Introduction :
General Discussion
The biochemical oxygen demand
(BOD) determination is an empirical test in which standardized laboratory
procedures are used to determine the relative oxygen requirements of
wastewaters, effluents, and polluted waters. The test has its widest
application in measuring waste loadings to treatment plants and in evaluating
the BOD-removal efficiency of such treatment systems. The test measures the
molecular oxygen utilized during a specified incubation period for the
biochemical degradation of organic material (carbonaceous demand) and the
oxygen used to oxidize inorganic material such as sulfides and ferrous iron. It
also may measure the amount of oxygen used to oxidize reduced forms of nitrogen
(nitrogenous demand) unless their oxidation is prevented by an inhibitor. The
seeding and dilution procedures provide an estimate of the BOD at pH 6.5 to
7.5.
Measurements of oxygen consumed
in a 5-d test period (5-d BOD or BOD5, 521 OB), oxygen consumed after 60 to 90
d of incubation (ultimate BOD or UBOD, 52IOC), and continuous oxygen uptake
(respirometric method, 5210D) are described here. Many other variations of
oxygen demand measurements exist, including using shorter and longer incubation
periods and tests to determine rates of oxygen uptake. Alternative seeding,
dilution, and incubation conditions can be chosen to mimic receiving-water conditions,
thereby providing an estimate of the environmental effects of wastewaters and
effluents.
The UBOD measures the oxygen
required for the total degradation of organic material (ultimate carbonaceous
demand) and/ or the oxygen to oxidize reduced nitrogen compounds (ultimate nitrogenous
demand). UBOD values and appropriate kinetic descriptions are needed in water
quality modeling studies such as UBOD: BOD5 ratios for relating stream
assimilative capacity to regulatory requirements; definition of river, estuary,
or lake deoxygenation kinetics; and instream ultimate carbonaceous BOD (UCBOD)
values for model calibration.
2. Carbonaceous Versus
Nitrogenous BOD
A number of factors, for example,
soluble versus particulate organics, settleable and floatable solids, oxidation
of reduced iron and sulfur compounds, or lack of mixing may affect the accuracy
and precision of BOD measurements. Presently, there is no way to include
adjustments or corrections to account for the effect of these factors.
Oxidation of reduced forms of
nitrogen, such as ammonia and organic nitrogen, can be mediated by microorganisms
and exert nitrogenous demand. Nitrogenous demand historically has been considered
an interference in the determination of BOD, as clearly evidenced by the
inclusion of ammonia in the dilution water. The interference from nitrogenous
demand can now be prevented by an inhibitory chemical.1 If an inhibiting
chemical is not used, the oxygen demand measured is the sum of carbonaceous and
nitrogenous demands.
Measurements that include
nitrogenous demand generally are not useful for assessing the oxygen demand
associated with organic material. Nitrogenous demand can be estimated directly from
ammonia nitrogen (Section 4500-NH3); and carbonaceous demand can be estimated
by subtracting the theoretical equivalent of the reduced nitrogen oxidation
from uninhibited test results. However, this method is cumbersome and is
subject to considerable error. Chemical inhibition of nitrogenous demand
provides a more direct and more reliable measure of carbonaceous demand.
The extent of oxidation of
nitrogenous compounds during the 5-d incubation period depends on the
concentration and type of microorganisms capable of carrying out this
oxidation. Such organisms usually are not present in raw or settled primary
sewage in sufficient numbers to oxidize sufficient quantities of reduced nitrogen
forms in the 5-d BOD test. Many biological treatment plant effluents contain
sufficient numbers of nitrifying organisms to cause nitrification in BOD tests.
Because oxidation of nitrogenous compounds can occur in such samples,
inhibition of nitrification as directed in 5210B.4e6) is recommended for
samples of secondary effluent, for samples seeded with secondary effluent, and for
samples of polluted waters.
