MICROBIOLOGICAL AND CHEMICAL TESTING FOR
TROUBLESHOOTING LAGOONS
Although highly useful, it must be remembered that the effluent TSS / BOD5 ratio may reflect a combination of problems, and differ from the above.
The effluent TSS / BOD5 ratio may change with season, which can be diagnostic. A high ratio in the warmer time of the year usually indicated algae overgrowth. A low ratio in the warmer time of the year usually indicates partial nitrification in the lagoon with continued nitrification in the BOD test bottle. Here, the CBOD is much less than the BOD5. A low ratio in the wintertime often indicates loss of untreated wastewater, due to low hydraulic detention time at cold temperature or short circuiting.
Effluent Soluble BOD5 Concentration
Typical domestic wastewater BOD5 is usually 40-50% soluble with the remainder being particulate (filterable). The soluble component of BOD5 is consumed by bacteria rapidly and preferentially and is reduced to a low amount (<5 mg/l) by the end of the first lagoon cell. The finding of a high soluble BOD5 in the lagoon effluent (more than 30% of the total) may indicate poor treatment; short circuiting; high ammonia concentration; or release of soluble BOD from digesting sludge. The soluble BOD5 concentration will be high throughout the lagoon cells in the first three cases above, but this will be low after the first lagoon cell and high thereafter for the last case. Another indication of problems with old sludge accumulation is an increase in ammonia concentration accompanying the increase in soluble BOD5 concentration, both derived from decaying sludge. The soluble BOD5 test is similar to the regular BOD5 test, with the sample being first filtered through a 0.45 um membrane filter before the BOD test.
SUMMARY
TROUBLESHOOTING LAGOONS
Michael Richard, Ph.D.
The Sear-Brown Group
209 S. Meldrum Street
Fort Collins, CO 80521
INTRODUCTION
Municipal wastewater treatment using lagoons is widespread in the U.S .with more than 6,000 in use, mostly by small communities. The lagoon treatment process usually works well for wastewater treatment in small communities; however, the regulated effluent quality is not always met. The most frequent problems are elevated BOD5, TSS and ammonia (where regulated), and high or low pH. Also, odor from lagoons can be a problem. Troubleshooting lagoons for effluent quality problems is not straightforward, as the biological and chemical processes involved are numerous and complex and not fully understood by most engineers and operators.
This paper presents a biology and chemistry based approach to troubleshooting lagoon systems that has proven successful in correcting lagoon treatment problems throughout the country. The information to collect and their interpretation is presented. The design, function and operation of lagoons, and corrective actions, are not covered in this paper. The reader is referred to the following reference for this information: Troubleshooting and Optimizing Wastewater Treatment in Small Communities: Lagoon Processes, Michael Richard and Richard Bowman, authors, National Environmental Training Center for Small Communities, West Virginia University, 1998.
CAUSES OF LAGOON EFFLUENT NONCOMPLIANCE
Lagoon effluent problems have a number of causes. The most common are: organic overloading and accompanying low dissolved oxygen conditions; short hydraulic detention time or short circuiting; algae or sulfur bacteria overgrowth; old sludge accumulation; and partial nitrification. An example of the causes of municipal lagoon effluent noncompliance for lagoon systems in Colorado is given in Table 1.
For these systems, no causes of organic overloading or incomplete treatment were found. The small incidence of low dissolved oxygen problems were in the wintertime with ice cover and frozen aerators. The most significant cause of effluent noncompliance was due to algae overgrowth followed by nitrification.
The Sear-Brown Group
209 S. Meldrum Street
Fort Collins, CO 80521
INTRODUCTION
Municipal wastewater treatment using lagoons is widespread in the U.S .with more than 6,000 in use, mostly by small communities. The lagoon treatment process usually works well for wastewater treatment in small communities; however, the regulated effluent quality is not always met. The most frequent problems are elevated BOD5, TSS and ammonia (where regulated), and high or low pH. Also, odor from lagoons can be a problem. Troubleshooting lagoons for effluent quality problems is not straightforward, as the biological and chemical processes involved are numerous and complex and not fully understood by most engineers and operators.
