How do sewer lagoons work

Improve your science content knowledge, demonstrate pedagogy that drives learning experiences and create applicable, standards-based curriculum. Skip to main content. How Does a Lagoon Work? A traditional lagoon system is a two part system. Contribute to odor problems during decay. Other less common problems reported were : 1. Certain types of algae clog effluent filters. A visual impact to the receiving water. Unfortunately, there is a common misconception among some operators that lagoons are mostly uncontrollable treatment systems that do what they do.

The results of our survey revealed that some of our lagoons systems are not being actively operated. On the other hand, there are also a significant number of lagoons that are being actively managed in an attempt to maximize treatment. The operators of these lagoons report that they do have some control over their systems and have some success in controlling the levels of algae. Although algae is a fundamental and natural part of the proper operation of lagoons, in many systems it does reach problem levels and can cause effluent violations.

Before any control action for algae is considered, its potential affect on other parameters needs to be evaluated first. For example, reducing detention time to prevent algae from developing to excessive levels in the first place, may have a negative affect on BOD removal.

The following actions are being used with varying degrees of success by operators in Maine to avoid the TSS problems that are caused by algae : 1. Controlling the loading rate within the system to prevent excessive algae growth or to control the type of algae that does grow. It was reported at a few facilities during our survey that certain types of algae prefer certain loading rates, hence the type and the amount could be controlled by manipulating the loading.

Although the literature on lagoon operation should be checked for guidance on this option, just what loading rate affects which algae is probably somewhat facility specific and may have to be determined experimentally on site. The loading can be increased or decreased to specific cells. This control procedure was attempted at these facilities through step feeding, bypassing certain cells or adjusting individual cell levels. Controlling the detention time within the system or within specific units.

This is related to the above action. Decreasing detention time can prevent excess algae from developing in the first place while increasing it can let it complete its life cycle and die away before it adversely affects the discharge.

This is usually accomplished by controlling the level of lagoons, putting or taking cells off line, and by discharging at varying rates to create or reduce detention times. Some operators have been able to control algae by recycling effluent with either designed recycle pumps or portable pumps.

Hold and release. Those lagoons which have adequate storage capacity, can monitor the effluent quality and then hold wastewater as necessary until the effluent TSS has improved.

The TSS levels in the effluent or in the individual cells can be monitored to determine hold or release times. Experience in operation and close observation can allow operators to predict when algae blooms usually occur so they can anticipate when such actions may be necessary.

Utilize Daphnia to consume excess algae. Some operators maintain a culture of Daphnia and add it at critical algae levels. It occurs naturally at sufficient levels at some facilities. Some operators distribute this natural Daphnia from one cell to another manually, by pumping or by recycling effluent.

Selecting which cell to discharge from. Often one unit, even an upstream one, may have a better TSS level than the final, traditional discharge point. If pumping to achieve this is not part of the design, a portable pump can be used. Varying the vertical level of the discharge draw off to draw from the zone of best water. This can be used to improve the discharge directly or to contain algae within certain units.

Although, there were no reports of success in actively culturing duckweed for this purpose, duckweed cover was reported to naturally shade out excessive algae at some facilities. Although using artificial covers to create shade has been reported to be of success in some other states, the only trial in Maine was ineffective.

There may be some potential in the use of shade to control algae in Maine if an inexpensive and practical way can be found to do it. Observation and records. Observing a particular lagoon system over time and recording the dates and other details regarding algae blooms and related phenomena may enable operators to take measures to control algae levels and TSS before they become a problem.

For example, some operators have determined when algae typically becomes a problem at their facility and release water ahead of time to create holding or detention time in anticipation of the event. An effective process control monitoring system can identify developing algae and TSS problems before they occur.

A n-microscope examination should be used on a regular basis to identify the types and amounts of algae. Odors caused by decaying algae are best controlled by preventing excess algae from growing in the first place. In some cases, increased mixing and outboard motor boats have been used to break up floating algae mats. Michael Richard believes that if C0 2 levels are controlled through the consumption of alkalinity in nitrification without the denitrification step to recover alkalinity then algae will not bloom.

