Biochemical Oxygen Demand Testing

Biochemical oxygen demand (BOD) measuresthe amount of oxygen consumed by microorganisms in decomposing organic matter in streamwater. BOD also measures thechemical oxidationofinorganic matter(the extraction of oxygen from water via chemical reaction). A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20°C). The rate of oxygen consumption in a stream is affected by a number of variables:temperature, pH, the presence of certain kinds of microorganisms, and the type of organic andinorganic materialin the water.

Biochemical oxygen demand directly affects the amount of dissolved oxygen in water bodies—the greater the BOD, the more rapidly oxygen is depleted in the water body, leaving less oxygen available to

higher forms of aquatic life. The consequences ofhigh BODare the same as those for low dissolved oxygen: Aquatic organisms become stressed, suffocate, and die. Most river waters used as water supplies have a BOD less than 7 mg/L; therefore, dilution is not necessary.

Sources of BOD include leaves and wood debris; dead plants and animals; animal manure; effluents from pulp and paper mills, wastewa-ter treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.

Note: To evaluate the potential use of raw water as a drinking water supply, it is usually sampled, analyzed, and tested for biochemical oxygen demand when turbid, polluted water is the only source available.

18.3.8.1 Sampling Considerations

Biochemical oxygen demand is affected by the same factors that affect dissolved oxygen. Aeration of streamwater (e.g., by rapids and waterfalls) will accelerate the decomposition of organic and inorganic material; therefore,BOD levelsat a sampling site with slower, deeper waters might be higher for a given column of organic and inorganic material than the levels for a similar site in highly aerated waters. Chlorine can also affectBOD measurement通过抑制或杀死微生物,decompose the organic and inorganic matter in a sample. If sampling in chlorinated waters (such as those below the effluent from a sewage treatment plant), neutralizing the chlorine with sodium thio-sulfate is necessary (see Standard Methods).

Biochemical oxygen demand measurement requires taking two samples at each site. One is tested immediately for dissolved oxygen; the second is incubated in the dark at 20°C for 5 days and then tested for dissolved oxygen remaining. The difference in oxygen levels (in mg/L) between the first test and the second test is the amount of BOD. This represents the amount of oxygen consumed by microorganisms and used to break down the organic matter present in the sample bottle during the incubation period. Because of the 5-day incubation, the tests are conducted in a laboratory.

Sometimes by the end of the 5-day incubation period, the dissolved oxygen level is zero. This is especially true for rivers and streams with a lot of organic pollution. Because knowing when the zero point was reached is not possible, determining the BOD level is also impossible. In this case, diluting the original sample by a factor that results in a final dissolved oxygen level of at least 2 mg/L is necessary. Special dilution water should be used for the dilutions (see Standard Methods).

Some experimentation is needed to determine the appropriate dilution factor for a particular sampling site. The result is the difference in dissolved oxygen between the first measurement and the second, after multiplying the second result by the dilution factor. Standard Methods prescribes all phases of procedures and calculations for BOD determination. ABOD testis not required for monitoring water supplies.

TABLE 18.2 BOD5 TEST PROCEDURE

1. Fill two bottles with BOD dilution water; insert stoppers.

2.Place sample in two BOD bottles; fill with dilution water; insert stoppers.

3. Test for dissolved oxygen.

4. Incubate for 5 days.

5. Test for dissolved oxygen.

6. Add 1 mL MnSO4 below the surface.

7. Add 1 mL alkaline KI below the surface.

8. Add 1 mL H2SO4.

9.Transfer 203 mL to flask.

10. Titrate with PAO or thiosulfate.

18.3.8.2 BOD5 Sampling, Analysis, and Testing

The approved BOD sampling and analysis procedure measures the DO depletion (biological oxidation of organic matter in the sample) over a 5-day period under controlled conditions (20°C in the dark). The test is performed using a specified incubation time and temperature. Test results are used to determine plant loadings, plant efficiency, and compliance with NPDES effluent limitations; however, the duration of the test makes it difficult to use the data effectively for process control.

