Lagoon
Microbiology
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Microorganisms found in aerated and
facultative ponds are more diverse than those observed in many other
biological treatment processes, due to the diverse growth environments present.
Both aerobic and anaerobic bacteria are involved as well as algae and some
higher life forms such as protozoans, rotifers, daphnia,
and insect larvae.
The aerobic bacteria that occur are similar to
those found in other treatment processes such as activated sludge. Three
functional groups occur: freely dispersed, single bacteria; floc-forming
bacteria; and filamentous bacteria. All function similarly to oxidize organic
carbon (BOD) to produce CO2 and new bacteria (new sludge).
Many bacterial species that degrade wastes grow as
single bacteria dispersed in the wastewater. Although these readily oxidize BOD,
they do not settle and hence often leave the lagoon system in the effluent as
solids (TSS). These tend to grow in lagoons at high organic loading and low
oxygen conditions. More important are the floc-forming bacteria, those
that grow in a large aggregate (floc) due to exocellular polymer production (the
glycocalyx). This growth form is important as these flocs degrade BOD and settle
at the end of the process, producing a low TSS effluent.
A number of fiIlamentous bacteria occur in
lagoons, usually at specific growth environments. These generally do not cause
any operational problems in lagoons, in contrast to activated sludge where
filamentous bulking and poor sludge settling is a common problem.
Most heterotrophic bacteria have a wide range in
environmental tolerance and can function effectively in BOD removal over a wide
range in pH and temperature. Aerobic BOD removal generally proceeds well from pH
6.5 to 9.0 and at temperatures from 3-4 oC to 60- 70°C (mesophilic bacteria are
replaced by thermophilic bacteria at temperatures above 35°C). BOD removal
generally declines rapidly below 3-4°C and ceases at 1-2°C.
A very specialized group of bacteria occurs to
some extent in lagoons (and other wastewater treatment systems) that can oxidize
ammonia via nitrite to nitrate, termed nitrifying bacteria. These bacteria are
strict aerobes and require a redox potential of at least +200 m V (Holt et al.,
1994). It was once thought that only two bacteria were involved in
nitrification: Nitrosomonas europaea, which oxidizes ammonia to nitrite, and
Nitrobacter winogradskyi, which oxidizes nitrite to nitrate. It is now known
that at least 5 genera of bacteria oxidize ammonia and at least three genera of
bacteria oxidize nitrite (Holt et al., 1994).
Besides oxygen, these nitrifying bacteria
require a neutral pH (7-8) and substantial alkalinity (these autotrophs use CO2
as a carbon source for growth). The pH effects on the growth of nitrifying
bacteria is shown in Figure 6-2. This indicates that complete nitrification
would be expected at pond pH values between pH 7.0 and 8.5. Nitrification ceases
at pH values above pH 9 and declines markedly at pH values below 7. This results
from the growth inhibition of the nitrifying bacteria.
Nitrification, however, is not a major pathway
for nitrogen removal in lagoons. Nitrifying bacteria exists in low numbers in
lagoons. They prefer attached growth systems and/or high MLSS sludge systems.
Anaerobic,
heterotrophic bacteria that commonly occur in lagoons are involved in methane
formation (acid-fonning and methane bacteria) and in sulfate reduction (sulfate
reducing bacteria). Anaerobic methane formation involves three different groups
of anaerobic bacteria that function together to convert organic materials to
methane via a three step process.
General anaerobic degraders - many genera of
anaerobic bacteria hydrolyze proteins, fats, and poly saccharides present in
wastewater to amino acids, short-chain peptides, fatty acids, glycerol, and
mono- and di-saccharides. These have a wide environmental tolerance in pH and
temperature.
Acid-forming bacteria
- this diverse group of bacteria converts products from above under anaerobic
conditions to simple alcohols and organic acids such as acetic, propionic, and
butyric. These bacteria are hardy and occur over a wide pH and temperature
range.
Methane forming bacteria - these bacteria
convert formic acid, methanol, methylamine, and acetic acid under anaerobic
conditions to methane. Methane is derived in part from these compounds and in
part from CO2 reduction. Methane bacteria are environmentally sensitive
and have a narrow pH range of 6.5- 7.5 and require temperatures > 14 o C.
Note that the products of the acid formers (principally
acetic acid) become the substrate for the methane producers. A problem at times
exists where the acid formers overproduce organic acids, lowering the pH below
where the methane bacteria can function (a pH < 6.5). This can stop methane
fonnation and lead to a buildup of sludge in a lagoon with a low pH. In an
anaerobic fennenter, this is called a "stuck digester". Also, methane
fennentation ceases at cold temperature, probably not occurring in most lagoons
in the wintertime in cold climates.
A number of anaerobic bacteria (14 genera reported to date
(Bolt et al., 1994)) called sulfate reducing bacteria can use sulfate as
an electron acceptor, reducing sulfate to hydrogen sulfide. This occurs when BOD
and sulfate are present and oxygen is absent. Sulfate reduction is a major cause
of odors in ponds.
Anaerobic, photosynthetic bacteria occur in all lagoons
and are the predominant photosynthetic organisms in anaerobic lagoons, The
anaerobic sulfur bacteria, generally grouped into the red and green sulfur
bacteria and represented by about 28 genera (Ehrlich, 1990), oxidize reduced
sulfur compounds (H2S) using light energy to produce sulfur and sulfate, Here,
H2S is used in place of H2O as used by algae and green plants, producing S04-
instead of O2.
All are either strict anaerobes or microaerophilic.
