Constituent Units
Average valuea
Daily Monthly
Arsenic mg/L 20
Cadmium mg/L 1.1
Chromium mg/L 11
Copper mg/L 4.9
Leadb mg/L 5.6
Mercury mg/L 2.1 0.012
Nickelb mg/L 7.1
Seleniumb mg/L 5.0
Silver mg/L 2.3
Zincb mg/L 58
Dieldrinc mg/L 0.0019 0.00014
Lindane mg/L 0.16 0.063
Tributyltin mg/L 0.01 0.005
PAHsd,e mg/L 0.049
a Limits apply to the average concentration of all samples collected during the averaging period (daily–24-h period; monthly-calendar month).
b Effluent limitation may be met as a 4-d average. If compliance is to be deter- mined based on a 4-d average, then concentrations of four 24-h composite samples must be reported as well as the average of four.
c Compliance will be based on the practical quantification level (PQL), 0.07 mg/L.
d PAHs = polynuclear aromatic hydrocarbons.
e Compliance will be based on the practical quantification level (PQL) for each PAH, 4 mg/L.
Source: Bay Area Regional Water Quality Control Board, Oakland, CA.
efficiency of some treatment processes, and (4) to determine compliance with wastewater discharge permits. Because it is likely that the BOD test will continue to be used for some time, it is important to know the details of the test and its limitations.
Basis for BOD Test. If sufficient oxygen is available, the aerobic biological decom- position of an organic waste will continue until all of the waste is consumed. Three more or less distinct activities occur. First, a portion of the waste is oxidized to end products to obtain energy for cell maintenance and the synthesis of new cell tissue. Simultaneously, some of the waste is converted into new cell tissue using part of the energy released during oxidation. Finally, when the organic matter is used up, the new cells begin to consume their own cell tissue to obtain energy for cell maintenance. This third process is called endog- enous respiration. Using the term COHNS (which represents the elements carbon, oxygen, hydrogen, nitrogen, and sulfur) to represent the organic waste and the term C5H7NO2 to represent cell tissue, the three processes are defined by the following generalized chemical reactions:
Energy reaction (oxidation)
COHNS 1 O21 bacteria S CO21 H2O 1 NH31 other end products 1 energy (2–55)
Synthesis reaction
COHNS 1 O21 bacteria 1 energy S C5H7NO2 (2–56)
New cell tissue
Endogenous respiration
C5H7NO21 5O2 S 5CO21 NH31 2H2O (2–57)
If only the oxidation of the organic carbon that is present in the waste is considered, the ultimate BOD is the oxygen required to complete the three reactions given above. This oxygen demand is known as the ultimate carbonaceous or first-stage BOD and is usually denoted as UBOD.
As will be discussed later, the ammonia produced in the energy reaction, Eq. (2–55), can be oxidized further to nitrite and nitrate. Thus, the BOD test only represents the amount of oxygen need to oxidize the carbonaceous material in a sample.
Description of BOD Test Procedure. The standard BOD test [see Fig. 2–21(a)]
involves placing a small sample of the wastewater in a BOD bottle (volume 5 300 mL).
The bottle is then filled with dilution water saturated in oxygen and containing the nutri- ents required for biological growth. To ensure that meaningful results are obtained, the sample must be suitably diluted with a specially prepared dilution water so that adequate nutrients and oxygen will be available during the incubation period. Normally, several dilutions are prepared to cover the complete range of possible values. Before stoppering the bottle, the oxygen concentration in the bottle is measured (see Fig. 2–22). When testing wastewaters with low concentrations of microorganisms, a seeded BOD test is conducted [see Fig. 2–21(b)]. The organisms contained in the effluent from primary sedimentation facilities are used commonly as the seed for the BOD test. Seed organisms can also be obtained commercially. When the sample contains a large population of microorganisms (e.g., untreated wastewater), seeding is not necessary.
The standard incubation period is usually five days at 20°C, but other lengths of time and temperatures can be used. After incubating for a period of 5-d at 20°C, the dissolved oxygen concentration is measured again. The BOD of the sample is the difference in the
2–6 Aggregate Organic Constituents 117
Figure 2–21
Procedure for setting up BOD test bottles: (a) with unseeded dilution water and (b) with seeded dilution water (Tchobanoglous and Schroeded, 1985).
