study of raw wastewater bod5 and cbod5 relationship yields ... · difference between raw wastewater...

Study of Raw Wastewater BOD5 and cBOD5 Relationship Yields Surprising Results

Woodie Mark Muirhead*, Greg Farmer**, Stacey Walker**, Leonard Robb**,

Holly Elmendorf***, Roger Matthews***, Rick Butler****, Henryk Melcer*****

*Brown and Caldwell 119 Merchant Street

Honolulu, Hawaii 96826

**Littleton-Englewood, Colorado *** Gwinnett County, Georgia **** King County, Washington

*****Brown and Caldwell

ABSTRACT

A study of the relationship of the raw wastewater BOD5:cBOD5 ratio was conducted at three wastewater treatment plants across the United States. Gwinnett County, Georgia; King County, Washington; and Littleton-Englewood, Colorado conducted a 3-month study using side-by-side samplers. The purpose of the study was to determine if cleaning wetted sampler parts routinely affected the ratio. One of the samplers was cleaned twice a week and the other was not cleaned. The results of the study show that cleaning the wetted parts of the sampler has a small but statistically insignificant impact on the BOD5:cBOD5 ratio. The study also showed a potential relationship of wastewater temperature to the BOD5:cBOD5 ratio, and also the possible influence of sampler type on raw wastewater characterization. KEYWORDS

Biochemical oxygen demand, carbonaceous biochemical oxygen demand, nitrification, inhibitor

INTRODUCTION

In the 1980s, many secondary treatment facilities had difficulty meeting effluent biochemical oxygen demand (BOD) limits due to exertion of nitrogenous oxygen demand in the BOD5 test. The Environmental Protection Agency (EPA) modified the definition of secondary treatment to permit the use of carbonaceous biochemical oxygen demand (cBOD) as an effluent limit in lieu of BOD5

(40 CFR 133.122). Many state regulatory agencies included the use of effluent cBOD5 in new National Pollutant Discharge Eliminations System (NPDES) permits, and consequently required facilities to perform raw wastewater cBOD5 to calculate 85 percent removal.

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Copyright 2006 Water Environment Foundation. All Rights Reserved©

Data from treatment facilities nationwide show raw wastewater BOD5 and cBOD5 can differ by over 20%, yet the cause of this difference is unknown. Raw wastewater BOD5 is a key analytical parameter for design and for regulatory compliance of wastewater treatment facilities. If the difference between raw wastewater BOD5 and cBOD5 is due to inhibition of heterotrophic organisms, use of raw wastewater cBOD5 might underestimate facility loading significantly. If nitrification is occurring in the BOD5 test, use of raw wastewater BOD5 might overstate facility loading since nitrogen loading is considered separately. Studies have shown that the nitrification inhibitor does not significantly affect the carbonaceous oxygen demand within the 5-day test period (Young, 1973; Slayton, 1978). Contrary to this, an analysis of BOD, cBOD, and chemical oxygen demand (COD) data from nine facilities concluded that the nitrification inhibitor suppresses the level of oxidation of carbonaceous matter by heterotrophic organisms resulting in a lower cBOD5 value (Albertson, 1995). In the late 1980s, a study determined that nitrifying organisms can grow in final effluent sample lines, seed the BOD test, and exert a nitrogenous oxygen demand (NOD) (Chapman, 1991). This caused many facilities to be in non-compliance with effluent BOD5 limits. Regular cleaning of the sample lines prevented nitrifier growth and reduced significantly the difference between BOD5 and cBOD5. In March 2005, Gwinnett County, Georgia, Littleton-Englewood, Colorado, and the South Plant, King County, Washington, conducted a study to determine if sample lines are the source of nitrifying organisms in the raw wastewater BOD5 test and if cleaning the sample lines affects the BOD:cBOD ratio. This paper will present the results of this cooperative study and the relationships that were identified.

