Using Traditional Measurements To Approximate Wastewater Characteristics For Modeling

As seen in the preceding section, characterization of a complex wastewater in a manner suitable for use with ASM No. 1 is quite involved and represents a significant investment of time and money. Consequently, such characterizations are not ordinarily done as part of the routine measurements made at wastewater treatment plants. Rather, in the United States, wastewaters are normally characterized in terms of the concentrations of TSS, VSS, five-day biochemical oxygen demand (BOD.), ammo-nia-N, total Kjeldahl nitrogen (TKN), and alkalinity. Total COD is also commonly measured, but the frequency is usually less than that of the other characteristics, although it is increasing. Furthermore, distinction is seldom made between soluble and particulate phases during measurements of BOD,, COD, and TKN. Because there are circumstances in which it would be advantageous to conduct preliminary modeling studies prior to conducting detailed treatability studies, it would be very useful to be able to translate the traditional data available in the records of wastewater treatment plants into a form that can be used with the models presented herein. Luckily, with a few simplifying assumptions, this can be done for domestic wastewaters. Such translations cannot be made for industrial wastewaters, however, because each is unique.

As indicated by Eq. 8.23, the total COD in a wastewater (COD,,,) is made up of four components: (1) particulate biodegradable COD (Xsu), (2) soluble biodegradable COD (Ssu), (3) particulate inert COD (X„,), and (4) soluble inert COD (S,„). (The subscript O in these symbols and the ones to follow designates that they are influent concentrations.) As a consequence, data on the total COD in the wastewater is essential to determination of the other constituents. If no COD data are available, the total COD of domestic wastewater can be approximated as:~"'

The wastewater biodegradable COD (COD,,,,) can be estimated from the ultimate BOD (BOD,,), which, in turn, can be estimated from the BOD^:

BOD,

where k is the BOD rate coefficient with units of day '. For domestic wastewater, the relationship between the ultimate BOD and the five-day BOD can be approximated as:"''

The biodegradable COD is greater than the ultimate BOD because the latter does not account for the electrons retained in the biomass debris formed during the BOD test. Consequently,

BODu

where Y,, is in COD/COD units and f,, can be assumed to have a value of 0.20 mg debris COD/mg biomass COD.1" Substituting Eq. 8.29 for ultimate BOD into Eq. 8.31 allows the biodegradable COD to be estimated from the 5-day BOD:

For wastewaters with a significant industrial component, both k and Y,, would have to be measured to allow the conversion, but for domestic wastewater, Y„ can be assumed to be 0.60,'" and Eq. 8.30 can be used as the relationship between the two types of BOD, giving:

The division of biodegradable COD into slowly and readily biodegradable fractions requires specific knowledge of the wastewater in question. This is necessary because slowly biodegradable substrate is not necessarily the same as particulate biodegradable substrate, even though it is considered to be particulate in the models. Rather, some of the slowly biodegradable substrate may pass through the filters used to determine VSS concentrations. As discussed earlier, the physical assay of Mamais et al.~4 provides a simple means of determining the readily biodegradable COD in a wastewater. If no other basis is available for making the division, its use is encouraged, even during preliminary studies. If that can't be done, a "best guess" division must be made based on experience.

The nonbiodegradable, or inert, COD (COD,,,) is the difference between the total COD and the biodegradable COD:

It must be partitioned into soluble (SK,) and particulate (XK1) forms. Experience suggests that 35 to 40 percent of the particulate organic matter in domestic wastewater is nonbiodegradable.1" 2" Particulate organic matter is represented by the VSS. If one assumes that the elemental composition of the inert particulate organic matter is similar to that of protein, which has a COD equivalent of 1.5 g COD/g protein (Table 3.1), and that protein is totally volatile in a volatile suspended solids test, then:

The soluble inert COD can be calculated by difference:

If one were going to use BOD as a measure of biodegradable organic matter, it would be better to measure the ultimate carbonaceous BOD (BODu) than to measure BODs, because the relationship expressed by Eq. 8.31 is subject to less error and variability than the relationship between biodegradable COD and BOD, expressed by Eq. 8.32, for which an assumed value of k is required. Although Eq. 8.33 can be used as a rough approximation for domestic wastewater, the relationship between biodegradable COD and BOD5 depends on the rate of oxygen consumption in the BOD test, as indicated by Eq. 8.32. Since that rate will be influenced by the nature of the organic chemicals present in the sample being tested, the presence of industrial discharges to a municipal wastewater treatment plant may well change the relationship from that associated with a strictly domestic wastewater. Thus, there is a high degree of uncertainty associated with Eq. 8.33 under that circumstance and it is better to use Eq. 8.32 with a measured value of the BOD rate coefficient, k. The best course of action, however, is to use COD as the measure of organic substrates, accounting for the nonbiodegradable material in the ways outlined earlier.

Some of the wastewater characteristics used in modeling are routinely measured directly: ammonia-N, nitrate-N, and alkalinity. The ammonia-N and nitrate-N concentrations can be used without conversion. Most domestic wastewaters contain no nitrate-N, although industrial wastewaters might. Alkalinity is typically measured as CaCO,, but is expressed as mM/L in ASM No. 1 and No. 2. Since the molecular weight of CaCO, is 100, the conversion is simple.