The standard BOD test does not differentiate between oxygen used to oxidize organic matter and that used to oxidize organic and ammonia nitrogen to more stable forms. Because many biological treatment plants now control treatment processes to achieve oxidation of the nitrogen compounds, it is possible that BOD test results for plant effluent and some process samples may produce BOD test results based on both carbon and nitrogen oxidation. To avoid this situation, a nitrification inhibitor can be added. When this is done, the test results are known ascarbonaceous BOD(CBOD). A second uninhibited BOD should also be run whenever CBOD is determined.

When taking a BOD sample, Key Point: Remember that all chemicals can be danger-no special sampling container is ous if not use,d p1^1^ and in acc°rdance with the

. . recommended procedures. Review the appropriate sec-

required. Either a grab or composite tions of the material safety data sheet (MSDS) for each sample can be used. BOD5 samples chemical to determine the proper methods for handling can be preserved by refrigeration at and for safety precautions that should be taken. or below 4°C (not frozen); composite

samples must be refrigerated during collection. The maximum holding time for preserved samples is 48 hours. Using the incubation of dissolved approved test method, a sample is mixed with dilution water in several different concentrations (dilutions). The dilution water contains nutrients and materials to provide optimum environment. Chemicals used include dissolved oxygen, ferric chloride, magnesium sulfate, calcium chloride, phosphate buffer, and ammonium chloride.

Sometimes it is necessary to add (seed) healthy organisms to the sample. The DO levels of the dilution and the dilution water are determined. If seed material is used, a series of dilutions of seed material

must also be prepared. The dilutions and dilution blanks are incubated in the dark for 5 days at 20 ± 1°C. At the end of 5 days, the DO level of each dilution and the dilution blanks is determined.

For the test results to be valid, certain criteria must be achieved. These test criteria are listed as follows:

1. Dilution water blank DO change must be <0.2 mg/L.

2.Initial DO must be >7.0 mg/L but <9.0 mg/L (or saturation at 20°C and test elevation).

3. Sample dilution DO depletion must be >2.0 mg/L.

4. Sample dilution residual DO must be >1.0 mg/L.

5. Sample dilution initial DO must be >7.0 mg/L.

6. Seed correction should be >0.6 but <1.0 mg/L.

The BOD5 test procedure consists of 10 steps (for unchlorinated water), as shown in Table 18.2.

Note: BOD5 is calculated individually for all sample dilutions that meet the criteria. The reported result is the average of the BOD5 of each valid sample dilution.

18.3.8.3 BOD5计算(种子)

Unlike the direct reading instrument used in the DO analysis, BOD results require calculation. Several criteria are used when selecting which BOD5 dilutions should be used for calculating test results. Consult a laboratory testing reference manual (such as Standard Methods) for this information.

Currently, there are two basic calculations for BOD5. The first is used for samples that have not been seeded. The second must be used whenever BOD5 samples are seeded. In this section, we illustrate the calculation procedure for unseeded samples:

TDOstart (mg/L) - DOflnal (mg/L)] x 300 mL

BOD5 (Unseeded) = ¡=-slt'" s -flnal 1 s JJ (18.8)

Sample Volume (mL)

■ Example 18.5

problem: The BOD5 test is completed. Bottle 1 of the test had a DO of 7.1 mg/L at the start of the test. After 5 days, bottle 1 had a DO of 2.9 mg/L. Bottle 1 contained 120 mg/L of sample.

Solution:

x,^ ,TT j ^ (7.1 mg/L - 2.9 mg/L) x 300 mL

BOD5 (Unseeded) = --^--= 10.5 mg/L

120 mL

18.3.8.4 BOD5 Calculation (Seeded)

If the BODs sample has been exposed to conditions that could reduce the number of healthy, active organisms, the sample must be seeded with organisms. Seeding requires use of a correction factor to remove the BOD5 contribution of the seed material:

, „ Seed Material BOD5 x Seed in Dilution (mL)

Seed Correction =-5 (18.9)

300 mL

BOD5 = {^DOstart (mg/L) - DOflnal (mg/L)] - Seed Correction} x 300 (18 10) (Seeded) Sample Volume (mL)

■ Example 18.6

Problem: Using the data provided below, determine the BODS:

BOD5 of seed material = 90 mg/L Dilution #1 seed material = 3 mL Sample = 100 mL Start DO = 7.6 mg/L Final DO = 2.7 mg/L

Solution:

„ J „ . 90 mg/L x 3 mL ___

Seed Correction =---= 0.90 mg/L

300 mL

|"(7.6 mg/L - 2.7 mg/L) - 0.90] x 300

BOD5 (Seeded) = ^---—-J-= 12 mg/L

100 mL

18.3.9 Solids Measurement

Solids in water are defined as any matter that remains as residue upon evaporation and drying at 103°C. They are separated into two classes: suspended solids and dissolved solids.