Report results as carbonaceous
biochemical oxygen demand (CBOD5) when inhibiting the nitrogenous oxygen
demand. When nitrification is not inhibited, report results as BOD5.
3. Dilution Requirements
The BOD concentration in most
wastewaters exceeds the concentration of dissolved oxygen (DO) available in an
air-saturated sample. Therefore, it is necessary to dilute the sample before
incubation to bring the oxygen demand and supply into appropriate balance.
Because bacterial growth requires nutrients such as nitrogen, phosphorus, and
trace metals, these are added to the dilution water, which is buffered to
ensure that the pH of the incubated sample remains in a range suitable for
bacterial growth. Complete stabilization of a sample may require a period of
incubation too long for practical purposes; therefore, 5 d has been accepted as the standard incubation period.
If the dilution water is of poor
quality, the BOD of the dilution water will appear as sample BOD. This effect
will be amplified by the dilution factor. A positive bias will result. The
methods included below (521 OB and 52IOC) contain both a dilution-water check
and a dilution-water blank. Seeded dilution waters are checked further for
acceptable quality by measuring their consumption of oxygen from a known
organic mixture, usually glucose and glutamic acid.
The source of dilution water is
not restricted and may be distilled, tap, or receiving-stream water free of
biodegradable organics and bioinhibitory substances such as chlorine or heavy
metals. Distilled water may contain ammonia or volatile organics; deionized waters
often are contaminated with soluble organics leached from the resin bed. Use of
copper-lined stills or copper fittings attached to distilled water lines may
produce water containing excessive amounts of copper.
B. 5-Day BOD Test
1. General Discussion
a. Principle: The method consists of filling
with sample, to overflowing, an airtight bottle of the specified size and
incubating it at the specified temperature for 5 d. Dissolved oxygen is
measured initially and after incubation, and the BOD is computed from the difference between initial and final DO.
Because the initial DO is determined shortly after the dilution is made, all
oxygen uptake occurring after this measurement is included in the BOD measurement.
b. Sampling and storage: Samples for BOD analysis may
degrade significantly during storage between collection and analysis, resulting
in low BOD values. Minimize reduction of BOD by analyzing sample promptly or by
cooling it to near-freezing temperature during storage. However, even at low
temperature, keep holding time to a minimum. Warm chilled samples to 20 ± 3°C before
analysis.
1) Grab samples—If analysis is begun within 2 h
of collection, cold storage is unnecessary. If analysis is not started within 2
h of sample collection, keep sample at
or below 4°C from the time of collection. Begin analysis within 6 h of collection;
when this is not possible because the sampling site is distant from the
laboratory, store at or below 4°C and
report length and temperature of storage with the results. In no case start
analysis more than 24 h after grab sample collection. When samples are to be
used for regulatory purposes make every effort to deliver samples for analysis
within 6 h of collection.
2) Composite samples—Keep samples at or below 4°C
during compositing. Limit compositing period to 24 h. Use the same criteria as
for storage of grab samples, starting the measurement of holding time from end
of compositing period. State storage time and conditions as part of the
results.
2. Apparatus
a. Incubation bottles'. Use glass bottles having 60 mL
or greater capacity (300-mL bottles having a ground-glass stopper and a flared
mouth are preferred). Clean bottles with a detergent, rinse thoroughly, and
drain before use. As a precaution against drawing air into the dilution bottle
during incubation, use a water seal. Obtain satisfactory water seals by
inverting bottles in a water bath or by adding water to the flared mouth of
special BOD bottles. Place a paper or plastic cup or foil cap over flared mouth
of bottle to reduce evaporation of the water seal during incubation.
b. Air incubator or water bath, thermostatically controlled at 20
± 1°C. Exclude all light to prevent possibility of photosynthetic production of
DO.
