Sample Seeding; Sample Temperature; Toxic Materials; Chlorine - Hach BODTrak II User Manual

When the oxygen requirement of a sample is more than 700 mg/L, dilute
the sample with high-quality distilled or deionized water.
Calculate the results to include the additional dilution factor. Example: if
the BOD of the sample is 1000 mg/L, dilute the sample 1:1 with distilled
or deionized water. The estimated BOD is now 500 mg/L. Use the
sample volume specified in the table for the 0 to 700 mg/L range of the
selected procedure. Multiply the instrument reading result by 2. If the
Hach Standard Method procedure is used, continue with the rest of the
calculations.

10.2 Sample seeding

Some types of BOD samples do not contain sufficient bacteria to oxidize
the organic matter in the sample. Many industrial wastes are of this type.
Some sewage treatment plant effluents are chlorinated and essentially
sterile. A BOD test cannot be done in the absence of viable bacteria. To
test such samples, seed each bottle from a source known to contain a
viable bacterial population.
Settled domestic wastewater plant influent or primary clarifier effluent are
the preferred sources of seed for most samples. Mixed liquor or
undisinfected effluent can be used for seed, but it is recommended to
include a nitrification inhibitor. Commercial seed sources are sometimes
suitable. To prepare, see the instructions from the manufacturer.

10.3 Sample temperature

Standard Methods for the Examination of Water and Wastewater, 21st
Ed., 2005 5210 D recommends an incubation temperature of 20 ±1 ºC
(68 ºF) for the BOD test. Put the instrument in an incubator that is
adjusted to 20 ±1 ºC. Warm or cool samples to 20 ±1 ºC.
Instrument performance has not been validated at temperatures other
than 20 ºC.

10.4 Toxic materials

Industrial and chlorinated samples often contain toxic substances and
require special considerations when running BOD tests. Toxic materials
in the sample will cause decreased BOD values. Dilute the sample to
minimize the toxic materials or their effects. Refer to Standard Methods
for the Examination of Water and Wastewater, 21st edition, 5210 D.
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10.5 Chlorine

Chlorine in the sample must be removed prior to testing. Keep the
sample at room temperature for 1 to 2 hours before a test to dissipate
low chlorine concentrations. If any chlorine remains after sitting for
2 hours, or if the chlorine concentration is high, add sodium thiosulfate to
remove the chlorine.
1. In a 250-mL Erlenmeyer flask, add 100 mL of sample.
2. Add 10 mL of 100 g/L potassium iodide solution and 10 mL of 0.02 N
sulfuric acid standard solution to the Erlenmeyer flask.
3. Add 3 droppers of starch indicator solution and swirl to mix.
4. Titrate from dark blue to colorless with 0.025 N Sodium Thiosulfate
standard solution.
5. Calculate the quantity of sodium thiosulfate standard solution
necessary to dechlorinate the remaining sample:
mL of sodium thiosulfate = (mL used)×(mL sample to be
dechlorinated)÷100
6. Add the necessary quantity of 0.025 N sodium thiosulfate standard
solution to the sample and mix fully. After 10 to 20 minutes, do the
BOD test.

10.6 pH effect

Low BOD test results occur when sample pH is outside the range of 6 to
8. Keep this pH to simulate source sample conditions or adjust the pH to
neutrality (buffered at pH 7). Use 1.0 N (or weaker) sulfuric acid to
neutralize caustic samples. Use 1.0 N (or weaker) sodium hydroxide to
neutralize acidic samples. When samples are pH adjusted, they should
also be seeded.

10.7 Supersaturation

Equilibrate supersaturated cold samples (samples that contain more
than 9 mg/L of dissolved oxygen at 20 ºC) to saturation.