Design, Operation and
of Lagoons in Maine
This section deals with
discussion of data, design considerations and regulatory
This section entails
general concepts of biological treatment and nitrification
in lagoons and ponds.
This is the executive
summary concluding findings of the Maine Lagoon Task Force
Lagoons are pond-like bodies of water or
basins designed to receive, hold, and treat wastewater for a
predetermined period of time. If necessary, they are lined with
material, such as clay or an artificial liner, to prevent leaks to the
In the lagoon, wastewater is treated
through a combination of physical, biological, and chemical processes.
Much of the treatment occurs naturally, but some systems use aeration
devices to add oxygen to the wastewater. Aeration makes treatment more
efficient, so that less land area is necessary. Aerators can be used
to allow existing systems to treat more wastewater.
Lagoons must be individually designed to
fit a specific site and use. Designs are based on such factors as type
of soil, amount of land area available, and climate. An important
design considerations for lagoons includes the amount and type of
wastewater to be treated and the level of treatment required by
regulations. Wastewater leaving a lagoon may require additional
treatment, or "polishing," to remove disease-causing organisms or
nutrients from the wastewater before it can be returned to the
environment. If surface applied to crops or grassland in Indiana, a
land application permit is needed from the Indiana Department of
There are several different terms for
lagoons. For example, the terms lagoon and pond are often used
interchangeably, and names, such as polishing, stabilization, and
maturation, can refer to a lagoon's particular role in treatment. This
can be very confusing for community leaders and homeowners trying to
evaluate lagoon systems.
The following is a brief overview of
some of the more common types of lagoons.
The word anaerobic means "without
oxygen", which describes the conditions inside this type of lagoon.
Anaerobic lagoons are most often used to treat animal wastes from
dairies and pig farms, commercial or industrial wastes, or as the
first treatment step in systems using two or more lagoons in a series.
Typically, anaerobic lagoons are
designed to hold and treat wastewater from 20 to 150 days.* They are
relatively deep (usually 8 to 15 feet) and work much like septic
Inside an anaerobic lagoon, solids in
the wastewater separate and settle into layers. The top layer consists
of grease, scum, and other floating materials. If not preceded with
septic tanks, the layer of sludge that settles at the bottom of an
anaerobic lagoon eventually accumulates and must be removed. The
wastewater that leaves an anaerobic lagoon will require further
Odor can be a problem with anaerobic
lagoons. However, in many cases odor can be managed through a variety
of methods, such as adding sodium nitrate, recirculating pond
effluent, and through regular maintenance.
Dissolved oxygen is present throughout
much of the depth of aerobic lagoons. They tend to be much shallower
than other lagoons, so sunlight and oxygen from air and wind can
better penetrate the wastewater. In general, they are better suited
for warm, sunny climates, where they are less likely to freeze.
Wastewater usually must remain in aerobic lagoons from 3 to 50 days to
receive adequate treatment.*
Wastewater treatment takes place
naturally in many aerobic lagoons with the aid of aerobic bacteria and
algae. Because they are so shallow, their bottoms need to be paved or
lined with materials that prevent weeds from growing in them.
Sometimes, the wastewater in aerobic
lagoons needs to be mixed to allow sunlight to reach all of the algae
and to keep it from forming a layer that blocks out the air and sun.
Aerial view of the
Veazie Sanitary District's aerated lagoon system.
Aerated lagoons are common in small
communities. These systems use aerators to mix the contents of the
pond and add oxygen to the wastewater. They are sometimes referred to
as partial-mix or complete-mix lagoons depending on the extent of
aeration. Partial-mix aerated lagoons are often anaerobic lagoons that
have been adapted and upgraded to receive more wastewater.
With the exception of wind-driven
designs, most aerators require energy to operate. However, energy
costs are almost always considerably less than those for other
mechanical treatment systems. Aeration makes treatment more efficient,
which offsets energy costs in some cases. Aerated lagoons require less
land area and shorter detention times.
*Exact detention times for wastewater
in lagoons are based on factors such as the particular design, the
amount of wastewater to be treated, and the level of treatment
Discharge Design: A
Design Feature That Can Distinguish Lagoons
Is How They Discharge Wastewater
Continuous Discharge Lagoons.
These lagoons release wastewater continuously to a holding pond, so
the rate of output roughly equals the rate of input. The hydraulic
flow pattern in the lagoon is designed so the wastewater remains in
the lagoon long enough to receive treatment before it reaches the
Controlled Discharge Lagoons.
In these lagoons, wastewater is discharged in controlled amounts,
usually once or twice per year. This method is common in cold
climates where discharges typically occur after spring thaw and
again in fall.