Total Solids = Suspended Solids (nonfilterable residue) + Dissolved Solids (filterable residue)

As shown above, total solids are dissolved solids plus suspended andsettleablesolids in water. In natural freshwater bodies, dissolved solids consist of calcium, chlorides, nitrate, phosphorus, iron, sulfur, and other ions—particles that will pass through a filter with pores of around 2 pm (0.002 cm) in size. Suspended solids include silt and clay particles, plankton, algae, fine organic debris, and other particulate matter. These are particles that will not pass through a 2-pm filter.

The concentration of total dissolved solids affects the water balance in the cells of aquatic organisms. An organism placed in water with a very low level of solids (distilled water, for example) swells because water tends to move into its cells, which have a higher concentration of solids. An organism placed in water with a high concentration of solids shrinks somewhat, because the water in its cells tends to move out. This in turn affects the organism's ability to maintain the proper cell density, making it difficult for it to maintain its position in the water column. It might float up or sink down to a depth to which it is not adapted, and it might not survive.

Higher concentrations of suspended solids can serve as carriers of toxics, which readily cling to suspended particles. This is particularly a concern where pesticides are being used on irrigated crops. Where solids are high, pesticide concentrations may increase well beyond those of the original application as the irrigation water travels down irrigation ditches. Higher levels of solids can also clog irrigation devices and might become so high that irrigated plant roots will lose water rather than gain it.

A high concentration of total solids will make drinking water unpalatable and could have an adverse effect on people who are not used to drinking such water. Levels of total solids that are too high or too low can also reduce the efficiency of wastewater treatment plants, as well as the operation of industrial processes that use raw water.

Total solids affect water clarity. Higher solids decrease the passage of light through water, thereby slowing photosynthesis by aquatic plants. Water heats up more rapidly and holds more heat; this, in turn, might adversely affect aquatic life adapted to a lowertemperature regime. Sources of total solids include industrial discharges, sewage, fertilizers, road runoff, and soil erosion. Total solids are measured in milligrams per liter.

18.3.9.1 Solids Sampling and Equipment Considerations

When conducting solids testing, many things can affect the accuracy of the test or result in wide variations in results for a single sample:

1. Drying temperature

2.Length of drying time

3. Condition of desiccator and desiccant

4. A lack of consistency among nonrepresentative samples in the test procedure

5. Failure to achieve constant weight prior to calculating results

Several precautions can be taken to improve the reliability of test results:

1. Use extreme care when measuring samples, weighing materials, and drying or cooling samples.

2.Check and regulate oven and furnace temperatures frequently to maintain the desired range.

3. Use an indicator drying agent in the desiccator that changes color when it is no longer good; change or regenerate the desiccant when necessary.

4. Keep the desiccator cover greased with the appropriate type of grease; this will seal the desiccator and prevent moisture from entering the desiccator as the test glassware cools.

5. Check ceramic glassware for cracks and glass fiber filters for possible holes. A hole in a glass filter will cause solids to pass through and give inaccurate results.

6. Follow manufacturers' recommendations for the care and operation of analytical balances.

Total solids are important to measure in areas where discharges from sewage treatment plants, industrial plants, or extensive crop irrigation may occur. In particular, streams and rivers in arid regions where water is scarce and evaporation is high tend to have higher concentrations of solids and are more readily affected by the human introduction of solids from land-use activities.

Total solids measurements can be useful as an indicator of the effects of runoff from construction, agricultural practices, logging activities, sewage treatment plant discharges, and other sources. As with turbidity, concentrations often increase sharply during rainfall, especially in developed watersheds. They can also rise sharply during dry weather if earth-disturbing activities occur in or near the stream without erosion control practices in place. Regular monitoring of total solids can help detect trends that might indicate increasing erosion in developing watersheds. Total solids are closely related to stream flow and velocity and should be correlated with these factors. Any change in total solids over time should be measured at the same site at the same flow.