Air Stone Glass container
(~20L)
Unseeded dilution
water
Dilution water (300 mL – )s
Test (waste) sample, , containing organic
matter and an adequate number of bacteria (volume of test
sample depends on estimated BOD)
Glass stoppered BOD bottle (volume = 300 mL)
BOD bottle filled with test sample plus unseeded dilution water (unseeded test sample)
Distilled water
Seeded dilution water
BOD bottle filled with seeded dilution water (seeded blank)
BOD bottle filled with test sample plus seeded dilution water (seeded test sample)
Test (waste) sample, , containing organic
matter and no bacteria or a limited
number of bacteria Air
Essential nutrients (N, P, K, Fe, etc.)
and other additives
Air
Dilution water (300 ml – )
s
Dilution water (300 ml) Bacteria
(seed) Essential nutrients and additives
V
Vs
Vs
V
(a)
(b)
Distilled water
Figure 2–22
Measurement of oxygen in BOD bottle: (a) with a DO probe equipped with a stirring mechanism and (b) close up of stirrer.
Stirrer
(a) (b)
dissolved oxygen concentration values, expressed in milligrams per liter, divided by the decimal fraction of sample used (Standard Methods, 2012). The computed BOD value is known as the 5-d, 20°C biochemical oxygen demand. The 5-d incubation period dates back to the use of the BOD test to assess river pollution in England the late 1800s. Because the maximum time of flow of any river in England from the headwaters to the ocean is 5 days, the 5-d period was, and is, utilized for the test.
Longer time periods (typically seven days), which correspond to work schedules, are often used, especially in small plants where the laboratory staff is not available on the weekends. The temperature, however, should be constant throughout the test. The 20°C temperature used is an average value for slow-moving streams in temperate climates and is easily duplicated in an incubator. Different results would be obtained at different tem- peratures because biochemical reaction rates are temperature dependent.
Modeling of BOD Reaction. The rate of BOD oxidation (“exertion”) is modeled based on the assumption that the amount of organic material remaining at any time, t, is governed by a first order function (see Chap. 1).
dBODr
dt 5k1BODr (2–58) Integrating between the limits of UBOD and BODt and t 5 0 and t 5 t yields:
BODr5UBOD (e2k1t) (2–59)
where BODr5 amount of waste remaining at time t (d) expressed in oxygen equivalents,
mg/L UBOD 5 the total or ultimate carbonaceous BOD, mg/L
k15 first-order reaction rate constant, 1/d
t 5 time, d Thus the BOD exerted up to time t is given by BODt5UBOD2BODr5UBOD2UBOD(e2k1t)5UBOD(12e2k1t) (2–60)
Equation (2–60) is the standard expression used to define the BOD for wastewater. The basis for this equation is discussed in Sec. 1–5 in conjunction with the analysis of a batch reactor. It should be noted that in the literature dealing with the characterization of waste- water, the terms L and BODu are often used to denote ultimate carbonaceous BOD (UBOD).
Biochemical oxidation theoretically takes an infinite time to go to completion because the rate of oxidation is assumed to be proportional to the amount of organic matter remaining. Within a 20-d period, the oxidation of the carbonaceous organic matter is about 95 to 99 percent complete, and in the 5-d period used for the BOD test, oxidation is from 60 to 70 percent complete.
BOD Reaction Rate Coefficients. The value of k1 for untreated wastewater is generally about 0.12 to 0.46 d21 (base e), with a typical value of about 0.23 d21. The range of k1 values for effluents from biological treatment processes is from 0.12 to 0.23 d21. For a given wastewater, the value of k1 at 20°C can be determined experimentally by observing the variation with time of the dissolved oxygen in a series of incubated samples. If k1 at 20°C is equal to 0.23 d21, the 5-d oxygen demand is about 68 percent of the ultimate first- stage demand. Occasionally, the first-order reaction rate constant will be expressed in log (base 10) units. The relationship between k1 (base e) and K1 (base 10) is as follows:
K1(base 10)5 k1(base e)
2.303 (2–61)
2–6 Aggregate Organic Constituents 119
As discussed above, the temperature at which the BOD of a wastewater sample is deter- mined is usually 20°C. It is possible, however, to determine the reaction constant k at a temperature other than 20°C using the following relationship developed in the discussion on the effects of temperature in Chap. 1, Sec. 1–6:
k2
k1
5u(T22T1) (1–44)
The value of the temperature coefficient u has been found to vary from 1.056 in the temperature range between 20 and 30°C to 1.135 in the temperature range between 4 and 20°C (Schroepfer et al., 1964). A value of u often quoted in the literature is 1.047 (Phelps, 1944), but it has been observed that this value does not apply at cold temperatures (e.g., below 20°C).