This paper presents a biology and chemistry based approach to troubleshooting lagoon systems that has proven successful in correcting lagoon treatment problems throughout the country. The information to collect and their interpretation is presented. The design, function and operation of lagoons, and corrective actions, are not covered in this paper. The reader is referred to the following reference for this information: Troubleshooting and Optimizing Wastewater Treatment in Small Communities: Lagoon Processes, Michael Richard and Richard Bowman, authors, National Environmental Training Center for Small Communities, West Virginia University, 1998.
CAUSES OF LAGOON EFFLUENT NONCOMPLIANCE
Lagoon effluent problems have a number of causes. The most common are: organic overloading and accompanying low dissolved oxygen conditions; short hydraulic detention time or short circuiting; algae or sulfur bacteria overgrowth; old sludge accumulation; and partial nitrification. An example of the causes of municipal lagoon effluent noncompliance for lagoon systems in Colorado is given in Table 1.
For these systems, no causes of organic overloading or incomplete treatment were found. The small incidence of low dissolved oxygen problems were in the wintertime with ice cover and frozen aerators. The most significant cause of effluent noncompliance was due to algae overgrowth followed by nitrification.
Table 1. Causes of Lagoon Noncompliance in Colorado
| Cause | Percent of Violations |
| Algae Overgrowth | 67.0 |
| Nitrification | 10.0 |
| Low Oxygen Concentration | 6.0 |
| Rotifer Bloom | 3.5 |
| Unknown | 13.5 |
* Based on a study of 24 municipal systems in 1990
Following is a brief discussion of the causes of effluent noncompliance
BOD5
High effluent BOD5 concentration can have a number of causes. These include incomplete wastewater treatment due to organic overloading, low oxygen concentration and low hydraulic detention time; physical short circuiting; high algae or sulfur bacteria growth; and sludge accumulation and loss of old sludge to the effluent. High effluent BOD5 can also be caused by high effluent ammonia concentration and nitrification in the BOD test bottle, yielding a high oxygen use in the BOD test when actual carbonaceous BOD is low.
TSS
High effluent TSS is usually caused by high algae or sulfur bacteria growth, the loss of old sludge to the effluent, or by organic overloading and dispersed bacterial growth.
BOD5
High effluent BOD5 concentration can have a number of causes. These include incomplete wastewater treatment due to organic overloading, low oxygen concentration and low hydraulic detention time; physical short circuiting; high algae or sulfur bacteria growth; and sludge accumulation and loss of old sludge to the effluent. High effluent BOD5 can also be caused by high effluent ammonia concentration and nitrification in the BOD test bottle, yielding a high oxygen use in the BOD test when actual carbonaceous BOD is low.
TSS
High effluent TSS is usually caused by high algae or sulfur bacteria growth, the loss of old sludge to the effluent, or by organic overloading and dispersed bacterial growth.
pH
High or low effluent pH problems can occur, for different reasons. Low effluent pH (<7.0) may be due to either organic overloading and low oxygen conditions, or due to nitrification when the lagoon alkalinity (buffer capacity) is low. High effluent pH is always due to extensive algae growth. Algae consume alkalinity (inorganic carbon) for growth and the pH increases as algae consume the alkalinity species in the order carbon dioxide, bicarbonate and carbonate.
High or low effluent pH problems can occur, for different reasons. Low effluent pH (<7.0) may be due to either organic overloading and low oxygen conditions, or due to nitrification when the lagoon alkalinity (buffer capacity) is low. High effluent pH is always due to extensive algae growth. Algae consume alkalinity (inorganic carbon) for growth and the pH increases as algae consume the alkalinity species in the order carbon dioxide, bicarbonate and carbonate.
Ammonia
Ammonia is primarily removed in lagoon systems by micro- biological nitrification to nitrate. Lagoons often support nitrification and have low effluent ammonia concentrations. However, one major problem in lagoons is wintertime low temperature due to the long hydraulic detention time and loss of the influent wastewater warmer temperature. Wastewater stays warm enough for nitrification year-round (5-8C) in the activated sludge process due to the relatively low hydraulic detention time (4-24 hours) in which the wastewater is treated. In colder climates, nitrification ceases in lagoons in the wintertime and early spring.
