hand, the pH decreases to 7 or below as carbon dioxide is produced by both bacterial and algal respiration. The long HRTs in facultative lagoons, coupled with the high pH values, result in excellent pathogen destruction. In fact, in some instances, facultative lagoons have been used for the disinfection of municipal wastewater. "-" Sedimentation of nematode eggs is another important pathogen removal mechanism.'

Significant variations in facultative lagoon performance occur because of ambient conditions, which vary on both a seasonal and a geographical basis. For example, ambient temperatures vary, and this affects the temperature in the lagoon. The availability of sunlight also varies seasonally and geographically. Thus, wide ranges in environmental conditions can exist within a lagoon, resulting in a wide range in allowable loadings."1 Consequently, care must be used in extrapolating al lowable loadings from one location to another. In some areas, lagoons freeze during the winter, which disrupts performance. This problem can be overcome by making the lagoon large enough to accumulate the wastewater during the portion of the year when performance is unsatisfactory, allowing its discharge only when the effluent quality is acceptable and the receiving water has sufficient assimilative capacity. Lagoons can also be designed to prevent surface water discharges; the water either seeps into the groundwater or evaporates.

Facultative lagoons can remove nitrogen and phosphorus from wastewaters. Nitrogen is removed by two mechanisms: nitrification and denitrification, and ammonia stripping. Because zones of both high and low oxygen concentration exist within a facultative lagoon, the environments required for both nitrification and de-nitrification are present. Ammonia stripping occurs because of the high pH in the aerobic zone of the lagoon. Elevated pH results in conversion of ammonium ion to free ammonia, as illustrated in Eq. 13.13. Free ammonia can be easily volatilized to the atmosphere. Although both mechanisms operate, their relative importance is not known.'424 Elevated pH can also result in the precipitation of phosphorus, thereby removing it from the liquid phase. Although these conversions occur in lagoons, they may not occur consistently. Consequently, effluent nutrient concentrations may fluctuate.

The organic loading on a facultative lagoon can be increased by providing additional oxygen by mechanical means. If only a low level of mixing energy is introduced by the oxygen transfer device, insufficient to completely mix the lagoon, the two zones will be maintained and light penetration will be sufficient for the algae to grow in the same fashion as in facultative lagoons. This provides the basis for facultative/aerated lagoons, which have operation and performance characteristics similar to facultative lagoons, but with somewhat higher allowable loadings.

Aerobic Lagoon. Aerobic lagoons (AELs) are designed and operated to exclude algae. This is accomplished by two means. First, sufficient mixing is used to keep all biomass in suspension, thereby providing turbidity that restricts penetration of light into the water column. The mixing also has the effect of making the SRT equal to the HRT. Second, the HRT is controlled to values less than the minimum SRT for algal growth (about 2 days).2* Because algae are excluded, oxygen must be delivered by mechanical means.

Aerobic lagoon systems can be designed to meet a variety of objectives, including the removal of biodegradable organic matter through its conversion to biomass, the stabilization of organic matter (including synthesized biomass) by aerobic-digestion, and the removal of synthesized biomass by gravity settling.2* Fig ure 14.3 illustrates these process options. Regardless of the objective, the first step in an AEL system is a completely mixed aerated lagoon (CMAL) where sufficient mixing is provided to keep all biological solids in suspension. Just as in activated sludge systems, aerobic bacteria oxidize a portion of the biodegradable organic material into carbon dioxide and water, and convert a portion into new biomass. Consequently, the overall waste stabilization accomplished is the difference in the oxygen demands of the original wastewater and the synthesized biomass. As discussed in Chapter 5, this is equal to 1 - Y, ,„„,., which typically represents stabilization of about 40%. As discussed in Section 9.3.2, nearly complete conversion of biodegradable organic matter into biomass can be accomplished aerobically at SRTs on the order a.

CMAL CMAL CMAL

Figure 14.3 Types of aerobic lagoons (vertical dimension exaggerated): a. stabilization through conversion of biodegradable organic matter; b. organic matter conversion and aerobic digestion; and c. organic matter conversion plus benthal stabilization.

of 2 to 3 days. Experience confirms this for a CMAL, where the HRT equals the

Further removal and stabilization of biodegradable organic matter can be accomplished in a variety of ways. One approach, illustrated in Figure 14.3b, is simply to provide a larger HRT to allow aerobic digestion of the synthesized biomass and any organic solids that entered via the influent. This can be accomplished by constructing a larger CMAL or by constructing several CMALs in series. Lagoons in series provide a slight benefit in terms of overall stabilization, as discussed in Section 12.2.5. Another approach, illustrated in Figure 14.3c, is to provide a lagoon with lower mixing energy in which the biosolids leaving the initial CMAL are removed by gravity sedimentation, stabilized by benthal processes, and stored for later disposal." Benthal stabilization involves anaerobic digestion and the end products are methane, carbon dioxide, organic acids, and nutrients such as ammonia-N. If an oxygen concentration of at least 2 mg/L is maintained in the clear water zone overlying the benthal layer, the reduced products (such as organic acids) will be oxidized as they pass through the upper portion of the settled solids." Steps must also be taken to minimize the growth of algae in the settling lagoon. This is generally accomplished by providing a minimal level of mechanical aeration and by limiting the HRT in the overlying clear water zone to a value less than the minimum SRT for algal growth.'"''"'

14.1.3 Comparison of Process Options

Table 14.1 summarizes the benefits and drawbacks of the various lagoon systems. Anaerobic lagoons are inexpensive, easy to operate, and effective. They can provide significant wastewater flow and load equalization for downstream treatment processes since their HRT is relatively large. Solids production is low, and methane is produced, which can be collected and used as an energy source. Effective destruction of pathogens is also obtained because of the relatively long SRTs, a benefit with many wastewater types. On the other hand, process control is poor because of the lack of mixing and biomass retention systems. Many ANL systems are not covered, except for scum and debris that accumulates at the lagoon surface, and thus they can be significant odor sources. Lagoons can be covered using membranes and other devices, but odor release can still occur from the reactor inlets and outlets. Furthermore, odors will be released when the cover is removed for periodic lagoon cleaning. Although significant removal of biodegradable organic matter can be accomplished with an anaerobic lagoon, effluent quality may be relatively poor, requiring further

Table 14.1 Lagoon Process Comparison

Process

Benefits

Drawbacks

Anaerobic lagoon (ANL)

• Simple construction

• Poor process control

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