In this case, the C0 2 available for algae growth is limited to that which can be transferred from the air. However, this operational scheme may cause a pH problem. This operational strategy was not observed during our lagoon survey. The only other reported significant cause of TSS violations in lagoons in Maine were attributed to the discharge of excessive levels of Daphnia. In most cases, however, Daphnia was reported to reduce TSS by controlling algae.

Daphnia populations usually increase in response to the algae. Because algae is one of its primary food sources, it usually increases in numbers after the algae has already started to bloom. In some cases, the Daphnia increases quickly enough to limit the amount of algae before it causes TSS effluent violations. In others, it is credited with reducing the magnitude of the TSS violations that do occur.

In a few cases, the Daphnia itself becomes so numerous in response to algae populations that it becomes the major constituent of the TSS in the effluent.

These violations are caused by its discharge in living and dead forms. Excess Daphnia in effluents and in the BOD test bottle can also contribute to BOD demand by using oxygen through respiration or in decomposition.

Use and Control of Daphnia Usually, the level of Daphnia is encouraged in lagoons rather than controlled. However, high nitrite levels can work against promoting the growth of Daphnia because it is toxic to them. Also, they may be prevalent in the spring time, but become low in numbers by mid summer when high numbers are expected.

Many operators seed and promote it for algae control. However, if the control of Daphnia levels does become necessary, it is best done indirectly by controlling the amount of algae.

Because an excess of Daphnia is caused by an excess of algae, some of those actions listed above for controlling algae will also be effective in controlling Daphnia.

BOD Related Problems BOD violations have been noted in operating facilities in all seasons and stem from a number of causes. Some of these violations may not be real, stemming from improper sample collection techniques or from improper testing procedure, or other operational factors.

A number of biochemical processes are at work in lagoon systems that can increase the likelihood of effluent violations. These can be influenced by; high strength wastes, partial nitrification, benthal release of high BOD materials and shifting or recycling BOD in the form of algae, daphnia, duckweed or other organisms. Recognition of these factors with monitoring and control to the extent possible can assist the operator in managing their lagoon facility to limit adverse impacts of these processes.

High Strength Wastes The addition of septage and trucked wastes to lagoon systems can exert a significant load on the process.

The BOD load can cause localized depression of the dissolved oxygen and the inability of the system to assimilate the load unless significant aeration is available and a long detention time is provided.

The TSS load increases the rate of sludge accumulation and leads to benthal BOD releases discussed below that can cause significant operating problems. Lagoon systems should not accept this waste without recognizing the possible impacts and developing the monitoring program necessary to track these systems, implementing the appropriate pretreatment program or addition system and without initiating the appropriate action when critical levels are approached.

Partial Nitrification and Denitrification Partial Nitrification : Nitrification is a biological process involving a unique group of organisms that oxidize ammonia to nitrite and then to nitrate, creating new generations of organisms in the process. This occurrence is normally restricted because the predominate organisms in treatment processes utilize organic material as a food source and are effective competitors for the oxygen necessary for assimilation of food and reproduction this competition restricts the growth potential of the nitrifying organisms.

But, once the majority of the organic material is utilized, this competition is reduced and the nitrification process can occur with less restrictions. So what's the problem, you might ask? True, nitrification can be seen as an indication that the assimilation of organic material has proceeded to the desired extent, but the nitrification reaction uses oxygen and can inadvertently be measured as BOD in the test procedure if nitrification takes place during the incubation period the wastewater added to the BOD bottle contains nitrifying organisms and therefore a "seed".

In reality the facilities that are exhibiting nitrification during their BOD tests are treating the wastewater to a higher degree than facilities that don't unless, of course the facility completely nitrifies during the course of treatment and no nitrogenous demand remains.