Other factors that limit nitrification in lagoons are low oxygen concentration and low alkalinity. Nitrification requires 2.0 mg/l or greater dissolved oxygen concentration for optimum performance. Alkalinity (inorganic carbon) is required by the nitrifying bacteria and nitrification becomes limited at a total carbonate alkalinity of <60-80 mg/L. One sign of an alkalinity limitation for nitrification is the build- up of nitrite about 1-2 mg/L. Nitrification can be increased by raising the dissolved oxygen concentration and by supplementing alkalinity (usually lime), but nothing can be done about low temperature.
Odor
Ammonia is primarily removed in lagoon systems by micro- biological nitrification to nitrate. Lagoons often support nitrification and have low effluent ammonia concentrations. However, one major problem in lagoons is wintertime low temperature due to the long hydraulic detention time and loss of the influent wastewater warmer temperature. Wastewater stays warm enough for nitrification year-round (5-8C) in the activated sludge process due to the relatively low hydraulic detention time (4-24 hours) in which the wastewater is treated. In colder climates, nitrification ceases in lagoons in the wintertime and early spring.
Other factors that limit nitrification in lagoons are low oxygen concentration and low alkalinity. Nitrification requires 2.0 mg/l or greater dissolved oxygen concentration for optimum performance. Alkalinity (inorganic carbon) is required by the nitrifying bacteria and nitrification becomes limited at a total carbonate alkalinity of <60-80 mg/L. One sign of an alkalinity limitation for nitrification is the build- up of nitrite about 1-2 mg/L. Nitrification can be increased by raising the dissolved oxygen concentration and by supplementing alkalinity (usually lime), but nothing can be done about low temperature.
Odor
Odor in lagoons is always due t low oxygen conditions where the bacteria use alternate electron acceptors to oxidize BOD; sulfate, producing hydrogen sulfide, and true fermentation of organic materials, producing odorous organic acids. This condition occurs at organic overloading and low oxygen conditions, and when sludge accumulation becomes excessive. Odor is common in lagoon systems with wintertime ice cover, when the ice melts in the springtime and the backlog of winter stored BOD is oxidized.
TESTING TO DETERMINE THE CAUSES OF EFFLUENT NONCOMPLIANCE
The tests needed for troubleshooting lagoon systems include:
1. Effluent BOD5 and TSS data for at least three years.
2. Effluent cBOD measurement
3. Effluent soluble BOD5 and total BOD concentrations
4. Effluent pH and its diurnal variation
5. Lagoon dissolved oxygen concentration and its diurnal variation
6. Microscopic examination results
7. The time of the year that effluent problems occur.
Microscopic Examination
Microscopic examination of the effluent using a phase contrast microscope can identify and quantitate specific organisms that cause effluent problems. The three groups of indicator organisms of most use are filamentous bacteria, algae and sulfur bacteria.
Filamentous bacteria are a large problem in activated sludge operation where high growth of these causes sludge bulking. About 20 different filamentous bacteria occur in activated sludge, each with a specific cause. Many of these filamentous bacteria occur in activated lagoons, where they don’t cause a bulking problem but are useful to diagnose the growth conditions in the lagoon and problems that are occurring. Two groups of filaments: Sphaerotilus natans, type 1701 and Haliacomwnobacter hydrossis; and (2) the filaments caused by septicity: Thiothrix I and II, type 0914, type 0411 and Beggiatoa spp. A large amount of these filaments in the lagoon effluent indicates either low oxygen conditions or septicity.
Algae are a normal and needed biological component in a lagoon, responsible for much of the oxygen used in BOD stabilization even if the system is mechanically aerated. However, these often overgrow and increase the effluent BOD5, TSS and pH. Many species of algae occur in lagoons, and each species may impact the effluent BOD5 > 30 mg/l (noncompliance). Causes of algae overgrowth are varied, and include long system hydraulic detention time, high mixing, and old sludge accumulation which releases algal growth nutrients.
The finding of a large amount of anaerobic sulfur bacteria in a lagoon effluent indicates a significant anaerobic environment in the lagoon. Seventeen genera of photosynthetic, anaerobic, sulfur- oxidizing bacteria have been identified in lagoons. The most common in municipal, lagoons are Thiocystis and Rhodococcus. These photosynthetic bacteria grow on organic acids in the presence of sunlight, and are anaerobic to microaerophilic (free oxygen inhibits these). At high amount these bacteria indicate an anaerobic environment in the lagoon.
