GENERAL CONCEPTS OF BIOLOGICAL
 TREATMENT AND NITRIFICATION
 IN LAGOONS AND PONDS



 

A.1 Introduction

A.2 Facultative Ponds

A.3 Partial Mix Aerated Lagoons

A.4 Complete Mix Aerated Ponds

A.5 Microorganisms

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A.6 Biochemical Interactions

A. 7 Environmental Controlling Factors

A. 8 Reaction Kinetics

A.9 Nitrification

A.10 Summary of Biological Activity

A. 1 Introduction

Lagoons and ponds refer broadly to basins constructed in, or on the ground surface, using earthen dikes to retain the wastewater within which natural stabilization processes occur with the necessary oxygen coming from atmospheric diffusion, photosynthetic and/or mechanical sources. More specifically, there are anaerobic lagoons, facultative ponds, complete mix aerated ponds, partial mix aerated ponds and various hybrids. The following discussion is limited to facultative ponds, complete mix aerated lagoons, and partial mix aerated lagoons.

A.2 Facultative Ponds

Facultative ponds are generally aerobic, however, these ponds do operate in a facultative manner and have an anaerobic zone. Facultative organisms function with or without dissolved oxygen. Treatment in a facultative pond is provided by settling of solids and reduction of organic oxygen demanding material by bacterial activity. Dissolved oxygen is supplied by algae living within the pond and atmospheric transfer through wind action. Facultative ponds are usually 4 - 8 feet in depth and can be viewed as having three layers (see Figure Al). The top six to eighteen inches is aerobic where aerobic bacteria and algae exist in a symbiotic relationship. Aerobic stabilization of BOD by aerobic bacteria occurs in the upper oxygenated layer. The aerobic layer is important in maintaining an oxidizing environment in which gases and other compounds leaving the lower anaerobic layer are oxidized. The middle two to four feet is partly aerobic and partly anaerobic, in which facultative bacteria decompose organic material. The bottom one to two feet is where accumulated solids are decomposed by anaerobic bacteria. BOD can be converted to methane by methane bacteria in the lower anaerobic layer. Maintaining a balance between the depth and surface area is important for facultative ponds to function properly. Aerobic reactions in facultative ponds are limited because they do not have mechanical aeration. Facultative and anaerobic reactions need more time than aerobic reactions to provide the same degree of treatment. The detention time of facultative ponds in Maine is typically over 120 days. The organic loading rate, limited by natural aeration capacity, is usually 15-35 pound BOD5/acre/day.

A.3 Partial Mix Aerated Lagoons

Partial mix aerated lagoons can be viewed as a combined biological process that oxidizes organic oxygen demanding material and a physical operation that allows settling of organic and inorganic solids. Mechanical aeration provides dissolved oxygen needed for aerobic organisms in the lagoon to convert and oxidize the organic material in the wastewater. It also provides the physical mixing necessary to distribute dissolved oxygen, suspend the organic material and bring the organisms into contact with the organic material. Mixing must not be so great at to prevent the settling of solids for both sedimentation and for facultative and anaerobic degradation. Partial mix aerated lagoons provided with adequate aeration can be deeper and smaller than facultative ponds. Typical partial mix aerated ponds are 10 to 16 feet deep and have a detention time of 30 to 60 days. The allowable organic loading is in the range of 50 - 100 pounds of BOD5/acre/day. The oxygen required 2- 4 pounds oxygen/pound BOD5 applied when the ammonia and sludge oxygen demands is included. maintaining the proper relationship between the oxygen supplied and detention time is important to the proper operation of partial mixed aerated ponds. Dissolved oxygen concentrations in ponds may vary significantly over a 24 hour period. The dissolved oxygen concentration can be 200-300% of saturation at mid afternoon due to algal photosynthesis. At night, dissolved oxygen concentrations drops to a minimum, due to bacterial and algal respiration.

