MICROBIOLOGICAL AND CHEMICAL TESTING
The Sear-Brown Group
209 S. Meldrum Street
Fort Collins, CO 80521
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
Percent of Violations
Low Oxygen Concentration
* Based on a study of 24 municipal systems in
Following is a brief discussion of the causes of
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.
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.
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 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 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
3. Effluent soluble
BOD5 and total BOD concentrations
4. Effluent pH and
its diurnal variation
5. Lagoon dissolved
oxygen concentration and its diurnal variation
7. The time of the
year that effluent problems occur.
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
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
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 Significance
TSS / BOD5 Ratio
1.0 – 1.5
Typical untreated wastewater
Soluble BOD in the effluent
Poor wastewater treatment
Nitrification in the BOD test
1.5 – 4.0
Low BOD5 but high TSS in the effluent
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
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 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
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.
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
Most of the lagoon
problems that affect effluent quality occur seasonally, and the time of year of
the problem can help diagnose the cause.
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.
Low Oxygen Conditions
|1 - 1.5
|1 – 1.5
|High amount of algae
Or no change
|Old Sludge Decomposition
In Settling Cell
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.