Lagoon Systems - 2...

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.

Aerial view of the Veazie Sanitary District's aerated lagoon system.

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 ?...

Continuous Discharge Lagoons : These lagoons release wastewater continuously to a holding pond, so the rate of output roughly equals the rate of input. The hydraulic flow pattern in the lagoon is designed so the wastewater remains in the lagoon long enough to receive treatment before it reaches the outlet.

Controlled Discharge Lagoons : In these lagoons, wastewater is discharged in controlled amounts, usually once or twice per year. This method is common in cold climates where discharges typically occur after spring thaw and again in fall.

Hydrograph Controlled Release Lagoons : This design can be used for lagoons that discharge directly to surface water. It includes devices that measure the level and quality of the wastewater and receiving water and the velocity of the receiving water to determine when conditions are most favorable for discharge. This method can sometimes eliminate the need for further treatment.

Complete Retention Lagoons : These lagoons are only practical in very dry climates where evaporation rate greatly exceeds rainfall amounts. Wastewater is never released from this type of lagoon. Instead it is allowed to evaporate.

Advantages and Disadvantages of Lagoon Systems...

• Lagoon systems can be cost-effective to design and construct in areas where land is inexpensive.
• They use less energy than most wastewater treatment methods.
• They are simple to operate and maintain and generally require only part-time staff.
• They can handle intermittent use and shock loadings better than many systems, making them a good option for campgrounds, resorts, and other seasonal properties.
• They are very effective at removing disease-causing organisms (pathogens) from wastewater.
• The effluent from lagoon systems can be suitable for irrigation (where appropriate), because of its high-nutrient and low pathogen content.
• Lagoon systems require more land than other treatment methods.
• They are less efficient in cold climates and may require additional land or longer detention times in these areas.
• Odor can become a nuisance during algae blooms, spring thaw in cold climates, or with anaerobic lagoons and lagoons that are inadequately maintained.
• Unless they are property maintained, lagoons can provide a breeding area for mosquitoes and other insects.
• They are not very effective at removing heavy metals from wastewater.
• Effluent from some types of lagoons contains algae and often requires additional treatment or "polishing" to meet local discharge standard.

Two, Three, or Four Lagoons Are Better Than One...

Many community systems are designed with more than one lagoon in a series, in parallel, or both. This is because two or more small lagoons can often provide better quality treatment than one large lagoon. Multiple lagoons are less common in systems designed for individual households. In systems that employ more than one lagoon, each lagoon cell has a different function to perform, and a different kind of lagoon design may be used for each 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.

Lagoons having completely mixing conditions must be designed in series ( at least 4, preferably 7 ). Because, their flow regime close to the plug flow regime. An example given below describes that how does it work, theoretically ?
( C : Effluent BOI 5 concentration. )

Data :

- Influent BOI 5 concentration = 250 mg / L
- Total detention time = 7 days
- First order kinetic coefficient = 1.30 1 / day

1 lagoon :

C = ( C O ) / [ 1 + ( k ) ( t ) ]
C = ( 250 ) / [ 1 + ( 1.30 ) ( 7 ) ] = 25 mg / L

4 lagoons in series :

C = ( C O ) / { [ 1 + ( k ) ( t / n ) ] } n
C = ( 250 ) / { [ 1 + ( 1.30 ) ( 7 / 4 ) ] } 4 = 2 mg / L

7 lagoons in series :

C = ( C O ) / { [ 1 + ( k ) ( t / n ) ] } n
C = ( 250 ) / { [ 1 + ( 1.30 ) ( 7 / 7 ) ] } 7 = 1 mg / L

Result :

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.

Design Considerations...


This Section deals with design considerations for all new and future upgrades of existing aerated lagoon wastewater treatment facilities. The following design issues are to be considered in addition to those standards presently established in TR- 16, Ten State Standards or any other published literature accepted by the DEP or EPA. 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.

Facility Planning...

The DEP and EPA should be contacted early in the process to determine treatment objectives and permit limits. The raw wastewater characteristics (BOD5, TSS, TKN, ammonia and alkalinity) and flows should be accurately defined. Infiltration / inflow allowances should be made for all new systems and upgrades. 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.

Lagoon Facilities...

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.)