The nitrification process also consumes alkalinity and can upset the pH balance within lagoons causing violations. Denitrification : In the absence of oxygen, facultative organisms can use the oxygen taken up during the nitrification process now in the nitrate form for their own growth.

They release gaseous nitrogen, add alkalinity and produce new cells as byproducts of this reaction. This process, known as denitrification, occurs in an anoxic environment low dissolved oxygen and require a source of carbon organic material or BOD to proceed. You can often observe very small bubbles rising to the surface when denitrification is taking place.

It resembles mist or light rain on the surface of the lagoon. If this reaction is occurring you know that nitrification is occurring in your system and that, in some locations, conditions are ideal for denitrification. Control of These Processes : Both of these reactions are temperature dependent, with increased activity at higher temperatures. Therefore, lagoons can cycle in and out of nitrification and denitrification seasonally. This can cause apparent violations of discharge parameters and other operating problems.

First, let's discuss nitrification. As noted above, it is a two part process, with the first step converting ammonia to nitrite and the second step converting nitrite to nitrate. The importance of this is that nitrite can interfere with chlorine based disinfection processes, causing ineffective disinfection at normal doses. Operators should recognize that they may experience seasonal nitrite increases that require an increased chlorine dose to achieve an effective kill, and either monitor the nitrite level or the effectiveness of their chlorination process as a control methodology.

The two step nitrification process also uses a lot of oxygen and alkalinity. For each gram of ammonia converted, 4. This oxygen utilization increases electrical costs and the alkalinity consumption can lead to effluent pH violations in wastewaters with low alkalinity. Operators have tried to increase the organic load at their facilities to limit the ability of the nitrification reaction to occur with mixed success. Others have increased the detention time and the aeration rate during the warmer months to attempt complete nitrification.

If the flexibility is available both techniques can reduce the operational problems associated with these processes.

Increasing the organic load by reducing the detention time will reduce the system operating cost and improve control, but if the flexibility is not available and the monitoring is not in place to track the system performance, effluent violations can result.

The second control philosophy can be an energy intensive process because, as you increase the detention time and increase the aeration rate to complete the nitrification reaction, you may increase your energy costs significantly. Benthal Release of BOD Benthal Release of High BOD Materials : As suspended solids settle and dead microorganisms accumulate, a sludge layer builds up on the bottom of the lagoons. This layer is decomposed by anaerobic and facultative organisms over time.

This process releases organic acids that are very high in BOD. Operating experience has shown that this release is often highest in the early spring after ice out when the anaerobic bacteria become active. This release can be a significant load on the treatment system at a time when biological activity is low and other factors are causing stress on the system e. Control Options Available : A number of techniques have been used by operators to minimize the impact of this load and are described in the following paragraphs.

Control Depth of Bottom Layer : The State of Vermont has evaluated the impacts of sludge layers and recommend that operators develop a monitoring program to track the build up of this layer.

They recommend that this program provide complete coverage of the lagoon bottom, recognize that blanket depth may vary with time of year therefore be consistent in the program and compare readings at similar times of year to gauge growth of the blanket , and they caution that a compacted layer may be difficult to measure accurately.

Their experience shows that some sludge layers will plug a sludge judge and that to get an accurate measurement you must include the difference between the water level in the sludge judge and the lagoon surface in penetrating this solid layer you can plug the judge and push the underlying material out of the way and this depth is represented by the water surface differential.

When the sludge depth reaches 10 inches they recommend removal of the material to limit adverse impacts to the system's operation. A yearly budget allocation is recommended to build a reserve account for this activity, as it can be very expensive. Limit the Solids Load on the System : Another technique is to limit the TSS load to the facility by eliminating trucked waste and septage additions to your system and by requiring pretreatment of wastes from users with high BOD or TSS loadings.