3. Reagents
Prepare reagents in advance but
discard if there is any sign of precipitation or biological growth in the stock
bottles. Commercial equivalents of these reagents are acceptable and different stock
concentrations may be used if doses are adjusted proportionally.
a. Phosphate buffer solution: Dissolve 8.5 g KH2PO4, 21.75 g K2HPO4>
33.4 g Na2HPO47H2O, and 1.7 g NH4C1 in about 500 mL distilled water and dilute
to 1 L. The pH should be 7.2 without further adjustment. Alternatively,
dissolve 42.5 g KH2PO4 or 54.3 g K2HPO4 in about 700 mL distilled water. Adjust
pH to 7.2 with 30% NaOH and dilute to 1 L.
b. Magnesium sulfate solution: Dissolve 22.5 g MgSO4-7H2O in
distilled water and dilute to 1 L.
c. Calcium chloride solution: Dissolve 27.5 g CaCl2 in
distilled water and dilute to 1 L.
d. Ferric chloride solution: Dissolve 0.25 g FeCl3-6H2O in
distilled water and dilute to 1 L.
e. Acid and alkali solutions, IN, for neutralization of caustic or
acidic waste samples.
1) Acid—Slowly and while stirring, add
28 mL cone sulfuric acid to distilled water. Dilute to 1 L.
2) Alkali—Dissolve 40 g sodium hydroxide
in distilled water. Dilute to 1 L. / Sodium sulfite solution: Dissolve
1.575 g Na2SO3 in 1000 mL distilled water. This solution is not stable; prepare
daily.
g. Nitrification inhibitor, 2-chloro-6-(trichloromethyl)
pyridine.*
h. Glucose-glutamic acid solution: Dry reagent-grade glucose and
reagent-grade glutamic acid at 103°C for 1 h. Add 150 mg glucose and 150 mg
glutamic acid to distilled water and dilute to 1 L. Prepare fresh immediately
before use.
* Nitrification Inhibitor,
Formula 2533, Hach Co., Loveland, CO, or equivalent.
i. Ammonium chloride solution:
Dissolve 1.15 g NH4C1 in about 500 mL distilled water, adjust pH to 7.2 with
NaOH solution, and dilute to 1 L. Solution contains 0.3 rng N/mL.
j. Dilution water: Use
demineralized, distilled, tap, or natural water for making sample dilutions.
4. Procedure
a. Preparation of dilution
water: Place desired volume of water (11 3/) in a suitable bottle and add 1
mL each of phosphate buffer, MgSO4, CaCl2, and FeCl3 solutions/L of water. Seed
dilution water, if desired, as described in H 4d. Test dilution water as
described in H 4h so that water of assured quality always is on hand.
Before use bring dilution water
temperature to 20 ± 3°C. Saturate with DO by shaking in a partially filled
bottle or by aerating with organic-free filtered air. Alternatively, store in
cotton-plugged bottles long enough for water to become saturated with DO.
Protect water quality by using clean glassware, tubing, and bottles.
b. Dilution water storage:
Source water (11 3j) may be stored before use as long as the prepared dilution
water meets quality control criteria in the dilution water blank (11 4h). Such
storage may improve the quality of some source waters but may allow biological
growth to cause deterioration in others. Preferably do not store prepared
dilution water for more than 24 h after adding nutrients, minerals, and buffer
unless dilution water blanks consistently meet quality control limits. Discard
stored source waterif dilution water blank shows more than 0.2 mg/L DO depletion
in 5 d.
c. Glucose-glutamic acid
check: Because the BOD test is a bioassay its results can be influenced
greatly by the presence of toxicants or by use of a poor seeding material.
Distilled waters frequently are contaminated with copper; some sewage seeds are
relatively inactive. Low results always are obtained with such seeds and
waters. Periodically check dilution water quality, seed effectiveness, and
analytical technique by making BOD measurements on a mixture of 150 mg
glucose/L and 150 mg glutamic acid/L as a "standard" check solution.