Disadvantages of Lagoon Systems
Lagoon systems can be cost-effective
to design and construct in areas where land is inexpensive.
They use less energy than most
wastewater treatment methods.
They are simple to operate and
maintain and generally require only part-time staff.
They can handle intermittent use and
shock loadings better than many systems, making them a good option
for campgrounds, resorts, and other seasonal properties.
They are very effective at removing
disease-causing organisms (pathogens) from wastewater.
The effluent from lagoon systems can
be suitable for irrigation (where appropriate), because of its
high-nutrient and low pathogen content.
Lagoon systems require more land than
other treatment methods.
They are less efficient in cold
climates and may require additional land or longer detention times
in these areas.
Odor can become a nuisance during
algae blooms, spring thaw in cold climates, or with anaerobic
lagoons and lagoons that are inadequately maintained.
Unless they are property maintained,
lagoons can provide a breeding area for mosquitoes and other
They are not very effective at
removing heavy metals from wastewater.
Effluent from some types of lagoons
contains algae and often requires additional treatment or
"polishing" to meet local discharge standard
Two, Three, or Four
Lagoons Are Better Than One
Many community systems are designed with
more than one lagoon in a series, in parallel, or both. This is
because two or more small lagoons can often provide better quality
treatment than one large lagoon. Multiple lagoons are less common in
systems designed for individual households. In systems that employ
more than one lagoon, each lagoon cell has a different function to
perform, and a different kind of lagoon design may be used for each
When lagoons operate in series, more of
the solid material in the wastewater, such as algae, has an
opportunity to settle out before the effluent is disposed of.
Sometimes serial treatment is necessary so the effluent from lagoon
systems can meet local requirements. Some lagoon systems are designed
to use more cells during the summer months when algae growth is
In parallel means that a system has more
than one cell that is receiving wastewater at the same stage of
treatment. This system design is particularly useful in cold climates
or where lagoons are covered with ice for parts of the year. Because
biological processes are involved, wastewater treatment slows down in
cold temperatures, making treatment less efficient. Parallel cells are
often used during winter months to handle extra loads.
Treat Wastewater Naturally
Like most natural environments,
conditions inside facultative lagoons are always changing. Lagoons
experience cycles due to variations in the weather, the composition of
the wastewater, and other factors. In general, the wastewater in
facultative lagoons naturally settles into three fairly distinct
layers or zones. Different conditions exist in each zone, and
wastewater treatment takes place in all three.
The top layer in a facultative lagoon is
called the aerobic zone, because the majority of oxygen is present
there. How deep the aerobic zone is depends on loading, climate,
amount of sunlight and wind, and how much algae is in the water. The
wastewater in this part of the lagoon receives oxygen from air, from
algae, and from the agitation of the water surface (from wind and
rain, for example). This zone also serves as a barrier for the odors
from gases produced by the treatment processes occurring in the lower
The anaerobic zone is the layer at the
very bottom of the lagoon where no oxygen is present. This area
includes a layer of sludge, which forms from the solids that settle
out of the wastewater. Here, wastewater is treated by anaerobic
bacteria, microscopic organisms, such as certain protozoa, and sludge
worms, all of which thrive in anaerobic conditions.
Names for the middle layer include the
facultative, intermediate, or aerobic-anaerobic zone. Both aerobic and
anaerobic conditions exist in this layer in varying degrees. Depending
on the specific conditions in any given part of this zone, different
types of bacteria and other organisms are present that contribute to
Throughout facultative lagoons,
physical, biological, and chemical processes take place that result in
wastewater treatment. Many of these processes are interdependent. For
example, on the surface, wind and sunlight play important roles.
Surface agitation of any kind adds oxygen to the wastewater. For this
reason, facultative lagoons are designed to make the best use of wind
in the area.
The amount of wind the lagoon receives
is not only important for the oxygen it contributes, but also because
it affects the overall hydraulic flow pattern of the wastewater inside
the lagoon, which is another physical factor that contributes to
Time is another important factor in
treatment. Facultative lagoons are designed to hold the wastewater
long enough for much of the solids in the wastewater to settle and for
many disease-causing bacteria, parasites, and viruses to either die
off or settle out. Time also allows treatment to reduce the overall
organic strength of the wastewater, or its biochemical oxygen demand
(BOD). In addition, some of the wastewater eventually evaporates.
