Facultative lagoons are designed to hold wastewater anywhere from 20 to 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.
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. Polson also began to operate its own lab to monitor the system.
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. In addition, the system won the U. Environmental Protection Agency Region 8 award for operation and maintenance. 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. 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 s, but its performance has deteriorated recently, and now the town faces some costly problems. In the s, Conrad constructed its first lagoon system consisting of two facultative lagoons. There must be a 2 X 2 ft concrete pad in the center of the lagoon directly below the opening of the outlet pipe to protect the integrity of the liner. Without the concrete pad, the force of the wastewater could erode the liner.
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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. 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. 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. 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 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 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.
Design Considerations This Section deals with design considerations for all new and future upgrades of existing aerated lagoon wastewater treatment facilities. These design considerations were established by the DEP Lagoon Task Force and based on the site visits of the task force members to each of the existing treatment facilities.
Industrial users and large commercial users must be evaluated for their impact on the system. Reaction rates can change significantly with substantial industrial or commercial wastes. Sludge may accumulate at a faster rate with certain industrial wastes. The heart and soul of any lagoon facility are the total treatment volume and the flexibility to increase or decrease the total detention time by varying the liquid level of each lagoon at any time of the year.
One of the few operational controls is detention time. Develop a site specific KI reaction rate coefficient by reviewing data from nearby lagoon facilities with similar climatic conditions, primarily in the winter months.
The three critical points are: 1 winter when temperatures and reaction rates are low, 2 spring turnover when benthic demand from sludge settled all winter is high, and 3 summer when temperatures and reaction rates are high. Consideration should be given to nitrification. The winter conditions normally control the lagoon volume and the second or third critical points will control aeration capacity. Volume for ice cover and sludge accumulation should be provided in the design.
Avoid small trapezoidal configurations with small bottom areas which leads to unfavorable aeration and nixing zones. The number of cells may have a significant effect on overall sizing. Normally three or four cells should be provided. At a minimum each cell must be removable from service while maintaining treatment. This allows the detention time of each cell to be increased or decreased independently.
Valves must function in any season and may require frost protection. Provide means to measure the water level in each lagoon this allows the operator to accurately measure the water level in each lagoon and assists in the operation of the facility throughout the year. Consider multiple draw off levels for all cells and especially for the final lagoon cell this allows for best type of effluent to be discharged to the receiving water.
Consider lagoon baffles to reduce short circuiting. The sizing of aeration equipment should consider future growth, benthal release, nitrification, standby equipment, and potential peak loads from domestic, commercial and industrial wastes users. Published literature typically recommends providing lbs.
This may consist of portable or permanent pumping facilities and allows for the effluent to be recirculated from one cell to another cell in order to assist in the treatment of wastewater. Pretreatment Facilities The recording of data would be assisted by the use of a computer system. Direct Purchase of Equipment Consider the direct purchase of the following items : Portable trash pump Trailer mounted emergency standby power unit.
Lagoon pontoon boat with trailer. Trailer mounted high pressure sewer flusher. Office furniture. Safety equipment. Utility truck with plow.
Utility tractor with brush hog attachment. Video inspection equipment for sewers. Maintenance tools and shop area. Laboratory equipment needed to perform process control and effluent monitoring functions.
Consider the purchase of a small portable dredge in larger facilities. Lawn mowing and grounds maintenance equipment. Phase contrast microscope. Operation Considerations The Lagoon Task Force has evaluated operating systems and found that effluent violations can occur for a number of reasons, including: BOD, TSS, pH, algae, partial nitrification leading to nitrification in the BOD bottle , inflow and infiltration and other problems.
It appears that causes of these violations include partial nitrification, benthal release, algae, winter cold, detention time, storage capacity, lack of knowledge of the dynamics of the processes at work, and recycling of BOD.
Operator's experience with these issues indicates that the following assessments and control strategies have improved performance in some facilities and may limit effluent violations. The common modes of entry are by roof drains, storm water drains, leaky collection system manholes, foundation drains, sump pumps and directly from the ground via leaky wastewater collection system piping. Although the water is relatively clean, the excess volume it creates contributes to sewer collection system overflows, bypasses and hydraulic related treatment problems at the wastewater treatment facility.
Because the volume and detention times are so much greater in lagoons and because there is usually not anything equivalent to a suspended MLSS, except for in complete mix types of lagoons, high flows do not commonly cause washouts of treatment type solids directly to the receiving water. In this relatively rural state where population and industrial growth is slow, few communities have actually outgrown their lagoon systems. On the other hand, it is a region of heavy rain and snow fall, high water tables and is prone to high seasonal runoff periods.
In addition, many collection systems are old and relatively extensive in comparison to the population served. Detention time : Excess flows reduce the time wastewater can be treated within the system. If it reduces the detention significantly or occurs during cold weather periods when treatment activity is low, it can especially impact BOD removal.
At these times, the wastewater in the lagoons is colder and biological processes are slower. Mgh flows reduce the time for treatment just when more treatment is needed. Although this was not commonly reported in our survey as a major problem, seasonal increases in influent flow and changes in its nature may affect the established process for awhile. Lagoons have periodic seasonal benthic release and pond turnover periods which usually take place in the spring and fall. Excess flows during these periods can result in pass through of excess wastes and nutrients to downstream units and can impact the final discharge.
Short circuiting : Although short circuiting was not identified as a common problem by operators during our survey, it was recognized as an important factor at a few facilities. Obviously, if a lagoon system is prone to short-circuiting, high flows will exacerbate this condition. Often short circuiting is associated with temperature stratification within the lagoons, especially in cold weather.
