Rate Of Biochemical Oxidations

For a great many years the BOD reaction was considered to have a rate constant k' equal to 0.23 per day at 20°C, This value was established by extensive studies on polluted river waters and domestic wastes in the United States and England. As application of the BOD test spread to the analysis of industrial wastes, and the use of synthetic chapter 23 Biochemical Oxygen Demand Table 23.2 I Significance of reaction rate constant k' upon BOD

Percent of total BOD exerted k' = 0.10

0.50

dilution waters became established, it was soon noted that k' values considerably in excess of 0.23 per day were involved and that an appreciable variation occurred for different waste materials. In addition, it was found that k' values for domestic wastes varied considerably from day to day and averaged about 0.40 per day, rather than 0.23 per day as originally determined. Also, the k' values for effluents from biological waste treatment plants were found to be significantly lower than those for the raw wastes. Another factor affecting BOD rates is temperature. The magnitude of this effect and a method for correction when projecting laboratory-determined rates to field conditions were discussed in Sec. 3.10. The importance of the reaction rate k' with respect to the BOD developed at any time is shown in Table 23.2.

The significance of k' in determining the course of the BOD reaction is illustrated in Fig. 23.3. For a waste having a given L0 value, the BOD values on any given day will vary widely until about 15 days have elapsed. In the past it was common practice to interpret 5-day BOD in terms of L0 values by assuming a k' value of

350 300

Q 150 O

M 100 50 0

5 10 15 20

Time f, days

Figure 23.3 Effect of rate constant k' on BOD for Lq value of 300 mg/L.

Q 150 O

M 100 50 0

5 10 15 20

Time f, days

Figure 23.3 Effect of rate constant k' on BOD for Lq value of 300 mg/L.

450 400

Figure 23.4 Effect of rate constant k' on the course of the BOD reaction with an assumed BODs of 200 mg/L.

450 400

5 10 15

Time t, days

Figure 23.4 Effect of rate constant k' on the course of the BOD reaction with an assumed BODs of 200 mg/L.

0.23 per day. Figure 23.4 shows how the L0 value of a sample with a 5-day BOD of 200 varies with the value of k'.

The variation in k' values leaves considerable room for speculation as to why such differences in rates of reaction occur. Two factors of major importance are involved: (1) the nature of the organic matter and (2) the ability of the organisms present to utilize the organic matter.

Organic matter occurring in domestic and industrial wastes varies greatly in chemical character and availability to microorganisms. That part which exists in true solution is readily available, but that part which occurs in colloidal and coarse suspension must await hydrolytic action before it can diffuse into the bacterial cells where oxidation can occur. The rate of hydrolysis and diffusion are probably the most important factors in controlling the rate of the reaction. It is well known that simple substrates, such as glucose, are removed from solution at very rapid rates, and k' values are correspondingly high. More complex materials are removed much more slowly, and k' values are lower. In a complex material such as domestic waste, reaction rates are modified greatly by the more complex substances, whereas in an industrial waste containing soluble compounds of simple character, the reaction rate is usually very rapid. Certain organic compounds, such as lignin, are very slowly attacked by bacteria. Some of the synthetic detergents also fall into this category.

A lag period is often noted in the BOD reaction with some industrial wastes, particularly those containing organic compounds of synthetic origin or with chemical structure not found in natural materials. The "seeding" organisms used in the BOD test may or may not have specific bacteria that can utilize the material as food. If not, the substance will not exert a BOD. Oftentimes only a few bacteria are present that can oxidize the substance, and the rate of oxidation is so slow for a period, possibly several days, that a measurable BOD cannot be detected. With sufficient time, however, the population of the specific bacteria will increase to levels at

Time t, days

Figure 23.5 Possible BOD change with time resulting when organic« requiring acclimation by organisms are present.

Time t, days

Figure 23.5 Possible BOD change with time resulting when organic« requiring acclimation by organisms are present.

which the oxidation progresses at normal rates. In cases of this kind, as illustrated in Fig. 23.5, the lag period can usually be overcome by using for "seeding" purposes water from a river into which such wastes are discharged. The water should be taken well downstream from the point of discharge. Growths attached to rocks downstream from the point of waste discharge sometimes will furnish an adequate seed. Adapted seed can sometimes be found in soils that have been exposed to the waste materials for a long period, perhaps through accidental spills. A properly adapted seed may also sometimes be developed in the laboratory by aerating for several days a mixture composed of the neutralized waste and a small portion of domestic wastewater. However, with some xenobiotic compounds, microorganisms with the ability to adapt to them may not be ubiquitous in the environment, and will need to be sought more broadly. Indeed, they may not exist!

The phenomenon of the lag period is sometimes explained on the basis that the "seeding" organisms do not have the proper enzyme systems to utilize the organic matter. With time, however, and if the right organisms are present, they adapt themselves to the new food supply and will then furnish the necessary enzymes. Another factor at times is the presence of toxic organic or inorganic compounds, which may cause delay in the onset of oxidation while bacteria adapt to the inhibition, or may even prevent the bacteria from growing and oxidizing the organic material present. The analyst needs to understand these limitations of the BOD test and how to deal with them.

The rate k' and extent Lq of biochemical reactions can be evaluated in a number of ways. Procedures for evaluation of k' and Lq, together with their uncertainties, through a series of BOD measurements with time were discussed in Sec. 10.9 on regression analysis. In order to determine carbonaceous BOD itself in such cases,.measurements need to be made of changes in nitrogen species with time in order to correct for oxygen consumption from nitrification. "Standard Methods" now has two proposed procedures specifically designed for determining oxygen uptake with time and for estimating i0. One uses a series of bottles, or alternately, a large reservoir for the diluted sample, and oxygen uptake measurements are made over time together with analyses for changes in nitrogen forms. Carbonaceous BOD at any time is determined by subtracting oxygen usage for nitrification from total oxygen consumption. Regression analysis is then used to estimate the ultimate BOD. The other approach uses a respirometer for continuous oxygen uptake measurements. Here, the objective is not necessarily to determine L0, but to obtain a better understanding of the characteristics of a wastewater. Observing oxygen uptake with waste dilution can help evaluate whether toxicants to biodégradation are present. Observations of oxygen uptake with time can help evaluate whether biological acclimation to organics is required. This is evident when resulting BOD curves are similar to that illustrated in Fig. 23.5. Corrections for nitrification need to be made here as this can cause similar late oxygen uptake as noted in Fig. 23.2. In any event, oxygen uptake measurements with time are essential when first characterizing a wastewater for biological treatment in order to determine waste characteristics that are crucial for process design.

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