Sunlight is also extremely important to
facultative lagoons because it contributes to the growth of green
algae on the water surface. Because algae are plants, they require
sunlight for photosynthesis. Oxygen is a byproduct of photosynthesis,
and the presence of green algae contributes significantly to the
amount of oxygen in the aerobic zone. The more warmth and light the
sun provides, the more green algae and oxygen there is likely to be in
The oxygen in the aerobic zone makes
conditions favorable for aerobic bacteria. Both aerobic and anaerobic
bacteria are very important to the wastewater treatment process and to
Bacteria treat wastewater by converting
it into other substances. Aerobic bacteria convert wastes into carbon
dioxide, ammonia, and phosphates, which, in turn, are used by the
algae as food. Anaerobic bacteria convert substances in wastewater to
gases, such as hydrogen sulfide, ammonia, and methane. Many of these
by-products are then used as food by both the aerobic bacteria and
algae in the layers above.
In addition, the sludge layer at the
bottom of the lagoon is full of anaerobic bacteria, sludge worms, and
other organisms, which provide treatment through digestion and prevent
the sludge from quickly accumulating to the point where it needs to be
removed. How often sludge must be removed from facultative lagoons
varies depending on the climate, the individual lagoon design, and how
well it is maintained. Sludge in all lagoons accumulates more quickly
in cold than in warm temperatures. However, many facultative lagoons
are designed to function well without sludge removal for 5 to 10 years
Lagoons Use Simple
Lagoons should be designed by qualified
professionals who have had experience with them. Permit requirements
and regulations concerning aspects of lagoon design vary, but there
are some design issues common to all lagoons. The following is a
description of some of the design details for facultative lagoons and
partial-mix aerated lagoons, two common lagoon designs used by small
Certain site-related factors, such as
the location of the water table and the composition of the soil,
always must be considered when designing lagoon systems. Ideally,
lagoons should be constructed in areas with clay or other soils that
won't allow the wastewater to quickly percolate down through the
lagoon bottom to the groundwater. Otherwise, lagoons must be
artificially lined with clay, bentonite, plastic, rubber, concrete, or
other materials to prevent groundwater pollution. Special linings
usually increase system costs.
Most areas in the U.S. have laws
concerning the siting of lagoons, including their distance from
groundwater below, and their distance from homes and businesses.
Lagoons also should be located downgrade and downwind from the homes
they serve, when possible, to avoid the extra cost of pumping the
wastewater uphill and to prevent odors from becoming a nuisance.
The amount and predominant direction of
wind at the site is another important factor, and helps to determine
the lagoon's exact position. Any obstructions to wind or sunlight,
such as trees or surrounding hillsides must be considered. Trees and
weed growth around lagoons should be controlled for the same reasons.
In addition, water from surface drainage or storm runoff should be
kept out of lagoons, if necessary install diversion terraces or drains
above the site.
Size and Shape
The exact dimensions of lagoons vary
depending on the type of processes they use for treatment, the amount
of wastewater that needs to be treated, the climate, and whether other
lagoons or other types of treatment are also being used. The size and
shape of lagoons is designed to maximize the amount of time the
wastewater stays in the lagoon. Detention time is usually the most
important factor in treatment.
In general, facultative lagoons require
about one acre for every 50 homes or every 200 people they serve.
Aerated lagoons treat wastewater more efficiently, so they tend to
require anywhere from one-third to one-tenth less land than
facultative lagoons. Many partial-mix aerated lagoons are simply
former facultative lagoons that have been adapted to receive more
Lagoons can be round, square, or
rectangular with rounded corners. Their length should not exceed three
times their width, and their banks should have outside slopes of about
three units horizontal to one unit vertical. This moderate slope makes
the banks easier to mow and maintain. In systems that have dikes
separating lagoon cells, dikes also should be easy to maintain.
Interior bank and dike slopes are determined by the size and depth of
the lagoon, potential wave action and other factors.
The bottoms of lagoons should be as flat
and level as possible (except around the inlet) to facilitate the
continuous flow of the wastewater. Keeping the corners of lagoons
rounded also helps to maintain the overall hydraulic pattern in the
lagoons and prevents dead spots in the flow, called short-circuiting,
which can affect treatment.
Facultative lagoons are designed to hold
wastewater anywhere from 20 to 150 days, depending on the discharge
method and the exact size and depth of the lagoon. Aerated lagoons
tend to require shorter detention times to treat the same amount of
wastewater. In cold weather, however, biological treatment processes
in all lagoons slow down, making longer detention times necessary.
Facultative lagoons are usually 3 to 8
feet deep, so they have enough surface area to support the algae
growth needed, but are also deep enough to maintain anaerobic
conditions at the bottom. Water depth in lagoons will vary, but a
minimum level should always be maintained to prevent the bottom from
drying out and to avoid odors.
Partial-mix aerated lagoons are often
designed to be deeper than facultative lagoons to allow room for
sludge to settle on the bottom and rest undisturbed by the turbulent
conditions created by the aeration process.