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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 |
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.
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