STUDY APPROACH

Each facility set up two samplers to collect flow-proportional samples of raw wastewater or primary influent. The sample line and wetted parts of one sampler (hereafter termed “Clean Sampler”) was cleaned twice weekly during the study and while the other (hereafter termed “Dirty Sampler”) was not cleaned. Figures 1, 2, and 3 show the sampler setups for each of the facilities. Samples were collected three times per week from March 1 through May 31, 2005, and analyzed for BOD5 and cBOD5. The nitrification inhibitor 2-chloro-6-(trichloromethyl pyridine), manufactured by HACH Chemical Company, was used in the cBOD5 analysis. Wastewater temperature data were also collected during the study.

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Figure 1. Gwinnett County Samplers

Clean Sampler

Brand ISCO/GLACIER

Type Peristaltic

Location Indoors

Sample line length 15’

Photograph

Dirty Sampler

Brand ISCO/6700 RF

Type Peristaltic

Location Indoors

Sample line length 12’

Photograph See photograph above.

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Figure 2. King County Sampler Set-up

Clean Sampler

Brand Sirco PVS 4100

Type Outdoors

Location Indoors or outdoors

Sample line length No data

Photograph

Dirty Sampler

Brand Sigma

Type Continuous flow, dipper

Location Indoors

Sample line length No data

Photograph

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Figure 3. Littleton-Englewood Samplers

Clean Sampler

Brand ISCO 3700 R

Type Peristaltic

Location Outdoors

Sample line length Total length = 23 feet; exposed = 3 feet

Photograph

Dirty Sampler

Brand ISCO 3700 R

Type Peristaltic

Location Outdoors

Sample line length Total length = 23 feet; exposed = 3 feet

Photograph See above photograph.

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VARIABILITY OF THE BOD5 AND CBOD5 TESTS

The BOD5 and cBOD5 analyses are differentiated by the use of a nitrification inhibitor in the cBOD5 test. All other aspects of the two analyses are the same and both tests are subject to the same quality control/quality assurance issues. Though there are differences in opinion about the cause of differences in BOD5 and cBOD5 results, the difference is solely associated with nitrification in the BOD5 test from a regulatory perspective. Two important issues that must be considered when analyzing BOD5 and cBOD5 relationships are the normal variability in the test, and the source of nitrifying organisms for nitrogenous oxygen demand (NOD) to be exerted in the BOD5 test. These issues are discussed in this section. The accepted accuracy of the BOD5 test is 198 +/- 30.5 mg/L (i.e., +/- 15%) when performed on a 2 percent dilution of 150 mg/L glucose and 150 mg/L glutamic acid (Standard Methods, 1998). Since this is accepted level of variability for a known concentration of oxygen demanding compounds, the amount of variability in the test for a raw wastewater sample with a wider range of organic compounds could be considerably higher. Consequently, BOD5 and cBOD5 values that vary more than 15% from one another might be normal test variability. Though the high variability in the test must be considered when analyzing BOD5 and cBOD5 discrepancies, we can make assumptions about the relationship subject to this variability:

• The BOD5 value should always be higher than or equal to the cBOD5, since the nitrification inhibitor is intended to prevent NOD, which is in excess of the oxygen demand exerted by heterotrophic oxidation of organic compounds.

• Neither the BOD5 or cBOD5 value should be consistently higher or lower than the other

value if normal variability is the sole cause of the discrepancy between the results for a given sample.

• The BOD5 value should be consistently higher than the cBOD5 value when nitrification is

the sole cause of the discrepancy between BOD5 and cBOD5 values.

• The nitrification inhibitor could result in cBOD5 values that are higher than the BOD5 values if the inhibitor is oxidized by heterotrophic organisms in the test.

• The nitrification inhibitor could result in BOD5 values that are higher than cBOD5 values

if the inhibitor interferes with the oxidation of organic compounds. For nitrification to occur in the BOD5 test, nitrifying organisms must be present in raw wastewater, or introduced during sampling and analysis. Since nitrifying organisms have a slower growth rate than heterotrophic organisms, they must also be present in sufficient numbers to exert an oxygen demand in the 5-day test.

• Nitrifying organisms are common in the soil and could enter raw wastewater through infiltration and inflow.