Glucose has an exceptionally high and variable oxidation rate but when it is
used with glutamic acid, the oxidation
rate is stabilized and is similar to that obtained with many municipal wastes.
Alternatively, if a particular wastewater contains an identifiable major
constituent that contributes to the BOD, use this compound in place of the
glucose- glutamic acid.
Determine the 5-d 20°C BOD of a
2% dilution of the glucose-glutamic acid standard check solution using the
techniques outlined in Us 4d-j. Adjust concentrations of commercial mixtures to give 3 mg/L glucose and 3 mg/L glutamic acid
in each GGA test bottle. Evaluate data as described in H 6, Precision and Bias.
d. Seeding:
1) Seed source—It is
necessary to have present a population of microorganisms capable of oxidizing
the biodegradable organic matter in the sample. Domestic wastewater,
unchlorinated or otherwise- undisinfected
effluents from biological waste treatment plants, and surface waters receiving
wastewater discharges contain satisfactory microbial populations. Some samples
do not contain a sufficient microbial population (for example, some untreated industrial
wastes, disinfected wastes, high-temperature wastes, or wastes with extreme pH
values). For such wastes seed the dilution water or sample by adding a
population of microorganisms. The preferred seed is effluent or mixed liquor
from a biological treatment system processing the waste. Where such seed is not
available, use supernatant from domestic wastewater after settling at room
temperature for at least 1 h but no longer than 36 h. When effluent or mixed
liquor from a biological treatment process is used, inhibition of nitrification
is recommended.
Some samples may contain
materials not degraded at normal rates by the microorganisms in settled
domestic wastewater. Seed such samples with an adapted microbial population
obtained from the undisinfected effluent or mixed liquor of a biological
process treating the waste. In the absence of such a facility, obtain seed from
the receiving water below (preferably 3 to 8 km) the point of discharge. When
such seed sources also are not available, develop an adapted seed in the
laboratory by continuously aerating a sample of settled domestic wastewater and
adding small daily increments of waste. Optionally use a soil suspension or
activated sludge, or a commercial seed preparation to obtain the initial
microbial population. Determine the existence of a satisfactory population by
testing the performance of the seed in BOD tests on the sample. BOD values that
increase with time of adaptation to a steady high value indicate successful
seed adaptation.
2) Seed control—Determine
BOD of the seeding material as for any other sample. This is the seed control.
From the value of the seed control and a knowledge of the seeding material
dilution (in the dilution water) determine seed DO uptake. Ideally, make dilutions
of seed such that the largest quantity results in at least 50% DO depletion. A
plot of DO depletion, in milligrams per liter,
versus milliters of seed for all bottles having a 2-mg/L depletion and a
1.0-mg/L minimum residual DO should present a straight line for which the slope
indicates DO depletion per milliliter of seed. The DO-axis intercept is oxygen
depletion caused by the dilution water and should be less than 0.1 mg/L (11
4/j). Alternatively, divide DO depletion by volume of seed in milliliters for
each seed control bottle having a 2-mg/L depletion and a 1.0-mg/L residual DO.
Average the results for all bottles meeting minimum depletion and residual DO
criteria. The DO uptake attributable to the seed added to each bottle should be
between 0.6 and 1.0 mg/L, but the amount of seed added should be adjusted from
this range to that required to provide glucose-glutamic acid check results in
the range of 198 ± 30.5 mg/L. To determine DO uptake for a test bottle,
subtract DO uptake attributable to the seed from total DO uptake .
Techniques for adding seeding
material to dilution water are described for two sample dilution methods.
e. Sample pretreatment: Check pH
of all samples before testing unless previous experience indicates that pH is
within the acceptable range.
1) Samples containing caustic
alkalinity (pH >8.5) or acidity (pH <6.0)—Neutralize samples to pH
6.5 to 7.5 with a solution of sulfuric acid (H2SO4) or sodium hydroxide (NaOH)
of such strength that the quantity of reagent does not dilute the sample by
more than 0.5%. The pH of dilution water should not be affected by the lowest
sample dilution. Always seed samples that have been pH-adjusted.