Microscopic examination of the effluent can also identify short circuiting and sludge accumulation problems. Here, raw sewage materials and especially toilet paper fibers are diagnostic of short circuiting. The observation of old sludge particles in the effluent indicates excess sludge accumulation and loss of this to the effluent.
Effluent TSS / BOD5 Ratio
The effluent TSS / BOD5 ratio can be diagnostic for specific problems that occur. This may be the only information available for many small systems. This data is usually available as effluent TSS and BOD5 concentration for a lengthy period of time. All that is needed is to make the ratio between these two parameters and plot this versus time. Following in Table 2 is the significance of the TSS / BOD5 ratio.
Table 2. Effluent TSS / BOD5 Ratios and Their SignificanceAlgae are a normal and needed biological component in a lagoon, responsible for much of the oxygen used in BOD stabilization even if the system is mechanically aerated. However, these often overgrow and increase the effluent BOD5, TSS and pH. Many species of algae occur in lagoons, and each species may impact the effluent BOD5 > 30 mg/l (noncompliance). Causes of algae overgrowth are varied, and include long system hydraulic detention time, high mixing, and old sludge accumulation which releases algal growth nutrients.
The finding of a large amount of anaerobic sulfur bacteria in a lagoon effluent indicates a significant anaerobic environment in the lagoon. Seventeen genera of photosynthetic, anaerobic, sulfur- oxidizing bacteria have been identified in lagoons. The most common in municipal, lagoons are Thiocystis and Rhodococcus. These photosynthetic bacteria grow on organic acids in the presence of sunlight, and are anaerobic to microaerophilic (free oxygen inhibits these). At high amount these bacteria indicate an anaerobic environment in the lagoon.
Microscopic examination of the effluent can also identify short circuiting and sludge accumulation problems. Here, raw sewage materials and especially toilet paper fibers are diagnostic of short circuiting. The observation of old sludge particles in the effluent indicates excess sludge accumulation and loss of this to the effluent.
Effluent TSS / BOD5 Ratio
The effluent TSS / BOD5 ratio can be diagnostic for specific problems that occur. This may be the only information available for many small systems. This data is usually available as effluent TSS and BOD5 concentration for a lengthy period of time. All that is needed is to make the ratio between these two parameters and plot this versus time. Following in Table 2 is the significance of the TSS / BOD5 ratio.
| TSS / BOD5 Ratio | Significance |
| 1.0 – 1.5 | Typical untreated wastewater |
| <1.0 | Soluble BOD in the effluentPoor wastewater treatment Nitrification in the BOD test |
| 1.5 – 4.0 | Low BOD5 but high TSS in the effluentAlgae overgrowth Loss of old sludge |
Although highly useful, it must be remembered that the effluent TSS / BOD5 ratio may reflect a combination of problems, and differ from the above.
The effluent TSS / BOD5 ratio may change with season, which can be diagnostic. A high ratio in the warmer time of the year usually indicated algae overgrowth. A low ratio in the warmer time of the year usually indicates partial nitrification in the lagoon with continued nitrification in the BOD test bottle. Here, the CBOD is much less than the BOD5. A low ratio in the wintertime often indicates loss of untreated wastewater, due to low hydraulic detention time at cold temperature or short circuiting.
Effluent Soluble BOD5 Concentration
Typical domestic wastewater BOD5 is usually 40-50% soluble with the remainder being particulate (filterable). The soluble component of BOD5 is consumed by bacteria rapidly and preferentially and is reduced to a low amount (<5 mg/l) by the end of the first lagoon cell. The finding of a high soluble BOD5 in the lagoon effluent (more than 30% of the total) may indicate poor treatment; short circuiting; high ammonia concentration; or release of soluble BOD from digesting sludge. The soluble BOD5 concentration will be high throughout the lagoon cells in the first three cases above, but this will be low after the first lagoon cell and high thereafter for the last case. Another indication of problems with old sludge accumulation is an increase in ammonia concentration accompanying the increase in soluble BOD5 concentration, both derived from decaying sludge. The soluble BOD5 test is similar to the regular BOD5 test, with the sample being first filtered through a 0.45 um membrane filter before the BOD test.