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• Recycle streams that enter the raw wastewater ahead of the influent sampler could introduce nitrifying organisms. It is common practice, if not a regulatory requirement, to sample raw wastewater upstream of any recycle streams.

• Nitrifying organisms can grow in the wetted parts of a raw wastewater sampler and seed

the BOD5 test.

RESULTS

Though the original intent of the study was to evaluate the impact of sample line cleaning on the BOD:cBOD ratio, the composite data of the study, as well as additional data collected subsequent to the study yielded some surprising results. These results are discussed in the following sections. General Observations All participants in the study expected to see consistently higher BOD5 results, yet the study showed the BOD5 concentration is not always higher than the cBOD5 concentration. Many analytical results from Gwinnett County and Littleton-Englewood showed cBOD5 values higher than BOD5 values. Only the analytical results from King County showed BOD5 concentrations consistently higher than cBOD5 concentrations. The higher cBOD5 results may be associated with the normal variation associated with the BOD testing procedure, or possibly with oxygen demand exerted by biodegradation of the nitrification inhibitor. If the latter is the case, the oxygen demand exerted by the nitrification inhibitor is not consistent or is masked in some tests by other impacts. Regardless, the large number of values showing higher cBOD5 results does suggest nitrification is not always occurring in the BOD5 test, nor is the nitrification inhibitor always interfering with heterotrophic oxidation in the cBOD5 test. The relationship between BOD5 and cBOD5 at each facility differed. Regression analyses were conducted on the BOD5 and cBOD5 data for the clean and dirty samplers. The results of the regression analysis are presented in Table 1. Gwinnett County showed a high correlation between the values from both the clean and dirty samplers. Figures 4 and 5 graphically present the data correlation for Gwinnett County. There is no statistical difference between the BOD5 and cBOD5 values for either sampler. These data suggest that nitrification is not occurring in the BOD5 test, and the nitrification inhibitor is not interfering with heterotrophic activity in the cBOD5 test. The relationships at King County and Littleton-Englewood were more highly variable and the same conclusions cannot be drawn from their results.

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Table 1. Regression Analysis Data for BOD5-cBOD5 Relationship

Facility Clean Sampler BOD5 versus cBOD5

Dirty Sampler BOD5 versus cBOD5

Gwinnett County y = 0.9533x – 0.8763 R2 = 0.9905

y = 0.7466x – 21.568 R2 = 0.9563

King County Y = 0.5979x + 30.981 R2 = 0.7802

Y = 0.7358x + 14.369 R2 = 0.5315

Littleton-Englewood y = 0.5991x + 69.892 R2 = 0.3934

y = 0.805x + 19.536 R2 = 0.5211

Figure 4. Gwinnett County Clean Sampler BOD5-cBOD5 Correlation

y = 0.9533x - 0.8763R2 = 0.9905

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D, m

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Figure 5. Gwinnett County Dirty Sampler BOD5-cBOD5 Correlation

y = 0.7466x + 21.568R2 = 0.9563

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Impact of Sample Line Cleaning Table 2 compares the BOD5 and cBOD5 results for the dirty and clean samplers. The results show the average BOD5 and cBOD5 values were lower in the clean sampler than in the dirty sampler. A Student’s T-test (p = 0.05) was performed on the data from all samplers. It showed that there was no significant difference in the BOD:cBOD ratio between the clean and dirty samplers. These data indicate that the wetted parts of a sampler are not a significant source of significant nitrifying organisms. In this case, cleaning a raw sewage sampler’s wetted parts will have limited impact on decreasing the BOD:cBOD ratio.

