AERATED LAGOON TECHNOLOGY

by
Linvil G. Rich
Alumni Professor Emeritus
Department of Environmental
 Engineering and Science
Clemson University - 
Clemson, SC 29634-0919 USA
Email: lrich@clemson.edu; 
Tel. (864) 656-5575; Fax (864) 656-0672

  It is useful to consider the differences between the technology and performance of facultative lagoons as they have been used to treat domestic wastewaters with those of the high performance aerated lagoons discussed in Technical Note Number 5. First, the oxygen needed for aerobic treatment in facultative lagoons is supplied primarily by algae, the cultivation of which is a major factor in the lagoon design. On the other hand, in high performance aerated lagoons, oxygen is provided by mechanical aeration (usually with mechanical surface aeration), and features to minimize algal growth are incorporated in their design. In fact, the technology of high performance aerated lagoons is more closely related to the activated sludge process than it is to that of facultative lagoons. Second, because algae is a major component of the effluent suspended solids and BOD, facultative lagoons can not, on a consistent basis, meet secondary effluent limits. Third, there is a spectrum of differences in technology and performance existing between the extremes of the facultative lagoon and the high performance aerated lagoon depending upon the extent of aeration and the size and physical configuration of the system. For this reason alone, it is useful to exam in detail the facultative lagoon. The technology and performance of aerated lagoons approach those of the facultative lagoon as the aerated lagoon becomes larger, shallower, and less aerated.
 

     Facultative lagoons used to treat domestic wastewaters provide an example of a highly stressed aquatic ecosystems. These lagoons consist of a shallow basin in which settleable solids introduced by the wastewater settle to the bottom to for a sludge layer that decomposes anaerobically. If oxygen is present in the water column, the biodegradable organic materials that do not settle are degraded aerobically. The term facultative describes the aerobic-anaerobic nature of the lagoon - an anaerobic bottom region covered by an aerobic top layer. The depth of the latter is in a state of constant fluctuation as the result of changing meteorological conditions. The dominant organisms in the system are algae and bacteria which function in a mutually beneficial relationship.
 

     Attention is directed to Fig. 1. Shown there is a diagram of the relationship between algae and bacteria in a facultative lagoon. Biodegradable organic carbon is introduced by the influent wastewater and converted by bacteria to biomass and carbon dioxide. The latter is utilized photosythetically by algae to form algal biomass and oxygen. The oxygen thus produced becomes available to the bacteria for the degradation of more organic carbon. Algal biomass, unlike bacterial biomass, resists gravity sedimentation. As a result, the effluent of a facultative lagoon generally consists of a high concentration of organic carbon in the form of algal biomass. Due to the presence of detergents and urinary products of humans, nitrogen and phosphorus are present in the lagoon in concentrations in excess to the needs of the growing organisms. Consequently, carbon is the element which limits growth in lagoons. Furthermore, the extreme fluctuations in environmental conditions that occur in facultative lagoons tend to minimize the development of higher organisms such as fish.

      Because of their shallow depths and large surface areas, meteorological conditions play a major role in determining the characteristics and behavior of facultative lagoons. Such conditions are especially important during the summer in influencing the changes taking place over a twenty-four hour period. A classic study conducted on a facultative lagoon located in the piedmont area of South Carolina revealed the extent to which these changes take place. The lagoon treated domestic wastewater and was loaded at 38 lb BOD/acre-day, a loading typical in the Southeast. The study was unique in that several parameters were monitored at different depths within the lagoon continuously over the 24-hour diurnal cycle (Meenaghan and Alley 1963).
 

     Figure 2 illustrates the type of changes that took place during the summer months. During the daylight period, solar radiation heated the lagoon causing thermal stratification, similar in some respects to the seasonal stratification of lakes. Photosynthesis was very active close to the water surface and, as a result, carbon dioxide was stripped from the top layers and the dissolved oxygen concentration increased to super saturation. The low carbon dioxide concentration in the upper layers of the lagoon was reflected by the abnormally high pH values. It is to be noted, however, that even when super saturation existed at the surface of with respect to dissolved oxygen, the lower portion the water column was devoid of dissolved oxygen. The depth where oxygen depletion occurred is labeled in the figure as the oxypause.
 

     After sundown photosynthesis ceased and cooling of the top layers of the lagoon occurred. Surface cooling imparted a greater density to the surface water causing instability and mixing in the water column. By early hours in the summer, the lagoon was unstratified and no gradient existed with respect to most of the water parameters. At sunup, stratification again occurred and the diurnal cycle repeated itself.
 

     Nuisance conditions in facultative lagoons generally occur during the summer period when high temperatures accelerate the oxygen-uptake activities taking place within the lagoon. For such a period, the information revealed in Fig. 2 leads to the following conclusions:

Distinct seasonal variations occur. During the winter months the diurnal changes in the lagoon were minimal compared to those observed during the summer.

     Facultative lagoons are often the source of odors. The odors are primarily caused by two factors - mats of dead algae decomposing at the surface and along the sides of the lagoon, and hydrogen sulfide evolving from the lagoon.
 

     Decaying algal mats give rise to a pig pen odor. Such mats are caused by the periodic occurrence of excessive quantities of filamentous, blue-green algae. This group of algae which flourishes in facultative lagoons during the summer months does not settle like green algae but floats and accumulates at the surface where it decays in the sun, giving off noxious odors. As stressed ecosystems, facultative lagoons are subject to periodic algal blooms.
 

     Hydrogen sulfide is formed from sulfates in wastewaters that become reduced in the anaerobic environment in the lagoon, as well as from the decomposition of proteinaceous materials in the bottom solids. Hydrogen sulfide, which has a rotten egg odor, ionizes in solution to form the equilibrium

     In as much as hydrogen ions are one of the ionic species involved in the equilibrium, the particular form in which the sulfide exists is dependent upon the pH of the system. This dependence is illustrated in Fig. 3. The combined form, H2S, is the only form of the sulfide that is not ionic and can be released from solution as a gas. From the figure it is noted that H2S exists in significant concentrations only at pH values less than 8. Since the top layer of a facultative lagoon during daylight hours in the summer is greater than 8, the layer functions as a lid to retain the sulfide in solution. Only during the night hours, when the pH of the top layer falls below 8 can hydrogen sulfide escape from the lagoon.

     Facultative lagoons have three major disadvantages: odors, variable effluent quality, and large land-area requirement. The sources of odors have been discussed above. Variable effluent quality is a characteristic of all facultative lagoons. The removal of BOD5 will vary from 50 to 95 percent, depending on how much algae is in the lagoon at the time. Only with additional treatment such as intermittent sand filtration can facultative lagoons consistently achieve BOD5 effluent limits now being imposed. Furthermore significant concentrations of nitrogen and phosphorus are found in the effluents. As for land area requirements, a wastewater flow of 1 mgd will require about 30 acres of lagoon, if the latter is loaded at 50 lb BOD5/acre-day.
 

     Consideration of the characteristics of facultative lagoons and their performance provide insight to the behavior of aerated lagoons that do not have the aeration, mixing, size, or depth that they should have. Such insight will provide a guide for design modifications that should be made to the aerated lagoon to achieve optimal performance.