USE OF THE TEST RESULTS IN DIAGNOSING THE CAUSE OF EFFLUENT NONCOMPLIANCE
Organic Overloading and Low Dissolved Oxygen Conditions
Organic overloading causes a low dissolved oxygen concentration (<1.0 mg/l) and limits treatment. This condition can be diagnosed by conducting a dissolved oxygen profile for the lagoon system (all cells). The dissolved oxygen concentration should not be <1.0 mg/l throughout the lagoon cells, measured at a representative location in the cells (not next to an aerator) and measured early in the day. Lagoon dissolved oxygen concentration is lowest at dawn, due to no algal oxygenation at night. Dissolved oxygen measurements in the afternoon can be very misleading, as these indicate the combination of both mechanical aeration and algal oxygenation combined. Low oxygen conditions can also de diagnosed by the finding of filaments caused by low oxygen or septicity, and by a high soluble BOD5 in the effluent. The effluent TSS/BOD5 in the effluent. The effluent TSS/BOD5 ratio at this condition is generally the same as for the influent wastewater, ranging from 1.0 to 1.5.
Algae Overgrowth
Algae overgrowth can be diagnosed by several methods. The most direct is the microscopic examination of the effluent and counting of the algae present. An algae concentration > 3-5 x 10^5/mL generally causes an effluent BOD5 concentration >30 mg/L. Other indications of algae overgrowth include: a significant pH increase through the lagoon system, often to >9.0 in the effluent; an increase in dissolved oxygen concentration through the lagoon system, often to supersaturating by the last lagoon cell; and an effluent TSS/BOD5 ratio >2. The effluent soluble BOD5 is usually quite low when algae overgrowth is occurring.
Short Circuiting
Short circuiting is the passage of untreated wastewater through the lagoon system in a short period of time. This can occur due to poor mixing and inappropriate location of lagoon inlet and effluent points. Short circuiting is most likely to occur in the wintertime when the influent wastewater and lagoon temperatures differ significantly. Observation of a lagoon cell effluent temperature higher than for the lagoon at a representative central point is a clear indication of short circuiting. Short circuiting can also be detected by the microscopic observation of raw sewage solids in the effluent, and by doing a dye tracer study. The TSS/BOD5 ratio in the effluent will be the same as for the influent wastewater, in the range 1.0-1.5.
High Effluent Ammonia Concentration
High effluent ammonia concentration has several causes. Effluent ammonia concentration would be expected to be high at cold temperature ,5C, due to inhibition of nitrification at cold temperature. High effluent ammonia concentration may also be due to organic overloading, low oxygen concentration, short hydraulic detention time, and release of ammonia from old digesting sludge, most common in the late summer and fall at warm lagoon temperature.
Partial NitrificationOrganic Overloading and Low Dissolved Oxygen Conditions
Organic overloading causes a low dissolved oxygen concentration (<1.0 mg/l) and limits treatment. This condition can be diagnosed by conducting a dissolved oxygen profile for the lagoon system (all cells). The dissolved oxygen concentration should not be <1.0 mg/l throughout the lagoon cells, measured at a representative location in the cells (not next to an aerator) and measured early in the day. Lagoon dissolved oxygen concentration is lowest at dawn, due to no algal oxygenation at night. Dissolved oxygen measurements in the afternoon can be very misleading, as these indicate the combination of both mechanical aeration and algal oxygenation combined. Low oxygen conditions can also de diagnosed by the finding of filaments caused by low oxygen or septicity, and by a high soluble BOD5 in the effluent. The effluent TSS/BOD5 in the effluent. The effluent TSS/BOD5 ratio at this condition is generally the same as for the influent wastewater, ranging from 1.0 to 1.5.
Algae Overgrowth
Algae overgrowth can be diagnosed by several methods. The most direct is the microscopic examination of the effluent and counting of the algae present. An algae concentration > 3-5 x 10^5/mL generally causes an effluent BOD5 concentration >30 mg/L. Other indications of algae overgrowth include: a significant pH increase through the lagoon system, often to >9.0 in the effluent; an increase in dissolved oxygen concentration through the lagoon system, often to supersaturating by the last lagoon cell; and an effluent TSS/BOD5 ratio >2. The effluent soluble BOD5 is usually quite low when algae overgrowth is occurring.