• Sizing of the treatment lagoons must be directly related to the climatic conditions and not dictated by a set 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.
• Recognize transitional periods of similar facilities such as spring to summer months and benthic release periods. 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.* Additional recommendations :
• Minimum of 10 foot depth for partial mix aerated lagoons; Multiple inlets and outlets (this minimizes short-circuiting of the wastewater and allows the wastewater to be evenly spread out across each lagoon); Provide bypass capabilities for each lagoon (this allows each lagoon to be taken out of service for periodic maintenance, process control, and discharge flexibility); Consider providing step feed in the first lagoon cell; Provide means to vary the water level in each lagoon (this may consist of a flow structure with an adjustable weir gate). 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.
• Aeration equipment shall be capable of maintaining a minimum dissolved oxygen of 2 mg/L at all times. 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 2-5 lbs. of oxygen per lb. of BOD loading.)
• The selection of aerator equipment should consider the present worth and future annual operation and maintenance costs.
• To improve operator control, provide timers, variable frequency drives and/or D.0, monitoring to control output of aeration equipment.
• Dedicate space for future plant expansion. This may consist of additional treatment lagoon cells, garages, sludge or spray/snow disposal areas, or other types of treatment facilities.
• Evaluate means and methods for sludge removal and resultant solids handling.
• Provide recirculation facilities. 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.
• Provide electrical service entrance capability and expansion space in the motor control center and aeration systems for easy expansion of the facility beyond the anticipated ultimate design loads.
• Review existing wastewater collection and pumping stations and upgrade as needed.
• Provide influent and effluent monitoring stations.

Pretreatment Facilities...

• Provide pretreatment facilities that consist of influent monitoring, mechanical screening, grit removal and influent sampling.
• Provide for grit and screenings removal and resultant solids handling.


• Provide adequate space for the storage of equipment, such as, safety, spare parts, laboratory, office furniture and supplies, plans, records and files.
• Provide adequate garage space for the storage of equipment, such as trucks, portable trash pumps, emergency generators, tractors or other utility vehicles.
• Provide adequate work bench space.
• Provide adequate space for the storage of chemicals.
• Provide adequate laboratory bench space.
• Provide adequate locker and lunch area for staff.

Seasonal Discharge/Stream Sensitive Discharge Facilities...

• Provide additional storage capabilities for treated effluent during periods of low stream flows or poor effluent quality.
• Provide additional treatment facilities such as polishing ponds, filters, sand filter beds or artificial wetlands to help achieve low effluent BOD, TSS, algae, ammonia and phosphorus levels when required by the discharge license limits.
• Provide a 24 hour recording stream gauging station to prorate the discharge of treated effluent to the receiving body.


• Provide 24 hour recording of influent flow, effluent flow, lagoon dissolved oxygen and effluent pH levels. 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.

Inflow & Infiltration...


Inflow and infiltration (I/I) is extraneous, nearly clean water that enters wastewater collection systems directly from rainfall events, snow melt, drainage of wet land areas and from ground water. 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. The impact of I / I on lagoons is somewhat different than the effect it can have on traditional activated sludge plants. 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 the task force's survey of lagoon operators, few of them identified I/I as the primary cause of effluent violations, however some of them said it contributed to treatment problems indirectly by affecting process control, detention times, storage capacity and the ability to control hold and release periods. I / I is the primary cause of flows that exceed the hydraulic design limits of lagoons in Maine. 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. Few lagoons in Maine have combined sewers, so most I/I is attributed to sewer line infiltration, manhole leaks, and roof, foundation and cellar drains.

I / I Problems...

I / I can impact the following aspects of lagoon operation :

1. 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.

2. Seasonal impacts : Often I/I is worse at certain times of the year, especially during the spring and in late fall. 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.

3. 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.

4. 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.

5. Storage : Obviously, excessive flows restrict storage options.

6. 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.

7. As in other types of systems :, I/I can impact headwork's performance, contribute to grit build- up within the system, cause excessive pumping, increased wear of equipment, bypasses etc.

I / I Reduction...

As with all treatment systems, removal of excessive I/I in the collection system is the most effective control method. However, certain types of I/I removal can be very expensive. 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. Tight controls on new sewer line construction and on new service connections can prevent the addition of more I/I and eliminate it in replacement projects.

