It is no wonder that one of the most popular methods for wastewater treatment around the world is also one of the simplest and least expensive. Lagoon systems use natural and energy-efficient processes to provide low-cost wastewater treatment.  They are one of the most cost-effective wastewater treatment options for many homes and communities.





What are lagoon systems?


There are several different types and names for lagoons and many possible system designs. Lagoon systems include one or more pond-like bodies of water or basins designed to receive, hold, and treat wastewater for a predetermined period of time. Lagoons are constructed and lined with material, such as clay or an artificial liner that will prevent leaks to the groundwater below. While in the lagoon, wastewater receives treatment through a combination of physical, biological, and chemical processes.  Much of the treatment occurs naturally, but some systems are designed to also use aeration devices that increase the amount of oxygen in the wastewater. Aeration makes treatment more efficient, so that less land area is necessary, and aerators can be used to upgrade some existing systems to treat more wastewater.


Every lagoon system must be individually designed to fit its specific site and use.  Designs are based on such factors as the type of soil, the amount of land area available, the climate, and the amount of sunlight and wind in an area.  Other important design considerations for lagoon systems include the amount and type of wastewater to be treated and the level of treatment required by state and local regulations. Depending on local standards and the final method of disposal chosen, wastewater leaving lagoon systems often requires additional treatment, or “polishing,” to remove disease-causing organisms or nutrients from the wastewater before it can be returned to the environment.






Lagoon Designs


Lagoons are not all the same.  Some employ different biological, chemical, and physical processes to treat the wastewater, while others may play a different role in overall treatment. Some lagoon designs provide adequate treatment for certain methods of discharge, while others should be used in combination with other lagoons or with additional treatment.  Complicating matters further, there can be several different names for the same type of lagoon. 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 and some of the terms used for them.





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 50 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. This layer keeps oxygen out, allowing bacteria and other organisms that thrive in anaerobic conditions to work to treat the wastewater.


As with septic tanks and most other lagoon designs, the layer of sludge that settles at the bottom of an anaerobic lagoon eventually accumulates and must be removed periodically. Also similar to a septic tank, the wastewater that leaves an anaerobic lagoon always requires 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.





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 aerobic lagoons with the aid of aerobicbacteria and algae. Because they are so shallow, their bottoms need to be either paved or lined with materials that will 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 completely.





Aerated Lagoons


Aerated lagoons are very 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 sometimes facultative 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 community treatment systems. Aeration makes treatment more efficient, which offsets energy costs in some cases. Aerated lagoons require less land area and shorter detention times for wastewater than other lagoons.





Facultative Lagoons


Both aerobic and anaerobic conditions exist in facultative lagoons, which also are called stabilization ponds, oxidation ponds, photosynthetic ponds, and aerobic-anaerobic ponds. They are the most common type of wastewater treatment lagoon used by small communities and individual households.


Facultative lagoons can be adapted for use in most climates, require no machinery, and treat wastewater naturally, using both aerobic and anaerobic processes. Because they are used so often by small communities.





Facultative Lagoons Treat Wastewater Naturally


Facultative lagoons can be fascinating places. Like other small bodies of water, they are constantly full of life and activity.  Wastewater is treated through natural processes with the help of bacteria, algae, and other organisms living in the lagoon.





How They Work


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.  However, in general, the wastewater inside 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 climate, the 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). 


Aerobic bacteria and other organisms live in the aerobic zone and contribute to wastewater treatment. 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 all the solids that settle out from the wastewater. In the anaerobic zone, 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 aerobicanaerobic 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.






How Treatment Occurs


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 waste-water. 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 and percolates very slowly through the soil below when site conditions are favorable.


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 byproducts 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.


In every lagoon, there are likely to be other plants and organisms that contribute to, and benefit from, the wastewater treatment processes taking place. These types of interdependent relationships are what make the treatment process in lagoons work






Lagoons Use Simple Design


Lagoons are relatively simple systems to design and construct. However, they always should be designed by qualified professionals who have had experience with them.


Permit requirements and regulations concerning aspects of lagoon design vary from state to state, 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.


In addition, 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 also must be considered. Sometimes trees and tree stumps need to be removed. 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.

Sometimes it is necessary to install diversion terraces or drains at 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.





Lagoon Operation and Maintenance


One of the advantages of lagoons is they usually require fewer staff hours to operate and maintain than 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, and they can stop sunlight and wind from penetrating the wastewater.