Short Circuiting
Short circuiting is the passage of untreated wastewater through the lagoon system in a short period of time. This can occur due to poor mixing and inappropriate location of lagoon inlet and effluent points. Short circuiting is most likely to occur in the wintertime when the influent wastewater and lagoon temperatures differ significantly. Observation of a lagoon cell effluent temperature higher than for the lagoon at a representative central point is a clear indication of short circuiting. Short circuiting can also be detected by the microscopic observation of raw sewage solids in the effluent, and by doing a dye tracer study. The TSS/BOD5 ratio in the effluent will be the same as for the influent wastewater, in the range 1.0-1.5.
High Effluent Ammonia Concentration
High effluent ammonia concentration has several causes. Effluent ammonia concentration would be expected to be high at cold temperature ,5C, due to inhibition of nitrification at cold temperature. High effluent ammonia concentration may also be due to organic overloading, low oxygen concentration, short hydraulic detention time, and release of ammonia from old digesting sludge, most common in the late summer and fall at warm lagoon temperature.
Partial Nitrification in a lagoon system can lead to both ammonia and nitrifying bacteria in the effluent. The nitrifying bacteria continue to oxidize ammonia in the BOD test bottle, elevating oxygen use and the resultant BOD5 value. Nitrification as a cause of elevated BOD5 can be determined by analyzing the effluent sample using the regular BOD5 test and the inhibited or carbonaceous BOD5 test (cBOD). These two test results are generally within 10-20% of each other when nitrification is not significant (the cBOD value is usually a little less than the BOD5 value). A high BOD5 value and a much lower cBOD value (often 2-3 fold difference) indicates nitrification and not carbonaceous BOD as the cause for the elevated BOD5. The effluent TSS/BOD5 ratio is usually quite low (0.2-0.6) when partial nitrification is occurring. Many systems that have this problem, which occurs only at the warmer time of the year, change their effluent standard from BOD5 to cBOD. Nitrification can also be encouraged in the lagoon and not the BOD test bottle by increasing aeration and by supplementing alkalinity, if this is low.
Seasonal Occurrence of Effluent Problems
Most of the lagoon problems that affect effluent quality occur seasonally, and the time of year of the problem can help diagnose the cause.
Organic overloading and low dissolved oxygen conditions can occur at any time , but are most pronounced at colder temperature when algae growth and algal oxygenation are low. Short circuiting can also occur at any time, but occurs most often in the wintertime when the lagoon temperature is low but the influent wastewater is warm. The warmer wastewater tends to no mix with the colder lagoon water and flows across the surface of the lagoon to the effluent without mixing.
A summary of key test parameters used to diagnose lagoon effluent problems discussed in this paper is given in Table 3.
TABLE 3. Summary of Key Test Parameters Used to Diagnose Lagoon Effluent Problems.| Cause | Microscopic Examination | Dissolved Oxygen | Soluble BOD5 | TSS/BOD5 Ratio | cBOD | pH Change |
Organic
Low Oxygen ConditionsOverloading | Filamentous Bacteria | Low | High | 1 - 1.5 | =BOD5 | May decrease |
| Short Circuiting | Raw sludge particles | Low | High | 1 – 1.5 | =BOD5 | No change |
| Algae Overgrowth | High amount of algae | High | Low | >2 | =BOD5 | Increase |
Sulfur Bacteria
Overgrowth
| Sulfur bacteria present | Low | May be High | 1 -1.5 | =BOD5 | May decrease Or no change |
| Old Sludge Decomposition In Settling Cell | Old Sludge particles | ---* | Variable | <1.0 | =BOD5 | No change |
Partial
Nitrification | ------------ | ------ | High** | <1.0 | <BOD5 | No change |
SUMMARY
Determining the cause(s) for municipal lagoon effluent noncompliance is often a difficult and confusing problem. The test outlined in this paper yield understandable and quantifiable information that pinpoints the specific problem(s) that are occurring. Once these problems are understood, they can be addressed directly and cost-effectively without wasting effort trying to fix what isn’t broke.
No comments:
Post a Comment