Increase Detention Time in the Spring : Some operators manage the release from their systems so that the storage potential is maximized at the time of spring flow. Increase Aeration Rate in the Spring : Some operators turn on additional aerators or blowers in the spring of the year to provide additional dissolved oxygen to increase the biological activity during high load period. Algae, Daphnia and duckweed growth in lagoon systems can cause operating problems, and in some cases, can offer operational advantages.

These are discussed in more detail in other sections of this manual. This segment will discuss the operational impacts of the death and recycling of these organisms. When adverse conditions are present in a system, these organisms will die and the remaining material may fracture or lyse, releasing the cell contents to the wastestrearn as BOD.

The heavy material will settle to the bottom. Often, these cells do not lyse and simply settle and accumulate on the bottom. In this way they become a sludge deposit that undergoes decomposition and causes the concerns outlined in the previous paragraphs. A few aspects of this process are worthy of note. First, these organisms are predominantly in the second, third or subsequent lagoons because the are able to develop only after the competing microorganisms have reduced the BOD available and died off , while TSS removal occurs largely in the first lagoon.

The importance of this is the understanding that there are mechanisms at work that develop solids layers in subsequent lagoons, causing the need to measure and track the development of this layer. Second, the final lagoons in a system often have less installed aeration potential. Therefore, if a significant benthal load is released in these lagoons, they are not as able to manage that impact without a violation. Finally, algae obtain the carbon necessary for growth from the atmosphere through a fixation process.

For safety and security and to prevent animals from entering the lagoon , fencing is installed on or immediately adjacent to the berm. A BC zero discharge lagoon works best in situations where the volume of wastewater is small due to low water use and the annual evaporation greatly exceeds precipitation. Key considerations when it comes to the design of these lagoons include preventing odour, mosquito breeding, disease transmission by insects, exposure to animals and managing vegetation over the long term tree roots, for instance, can cause leakage.

Lagoon systems that use two or three smaller cells rather than one large cell tend to provide more effective treatment. In brief, this is because each cell can have a slightly different function and design, purifying the wastewater to an increasingly better quality as the water moves along the series.

When lagoon cells are designed to operate in series, there is more time for the solid material in the wastewater grit, sand, food scraps, algae etc. Sometimes, a serial design is the best or only way to ensure that effluent from the lagoon septic systems can meet local requirements. Another consideration is algae growth: some lagoon systems use more cells during the summer months when algae growth is highest.

Lagoons in a series are a common design for community systems because they usually require more land than an individual household has to spare. More land allows for greater wastewater capacity, more cells and treatment that meets regulated standards.

Low-cost construction — Lagoon systems can be cost-effective to build in areas where land is available and inexpensive. Minimum operating cost — These systems use less energy than most other septic systems. Reduced labour — Lagoon systems are easy to operate and maintain. This means they require less labour than other methods and are therefore a great option in areas where labour is limited.

Can handle shock loads — Lagoon systems can handle sporadic use better than many other systems, which makes them a great option for seasonal properties like campgrounds and hotels. Eliminate pathogens — Lagoon systems are very effective at removing disease -causing organisms pathogens from wastewater.

Suitable for irrigation — Because of its high-nutrient and low pathogen content, the effluent from lagoon systems is often suitable for irrigation. Less effective in cold climates — Lagoon systems are less effective in cold climates and may require extra land or a longer retention time period in these regions. Ineffective treatment of heavy metals — Lagoon systems are not effective at removing heavy metals from wastewater.

Bad Odor — Even a properly functioning anaerobic lagoon can produce unpleasant smells. Size — Lagoon systems require much more land than other wastewater treatment methods. May form mosquito habitat — Unless they are properly maintained, lagoons can provide a breeding area for mosquitoes and other insects.

This site uses Akismet to reduce spam. Learn how your comment data is processed. Skip to content Search for: Search Close. 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. 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. 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. 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. 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 , when Polson built its first lagoon system, the city used a series of septic tanks and chlorination to treat its wastewater. Located on Flathead Lake in northwest Montana, the city was incorporated in 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, The system built in 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 with three aerated lagoons and one polishing lagoon.