Fundamentals
of Lagoon Aeration
For years
wastewater lagoon systems have provided secondary treatment performance to
many small to medium sized communities. The attributes of these processes have
been attractive as cost effective options for the treatment of municipal
wastewater. The aeration segment in these systems is the most critical
component and is the core of their biological treatment process. A lagoon
systems ability to aerate the incoming sewage has a direct impact on the level
of wastewater treatment it achieves. This page will focus on the heart and soul
of these systems - aeration.
An
ample oxygen supply in a wastewater pond system is the key to rapid and
effective wastewater treatment. Oxygen is needed by the bacteria to allow their
respiration reactions to proceed rapidly. The oxygen is combined by the bacteria
with carbon to form carbon dioxide. Without sufficient oxygen being present,
bacteria are not able to quickly biodegrade the incoming organic matter. In the
absence of dissolved oxygen, degradation must occur under septic conditions
which are slow, odorous and yield incomplete conversions of pollutants. Under
septic conditions, some of the carbon will be react with hydrogen and sulfur to
form sulfuric acid and methane. Other carbon will be converted to organic acids
that create low pH conditions in the ponds and make the water more difficult to
treat. For example, treated ponds designed to biodegrade wastewater pollutants
without oxygen often must hold the incoming sewage for six months or longer to
achieve acceptable levels of pollution removal. This is because the
biodegradation of organic matter in the absence of oxygen is a very slow kinetic
process.
The
designers of wastewater lagoon systems must take into account the fate of the
biological cells, called sludges, that would eventually settle to the pond's
bottom. One advantage to using system to treat wastewater is the lack of sludge
volumes that require treatment. Unlike conventional wastewater treatment plants
that must remove excess sludges daily, a pond type system can go from ten to
twenty years without ever needing cleaning. This is because, in the presence of
sufficient oxygen, bacterial cells that settle to the ponds bottom are
eventually biodegraded into carbon dioxide and inert materials. Since a large
portion of municipal wastewater consists of biodegradable organic carbon matter,
much of the settled sludge in the lagoon can be quickly decomposed by the
remaining active bacteria. If sufficient oxygen is not present in the ponds, the
sludge layer will accumulate faster than it can be biodegraded. When this
occurs, the sludges may build up to a point that it must be removed at a faster
rate than would be expected for a pond. This effectively eliminates the main
advantage of a lagoon system which is supposed to be an infrequent sludge
removal need.
Adequate aeration is also an important element in keeping the lagoons content
mixed and in suspension. Even in a partially-mixed hydraulic regime, mixing is
very important to the overall treatment process. With adequate mixing, incoming
pollutants and wastewater are better distributed throughout the entire lagoon
volume. This results in more uniform and efficient treatment. In addition,
solids that settle can be re-suspended by the aerator's mixing action and
brought back into contact with the microbial population floating throughout the
pond. Poor mixing has the effect of creating thick solids deposits that fall to
the lagoon floor before proper treatment has occurred. This causes improperly
treated solids to fall away from the active, overhead treatment process. It also
creates septic conditions on the lagoon bottom which, in themselves, pull
available oxygen out of the upper layers of the pond and reduce the
effectiveness of treatment in the upper zone.
Clearly the proper aeration and mixing of a lagoon is critical if the system is
to properly treat the influent wastewater pollutants. A plant that has never had
proper aeration and mixing will often result in a lot of money spent on
electrical power for aeration purposes with little lagoon oxygen to show for
this expenditure as well as deep accumulations of partially treated sludge on
the lagoons floors which requires frequent removal.
The
proper oxygenation of a treatment system also has important implications for an
emerging wastewater treatment plant toxicity issue. Raw sewage contains large
amounts of ammonia (NH3). The typical ammonia concentration of raw sewage is 30
ppm. This ammonia is present as a natural consequence of the degradation of
protein-based compounds in wastewater. Nitrogen is a major building block of
protein which is present in wastewater due to the discharge of protein based
wastewater materials. As the organic compounds that contain protein are
degraded, ammonia is released into the raw sewage stream. This is problematic
because ammonia has been found to be toxic to aquatic organisms in the water
body. In the presence of adequate oxygen, the nitrogen in ammonia can combine
with the oxygen to form non-toxic nitrate compounds (NO3-). In order for this
reaction to occur, the plant must have sufficient dissolved oxygen available.
Both
coarse and
fine bubble diffused
aeration systems transfer oxygen into the water by creating small bubbles. As
the bubbles travel through the water, oxygen is transferred across the bubble's
surface and into the water.
Mechanical aerators work in the opposite way by creating small
droplets of water using a mixer. These droplets are propelled through the
atmosphere above the ponds surface. Oxygen in the air is transferred into the
small water droplets which then fall back into the water.
Factors to Consider
There are many factors that will act to hinder the transfer of the oxygen load
in a wastewater lagoon system. All of these factors must be considered to ensure
that sufficient air is added to allow the necessary pounds of oxygen per day to
be transferred. Some of these factors include:
The alpha
factor in oxygen transfer relates how well oxygen will diffuse into wastewater
as compared to clean tap water. This is an important consideration because most
aeration equipment is tested and rated in clean water laboratory testing and
these results must be correlated to actual wastewater applications. An alpha
factor of 0.80 is common for fine bubble aeration in wastewater lagoons.