Most common are Chromatium, Thiocystis, and Thiopedia, which can grow in
profusion and give a lagoon a pink or red color. Finding them is most often an
indication of organic overloading and anaerobic conditions in an intended
aerobic system. Conversion of odorous sulfides to sulfur and sulfate by these
sulfur bacteria is a significant odor control mechanism in facultative and
anaerobic lagoons, and can be desirable.
Algae are aerobic organisms that are
photosynthetic and grow with simple inorganic compounds CCO2, NH3, NO3-, and
PO4-- ) using light as an energy source.
**Note that algae produce oxygen during the
daylight hours and consume oxygen at night.
Algae are desirable in lagoons as they generate oxygen
needed by bacteria for waste stabilization. Three major groups occur in lagoons,
based on their chlorophyll type: brown algae (diatoms), green algae, and red
algae. The predominant algal species at any given time is dependent on growth
conditions, particularly temperature, organic loading, oxygen status, nutrient
availability , and predation pressures.
A fourth type of "algae" common in lagoons is the cyano-bacteria
or blue-green bacteria. These organisms grow much as the true algae, with
the exception that most species can fix atmospheric nitrogen. Blue-green
bacteria often bloom in lagoons and some species produce odorous and toxic
by-products. Blue-green bacteria appear to be favored by poor growth conditions
including high temperature, low light, low nutrient availability (many fix
nitrogen) and high predation pressure. Common blue-green bacteria in waste
treatment systems include Aphanothece, Microcystis, Oscillatoria and Anabaena.
Algae can bloom in lagoons at any time of the year (
even under the ice) ; however, a succession of algal types occurs over the
season.
There is also a shift in the algal species present in a
lagoon through the season, caused by temperature and rotiter and Daphnia
predation. Diatoms usually predominate in the wintertime at temperatures
<60°F. In the early spring when predation is low and lagoon temperatures
increase above 60°F, green algae such as Chlorella, Chlamydomonas, and Euglena
often predominate in waste treatment lagoons. The predominant green algae change
to species with spikes or horns such as Scenesdesmus, Micractinium, and
Ankistrodesmus later in the season when Rotifers and Daphnia ( photo to right )
are active (these species survive predation better).
Algae grow at warmer temperature, longer detention
time, and when inorganic minerals needed for growth are in excess. Alkalinity
(inorganic carbon) is the only nutrient likely to be limiting for algal growth
in lagoons. Substantial sludge accumulation in a lagoon may become soluble upon
warming in the spring, releasing algal growth nutrients and causing an algal
bloom. Sludge resolution of nutrients is a major cause of high algal growth in a
lagoon, requiring sludge removal from the lagoon for correction.
The pH at a treatment lagoon is determined by the
various chemical species of alkalinity that are present. The main species
present are carbon dioxide (COJ, bicarbonate ion (HCO3), and carbonate ion
(CO3=). Figure 6-2 illustrates how alkalinity and pH affect which species will
be present. High amounts of CO2 yield a low lagoon pH, while high amounts of
CO3= yield a high lagoon pH.
Bacterial growth on BOD releases CO2 which subsequently
dissolves in water to yield carbonic acid (H2CO3). This rapidly dissociates to
bicarbonate ion, increasing the lagoon alkalinity . Bacterial oxidation of BOD
causes a decrease in lagoon pH due to CO2 release.
Algal growth in lagoons has the opposite effect on lagoon pH,
raising the pH due to algal use for growth of inorganic carbon (CO2 and HCO31.
Algal growth reduces the lagoon alkalinity which may cause the pH to increase if
the lagoon alkalinity (pH buffer capacity) is low. Algae can grow to such an
extent in lagoons (a bloom) that they consume for photosynthesis all of the CO2
and HCO3-present, leaving only carbonate (CO3=) as the pH buffering species.
This causes the pH of the lagoon to become alkaline. pH values of 9.5 or greater
are common in lagoons during algal blooms, which can lead to lagoon effluent pH
violations (in most states this is pH = 9). It should be noted that an increase
in the lagoon pH caused by algal growth can be beneficial. Natural disinfection
of pathogens is enhanced at higher pH. Phosphorus removal by natural chemical
precipitation is greatly enhanced at pH values greater than pH = 8.5. In
addition, ammonia stripping to the atmosphere is enhanced at higher pH values
(NH3 is strippable, not NH4+).
Many higher life forms (animals) develop in lagoons. These
include protozoans and microinvertebrates such as rotifers, daphnia, annelids,
chironomids (midge larvae), and mosquito larvae ( often termed the zooplankton).
These organisms playa role in waste purification by feeding on bacteria and
algae and promoting flocculation and settling of particulate material.
Protozoans
are the most common higher life forms in lagoons with about 250 species
identified in lagoons to date (Curds, 1992). Rotifers and daphnia are
particularly important in controlling algal overgrowth and these often "bloom"
when algal concentrations are high. These microinvertebrates are relatively slow
growing and generally only occur in systems with a detention time of >10 days.
Mosquitos grow in lagoons where shoreline vegetation is not removed and
these may cause a nuisance and public health problem. Culex tarsalis, the vector
of Western Equine Encephalitis in the western U.S., grows well in wastewater
lagoons (USEPA, 1983). The requirement for a minimum lagoon bank slope and
removal of shoreline vegetation by most regulatory agencies is based on the
public health need to reduce mosquito vectors.
Ceriodaphnia image
courtesy of
Marinco Bioassy Laboratory
and James Sweiderk of TheyWill.com