Total solids are measured by weighing the amount of solids present in a known volume of sample; this is accomplished by weighing a beaker, filling it with a known volume, evaporating the water in an oven and completely drying the residue, then weighing the beaker with the residue. The total solids concentration is equal to the difference between the weight of the beaker with the residue and the weight of the beaker without it. Because the residue is so light in weight, the lab needs a balance that is sensitive to weights in the range of 0.0001 g. Balances of this type are called analytical or Mettler balances, and they are expensive (around $3000). The technique requires that the beakers be kept in a desiccator, a sealed glass container containing material that absorbs moisture and ensures that the weighing is not biased by water condensing on the beaker. Some desiccants change color to indicate moisture content. Measurement of total solids cannot be done in the field. Samples must be collected using clean glass or plastic bottles or Whirl-Pak® bags and taken to a laboratory where the test can be run.

18.3.9.2 Total Suspended Solids

The term solids refers to any material suspended or dissolved in water and wastewater. Although normal domesticwastewater containsa very small amount of solids (usually less than 0.1%), most treatment processes are designed specifically to remove or convert solids to a form that can be removed or discharged without causing environmental harm. In sampling for total suspended solids (TSS), samples may be either grab or composite and can be collected in either glass or plastic containers. TSS samples can be preserved by refrigeration at or below 4°C (not frozen); however,composite samplesmust be refrigerated during collection. The maximum holding time for preserved samples is 7 days.

18.3.9.2.1 Test Procedure

进行分时系统测试,摊位sampl来衡量e is poured into a filtration apparatus and, with the aid of a vacuum pump or aspirator, is drawn through a preweighted glass fiber filter. After filtration, the glass filter is dried at 103 to 105°C, cooled, and reweighed. The increase in weight of the filter and solids compared to the filter alone represents the total suspended solids. An example of the specific test procedure used for total suspended solids is given below.

1. Select a sample volume that will yield between 10 and 200 mg of residue with a filtration time of 10 minutes or less.

Note: If filtration time exceeds 10 minutes, increase filter area or decrease volume to reduce filtration time.

Note: For nonhomogeneous samples or samples with very high solids concentrations (e.g., raw wastewater or mixed liquor), use a larger filter to ensure that a representative sample volume can be filtered.

2.Place the preweighed glass fiber filter on the filtration assembly in a filter flask.

3. Mix the sample well, and measure the selected volume of sample.

4. Apply suction to the filter flask, and wet the filter with a small amount of laboratory-grade water to seal it.

5. Pour the selected sample volume into the filtration apparatus.

6. Draw the sample through the filter.

7. Rinse the measuring device into the filtration apparatus with three successive 10-mL portions of laboratory-grade water. Allow complete drainage between rinsings.

8. Continue suction for 3 minutes after filtration of the final rinse is completed.

9.删除的玻璃过滤器过滤的屁股embly (membrane filter funnel or clean Gooch crucible). If using the large disks and membrane filter assembly, transfer the glass filter to a support (aluminum pan or evaporating dish) for drying.

10. Place the glass filter with solids and support (pan, dish, or crucible) in a drying oven.

11. Dry the filter and solids to a constant weight at 103 to 105°C (at least 1 hour).

12. Cool to room temperature in a desiccator.

13. Weigh the filter, solids, and support, and record the constant weight in the test record.

18.3.9.2.2 TSS Calculations

To determine the total suspended solids concentration in milligrams per liter, we use the following equations:

1. To determine the weight of dry solids in grams:

(18.11)

Dry Solids (g) = Weight of dry solids and filter (g)

- Weight of dry filter (g)

2.To determine the weight of dry solids in milligrams:

Dry Solids (mg) = Weight of dry solids and filter (mg)

(18.12]

- Weight of dry filter (mg)

3. To determine the TSS concentration in milligrams per liter:

TSS (mg/L) = Dry Solids (mg) x1000mL (18.13)

Sample Volume (mL)

■ Example 18.7

Problem: Using the data provided below, calculate total suspended solids (TSS):

Sample volume = 250 mL

Weight of dry solids and filter = 2.305 g

Weight of dry filter = 2.297 g

Solution:

Dry Solids (g) Dry Solids (mg)