A.4 Complete Mix Aerated Ponds

Complete mix aerated ponds can be viewed as extended aeration systems without sludge recycle. Mechanical aeration provides dissolved oxygen needed to maintain an aerobic system and mixing necessary to maintain solids in suspension. The detention time in a complete mix aerated pond is typically 7 to 10 days. The degree of treatment is a function of the mass of biological solids in suspension and detention time.

A.5 Microorganisms

Lagoons and ponds are similar to activated sludge systems in function, however, the mass of biological solids is much less. Lagoons and ponds typically have 50-200 mg/L dry weight biomass compared to activated sludge systems which typically have 1000-5000 mg/L. The overall mass of biological solids determines the rate of biological reaction in a waste treatment system. Lagoons and ponds function 10-20 times slower than activated sludge systems because they have less biomass. The microorganisms responsible for biological treatment in lagoons and ponds are interrelated. Organic material in wastewater entering a lagoon or pond contains organic carbon used as an energy source. Bacteria decompose the organic material and convert it into new bacterial cells and carbon dioxide. The carbon dioxide produced by this process and atmospheric carbon dioxide is used by algae to generate new alga cells and oxygen (during the sunlight period). microscopic animals, called herbivores, graze on the algae and bacteria. Larger animals, called carnivores, graze on the herbivores. Organisms that use free dissolved oxygen are called aerobes. Most of the microorganisms in aerated systems convert food to energy in the presence of free dissolved oxygen. This process is called aerobic respiration. Anaerobes obtain oxygen from chemically bound oxygen compounds such as nitrate and sulfate. This process is called anaerobic respiration. Facultative organisms use either free dissolved oxygen or chemically bound oxygen. Freely dispersed, floc-forming and filamentous bacteria, similar to those found in activated sludge systems, are found in lagoons and ponds. Many of the bacteria found in ponds are motile. The aerobic bacteria oxidize organic carbon to produce carbon dioxide and new bacterial cells that eventually become sludge. Typically, 20-40 percent of the applied organic load becomes sludge. Bacteria are tiny (0.05 microns by 1.0 - 5.0 microns), single-celled organisms. Bacteria get carbon from either organic material or inorganic material. Organisms that get their carbon from organic material are called heterotrophic. Organisms that get their carbon form inorganic sources are called autotrophic. Bacteria are responsible for the majority of the activity in a lagoon system. Protozoa are single-celled organisms. They are larger than bacteria (10 - 200 microns) and more complex. Protozoa are strict aerobes. There are four types of protozoa identified on the basis of locomotion. The amoebae have no defined shape and move by use of finger-like or foot-like protrusions, called pseudopodia (false feet). Free-swimming ciliates propel themselves through the liquid using a rapid rhythmic movement of cilia The third group have flagella rather than cilia. Suctorians have an early pre-swimming stage and an adult stalked stage. Fungi are multicellular, non-photosynthetic, heterotrophic protista. They are strict aerobes. Rotifers and nematodes (worms) are animals found in lagoon systems. Rotifers are multicellular animals and are generally strict aerobes. Anaerobic bacteria that are found in lagoon systems include methane bacteria and sulfur bacteria. Two different groups of anaerobic bacteria convert organic material to methane in a two-step process. Acid-forming bacteria convert organic materials to simple alcohols and organic acids such as acetic, propionic and butyric. Acid-forming bacteria are hardy and function over a wide pH range. Methane-forming bacteria convert the acetic acid to methane. Methane-forming bacteria are environmentally sensitive and function in a narrow pH range of 6.8-7.4. The anaerobic sulfur bacteria oxidize reduced sulfur compounds using light energy to produce sulfur and sulfate. An excess of these bacteria will result a lagoon having a pink or red color. Algae are aerobic organisms that use light as an energy source and grow with simple inorganic compounds such as carbon dioxide, nitrate, nitrite and phosphate. Algae are unicellular or multicellular, autotrophic, photosynthetic protista. They are motile or immotile. Three major groups of algae are found in lagoons, based on their chlorophyll type. They are brown algae (diatoms), green algae, and red algae. The type of algae present in a lagoon is dependent upon environmental growth factors such as temperature, organic loading, oxygen, and nutrient availability. Blue-green bacteria (cyanobacteria) grow like algae except that they can fix atmospheric nitrogen. A common growth pattern for algae is shown in Table 3.