Equation (2–60), along with Eq. (1–44), makes it possible to convert test results from different time periods and temperatures to the standard 5-d 20°C test, as illustrated in Example 2–9.
EXAMPLE 2–9 Calculation of Different BOD Values Determine the 1-d BOD and ultimate first-
stage BOD for a wastewater whose 5-d, 20°C BOD is 200 mg/L. What would have been the 5-d BOD if the test had been conducted at 25°C? The reaction constant k (base e) 5 0.23 d21, and u5 1.047.
Solution
1. Determine the ultimate carbonaceous BOD.
BOD55UBOD2BODr5UBOD(12e2k1t) 2005UBOD(12e20.2335)5UBOD(120.317) UBOD5293 mg/L
2. Determine the 1-d BOD.
BODt5UBOD(12e2k1t) BOD15293(12e20.2331)5293(120.795)560.1 mg/L
3. Determine the 5-d BOD at 25°C.
k1T5k120(1.047)T220
k12550.23(1.047)2522050.29 d21 BOD55UBOD(12e2k1t)5293(12e20.2935)5224 mg/L
For polluted water and wastewater, a typical value of k1 (base e at 20°C) is 0.23 d21 (K1, base 10, 5 0.10 d21). The value of the reaction rate constant varies significantly, how- ever, with the type of waste. The range may be from 0.05 to 0.3 d21 (base e) or more. For the same ultimate BOD, the oxygen uptake will vary with time and with different reaction rate constant values (see Fig. 2–23).
Nitrification in the BOD Test. Noncarbonaceous matter, such as ammonia, is
produced during the hydrolysis of proteins. It is now known that a number of bacteria are capable of oxidizing ammonia to nitrite and subsequently to nitrate. The generalized reac- tions are as follows:
Conversion of ammonia to nitrite (as typified by Nitrosomonas):
NH31 3/2O2S HNO21 H2O (2–62)
Conversion of nitrite to nitrate (as typified by Nitrobacter):
HNO21 1/2O2S HNO3 (2–63)
Overall conversion of ammonia to nitrate:
NH31 2O2S HNO3 1 H2O (2–64)
The oxygen demand associated with the oxidation of ammonia to nitrate is called the nitrogenous biochemical oxygen demand (NBOD). The normal exertion of the oxygen demand in a BOD test for a domestic wastewater is shown on Fig. 2–24. Because the reproductive rate of the nitrifying bacteria is slow, it normally takes from 6 to 10 d for them to reach significant numbers to exert a measurable oxygen demand. However, if a sufficient number of nitrifying bacteria is present initially, the interference caused by nitrification can be significant.
When nitrification occurs in the BOD test, erroneous interpretations of treatment oper- ating data are possible. For example, assume the effluent BOD from a biological treatment process is 20 mg/L without nitrification and 40 mg/L with nitrification. If the influent BOD to the treatment process is 200 mg/L, then the corresponding BOD removal efficiency
Figure 2–23
Effect of the rate constant k1 on BOD (for a unit UBOD value).
BOD / UBOD
1
0 0.2
0 0.4 0.6 0.8
5 10 15 20
Time, d
25 BOD = UBOD(1 – e–kt) Unit BOD = 1.0(1 – e–kt) k = 0.3/d
k = 0.2/d k = 0.1/d
Figure 2–24
Definition sketch for the exertion of the carbonaceous and nitrogenous biochemical oxygen demand in a waste sample.
Time, d
Oxygen demand, mg/L
Nitrogenous biochemical oxygen demand, NBOD
Carbonaceous biochemical oxygen demand, BOD or
CBOD Where a sufficient number of nitrifying
organisms are present, nitrification can occur as shown by the dotted curve.
Nitrification is usually observed to occur from 5 to 8 d after the start of the BOD incubation period.