TSS

= 2.305 g - 2.297 g = 0.008 g = 0.008 g x 1000 mg/g = 8 mg 8.0 x 1000 mL/L

250 mL

= 32.0 mg/L

18.3.9.3Volatile Suspended SolidsTesting

When the total suspended solids are ignited at 550 ± 50°C, the volatile (organic) suspended solids of the sample are converted to water vapor and carbon dioxide and are released to the atmosphere. The solids that remain after the ignition (ash) are the inorganic or fixed solids. In addition to the equipment and supplies required for the total suspended solids test, you need the following:

1. Muffle furnace (550 ± 50°C)

2.Ceramic dishes

3. Furnace tongs

4. Insulated gloves

18.3.9.3.1 Test Procedure

An example of the test procedure used for volatile suspended solids is given below:

1. Place the weighed filter with solids and support from the total suspended solids test in the muffle furnace.

2.Ignite the filter, solids, and support at 550 ± 50°C for 15 to 20 minutes.

3. Remove the ignited solids, filter, and support from the furnace, and partially air cool.

4. Cool to room temperature in a desiccator.

5. Weigh the ignited solids, filter, and support on an analytical balance.

6. Record the weight of the ignited solids, filter, and support.

18.3.9.3.2 Total Volatile Suspended Solids Calculations

Calculating the total volatile suspended solids (TVSS) requires knowing the weight of the dry solids, filter, and support in grams (A) and the weight of the ignited solids, filter, and support in grams (C):

TVSS (mg/L) = (A - C) x1000 mg/g x1000mL/L (18.14,

Sample Volume (mL)

■ Example 18.8

Problem: Using the data provided below, calculate the total volatile suspended solids:

Weight of dried solids, filter, and support = 1.6530 g Weight of ignited solids, filter, and support = 1.6330 g

Solution:

TVSS =

(1.6530 g -1.6330 g) x 1000 mg/g x 1000 mL 100 mL

0.02 x 1,000,000 mg/L 100

200 mg/L

Note: Total fixed suspended solids (TFSS) is the difference between the total volatile suspended solids (TVSS) and the total suspended solids (TSS) concentrations:

■ Example 18.9

问题:计算tfs给出下列数据:

Total suspended solids (TSS) = 202 mg/L

Total volatile suspended solids (TVSS) = 200 mg/L

Solution:

18.1 How soon after the sample is collected must the pH be tested?

18.2 The operator titrates a 200-mL dissolved oxygen sample. The buret reading at the start of the titration was 0.0 mL. At the end of the titration, the buret read 7.1 mL. The concentration of the titration solution was 0025. What is the DO concentration in mg/L?

18.3 What is agrab sample?

18.4 Dissolved oxygen samples collected from the aeration tank and carried back to the lab for testing must be preserved by adding

18.5 When is it necessary to use a grab sample?

18.6 If a grab sample is to be used to evaluate plant performance, when should theinfluent and effluentsamples be collected?

18.7 What is a composite sample?

18.8 Why is a composite sample more representative of the average characteristics of the wastewater?

18.9 List three rules for sample collection.

18.10 The approved method for pH testing requires__

18.11 Who specifies the sample type, preservation method, and test method for effluent samples?

TFSS (mg/L) = TSS - TVSS

(18.15)

TFSS = 202 mg/L - 200 mg/L = 2 mg/L

18.4 CHAPTER REVIEW QUESTIONS

18.12 The Guidelines Establishing Test procedures for the Analysis of

pollutants under the Clean Water act is a/an_

regulation.

18.13 The average daily flow is 7.66 MGD, and the effluent testing will require 3000 mL of sample. What is the proportioning factor if a 24-hour composite sample is to be collected?

18.14 The proportioning factor is 100, and the flow at the time the sample is collected (7 a.m.) is 4.66 MGD. How many milliliters of sample should be added to the composite sample container at 7 a.m.?

18.15 What is the maximum holding time and recommended preservation technique for BOD5 samples?

18.16 The dissolve oxygen meter requires calibration at least once per

18.17有什么区别BOD5和the CBOD5 test?

18.18 Why is seeding required for samples with high or low pH or chlorinated samples?

18.19是什么the acceptable range of seed correction?

18.20 What is the acceptable preservation method for suspended solids samples?

18.21 Most solids test methods are based on changes in weight. What can cause changes in weight during the testing procedure?

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