Table 3.(2.)

 Season Growth Conditions Main Organisms Winter low light, low temperature none Spring high light, low temperature diatoms Summer high light, low temperature green algae Fall low light, high temperature blue-green bacteria   During a bloom, algae can grow and consume all of the carbon dioxide and bicarbonate causing the pH to become alkaline. The pH can exceed 9.5 during a bloom.

 A.6 Biochemcial Interactions

Photosynthesis is the process whereby organisms are able to grow utilizing the sun's radiant energy to power the fixation of atmospheric C02 and subsequently provide the reducing power to convert the C02 to organic compounds. The following represents the biochemical reactions that occur during photosynthesis and respiration by algae.

Photosynthesis C02 + 2H20 ---> CH20 + H20 + 02 Carbon Dioxide + Water -> Cells + Water + Oxygen

Respiration CH20 + 02 ---- > C02 + H20 Cells + Oxygen -> Carbon Dioxide + Water

In the presence of light, respiration and photosynthesis can occur simultaneously in algae. However, the respiration rate is low compared with the photosynthesis rate, resulting in a net consumption of carbon dioxide and production of oxygen. In the absence of light, algal respiration continues while photosynthesis stops, resulting in a net consumption of oxygen and production of carbon dioxide. During a standard BOD test algae exert an oxygen demand due to their respiration, decomposition, and stimulation of bacterial respiration when used as food by the bacteria. exact oxygen demand of algae is not known and dependent on the type of algae and initial "health" of the algae. Table 4 shows the BOD depletion versus various algal species.

Table 4(2)

 The intensity and composition of light penetrating a pond surface significantly affects the microbial activity. The available light determines, to a large degree, the level of photosynthetic activity and hence, oxygen production. In general, photosynthetic activity increases with increasing light intensity until the photosynthetic system becomes light saturated. The quality and quantity of light penetrating the pond surface to any depth depends on the presence of dissolved and particulate matter as well as the water absorption characteristics. Because of these restrictions, photosynthesis is significant only in the upper pond layer. Light intensity varies with the time of day and difference in latitude. In Maine, light penetration is drastically reduced by ice and snow cover seasonally. Respiration is a physiological process in which organic compounds are oxidized mainly to carbon dioxide and water. However, respiration does not only lead to the production of carbon dioxide, but to the synthesis of cell material as well. Respiration is an orderly process, catalyzed by enzymes such as the cytochromes and consisting of many integrated step reactions terminating in the reduction of oxygen to water. Aerobic respiration common to species of bacteria, protozoa and higher animals, may be represented by the following simple equation:

enzymes C2H1206 + 602 ------ > 6CO2 + 6H20 + new cells

A. 7 Environmental Controlling Factors

Temperature has a tremendous effect on the rate of cell growth. An increase in temperature of 10*C (within the range of temperature that bacteria can grow) doubles the rate of microbial growth. Aerobic BOD removal works well at temperatures from 3-4'C to 60-70*C (thermophilic bacteria replace mesophilic bacteria at temperatures above 35*C) and declines rapidly below 34*C and ceases at 1-2'C. This accounts, in large part, for the decrease in BOD removal efficiency by lagoons and ponds during the winter months. The growth of organisms is temperature dependent. Table 5.2 shows the required temperature for proper growth.

