Lagoon Systems Can Provide Low-Cost Wastewater Treatment

From Pipeline* by National Small Flows Clearinghouse
This publication has been adapted for use in Indiana by Purdue University**

What are lagoon systems?, Advantages and Disadvantages of Lagoon Systems, Two, Three or Four Lagoons are Better than One,

Facultative Lagoons Treat Wastewater Naturally, Lagoons Use Simple Design, Lagoons Need Proper Operation, Maintenance,

Common Lagoon Problems, Two Montana Towns Use Lagoons, For More Information

Lagoons are one of the most popular methods for wastewater treatment around the world they are also among the simplest and least expensive. Lagoon systems use natural and energy-efficient processes to provide low-cost wastewater treatment for many homes and rural communities in the U.S.

Lagoons are especially well suited to small communities because they can cost less to construct, operate, and are simpler to maintain than other systems. They require more land than other wastewater treatment methods, but land is usually more available and less expensive in rural areas. Lagoons work well for many seasonal rental properties and recreational areas, because they are able to handle intermittent periods of light or heavy use.

What are lagoon systems?

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 groundwater below.

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 Environmental Management.

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.

Anaerobic 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 tanks.

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 treatment.

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.

Naturally Aerobic Lagoons

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.

Aerated Lagoons

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 desired.

Discharge Design: A Design Feature That Can Distinguish Lagoons Is How They Discharge Wastewater

Advantages and Disadvantages of Lagoon Systems

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 cell.

In Series

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 highest.

In Parallel

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.

Facultative Lagoons 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 layers.

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 wastewater treatment.

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 treatment.

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 lagoon.

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 each other.

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 or more.

Lagoons Use Simple Design

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 communities

Site Conditions

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 wastewater.

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.


Wastewater enters and leaves the lagoon through inlet and outlet pipes. Modern designs place the inlet as far as possible from the outlet, on opposite ends of the lagoons, to increase detention times and to prevent short-circuiting. Some lagoons have more than one inlet. Outlets are designed depending on the method of discharge. They often include structures that allow the water level to be raised and lowered.

Aerators, which are used instead of algae as the main source of oxygen in aerated lagoons, work by releasing air into the lagoon or by agitating the water so that air from the surface is mixed in. Aeration always causes turbulence and mixing in the lagoon. Different aerator designs produce either fine or coarse bubbles, and work either on the water surface or submerged. Subsurface aerators are preferable in climates where the lagoon is likely to be covered by ice for part of the year.

Safety Is Important With Lagoon Systems

Lagoons can attract children, pets, and unsuspecting adults, who may think they look like good places to play and even swim. Lagoon bottoms can be both very slick and sticky in places from linings, slime, clay, and sludge, which make it difficult for anyone who has entered a lagoon to get out. Safety training should be made available for homeowners, operators, and anyone else working with these systems. Laws in most areas require lagoons to be surrounded by high fences with locking gates and have warning signs clearly posted.

Lagoons Need Proper Operation, Maintenance

One of the advantages of lagoons is that they require fewer staff hours to operate and maintain than most other systems. However, this doesn't mean they can be neglected. Routine inspections, testing, record keeping, and maintenance are required by local and state agencies, and are all necessary to ensure that lagoons continue to provide good treatment.

Routine Inspections

How often lagoons should be inspected depends on the type of lagoon, how well it functions, and local and state requirements. Some lagoons need more frequent checking in the spring and summer, when grass and weeds grow quickly and when seasonal rental properties are occupied.

Systems with more than one lagoon operated in parallel or series may need operators to check and adjust flow levels or divert flows to and from certain lagoon cells to optimize performance. With aerated systems, mechanical components need to be checked and serviced as needed and according to manufacturer recommendations.

Most inspection visits include brief checks of the banks, dikes, grounds around the lagoon, inlet and outlet pipes, and the appearance, level, and odor (if any) of the water. Records should be kept of every visit and all observations, including information about the weather or other factors that may be influencing lagoon conditions. More extended inspections and formal sampling and testing are periodically necessary.

With regular inspections, testing, and record keeping, operators become familiar with the natural cycles and particular requirements of a system, as well as what factors tend to influence its performance.


Tests required for lagoons include those that measure the wastewater's temperature, pH, and the amount of dissolved oxygen, solids, nitrogen, and disease-causing organisms in the effluent.

Regulatory agencies use water quality measures as indicators of treatment system performance. Among the most important indicators are biochemical oxygen demand (BOD) and total suspended solids (TSS). BOD is important because it measures how much oxygen organisms in the wastewater would consume when discharged to receiving waters. TSS measures the amount of solid materials in the wastewater. If BOD or TSS levels in the effluent are too high, they can degrade the quality of receiving waters.

Together, the results of all these tests can provide a picture of the conditions inside the lagoon and show how well it was performing at the time the tests were taken. But because lagoon conditions change constantly, most tests must be performed several times, and sometimes at specific intervals or times of the day, to get an accurate overall view of the lagoon's health.

Operators can be trained to take samples and perform some or all of the tests themselves. It is usually more practical for part-time operators of small systems to send samples out to a lab to be tested.


Mowing grass and controlling weed growth in and around the lagoon is one of the easiest and most important tasks in lagoon maintenance. Long grass and weeds block wind and provide breeding areas for flies, mosquitoes, and other insects. Weeds also can trap trash, grease, and scum, which cause odors and attract insects. Weeds are used as food by burrowing animals, who can cause damage to banks and dikes. In addition, dead weeds may contribute to increased BOD levels.

It is also important to control weeds that grow on the water surface, like duckweed and watermeal. These weeds take up valuable space that should be occupied by algae, they can stop sunlight from penetrating the wastewater, and slow mixing by the wind.