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.
2. 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. Consider redirecting flows with aerators and/or mixers. 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.
3. Increasing or decreasing mixing may have some impact on short-circuiting.
4. The use of curtains within some lagoons has helped in handling high flows and in reducing short-circuiting.
5. Periodic measurement and removal of bottom deposits of grit, sludge etc. as necessary, especially near inlet structures, helps to preserve lagoon volume and prevent short-circuiting by removing obstructions that may shunt flows in undesirable directions and/or by recreating proper operating depths.
6. If the system has these options, put more cells on line or split flows differently during high flow periods.
7. 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.
8. 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.O. demand through respiration and the process of decay. Receiving waters are especially sensitive to this during June through September when temperatures are high and water levels can be low.

Algae Blooms...

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. Dissolved oxygen in lagoons can rise to very high levels during this period, often exceeding 10 mg/l or more. Because algae utilize dissolved C02 in photosynthesis (C02 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 C02 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.O. levels in lagoons. Also, the decay of dead algae within the lagoon system uses up some measure of D.O. Overall though, algae are thought to produce more oxygen in lagoons than they use.

There are many types of algae and not all of them are green. Some are shade tolerant, some are single celled and others form long filaments. Blue green algae can be particularly noxious when it blooms and can form large slimy mats of decaying algae after it dies. Despite the many differences in algae, most of them contribute to the operation and the problems associated with TSS in lagoons in much the same way.

Algae Problems...

During the lagoon visits and surveys made by the lagoon task force in preparation of this document, operators reported the following problems associated with excess algae :

1. TSS effluent violations.
2. Depletion of oxygen levels at night.
3. The algae die and cause a BOD demand which can contribute to BOD violations.
4. Cause high pH problems and pH violations, destroy alkalinity.
5. Contribute to odor problems during decay.

Other less common problems reported were :

1. Certain types of algae clog effluent filters.
2. A visual impact to the receiving water.

Control of Algae...

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.

2. 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.

3. 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. Other facilities can create short periods of holding time by discharging at higher rates previous to anticipated algae blooms, then holding until die off or until Daphnia reduces the algae/TSS level. Creating holding times of as little as seven days has been reported to be effective in algae/TSS control. 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.

4. 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.

5. 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.

6. 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.

7. Shade. 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.

8. 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.

9. An effective process control monitoring system can identify developing algae and TSS problems before they occur. Tracking TSS , algae, Daphnia, D.O., pH and other trends in the discharge and within the system in graphic form can alert operators ahead of time to developing problems. A n-microscope examination should be used on a regular basis to identify the types and amounts of algae.

10. 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.

11. Dr. Michael Richard believes that if C02 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 C02 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. These wastes, by their nature, are extremely high in BOD and TSS. 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, 4.33 grams of oxygen are used and 7.14 grams of alkalinity are consumed. 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.g., inflow, infiltration, slowly increasing temperature, etc.)

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.

1. 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.

2. 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.

3. 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. This is often done to capture the high spring flows caused by I/I but, also creates the opportunity to store and treat this higher strength waste for a longer period.

4. 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.

Recycling BOD...

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. In this fashion they are adding BOD to the treatment system. However, they also produce oxygen to satisfy some of their demand, so we need to recognize that facilities that exhibit algal growth may be achieving excellent BOD removal and treatment of the wastewaters.



Operators that live and work in Maine have to contend with and prepare for the different seasons. Operators of lagoon systems must change the operation of the system along with the changing seasons. Lagoon systems perform differently during summer months than winter months, plus, changes take place during the spring and fall. From conversations with operators during the site visits, the Lagoon Task Force found that the difference from one season to another is part of the challenge for lagoon operators.

Reduced Treatment (Cold)...

The rate of biological metabolism is influenced by a number of factors and one that causes a significant impact is temperature. During the colder months there will be less biological activity and so treatment of the wastewater will be reduced. One technique to minimize the impact of this phenomenon is to fill the ponds to maximum depth to take full advantage of available space and maximize detention time. Because colder water dissolves more oxygen and biological activity is reduced, less aeration is needed at colder temperatures.

Aerator Maintenance (Winter)...