Table 5(2)

 Temperature changes in nonaerated ponds result in vertical stratification during certain seasons of the year. Stratification results because of an increase in water density with depth caused by a decrease in temperature. During the summer, the upper waters are warmed and the density decreases, and stratification results. The temperature of the upper layer of water is relatively uniform because of n-mixing by the wind. Temperatures change rapidly in the thermocline (transition zone between warmer and colder layers), and the zone is very resistant to mixing. As temperatures decrease during the fall, stratification is decreased and the pond is mixed by wind action. This phenomenon is referred to as the fall overturn. The density of water decreases as the temperature falls below 4*C, and a winter stratification can occur. As ice cover breaks up and the water warms, a spring turnover can also occur. The microorganisms in a lagoon system will respond to the type and quantity of food present. The growth of bacteria can be represented by five distinct phases on a growth curve (see Figure A4). The adaptation or lag phase represents the time required for the organisms to acclimate themselves to the organic material present in the wastewater. Once the bacteria have adapted, the rate of growth becomes logarithmic until food becomes limiting. As food becomes limiting, the rate of growth declines and eventually becomes stationary. When the supply of food becomes insufficient to maintain bacterial mass, the microorganisms use organic matter within their own cells as a source of energy. This is known as endogenous respiration. Growth and, to some extent, activity of microorganisms is controlled by the availability of essential nutrients such as carbon, nitrogen, phosphorus, and sulfur. Nitrogen can be a limiting nutrient for primary productivity involving algae. Figure A5 represents the various forms that nitrogen typically assumes over time in a lagoon or pond. The conversion of organic nitrogen to various other nitrogen forms results in a net loss in total nitrogen. This nitrogen loss may be due to either algal uptake for metabolic purposes or to bacterial action. It is likely that each mechanism contributes to the overall total nitrogen reduction. Phosphorus is often the growth-limiting nutrient in aquatic environments. However, municipal wastewater is normally rich in phosphorus. There are four major sources of oxygen (electron acceptors) available to the bacteria in lagoons, and ponds. The four major sources (free molecular oxygen, nitrate, sulfate and carbon dioxide) are shown below with the redox potential where each can be used by bacteria.

Table 6(2)

Oxygen  Source Reaction Redox  Potential Redox  Potential Status O2 aerobic metabolism >200 MV aerobic NO3 denitrification + 200 mV anoxic SO4 sulfate reduction 0 MV anaerobic SO2 methane -200 mV anaerobic The biological reactions that occur in lagoon systems are related to the food chain as shown in Figure A6. The incoming wastewater contains settleable and dissolved organic matter. Part of the organic matter settles and becomes part of the sludge layer. Part of the dissolved organic matter is digested by heterotrophic (use organic carbon) bacteria using free oxygen to form new cells and release carbon dioxide. Organic nitrogen is converted to ammonia. Autotrophic (use inorganic carbon) bacteria convert the ammonia to nitrite and nitrate. Algae use sunlight, carbon dioxide, nitrogen, and phosphorus to generate new algal cells and release oxygen during the daytime. At night the algae respire and use oxygen and release carbon dioxide. Anaerobic reactions occur in the sludge layer which release hydrogen sulfide and methane. Denitrification (an anoxic reaction) occurs in the sludge layer and releases nitrogen gas.

A. 8 Reaction Kinetics   

BOD removal for partial mix aerated lagoons can be estimated using a complete mix hydraulic model and first order reaction kinetics: Cn/Co = (1/(I+kT/n)n) Where, Cn = effluent BOD Co = influent BOD k = reaction rate T = total hydraulic detention time n = number of cells in series k = k20()(t-20) Where, t = temperature of pond at design (winter) 0 = temperature factor, = 1.036 k = reaction rate at design temperature k20=reaction rate at 20 Degrees Celcuis Taken together, the mathematical expressions suggest that the effluent BOD is a function of the influent concentration, the lagoon operating temperature, the number of cells on-line, and the overall detention time. The effluent concentration will decrease as the influent decreases, given the same operating temperature, number of cells and detention time. As the detention time increases, the effluent concentration decreases. As the temperature increases, the effluent concentration decreases. More cells operated in series will produce a lower effluent concentration given the same overall detention time and operating temperature.