2) Samples containing residual
chlorine compounds—If possible, avoid samples containing residual chlorine
by sampling ahead of chlorination processes. If the sample has been chlorinated
but no detectable chlorine residual is present, seed the dilution water. If
residual chlorine is present, dechlorinate sample and seed the dilution water
(H 4f). Do not test chlorinated/dechlorinated samples without seeding the
dilution water. In some samples
chlorine will dissipate within 1 to 2 h of standing in the light. This often occurs during
sample transport and handling. For samples in which chlorine residual does not dissipate in a
reasonably short
time, destroy chlorine residual by adding Na2SC>3 solution. Determine required volume of
Na2SO3 solution on a 100- to
1000-mL portion of neutralized sample by adding 10 mL of 1 + 1 acetic acid or 1 +50
H2SO4, 10 mL potassium iodide (KI)
solution (10 g/100 mL) per 1000 mL portion, and titrating with Na2SO3 solution to the
starch-iodine end point for residual. Add to neutralized sample the relative volume of Na2SO3
solution determined
by the above test, mix, and after 10 to 20 min check sample for residual chlorine.
(NOTE: Excess Na2SO3 exerts an oxygen
demand and reacts slowly with certain organic chloramines compounds that may be present in
chlorinated samples.)
3) Samples containing other
toxic substances—Certain
industrial wastes, for example, plating wastes, contain toxic metals. Such
samples often require special study and treatment.
4) Samples supersaturated with
DO—Samples
containing more than 9 mg DO/ L at 20°C may be encountered in cold waters or in
water where photosynthesis occurs. To prevent loss of oxygen during incubation
of such samples, reduce DO to saturation at 20°C by bringing sample to about
20°C in partially filled bottle while agitating by vigorous shaking or by
aerating with clean, filtered compressed air.
5) Sample temperature
adjustment—Bring
samples to 20 ± 1°C before making dilutions.
6) Nitrification inhibition—If nitrification inhibition is
desired add 3 mg 2-chloro-6-(trichloro methyl) pyridine (TCMP) to each 300-mL
bottle before capping or add sufficient amounts to the dilution water to make a
final concentration of 10 mg/L. (NOTE: Pure TCMP may dissolve slowly and can
float on top of the sample. Some commercial formulations dissolve more readily
but are not 100% TCMP; adjust dosage accordingly.) Samples that may require
nitrification inhibition include, but are not limited to, biologically treated
effluents, samples seeded with biologically treated effluents, and river
waters. Note the use of nitrogen inhibition in reporting results.
F. Dilution technique: Make several dilutions of sample
that will result in a residual DO of at least 1 mg/L and a DO uptake of at
least 2 mg/L after a 5-d incubation. Five dilutions are recommended unless
experience with a particular sample shows that use of a smaller number of
dilutions produces at least two bottles giving acceptable minimum DO depletion
and residual limits. A more rapid analysis, such as COD, may be correlated
approximately with BOD and serve as a guide in selecting dilutions. In the absence of prior knowledge, use the
following dilutions: 0.0 to 1.0% for strong industrial wastes, 1 to 5% for raw
and settled wastewater, 5 to 25% for biologically treated effluent, and 25 to 100%
for polluted river waters.
Prepare dilutions either in
graduated cylinders or volumetric glassware, and then transfer to BOD bottles
or prepare directly in BOD bottles. Either dilution method can be combined with
any DO measurement technique. The number of bottles to be prepared for each
dilution depends on the DO technique and the number of replicates desired.