Since less aeration is required during colder months than warmer months, operators with aspirating aerators should remove aerators that will not be needed during winter months and place them in a storage building. Winter months are a good time to inspect aerators and do any needed maintenance. It is not a good idea to leave non-operating aerators out in the harsh winter weather. Using more aerators than needed will waste power (money), can cause excess foaming and unnecessary wear of equipment. Some operators leave all aerators operating in the event that if some aerators freeze and kick out, they will still have some aeration at the end of the winter season. Some operators take unneeded aerators out for maintenance. If aerators freeze and kick out, or additional aeration is needed, the operator can cut the ice with an ice chisel, or chainsaw to remove the aerator and replace it. Many times, anti-freeze poured down the draft tube will thaw the ice in the draft tube allowing the aerator to be started. A flat bottom boat can be used and will easily slide across. the ice to get to the aerator needing attention. (This should never be done alone nor without proper safety equipment.)

Algae, Daphnia...

Algae and daphnia (a very small crustacean, also known as a water flea) are a common cause for TSS violations at a lagoon system and they both occur during warm weather. When the water warms up, the algae start growing. Usually after there is an algae bloom, the daphnia will start to show. Sometimes there will be so many daphnia, the water or side of the lagoon will turn a reddish color. The daphnia will consume the algae and once the algae is gone most of the daphnia will die off. In early spring, some operators will scrape the sides of the liner, or pick some rocks from the side of the lagoon. They will then bring this to the building and place it in a bucket, or aquarium with an air pump to start daphnia growing before the algae bloom starts. Then when the water warms up and the algae starts to grow, the operator will seed the pond with the daphnia to eat the algae before the algae becomes a problem. Some operators have had some success with the use of daphnia, others have not. There are many things an operator can try to produce a good effluent, usually the operator will need to do several things at a time to produce a good effluent. See the section on TSS and BOD for more information.


Nitrification occurs during the summer season and into early fall. Nitrification starts when the water temperature in the ponds reaches 12 to 15 O C. It usually starts in the first pond and can be tracked to the other ponds. Nitrification happens when ammonia nitrogen is convened to nitrite by Nitrosomonas bacteria and nitrite is convened to nitrate by Nitrobacter bacteria. An operator can track the nitrification process by doing the BOD 5 and C - BOD 5 tests, ammonia, nitrite, nitrate and alkalinity on each pond effluent. If nitrification is occurring, the D.O. and pH levels drop, BOD 5 test results may be elevated and if nitrification is not completed, a considerable increase in chlorine demand will occur. The operators should check the effluent nitrite levels if the chlorine demand keeps increasing. Partial nitrification and high nitrite will increase the demand. The operator should also test alkalinity, ammonia nitrogen, nitrite and nitrate on each pond effluent to see how they change and how nitrification moves through your system. As nitrification takes place, alkalinity is used and the pH will drop. See the section on BOD for more information on nitrification.

Rising Sludge...

Rising sludge usually happens during spring turnover when the water temperature warms up, clumps of sludge will rise to the surface. Rising sludge usually happens in the first pond, as this is where most of the sludge is typically concentrated. Add aeration if possible for mixing and for extra D.O.

Benthal Release...

Benthal release occurs during the spring, as the water temperature warms up. During the winter months when the temperatures are low, there is little biological activity taking place in the lagoon, including the sludge layer. When the water temperature warms up in the spring, biological activity increases in both the water column and in the sludge layer. This increase in activity creates a high oxygen demand. At this time you want to keep a close eye on D.O.'s and have available aeration in place so as to maintain a D.O. of around 2.0 mg/L in the early morning.

Dissolved Oxygen...

A D. 0. monitoring program is critical to understanding the seasonal shifts occurring in a lagoon system. D.O. in lagoon systems is impacted by 1) benthal release, 2) spring turnover, 3) organically overloaded pond, 4) midnight dumping, 5) algal respiration, and 6) nitrification. From spring to early fall the D.O. should be checked with a field probe twice a day at the same place and depth, early morning and again in the afternoon. If there is algae in the lagoons, the D.O. can be around 1.0 mg/L in the morning and greater than 10.0 mg/L in the afternoon. During the colder months less aeration is needed, but D.O. readings should be checked twice per week.


With a well designed, well operated system, odors should not be a problem. However, some odors may occur during the spring, when temperatures start warming up and biological activity increases. If odors become a problem in the spring, check D.O.s and add aeration and mixing if possible. Duckweed can also cause odors, but normally they are not strong enough to cause a problem. Other causes for odors can also occur if your influent is pumped to the plant and the influent sits in a wet well, or force main for long periods between pumping. To correct an odor problem you first need to identify the source.