A.9 Nitrification

The forms of nitrogen most often found in wastewater are ammonia (as NH4+), nitrate (as N03-) and organic nitrogen in the form of amines and other nitrogenated compounds. Most of the organic nitrogen is converted to ammonia by ammonifying bacteria. Ammonia is removed in lagoons and ponds by 1) stripping to the atmosphere (this can be significant at pH above 8 which may be induced by algae), 2) assimilation into bacteria and alga cells, and 3) bacterial nitrification (which may be followed by denitrification). Nitrification is a naturally occurring, two-step aerobic biological process through which autotrophic bacteria oxidize the ammonium ion to nitrite or nitrate. In the first step, ammonia is oxidized to nitrite by Nitrosomonas bacteria.

Step 1 NH4+ + 3/2 02 ------ > 2H+ + H20 + N02-

Step 2: Next, the nitrite is oxidized to nitrate by Nilrobacter bacteria. N02- + 1/202 ------ > N03-

The overall energy reaction is: NH4+ + 202 ------ > N03- + 2H+ + H20

Along with obtaining energy, however, some of the ammonium ion is assimilated into cell tissue. The equations for the synthesis of Nitrosomonas and Nitrobacter are shown in the following two equations. 13NH4+ + 15CO2 ------ > N02- + 3C5H7NO2 + 23H+ + 4H20 Nitrosomonas 1ON02- + 5CO2 + NH4+ + 2H20 ------ @' I ON03- + C5H7NO2 + H+(Nitrobacter) Combining the energy and synthesis equations, the overall reaction describing complete nitrification is: I.OONH4+ + 1.8902 + 0.0805CO2 ------ > 0.0161C5H7NO2 + 0.952H20 + 0.984NO3- + 1.98H+ The implications of the equation are significant. The stoichiometric coefficients imply that per mole of ammonium removed, the nitrification process requires a significant amount of oxygen, produces a small amount of biomass, and results in substantial destruction of alkalinity through the production of hydrogen ions. For design purposes the oxygen utilization coefficient is 4.6 g 02 required per g NH4+-N. The biomass yield is 0. 1 g VSS produced (as nitrifiers) per g NH4+- N. Nitrifying bacteria obtain carbon as a structural component for growth and reproduction through bicarbonate alkalinity. In the first biochemical reaction nitrous acid is produced. The combined alkalinity destroyed is 7.1 g alkalinity (as CaC03) per g NH4+-N. The factors that influence nitrification include: influent characteristics, dissolved oxygen, BOD loading, detention time, pH, alkalinity, temperature, nitrifier mass, and lack of toxins. Wastewaters containing high concentrations of ammonia tend to nitrify to a greater degree. As was mentioned previously, nitrification consumes large amounts of oxygen. In order for uninhibited nitrification to occur, an operating D.O. level of 2.0 mg/L is suggested (see Figure A8). Another factor that influences nitrifying bacteria is that they do not compete well against heterotrophic bacteria for D.O. and nutrients. Therefore, before nitrification can take place, the soluble BOD must be sufficiently reduced to eliminate this competition, generally down to 20-30 mg/L. This condition is usually achieved in the final cells of a lagoon system. In general, the longer the detention time, the more likely that nitrification will occur. Activated sludge plants are able to nitrify in 6 - 48 hours. Most lagoons and ponds have detention times of 30 days or longer. Nitrification is enhanced at higher pH's where 7.5 - 8.5 is ideal (see Figure A9), although nitrifying bacteria can adapt outside this range. Nitrification tends to produce acids and alkalinity is also consumed at a rate of 7.14 lbs CaC03/lb NH3 oxidized. Therefore, sufficient alkalinity must be present to buffer the acids produced during nitrification. The minimum concentration of alkalinity is 50-60 mg/L and less than 150 mg/L inhibits both nitrification and algae. Some lagoons and ponds may be alkalinity-limited for nitrification, particularly in the summertime when algae compete for the available alkalinity. The rate of nitrification is greatly influenced by temperature. As the temperature increases, the rate of nitrification increases. Temperatures greater than 20 degrees up to about 3 5 degrees C enhance nitrification (see Figure A 10). Nitrification slows down dramatically or may stop altogether at around 5 degrees C. In the winter lagoons and ponds get down to 0 degrees C. Nitrobacter is more temperature sensitive than Nitrosomonas. With decreasing temperature, Nitrobacter growth decreases resulting in decreased nitrification and a build-up of N02-. A very important factor is that a sufficient population of nitrifying bacteria must be present in order to nitrify. These bacteria are attached growth organisms, meaning that they must attach themselves to the surface of an object. In an activated sludge plant, the surface is a floc particle. In a trickling filter or RBC, the surface is the artificial media. In lagoons and ponds, it is believed that nitrifiers may attach to side slopes, baffles and algal particles. Nitrifying bacteria are more sensitive to inhibitory compounds, such as heavy metals, than are the BOD reducing bacteria, thus the nitrifying bacteria would be the first ones to die off. Nitrifying bacteria are found in the soil and enter the waste treatment system through infiltration and inflow. During the winter, when the ground is frozen, less nitrifying bacteria will be coining into the system. Should any of the factors necessary for complete nitrification be missing or in limited supply, the nitrification cycle may not go to completion during the time the wastewater is contained in the treatment process. This phenomenon, called "partial nitrification", leaves ammonia and/or residual nitrites in the effluent. There are four possible different forms of partial nitrification. The first form is possible but unlikely and occurs when 1) effluent NH4 is < I mg/L, 2) effluent N02 is high, and 3) effluent N03 is < I mg/L. The second form is due to a temporary limiting factor, e.g., low D.O. or a slug discharge. It occurs when effluent NH4 is > I mg/L, but not as high as possible, 2) effluent N02 is < I mg/L, and 3) effluent N03 is > I mg/L, but not as high as possible. The third form is usually due to colder wastewater temperature. It occurs when 1) effluent NH4 is < I mg/L, 2) effluent N02 is > I mg/L, but not as high as possible, and 3) effluent N03 is >1 mg/L, but not as high as possible. The fourth form may be due to a limiting factor, but is usually due to colder wastewater temperature. It occurs when 1) effluent NH4 is > I mg/L, but is not as high as possible, 2) effluent N02 is > I mg/L, but not as high as possible, and 3) effluent N03 is > I mg/L, but not as high as possible. Effluent nitrite concentrations over 0.5 mg/L are considered above normal. "Incipient nitrification" occurs when the nitrifying bacteria just begun to establish themselves, but the detention time is not adequate for detectable ammonia reduction. Lagoons and ponds are not nitrifying, unless they can demonstrate the production of N02 or N03. The proliferation of duckweed may indicate that nitrification is occurring because the duckweed use nitrate to grow.