In these circumstances, high influent flows of a higher temperature can flow across the top layer of the lagoon above the colder, deeper, heavier layers thus receiving only partial treatment in the passing. At times influent waters can be warmer than the deeper lagoon layers due to changes in the seasons, heated sources of water from industries, homes and businesses and due to the lagoon cooling affects of mixing and aeration during colder ambient air conditions.
Stratification disruption : Many lagoons are designed to stratify into zones of aerobic and anaerobic treatment. Aerobic decomposition takes place in the top layer where there is sufficient oxygen and anaerobic decomposition takes place in the lower water and sludge layers where oxygen is lacking. There is an interchange between the layers through settling and benthic release. This relationship allows extended treatment through aerobic, anaerobic and facultative processes.
Excessive flows, especially of a different temperature, can disrupt this stratification, causing partial treatment. Colder, more dense influent flows can disrupt the bottom anaerobic treatment layer while warmer ones can skim across the top inhibiting zonal treatment interchanges.
Storage : Obviously, excessive flows restrict storage options. Process control : The biggest impact of excessive flows reported by operators in our survey was its affect on their process control options. Many operators actively operate their lagoon systems by controlling detention times, lagoon levels, individual cell loadings and through step feeding. Some operators put individual lagoon cells on or off line, store seasonally, operate to promote Daphnia, store during poor water quality periods, manage lagoon loading and holding times to control algae growth and algae die off, etc.
Excessive flows can disrupt these treatment strategies by using up the extra capacity needed to make them possible. For example, controlling detention times and individual cell loadings can be impossible under high flow conditions.
Lagoons licensed only for seasonal discharges can run out of storage and be forced to discharge during unlicensed periods or when effluent quality limits are not being met. Immediate replacement of leaky sewer lines is beyond the economic capability of many communities. A long term upgrade and replacement program needs to be developed to meet these long term needs.
At the least, it is important to get such a program started just to prevent the existing problem from worsening.
Some extraneous water can be eliminated more quickly and economically. Roof drains, leaky manholes and cellar drains and sump pumps can be removed in a short period of time through an aggressive local removal program, by providing alternate discharge options and by more vigorous implementation of existing local codes.
Inflow protectors can be installed under leaky manhole covers. Handling Excess Flows Within the Lagoon There are only a few options in handling excessive flows within lagoon systems : 1.
Draw down during low flow periods in anticipation of seasonal high flows. Some facilities lower levels in anticipation of the springtime surge.
Determine if short-circuiting is a significant problem. This can be done through dye studies, conducting vertical temperature profiles, observing flow patterns, measuring sludge deposit patterns and by reviewing the hydraulic design of the facility length to width ratio, depth, etc.
If short-circuiting is found to be a significant problem, evaluate the inlet and outlet configurations of the system. Upgrade baffling arrangements if necessary. Remove lagoon deposits that may be misdirecting flows. Experiment with running cells in different flow schemes that might overcome inadequacies in design, such as, splitting flows to individual units differently, varying lagoon feed and draw off levels, altering individual cell operating levels, etc.
Increasing or decreasing mixing may have some impact on short-circuiting. The use of curtains within some lagoons has helped in handling high flows and in reducing short-circuiting. Periodic measurement and removal of bottom deposits of grit, sludge etc. If the system has these options, put more cells on line or split flows differently during high flow periods. Observe, track and record the hydraulic characteristics of a specific facility so that high flow problems can be anticipated in the future and preventative actions taken.
Develop a written high flow response plan and revise it as necessary. Total Suspended Solids Many lagoon systems have effluent and operational problems caused by excessive TSS within their systems. Unlike the TSS problems that often occur at activated sludge plants, the source of the TSS in lagoons is usually not caused by a loss of MLSS or a direct pass through of other forms of partially treated wastewater solids.
In most cases, the TSS in lagoons is in the form of algae or, less frequently, in the form of Daphnia. Additional TSS in effluents can derive from rising sludge deposits, pond turnover situations or in short-circuiting.
However, these sources have rarely been reported to be the major causes of TSS violations in Maine. Although some oxygen is obtained through the interface between air and water, most kinds of lagoons, especially aerobic, facultative and partially mixed ones, depend on algae to produce a portion of the oxygen used by the bacteria and other microorganisms in breaking down treating the wastewater.
Even though algae is a vital component of these kinds of lagoons and needs to be promoted within the system, in excess it can cause significant effluent compliance problems and once in the receiving water it can exert a D.
Receiving waters are especially sensitive to this during June through September when temperatures are high and water levels can be low. Algae proliferates in lagoons because of the ample supply of nutrients provided by the influent wastewater stream and the good conditions of light.
Because lagoons are relatively shallow with a large surface area and the water in them is relatively clear, sunlight gets good penetration. Most algae get their energy for growth from sunlight through the action of the chlorophyll that exists within its cells.
Chlorophyll is green. This is why the intensity of the green color that occurs in lagoons as well as in lakes is usually a good indicator of the amount of algae that exists within these system.
Large populations of algae, often accompanied by an intense green color are called "algae blooms". Because the light is more intense in the spring, summer and early fall and temperatures are more amenable, most blooms occur at this time of year. Often a good supply of nutrients for algae occurs during the spring and summer benthic release periods. Although, blooms are not common in the winter, some lagoons in Maine have been known to bloom profusely under the ice in late winter and early spring.
In the daytime, when algae is utilizing light, it produces and releases oxygen. Because algae utilize dissolved C0 2 in photosynthesis C0 2 is a factor in the acid level in the water , the pH of the water can reach high levels of 10 or more. At night, however, the process is reversed. Then algae use oxygen during respiration and release C0 2 instead. This can deplete the supply of oxygen in the lagoon and may lower the pH if the alkalinity is low.
Because of these differences between day and night, algae can produce dramatic diurnal effects on the D. Also, the decay of dead algae within the lagoon system uses up some measure of D.
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