It is very important that any treatment system, set up and maintain a monitoring program. Over time, as the operator becomes familiar with the system, he or she will be able to use the program to anticipate changes in the system and take timely action to minimize their impact. The following is a suggested monitoring plan. It includes both permit compliance monitoring and operational monitoring.

Monitoring Plan...

The following table lists recommended monitoring considerations to monitor and evaluate lagoon performance. These recommendations should be used in conjunction with normal operating schedules and licensing requirements for effluent monitoring.


In addition to the routine monitoring suggested above, the operator should measure the sludge depth in each lagoon, percent solids and percent volatile solids of the sludge in early spring and fall. Air patterns should be observed daily. The pressure gauges on the air blowers should be read and recorded daily. Data from individual tests can be graphically displayed on trend charts so that the process status can be seen at a glance.

Regulatory Considerations...

Regulatory Provisions...

Authority for the Secondary Treatment Information regulation is based on sections 301 and 304 of the Federal Water Pollution Control Act, commonly known as the Clean Water Act (the Act), as amended. Limits for POTWs are based on two major considerations :

1. Water quality standards : Permit limits based on water quality standards are evaluated on a case-by-case basis to assure that a discharge does not cause or contribute to water quality standard violations. Where water quality concerns are present, limits must be set to protect all uses and criteria of the receiving water.

2. Technology based limits : These apply to all facilities and establish the minimum standards. For POTWs they are set forth in 40 CFR Part 133. The 1987 amendments to the CWA allow alternate limits for treatment for waste stabilization ponds, trickling filters, and others referred to as "equivalent to secondary treatment". Those limits are also described in 40 CFR Part 133. The permit writer is given some latitude to apply these alternate limits on a case-by-case or alternative State requirements that conform to the BOD5 and TSS consistently achievable through proper operation and maintenance by the median (50th percentile) facility in a representative sample.

The legislative history for secondary treatment recognizes that certain biological treatment processes such as trickling filters and oxidation ponds, lagoons and ditches are effective in achieving significant reductions in BOD and TSS, when properly designed. For the most part, these treatment methods, are easier to operate, and are particularly useful in smaller communities. Trickling filters and waste stabilization ponds have long been regarded as appropriate secondary processes for municipal wastewater. Nevertheless, POTWs that use these treatment processes may not consistently meet the current requirements for secondary treatment, due largely to varying geographical/climatic and seasonal conditions. Thus, the language of the Act and the legislative history explicitly allow the use of certain biological treatment facilities to meet secondary treatment requirements, regardless of their capability to consistently provide for 85 percent removal or 30 mg/L of BOD5 and TSS on a 30- day average.

The current federal regulation describes the minimum level of effluent quality attainable by facilities eligible for treatment equivalent to secondary treatment in terms of the parameters - BOD, TSS and pH. The 30-day average BOD5 and TSS shall not exceed 45 mg/L. The 7-day average BOD5 and TSS shall not exceed 65 mg/L. The 30-day average percent removal shall not be less than 65 percent. The effluent values for pH shall be maintained within the limits of 6.0 to 9.0. Where data are available to establish C-BOD limitations for a treatment works, the permitting authority may substitute the parameter C-BOD5 for the BOD5, on a case-by-case basis provided that the levels of C-BOD5 are not less than the following : ( i ) The 30-day average shall not exceed 40 mg/L, ( ii ) The 7-day average shall not exceed 60 mg/L, ( iii ) The 30-day average percent removal shall not be less than 65 percent. Alternative state requirements may be authorized after notice and opportunity for public comment and subject to EPA approval. Alternative requirements shall conform to the BOD5 and TSS effluent concentration consistently achievable through proper operation and maintenance by the median (50th percentile) facility in a representative sample of facilities within a State or contiguous geographical area that meet the definition of facilities eligible for treatment equivalent to secondary treatment. Where data are available, the parameter C-BOD5 may be used for effluent quality limitations. Where concurrent BOD effluent data are available, they must be submitted with the C-BOD data as part of the approval process. Permit adjustments shall require, more stringent limitations when adjusting permits if. ( 1 ) For existing facilities the permitting authority determines that the 30-day average and 7-day average BOD5 and TSS effluent values that could be achievable through proper operation and maintenance of the treatment works, based on an analysis of the past performance of the treatment works, would enable the treatment works to achieve more stringent limitations, or ( 2 ) For new facilities, the permitting authority determines that the 30-day average and 7-day average BOD5 and TSS effluent values that could be achievable through proper operation and maintenance of the treatment works, considering the design capability of the treatment process and geographical and climatic conditions, would enable the treatment works to achieve more stringent limitations.