2–6 Aggregate Organic Constituents 121
would be reported as 90 and 80 percent without and with nitrification, respectively. Thus, if nitrification is occurring but is not suspected, it might be concluded the treatment process is not performing well, when in actuality it is performing quite well.
Carbonaceous Biochemical Oxygen Demand. When nitrification occurs, the measured BOD value will be higher than the true value due to the oxidation of carbona- ceous material (see Fig. 2–25). If a given percentage of carbonaceous biochemical oxygen demand (CBOD) removal must be achieved to meet regulatory permit limits, early nitrifica- tion can pose a serious problem. The effects of nitrification can be overcome either by using
Figure 2–25
Functional analysis of the BOD test: (a) interrelationship of organic waste, bacterial mass (cell tissue), total organic waste, and oxygen consumed in BOD test and (b) idealized representation of the BOD test (Tchobanoglous and Schroeder,
1985). Oxygen consumed
(e.g., BOD exerted) Point at which organic matter remaining
in BOD bottle is essentially all in the form of cell tissue
Bacterial mass (cell tissue) Ultimate carbonaceous biochemical oxygen
demand of waste sample (BODu)
Total amount of organic matter remaining in BOD bottle
Organic waste, bacterial mass (cells), total organic matter, and oxygen consumed expressed in BOD units
Time, BODr (remaining)
Death phase Stationary growth phase Log
growth phase Lag
phase Amount of
organic waste remaining
(a)
(b)
BOD = BODu – BODr Amount of organic matter remaining
(actual and idealized) Idealized BOD curve BODu
BODr t BODr Organic waste remaining and oxygen consumed expressed in BOD units
Time, t t
Idealized Actual
various chemicals to suppress the nitrification reactions, or by treating the sample to elimi- nate the nitrifying organisms (Young, 1973). Pasteurization and chlorination/dechlorination are two methods that have also been used to suppress the nitrifying organisms.
When the nitrification reaction is suppressed, the resulting BOD is known as the car- bonaceous biochemical oxygen demand (CBOD). In effect, the CBOD is a measure of the oxygen demand exerted by the oxidizable carbon in the sample. The CBOD test, in which the nitrification reaction is suppressed chemically, should only be used on samples that contain small amounts of organic carbon (e.g., treated effluent). Errors have sometimes been observed in the measured BOD values when the CBOD test is used on wastewater containing significant amounts of organic matter such as untreated wastewater.
Analysis of BOD Data. The value of k is needed if the BOD5 is to be used to obtain UBOD, the ultimate or 20-d BOD. The usual procedure followed when these values are unknown is to determine k1 and UBOD from a series of BOD measurements. There are several ways of determining k1 and UBOD from the results of a series of BOD measure- ments, including the method of least-squares, the method of moments (Moore et al., 1950), the daily-difference method (Tsivoglou, 1958), the rapid-ratio method (Sheehy, 1960), the Thomas method (Thomas, 1942, 1950) and the Fujimoto method (Fujimoto, 1961). The least-squares and Fujimoto method are illustrated in the 4th edition of this textbook (Tchobanoglous et al., 2003).
Effect of Particle Size on BOD Reaction Rates. If a separation and analysis
technique, such as membrane filtration (see Figs. 2–4 and 2–8), is used to quantify the size distribution of the solids in the influent wastewater, the various size fractions can be cor- related to observed oxygen (BOD) uptake rates, determined using a respirometer. As reported in Table 2–14, the observed BOD reaction rate coefficients are affected signifi- cantly by the size of the particles in wastewater. Based on the data given in Table 2–14, it is clear that the treatment of a wastewater can be affected by modifying the particle size distribution. Further, wastewaters with significantly different particle size distributions will respond differently, depending on the method of treatment (e.g., in constructed wetlands).
Limitations in the BOD Test. The limitations of the BOD test are as follows: (1) a high concentration of active, acclimated seed bacteria is required; (2) pretreatment is needed when dealing with toxic wastes, and the effects of nitrifying organisms must be reduced; (3) only the biodegradable organics are measured; (4) the test does not have stoi- chiometric validity after the soluble organic matter present in solution has been used (see Fig. 2–25); and (5) the relatively long period of time required to obtain test results. Of the above, perhaps the most serious limitation is that the 5-d period may or may not corre- spond to the point where the soluble organic matter that is present has been used. The lack of stoichiometric validity at all times reduces the usefulness of the test results.
Table 2–14