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Table 2. Sample Line Cleaning Study Results Summary

Average Difference BOD5 between cBOD5, mg/L

Average BOD:cBOD Ratio Facility Clean Sampler Dirty Sampler Clean Sampler Dirty Sampler

Gwinnett County 7 11 1.05 1.09

King County 48 54 1.27 1.35

Littleton-Englewood 15 22 1.08 1.11

Impact of Raw Wastewater Temperature Though the original study lasted for only three months, Littleton-Englewood continued conducting analyses of BOD5 and cBOD5 on raw wastewater. Near the end of May 2005, the raw wastewater BOD:cBOD ratio decreased significantly and abruptly in both the clean and dirty samplers. As a result of this significant change, Littleton-Englewood continued to perform comparative analyses and calculate the BOD:cBOD ratio. Though the cause of this relationship change is unknown, the data are summarized based on temperature due to the abruptness of the change at approximately 18 degrees Celsius. Littleton-Englewood continued to perform comparative BOD5-cBOD5 analyses on the raw wastewater sampler used for compliance testing through April 2006. Table 3 shows the relationship for the compiled data from January 1, 2005 through April 19, 2006. Figure 6 shows the BOD5, cBOD5, and temperature values. The dashed, BOD:cBOD ratio trend line in Figure 6 graphically illustrates an inverse trend of the BOD-cBOD ratio and temperature. Table 3. Littleton-Englewood Temperature-Related BOD-CBOD Data

Temperature and Values

Average Difference, mg/L

Standard Deviation

Average BOD:cBOD Ratio

All 19 24.0 1.10

< 18º C 31 19.8 1.16

> 18º C 1 18.2 1.01

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Figure 6. Littleton-Englewood BOD:cBOD Ratio and Temperature for 1/1/05 to 4/19/06

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0.90

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1.10

1.20

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1.40

1.50

BO

D:c

BO

D R

atio

Degrees C BOD:cBOD Ratio BOD:cBOD Trendline

As a result of this apparent relationship to temperature, data from the original study for Gwinnett County and King County was reviewed.

• King County had the highest overall BOD:cBOD ratio (1.27 for the clean sampler and 1.35 for the dirty sampler) and the wastewater temperature was below 18 degrees Celsius for all but the last three days of the study. The raw wastewater temperature averaged 16.8 degrees C and ranged from 15 to 18.3 degrees C.

• Gwinnet County had the lowest overall BOD:cBOD ratio (1.05 for the clean sampler and

1.09 for the dirty sampler) and the wastewater temperature was above 18 degrees Celsius except for 12 of the 35 sample days. The raw wastewater temperature averaged 19 degrees C and ranged from 16.2 to 22.0 degrees C.

Data from all three plants suggest a relationship between the BOD:cBOD ratio and temperature. Since the BOD5 and cBOD5 tests are conducted at a standard temperature, these data suggest that raw wastewater temperature indirectly impacts the analysis, by affecting the wastewater characteristics prior to analysis. Indirect impacts of temperature on wastewater characteristics and possibly the BOD:cBOD ratio include:

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• More nitrification possibly occurs in the collection system during the winter months when oxygen solubility is higher, which seeds the raw wastewater BOD5 test with nitrifying organisms. In the presence of sufficient oxygen, nitrification is more rapid at warm temperatures, but if oxygen is absent, sewer nitrification will not occur. In the colder winter months, higher dissolved oxygen concentrations may support the nitrifiers, which are obligate aerobes. Note that under all conditions the wastewater was still relatively warm; none of the wastewater temperatures recorded during the study was below 15 degrees C.

• Larger heterotrophic populations occur at higher temperatures. Since a constant

concentration of the nitrification inhibitor is added in the BOD test, the larger populations of heterotrophic organisms might mitigate the inhibitory effect of the nitrification inhibitor on heterotrophic organisms.

• The nitrification inhibitor interferes with the oxidation of large organic compounds in the

BOD test. These compounds are more readily hydrolyzed in the wastewater collection system at higher wastewater temperatures, resulting in more readily biodegradable organics in the raw wastewater sample. At lower wastewater temperatures, large organics are not readily hydrolyzed in the collection system, and are more abundant in the raw wastewater sample. The nitrification inhibitor could possibly bind with the large organics, or could interfere with heterotrophic enzyme formation and activity.

A more detailed study will be needed to determine the impact of temperature on the BOD:cBOD ratio.