Analysis of Lagoon and Pond Capabilities...

Consistent with the Federal program, the Lagoon Task Force carried out an analysis of treatment process capabilities and plant performance for lagoons and ponds that are described below. Lagoons and ponds are considered to be basins within which natural stabilization processes occur with the necessary oxygen coming from atmospheric diffusion, photosynthetic and/or mechanical sources. An empirical approach using data from existing facilities was used to assess the effects of climatic and seasonal variation on process capabilities for lagoons and ponds in Maine. In selecting the sample of facilities for the plant performance analyses, consideration was given to assure that the treatment capabilities of the selected POTWs were representative of the true treatment capabilities of lagoons and ponds in Maine. Facilities were included in the sample if it was determined that the facility design, operation and maintenance conformed to generally accepted principles of engineering and standard practice. Additionally, facilities were excluded where exceptional performance resulted from augmentation of the basic unit process by "add-on" processes which go beyond secondary treatment. The data for each POTW in the sample include monthly average values for effluent quality. This approach is consistent with the Federal regulatory requirements. In order to determine treatment capabilities of lagoons and ponds, the task force assumed that the value should reflect an effluent quality that is attainable by median of POTWs using that process. The following plant performance data from the sample of well-designed, operated, and maintained lagoons and ponds represent interpolated values for BOD5 and TSS effluent quality based on monthly averages that were reliably, i.e., 95 percent of time, achieved by a given percentage of POTWs in the sample ( Sample size, n = 19 ).

The data from the plant performance analyses indicate that half of the POTWs in the sample using lagoons and ponds processes achieved a BOD5 effluent quality of a least 25 mg/L and a TSS of at least 22 mg/L.

Recommended Regulatory Changes for BOD5 and TSS...

The analysis does not support adjusting BOD5 and TSS effluent limitations across the board for lagoons and ponds in Maine. The analysis does support adjusting BOD5 and TSS effluent limitations on a case-by-case basis. No adjustments would be allowed where the adjusted effluent concentration would have an adverse effect on water quality, public health, or designated uses of receiving waters. The test for determining adverse effects on receiving waters must involve adequate modeling analyses. Effluent BOD5 and TSS concentrations for eligible facilities could range up to a maximum allowable value of 45 mg/L in a period of 30-day average.

To account for variations in lagoon and pond performance within the range e.g., 30 to 45 mg/L, which may occur due to differences in design, wastewater characteristics, climate, seasonal and unique local factors, the task force proposes that the adjusted permit limitations for lagoons and ponds be set based on an individual facility's performance capability. The task force is recommending that a facility not be allowed to obtain effluent limitations that are any less stringent than the level of effluent quality that a facility is capable of achieving. But not withstanding water quality considerations, no facility would receive 30-day average limits of less than 30 mg/L. This will help to minimize additional pollutant loadings and help ensure that facilities continue to operate in accordance with their design capabilities. The facility must have been designed and operated properly to obtain adjusted effluent limitations.

For facilities designed after 1980, the design should be based on accepted design standards such as TR-16 "Guides for the Design of Wastewater Treatment Works", 1980 Edition. In general for facultative stabilization ponds, 1) the organic loading of BOD5 may vary from 15 to 35 lbs. per acre per day, and 2) the pond should be designed to normally operate within overall depths of 3 feet minimum to 5 feet maximum. For partial mix aerated facultative ponds, 1) the detention time should be based on many variables such as waste strength, volume, temperature and nutrient balance and intended removal efficiencies. With normal biological reaction rates and removal requirements, the hydraulic detention time generally should be at least 20 to 30 days. An additional volume for sludge storage of approximately 10% should be provided and the volume occupied by ice should also be taken into account. The water depth should be 10 to 20 feet. The system should be capable of providing for normal oxygen requirements of 2 lbs 02 per lb of BOD5 applied with the capability of transferring 3 lbs 02 per lb of BOD5 for periodic high oxygen demand. The aeration and mixing system should be capable of maintaining a 2 mg/L dissolved oxygen level at any point in the basin.