The beta factor
in oxygen transfer considers how dissolved solids in the wastewater will hinder
the diffusion of oxygen as compared to being in clean water with few dissolved
solids. This is an important consideration because aeration equipment is usually
tested and rated in clean water. A typical beta factor of 0.95 is common for
most wastewater applications.
The theta
factor in oxygen transfer relates how temperature changes will affect the rate
at which oxygen can be transferred into the water. This is important because
most aeration equipment is sized and rated at standard temperature conditions of
20 degrees Celsius. It must then be adjusted using the theta factor for the
expected worst case field temperature conditions. Typically, a theta factor of
1.024 is used and the hottest temperatures of the year are considered since it
is more difficult to transfer oxygen into hot water than cold water.
Biological
activity in the ponds is optimized when a minimum dissolved oxygen saturation
concentration of 2.0 ppm is maintained at all times. The aeration equipment
should be sized for this basis.
The atmospheric
pressure at the treatment plant site is an important factor in determining how
much oxygen can be transferred. It is more difficult to transfer oxygen at
higher elevations than at sea level because of changes in the local air
pressure.
The maximum
allowable oxygen saturation concentration that can occur at the field
temperature and lagoon depth conditions must be considered. The maximum amount
of oxygen that water can hold at 20 degrees Celsius is 9.09 ppm. As the water
temperature increases in the summer months, lesser concentrations of oxygen can
be held by the warmer water.
All
of the above factors must be considered collectively to properly size aeration
equipment for any treatment system. These factors can be mathematically related
to match the actual oxygen requirement (AOR) needed to meet field conditions
with the standard oxygen requirements (SOR) at which aeration equipment is rated
in the laboratory. The relationship between AOR and SOR is very important in
properly designing a pond's aeration system. AOR represents the actual amount of
oxygen that needs to be added to the ponds under full design loading, full
ammonia conversion, and worst case temperature conditions. SOR represents the
excess oxygen that must be added in order to make sure that the AOR will be met.
Blowers and diffusers are sized on the basis of SOR which represents clean water
conditions at 20 degrees Celsius and sea level air pressures. They are purchased
on the basis of SOR after first relating the required AOR conditions to SOR
conditions.
Having determined the standard oxygen requirement of a pond system, it is next
important to consider how much air volume in SCFM (standard cubic feet per
minute) will be needed to deliver that mass of oxygen. Each cubic foot of air
added to the lagoon will contain about 0.0173 pounds of oxygen. The oxygen
transfer efficiency (OTE) of a diffuser system is a function of its depth in the
ponds. Typically, an OTE of about 1.6% per foot of depth is found for fine
bubble diffusers in a pond setting. For a lagoon with ten feet of depth, a
transfer efficiency of about 16% could be expected. This means that 16% of the
air added at a depth of ten feet will actively be transferred into the water
while eighty-four percent will be excess and will bubble to the surface. This
seems like an excessive air loss rate, but it is the best now available with
current technology.
Would your wastewater treatment plant benefit by replacing an older aeration
system such as coarse bubble diffusion with fine bubble technology? It's well
worth investigating. It is important to understand some critical terms, such as
oxygen transfer efficiency and alpha, before beginning your evaluation. The
oxygen transfer efficiency of an aeration system is the ratio of the amount of
oxygen that actually dissolves into the water to the total amount of oxygen
pumped into the water. Only the dissolved oxygen is available for treatment, and
any portion of oxygen that does not dissolve is a waste of energy and,
therefore, money.
Finally, aeration basin and aerator layout geometry can dramatically alter
oxygen transfer efficiency. Standard clean water transfer efficiency tests are
usually based on full floor aerator coverage. This creates a nearly ideal
situation for oxygen transfer. A turbulent counter-current flow regime is
established with a volume of water being dragged upward with the rising bubbles,
being opposed by an equal flow of water traveling downward. In large basins,
such as lagoons, full floor coverage is not normally needed to meet the
biological aeration demands. As a result of incomplete coverage, large scale
currents can form a spiral roll in the basins. The air bubbles flow with the
water, which significantly reduces the transfer efficiency. Compared with the
full floor configuration, many more air bubbles short-circuit their way to the
top of the aeration basin.
Although it may be true that fine bubble systems can
provide an appreciable savings in many situations, those who purchase and
operate these systems without verification do so at the peril of their own
bottom line cost. How can you avoid this pitfall? First, test the system
proposed with your wastewater. Do not trust a vendor who does not recommend
oxygen transfer testing. Second, make sure that oxygen transfer testing mimics
the actual conditions, including the configuration of your basin. The cost of
aeration system testing can be high and may not be affordable for many plants.
Before testing, the potential cost savings of switching to fine bubble aeration
should be evaluated. Literature, vender, and EPA values for oxygen transfer
efficiently and alpha can be used. Find plants with similar wastewater to refine
from their experiences the values and contact plants to learn from their
experience. If the investment in fine bubble diffusion appears favorable,
consider testing before replacing the system.
Whether it be a plant upgrade or a new facility, a lagoon system will not work
well without proper aeration. As sludge disposal regulations become stricter and
disposal sites become scarce, proper aeration will reduce the amount of solids
that may have to be removed from the lagoons since more of the incoming
pollutants will be oxidized.