Without treatability studies, the other nitrogen forms used in modeling must be deduced from the routine measurements made at wastewater treatment plants. The total organic nitrogen concentration (ONTO) is the difference between the TKN and ammonia-N concentrations:

Furthermore, the wastewater total organic nitrogen can be divided into soluble, particulate, biodegradable, and inert fractions:

The concentration of soluble, inert organic nitrogen (SMO) in domestic wastewaters typically ranges from 1 to 2 mg/L as N,"' suggesting that a value of 1.5 mg/L as N can be assumed for preliminary modeling without fear of gross error. The particulate inert organic nitrogen is associated with the particulate inert organic matter. The nitrogen content of this material can be assumed to be equal to iN M„ the nitrogen content of biomass debris. Consequently, the concentration of particulate inert organic nitrogen (XNI, ,) can be approximated as:

When possible, distribution of biodegradable organic nitrogen between the soluble and particulate phases should be based on data. In the absence of specific data, the biodegradable organic nitrogen is often distributed into particulate and soluble fractions in the same proportions as the slowly and readily biodegradable fractions of the COD, as indicated in Eq. 8.25.

Example 8.6.1

Conventional characterization of a domestic wastewater following primary clarification is given in the upper portion of Table E8.4. Translate that information into a form that can be used in ASM No. 1.

a. The first task is to estimate the concentration of biodegradable COD. This is done by using the BOD, and Eq. 8.33:

b. The inert COD concentration can then be calculated from the total COD using Eq. 8.34:

Table E8.4 Translation of Traditional Wastewater'' Characteristics into a Form Suitable for Modeling

Component

Concentration

Conventional wastewater characterization TSS VSS BOD, Total COD Ammonia-N

Total Kjeldahl nitrogen (TKN)

Nitrate-N

Alkalinity

Characterization as required for use in ASM No. 1 Particulate inert organic matter Soluble inert organic matter Slowly biodegradable substrate Readily biodegradable substrate Oxygen

Soluble nitrate nitrogen

Soluble ammonia nitrogen

Soluble biodegradable organic nitrogen

Particulate biodegradable organic nitrogen

Alkalinity

82 mg/L 61.5 mg/L 155 mg/L 325 mg/L as COD 25 mg/L as N 43.5 mg/L as N 0.0 mg/L as N 200 mg/L as CaCO,

35 mg/L as COD 25 mg/L as COD 150 mg/L as COD 115 mg/L as COD 0 mg/L as O. 0 mg/L as N 25 mg/L as N 6.5 mg/L as N 8.5 mg/L as N 2 mM/L

"The wastewater is considered to be typical of domestic wastewater that has undergone primary sedimentation.

c. The concentration of particulate inert COD can be estimated from the VSS concentration using Eq. 8.35:

d. This, in turn allows the soluble inert COD concentration to be calculated from Eq. 8.36:

e. Partitioning of the biodegradable COD into slowly and readily biodegradable fractions requires some knowledge of the nature of the wastewater. Additional information suggests that 43% of the biodegradable COD is readily biodegradable. Consequently,

f. The concentration of organic nitrogen in the wastewater can be obtained as the difference between the TKN and the ammonia-N concentrations as expressed in Eq. 8.37:

g. The biodegradable organic nitrogen concentration must be obtained by subtracting the concentrations of the soluble and particulate inert organic nitrogen. The concentration of soluble inert organic nitrogen can be assumed to be 1.5 mg N/L, as discussed previously. The concentration of particulate inert organic nitrogen can be estimated with Eq. 8.39 by assuming a value for is xi). A reasonable value 0.06 mg N/mg COD, as indicated in Table 6.3 Consequently:

Use of Eq. 8.38 gives the biodegradable organic nitrogen concentration:

h. Partitioning of the biodegradable organic nitrogen in accordance with Eq. 8.25 gives:

Consequently,

The estimated characteristics of the wastewater are listed in the lower portion of Table E8.4. Comparison of them to the values in Table 6.6 shows that with the exception of alkalinity, they are the same. The alkalinity is a very site-specific characteristic, being dependent on the nature of the carriage water.

So far in this book, the concentrations of all organic constituents, both substrate and biomass, and the values of all kinetic and stoichiometric parameters have been presented as COD. This has been done because it simplifies the equations and makes the computation of COD, i.e., electron, balances very straightforward. Furthermore, there are strong arguments for using biodegradable COD routinely as the measure of organic substrate concentrations, with the result that there is an increasing trend toward this practice throughout the world. Consequently, COD will be used as the measure of organic substrate concentration almost exclusively in this book, and the majority of the kinetic and stoichiometric coefficients will be presented in terms of it. We recognize that the BOD test is still widely used in the United States and that most historical records are expressed in terms of it. Thus, most historical data on kinetic and stoichiometric coefficients use BOD as the measure of organic substrate. Conversion of half-saturation coefficients to COD units can be done with the expressions presented in Section 8.6. In this section, we present the equations for converting yield values so that substrate is expressed as COD and biomass is expressed either as COD or as TSS.

Conversion between COD units and TSS or VSS units is straightforward for biomass and biomass debris because there is little variability associated with the COD equivalents of those constituents. Consequently, biomass concentrations may h. Partitioning of the biodegradable organic nitrogen in accordance with Eq. 8.25 gives:

Consequently,

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