Proper operation means that the facility was operated as designed and described in its approved operation and maintenance (O&M) manual. In general, stabilization ponds should be operated at a minimum operating depth of 3 feet and a maximum of 5 feet and the organic loading should not exceed 35 lbs BOD5 per acre. A detention time of 90 to 120 days should be provided. For partial mix aerated ponds, the hydraulic detention time should be at least 20 days and the mechanical aeration equipment should provide 2 lbs 02 per lb of BOD5 applied. In addition, the number of cells on-line should be operated as provided by the design and described in the O&M manual.

The task force recommends that effluent BOD5 and TSS concentrations could range up to a maximum of 45 mg/L as a 30-day average when :

( a ) the facility cannot consistently achieve secondary treatment defined as a 30-day average of 30 mg/L 95 percent of the time based on at least three years of monthly average data,
( b ) the facility provided information and data to demonstrate that the problem is uncontrollable while using a properly designed and operated lagoon or pond as the principal biological treatment process, and
( c ) there are no extenuating circumstances such as overloading or industrial wastes.

The task force recognizes that the performance of "equivalent treatment processes" may be affected by differences in temperature, and that such facilities may exhibit variation in performance depending on geographical, climatic or seasonal factors. For a given facility, there may be significant differences in performance from one period of the year to another. The task force suggests that DEP permit writers consider the development of seasonal permits that would reflect such differences in performance where the differences are significant. If a seasonal permit is developed, a specific time period, during which the different effluent limitations would apply, should be established during the permit revision process based on historical records for either mean monthly ambient air temperature or effluent wastewater temperature.

BOD5 test versus C-BOD5...

The task force examined the appropriateness of the current BOD5 test for analyzing the effluent quality of lagoons and ponds. Where a treatment process provides full or partial oxidation of ammonia (nitrification), the BOD5 test will measure a combined effect of two types of BOD: carbonaceous biochemical oxygen demand (C-BOD) and nitrogenous oxygen demand (NOD). An analysis of the lagoons and ponds in Maine showed that some systems provide full or partial nitrification, especially during warm weather months, while others do not. The data show that warm weather effluent C-BOD5 concentrations typically range from 10 to more than 90 percent less than the measured BOD values.

The effect of nitrifying bacteria exerting NOD in a test of an effluent sample will not necessarily bear any relationship to in-stream NOD effects; those effects will be governed by the presence or absence of nitrifying bacteria in the stream itself as well as stream conditions, such as depth, surface area for attachment, etc. Where in-stream nitrification is significant, water quality analyses should account for the effects of C-BOD and NOD separately. For water quality limited segments and water quality based permits, it is appropriate that the effluent limitations provide for separate C-BOD and NOD controls; in such cases., the use of a BOD5 test may be inappropriate.

Where in-stream nitrification is not occurring and there are no ammonia toxicity impacts, a separate NOD control is not necessary as long as existing ammonia discharge levels are maintained. The task force supports the concept of using the C-BOD5 test in place of the standard BOD5 test seasonally on a case-by-case basis. The task force has not proposed an across-the- board substitution because it believes that problems arising from the use of the BOD5 parameter may not be experienced in all cases. The task force recommends that a technology-based 30-day average C-BOD5 limit should be 25 mg/L or may be increased on a case-by-case basis up to 40 mg/L based on historical performance (water quality may take precedence and require addition tests). C-BOD substitution will be allowed when :

( a ) parallel C-BOD5 and BOD5 data (at the permit frequency for a time period of April through November) have been provided and show a problem with BOD5 compliance due to nitrification in the BOD5 test results and that the C-BOD5 is not directly correlated with the BOD5 test results, and

( b ) baseline influent and effluent ammonia, nitrite and nitrate data (same frequency and duration as the parallel C-BOD5 and BOD5 data) have been provided.

Prior to relicensing, the licensee and DEP should develop a procedure to determine whether nitrification is still occurring. The task force recognizes that nitrification is affected by differences in temperature, and that lagoons and ponds may exhibit variation in performance depending on seasonal factors. For a given facility, there may be significant differences in nitrification from one period of the year to another. The task force suggests that DEP permit writers develop seasonal C-BOD permits for a specific time period, during which the C-BOD effluent limitations would apply. The time period should be established during the permit revision process based on historical records or a default time period of April through November may be assigned.