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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 |
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)
Algal
Species
|
5
Day Oxygen use (mg DO/ mg SS) |
Chlorella
|
0.05
- 0.19
|
Scenesdesmus
|
0.13
|
Nostoc
|
0.09
|
Glocotrichia
|
0.40
|
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)
heterotrophic
bacterial oxidation of BOD
|
5
- 350C |
algal
growth and oxygen
production
|
5
- 350C |
anaerobic
fermentation
|
>140C |
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 |
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.
Executive
Summary
This is the executive
summary concluding findings of the Maine Lagoon Task Force
 |