Scum that collects on the water surface should be removed for the same reasons as duckweed, but also to control odors and insects and to prevent inlet and outlet clogging. Trash, leaves, and branches around the lagoon should be picked up because they can also clog inlet and outlet pipes.

Finally, the depth of the sludge layer in lagoons should be checked at least once per year, usually from a boat using a long stick or hollow tube. In most lagoon systems, sludge eventually accumulates to a point it must be removed, although this may take years. Performance will suffer if too much sludge is allowed to accumulate.

Common Lagoon Problems

Two Montana Towns Use Lagoons


Before 1962, when Polson built its first lagoon system, the city used a series of septic tanks and chlorination to treat its wastewater.

"The disinfected septic tank effluent was discharged directly into Poison Bay and the Flathead River," says John Campbell, water and sewer superintendent for the city of Polson. "Lagoons were an improvement then, and they still work well today."

Located on Flathead Lake in northwest Montana, the city was incorporated in 1910 and has experienced slow, steady growth over the years. Recently, the growth rate has increased to about five percent per year, bringing the current population to about 4,300.

The system built in 1962 consisted of two facultative lagoons. Flows were simply diverted from one lagoon to the other every six months. To accommodate growth, the city built a new system in 1981 with three aerated lagoons and one polishing lagoon. Polson also began to operate its own lab to monitor the system.

"We decided on the aerated system based on recommendations from our engineers, public hearings, and the low operation and maintenance costs," says Campbell. "We're still using the same system today, with some improvements. We've added a wind-powered aerator and mixer that works quite well, and three floating aerators. The only weak points in the system are the original fine bubble aerators, which lie on the bottom and are very prone to clogging."

According to Campbell, residents seem happy with the lack of odor from the system and its low cost. Sewer rates are around $6.50 a month per household, but of that, actual treatment costs are only $8.25 a person per year. In addition, the system won the 1989 U.S. Environmental Protection Agency Region 8 award for operation and maintenance. Since January 1995, effluent biochemical oxygen demand (BOD) levels have averaged 16 mg/L, and total suspended solids (TSS) levels have averaged 38 mg/L.

Currently, Polson is considering ways to upgrade its facilities again, because the system is getting very close to meeting its design flows. Some of the options being considered are replacing the current system with a fully mechanized plant, or expanding the system and adding land application and disinfection. The city will continue to upgrade its collection system.

"Our goal for the future is to construct the most effective, long-lasting system we can afford by 1999."


Officials in the city of Conrad in north central Montana also are deciding about the future of their lagoon system. Parts of Conrad's system have been in use since the 1950s, but its performance has deteriorated recently, and now the town faces some costly problems.

In the 1950s, Conrad constructed its first lagoon system consisting of two facultative lagoons. In 1972, the system was upgraded and an aerated lagoon was added as the primary cell. The town, which currently has about 3,000 residents, relies on this same three-cell lagoon system today. According to Steve Ruhd, Conrad's public works director, the system seemed to work well until 1993, when monitoring showed the quality of the effluent was getting close to being out of compliance.

"We did a sludge judge test in 1994 that showed the aerated cell about 50 percent full of sludge, the second cell about 33 percent full, and the third cell about 25 percent full! This totals to about 97,540 yards of sludge." As far as Ruhd knows, sludge has never been removed from the system.

Now Conrad is trying to find cost effective ways to fix the problem. The city retained an engineering firm to evaluate their options, but proposals for removing and disposing of all the sludge at once ranged from $1.4 to $3.2 million. This approach would have doubled or tripled current rates of $7 to $8 per household per month.

"There is also $400,000 worth of work that needs to be done to the collection system," adds Ruhd.

Having decided that the above solution would be too expensive, Conrad is currently trying to find ways to remove the sludge gradually over a period of years. They hope to form inter-local agreements with other communities in the area to purchase equipment and hire staff to perform regular maintenance and annual sludge removal.

For More Information

Note: Lagoons may be a good alternative for community and cluster systems. They are not recommended for individual residences in Indiana. Additionally, Indiana state law prohibits wastewater discharge from individual residential dwellings.

Health Departments: If you would like more information about lagoon systems or are interested in utilizing one, contact your local health department or the Indiana State Department of Health at (317) 233-7177 for assistance. (Local health department phone numbers are usually listed in the government section of local phone directories.)

National Small Flows Clearinghouse (NSFC): The National Small Flows Clearinghouse (NSFC), which specializes in on-site technology, operation, maintenance, regulations, management, finance, and education, has a variety of free and low-cost products available. NSFC can be reached at (800) 624-8301.

Extension Service: Extension service offices can provide assistance and information about many of the wastewater treatment issues discussed. To locate the extension office in your area, call Purdue University at (888) 398-4636, the U.S. Department of Agriculture at (202) 720-3377 or NSFC.

Rural Community Assistance Program (RCAP): This network of nonprofit organizations can provide assistance to rural and low-income communities with almost every aspect of planning wastewater treatment projects. Call their national office at (703) 771-8636, or call the NSFC for the number of your regional office.

National Rural Water Association (NRWA): NRWA is a nonprofit association organized to represent small water and wastewater utilities in each state and to meet their needs with operation, maintenance, management, funding, and political concerns. It offers a variety of assistance and services. Contact the NSFC for the number of your state RWA office.

*Spring 1997. Vol. 8, No.2.

*Catherine Taylor and Joseph Yahner, Agronomy (765) 494-4773, Don Jones, Agricultural Engineering (765) 494-1167, Purdue University and Alan Dunn, Indiana State Department of Health (317) 233-7177. The On-Site Project is in cooperation with the Indiana State Department of Health.