Impact of Sampler Type

Each of the facilities except King County used peristaltic samplers for the dirty and clean samplers. King County used a vacuum sampler for the clean sampler and a continuous flow, dipper-type sampler for the dirty sampler. Both samplers collected samples from a very well mixed location. Table 4 presents a summary of data from the samplers. Though the BOD:cBOD ratio was not significantly different between the samplers, the BOD and cBOD values between the two samplers were significantly different. The BOD and cBOD analytical results from the continuous flow, dipper-type sampler were 13 percent and 23 percent lower, respectively, than the results from the vacuum sampler. Similar results were shown for total suspended solids.

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Table 4. King County Sampler Comparison

Parameter

Intermittent Flow Vacuum Sampler

Continuous Flow Dipper Sampler

Sampler Ratio

Average BOD, mg/L 271 238 1.13

Average cBOD, mg/L 212 173 1.23

cBOD/BOD Ratio .78 .73 1.07

DISCUSSION AND CONCLUSIONS Resolution of raw wastewater BOD5-cBOD5 discrepancies is important because of the potential impact on design and compliance. If the nitrification inhibitor used in the cBOD5 test inhibits the oxidation of organic matter, use of raw wastewater cBOD5 might underestimate facility loading significantly. If nitrification is occurring in the BOD5 test, use of raw wastewater BOD5 might overstate facility loading and, in some cases, make compliance with 85% removal requirements more difficult.. Though the initial purpose of this study was to determine the impact of sample line cleaning on the BOD:cBOD ratio, the overall results have greater implications. The data show that the relationship between raw wastewater BOD5 and cBOD5 is plant specific, and possibly seasonal. The study could not conclude whether or not nitrification occurs in the BOD5 test or whether the nitrification inhibitor interferes with the cBOD5 test. However, the study results show that nitrification or interference does not occur at all times and are not consistent from plant-to-plant or seasonally. Conclusions of the study include: o Sample line cleaning has a small but statistically insignificant impact on the BOD:cBOD

ratio. Since data from all plants showed that the ratio from the clean sampler is always lower than the dirty sampler, regular sample line cleaning is necessary for representative sampling.

o The wetted parts of a raw wastewater sampler are not a significant source of nitrifying

organisms in a sample. o The BOD5 concentration is not always greater than the cBOD5 concentration. Though this

likely reflects normal variability in the test, it does suggest that nitrification is not always occurring nor that the nitrification inhibitor consistently interferes with carbonaceous oxidation.

o There is a temperature relationship with the BOD:cBOD ratio. Since the analytical tests are

conducted at a standard temperature (i.e., 20 +/-1 ○C), the characteristic of an oxygen demanding substance must be affected in the wastewater collection system.

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o The type of sampler impacts potentially the BOD5 and cBOD5 concentration, but not the

BOD:cBOD ratio. The more representative of the two samplers cannot be determined from the limited data.

o Use of raw wastewater cBOD5 possibly understates the organic load for some facilities and

might result in inadequate designs. Additional study is needed to correlate additional wastewater characteristics with the BOD5 and cBOD5 analyses. These characteristics include soluble, colloidal, and suspended particulate fractionation of the oxygen demanding substances, assessment of nitrogen species during the 5-day test, and other chemical and physical parameters that affect the viability of autotrophic and heterotrophic organisms.

REFERENCES

Albertson, Orrie, “Is CBOD5 Test Viable for Raw and Settled Wastewater?”, Journal of Environmental Engineering, July 1995. Chapman, Keith; James, Harley; and Muirhead,Woodie. “Minimizing the Impact of Nitrification in the BOD5 Test.” Operations Forum, Vol. 8, No. 9, September 1991. Code of Federal Regulations, 40 CFR §133.102. Slayton, Joseph Lee and Trovato, E. Ramona, “Simplified N.O.D. Determination,” Environmental Protection Agency Field Report, Annapolis Field Office, March 1978. Standard Methods for the Examination of Water and Wastewater, 20th Edition; American Public Health Association, American Water Works Association, and the Water Environment Federation; 1998. Young, James C., “Chemical Methods for Nitrification Control”, 24th Purdue Industrial Waste Conference, Purdue University, May, 8, 1969.

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study of raw wastewater bod5 and cbod5 relationship yields ... · difference between raw wastewater...

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