When using graduated cylinders
or volumetric flasks to prepare dilutions, and when seeding is necessary, add
seed either directly to dilution water or to individual cylinders or flasks
before dilution. Seeding of individual cylinders or flasks avoids a declining ratio
of seed to sample as increasing dilutions are made. When dilutions are prepared
directly in BOD bottles and when seeding is necessary, add seed directly to
dilution water or directly to the BOD bottles. When a bottle contains more than
67% of the sample after dilution, nutrients may be limited in the diluted sample
and subsequently reduce biological activity. In such samples, add the nutrient,
mineral, and buffer solutions (H 3a through e) directly to
individual BOD bottles at a rate of 1 mL/L (0.33 mL/ 300-mL bottle) or use
commercially prepared solutions designed to dose the appropriate bottle size.
1) Dilutions prepared in
graduated cylinders or volumetric flasks—If the azide modification of the
titrimetric iodometric method (Section 4500-O.C) is used, carefully siphon
dilution water, seeded if necessary, into a 1- to 2-L-capacity flask or
cylinder. Fill half full without entraining air. Add desired quantity of
carefully mixed sample and dilute to appropriate level with dilution water. Mix
well with a plunger-type mixing rod; avoid entraining air. Siphon mixed dilution
into two BOD bottles. Determine
initial DO on one of these
bottles. Stopper the second bottle tightly, water-seal, and incubate for 5 d at
20°C. If the membrane electrode method is used for DO measurement, siphon dilution
mixture into one BOD bottle. Determine initial DO on this bottle and replace
any displaced contents with sample dilution
to fill the bottle. Stopper
tightly, water-seal, and incubate for 5 d at 20°C.
2) Dilutions prepared directly
in BOD bottles—Using
a wide tip volumetric pipet, add the desired sample volume to individual BOD
bottles of known capacity. Add appropriate amounts of seed material either to
the individual BOD bottles or to the dilution water. Fill bottles with enough
dilution water, seeded if necessary, so that insertion of stopper will displace
all air, leaving no bubbles. For dilutions greater than 1:100 make a primary
dilution in a graduated cylinder before making final dilution in the bottle. When
using titrimetric iodometric methods for DO measurement, prepare two bottles at
each dilution. Determine initial DO on one
bottle. Stopper second bottle tightly, water-seal, and incubate for 5 d
at 20°C. If the membrane electrode method is used for DO measurement, prepare
only one BOD bottle for each dilution. Determine initial DO on this bottle and
replace any displaced contentswith dilution water to fill the bottle. Stopper
tightly, waterseal, and incubate for 5 d at 20°C. Rinse DO electrode between determinations
to prevent cross-contamination of samples.
Use the azide modification of
the iodometric method (Section 4500-O.C) or the membrane electrode method
(Section 4500- O.G) to determine initial DO on all sample dilutions, dilution water
blanks, and where appropriate, seed controls.
If the membrane electrode method
is used, the azide modification of the iodometric method (Method 4500-O.C) is
recommended for calibrating the DO probe.
g. Determination of initial DO: If the sample contains materials that react rapidly with DO, determine initial
DO immediately after filling BOD bottle with diluted sample. If rapid initial
DO uptake is insignificant, the time period between preparing dilution and
measuring initial DO is not critical but should not exceed 30 min.
h. Dilution water blank: Use a dilution water blank as a
rough check on quality of unseeded dilution water and cleanliness of incubation
bottles. Together with each batch of samples incubate a bottle of unseeded
dilution water. Determine initial and final DO as in Us 4g and j. The DO
uptake should not be more than 0.2 mg/L and preferably not more than 0.1 mg/L
Discard all dilution water having a DO uptake greater than 0.2 mg/L and either eliminate source of contamination or select an
alternate dilution water source.
i. Incubation: Incubate at 20°C ± 1°C BOD bottles
containing desired dilutions, seed controls, dilution water blanks, and
glucose- glutamic acid checks.
j. Determination of final DO: After 5 d incubation determine DO
in sample dilutions, blanks, and checks as in U 4g.