A. 10 Summary of Biological Activity

 In summary, the removal of organic oxygen demanding material (BOD) and the changes in the forms of nitrogen that occur in lagoons and ponds is dependent on the interrelationship of heterotrophic and autotrophic bacterial activity and algal activity. The aerobic stabilization of carbonaceous BOD is primarily dependent on heterotrophic bacterial activity. Heterotrophic bacterial activity is primarily a function of temperature and oxygen availability. The changes in the forms of nitrogen that occur in lagoons and ponds is, among other factors, related to the degree of autotrophic bacterial nitrification . Nitrification is a function of the availability of free oxygen, the temperature, the BOD loading, and the availability of attachment sites. It has been suggested that algae provide, in part, the attachment sites necessary for bacterial nitrification. The growth of algae is a function of light, temperature, and BOD loading. Figure Al I shows the typical patterns of microorganism growth and activity in lagoons and ponds with the corresponding BOD removal and changes in the forms of nitrogen. Figure Al I also shows a typical BOD curve over a one year period for lagoons and ponds that are nitrifying. The standard BOD test will measure the carbonaceous oxygen demand, nitrogenous oxygen demand, algal respiration and algal die-off.

Click here to get the second section which includes
 the sections listed below.

A.6 Biochemical Interactions

A. 7 Environmental Controlling Factors

A. 8 Reaction Kinetics

A.9 Nitrification

A.10 Summary of Biological Activity

Executive Summary
This is the executive summary concluding findings of the Maine Lagoon Task Force