5. Calculation
For each test bottle meeting the
2.0-mg/L minimum DO depletion and the 1.0-mg/L residual DO, calculate BOD5 as
follows:
When dilution water is not
seeded:
BOD5, mg/L =(D1-D2)/P
When dilution water is seeded:
BOD3, mg/L = [(D1-D2)-(B1-B2)f]/P
where:
D1 = DO of diluted sample
immediately after preparation, mg/L,
D2 = DO of diluted sample after 5 d
incubation at 20°C, mg/L,
P = decimal volumetric fraction of
sample used,
B1 = DO of seed control before
incubation, mg/L (1 4d),
B2 = DO of seed control after
incubation mg/L (K 4d), and
f = ratio of seed in diluted
sample to seed in seed control seed in diluted sample)=(% seed in seed
control). = (%
If seed material is added directly to sample
or to seed control bottles:
f= (volume of seed in diluted
sample)/(volume of seed in seed control)
Report results as CBOD5 if
nitrification is inhibited.
If more than one sample dilution
meets the criteria of a residual DO of at least 1 mg/L and a DO depletion of at
least 2 mg/L and there is no evidence of toxicity at higher sample
concentrations or the existence of an obvious anomaly, average results in the acceptable
range.
In these calculations, do not
make corrections for DO uptake by the dilution water blank during incubation.
This correction is unnecessary if dilution water meets the blank criteria
stipulated above. If the dilution water does not meet these criteria, proper corrections
are difficult ; do not record results or, as a minimum, mark them as not meeting quality control
criteria.
6. Precision and Bias
There is no measurement for
establishing bias of the BOD procedure. The glucose-glutamic acid check
prescribed in H 4c is intended to be a reference point for evaluation of
dilution water quality, seed effectiveness, and analytical technique.
Single-laboratory tests using a 300-mg/L mixed glucose-glutamic acid solution provided
the following results:
Number of months: 14
Number of
triplicates: 421
Average monthly
recovery: 204 mg/L
Average monthly
standard deviation: 10.4 mg/L
In a series of interlaboratory
studies,' each involving 2 to 112 laboratories (and as many analysts and seed
sources), 5-d BOD measurements were made on synthetic water samples containing a
1:1 mixture of glucose and glutamic acid in the total concentration range of
3.3 to 231 mg/L. The regression equations for mean value, X, and
standard deviation. 5, from these studies were:
X= 0.658 (added level, mg/L) +
0.280 mg/L
S = 0.100 (added level, mg/L) +
0.547 mg/L
For the 300-mg/L mixed primary
standard, the average 5-d BOD would be 198 mg/L with a standard deviation of
30.5 mg/L. When nitrification inhibitors are used, GGA test results falling
outside the 198 ± 30.5 control limit quite often indicate use of
incorrect amounts of seed. Adjust amount of seed added to the GGA test to
achieve results falling within this range.
a. Control limits: Because of many factors
affecting BOD tests in multilaboratory studies and the resulting extreme
variability in test results, one standard deviation, as determined by interlaboratory
tests, is recommended as a control limit for individual laboratories. Alternatively,
for each laboratory, establish its control limits by performing a minimum of 25
glucose-glutamic acid checks (114c) over a period of several weeks or months
and calculating the mean and standard deviation. Use the mean ± 3 standard
deviations as the control limit for future glucose-glutamic acid checks.
Compare calculated control limits to the single-laboratory tests presented
above and to interlaboratory results. If control limits are outside the range
of 198 ± 30.5, re-evaluate the control limits and investigate source of the
problem. If measured BOD for a glucose-glutamic acid check is outside the
accepted control limit range, reject tests made with that seed and dilution
water.
b. Working range and detection
limit: The working range is equal to
the difference between the maximum initial DO (7 to 9 mg/L) and minimum DO
residual of 1 mg/L multiplied by the dilution factor. A lower detection limit
of 2 mg/L is established by the requirement for a minimum DO depletion of 2
mg/L.
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