CHEMICAL SCIENCES, ENGINEERING AND TECHNOLOGY RESOURCES - SAMPLE CHAPTERS
CHEMISTRY OF WASTEWATER
Timothy G. Ellis,
Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, Iowa, USA
Keywords: wastewater, biochemical oxygen demand, chemical oxygen demand, suspended solids, turbidity, nitrogen, phosphorus, nitrification, denitrification, surfactant, endocrine disruptors, Ascaris, nitrosomonas, nitrobacter, Schistosomiasis, hypoxic zone, Giardia, Cryptosporidium, olfactometer
In order to measure the extent to which a wastewater discharge will impact the dissolved oxygen concentration in a receiving stream (i.e. the body of water that the wastewater is discharged into), a measure of the oxygen consuming organic matter must be determined. One way to determine this is to measure the disappearance of oxygen from a bottle containing oxygen saturated water, a prescribed volume of the wastewater sample, a small amount of active biomass (primarily bacteria) seed, and any necessary nutrients for biomass growth. This is a direct measurement of the oxygen consuming capacity of the wastewater and is termed the biochemical oxygen demand (BOD) test (see Biochemical Oxygen Demand).
The general form of the equation for decomposition of organic matter during the BOD test is:
It should be noted that oxygen is consumed in the reaction and biomass is the catalyst for the reaction. Actually, the organic matter is the growth substrate (carbon and energy source) for the generation of new biomass, an end product of the reaction. Essential macro- and trace nutrients are also required for sustained growth of the microorganisms. Using glucose as an example (and ignoring the substrate being incorporated into biomass):
The theoretical BOD of glucose can be calculated as:
As the biodegradation of organic matter proceeds, its remaining oxygen demand decreases. At any given time the rate of biodegradation can be modeled as a first order reaction (i.e. as a function of the remaining oxygen demand). A portion of the organic matter is also converted into cell material, or biomass, so its concentration is changing as well. If we were to look at all three events on the same plot of relative oxygen concentration versus time, it might look like that in Figure 1.
Figure 1. Theoretical plot of substrate disappearance, biomass growth, and oxygen consumption. Plot was generated with kinetic parameters similar to those typically measured for xenobiotic compounds and assumes a significant population of acclimated biomass.
The actual shape of plot will be determined by the experimental conditions at the beginning of the batch test (e.g. quantity of biomass seed, nature of substrate, acclimation of biomass, etc.). It should also be noted that, in this case, the initial oxygen demand of the substrate equals the sum of the oxygen consumed during the test and the concentration of biomass produced in oxygen units. In other words, one could perform a mass balance in terms of oxygen units. In practice, a mass balance on BOD is not very practical due to the high degree of variability in the BOD test.
In the laboratory procedure for determining the BOD concentration, the amount of biomass generated is considered negligible (or inconsequential), and only the two events (substrate depletion and oxygen consumption) are therefore considered. Depending on the biomass yield (miligrams of biomass formed per miligram of substrate utilized) and the experimental conditions, the actual measured concentration of BOD will vary. In fact, several measurements of the same sample (at the same dilution) are usually required to obtain reliable and reproducible results from the BOD test.
One of the potential difficulties in the BOD test is the interference of nitrification. The normal BOD test determines the carbonaceous BOD and excludes the effects of nitrification. Nitrification is the two-step process whereby ammonia is transformed to nitrate by autotrophic organisms (those that use an inorganic energy source and CO2 for growth) as follows:
As can be seen from these two equations, the amount of oxygen required for nitrification is significant (and can actually exceed the amount of oxygen required to satisfy the BOD). Stoichiometrically, each miligram of nitrogen from ammonia (NH3-N) requires 4.57 mg of oxygen to transform it biochemically to nitrate. If a wastewater contains 200 mg L-1 BOD5 and 44 mg L-1 NH3-N, the oxygen required for BOD removal and nitrification will be approximately the same. In practice, the total oxygen requirement is not realized since some of the ammonia is converted into new cell material, and the actual demand is usually closer to 4.3 mg O2 per mg NH3-N. In practice, however, the value of 4.57 mg O2 per mg NH3-N is often used as a conservative estimate.
In the BOD test, the carbonaceous BOD is usually the desired measurement since ammonia can be measured quickly and directly (e.g. using an ammonia probe). On the other hand, the BOD test is typically run for 5 days (designated as BOD5). The reason for the 5-day BOD dates back to the early 19th Century in England where it was determined that it took approximately 5 days for wastewater to travel from London to the mouth of the Thames River. Thus, by the time the wastewater traveled to the mouth of the river, its 5-day oxygen demand had been realized, and the impact on the river could be accounted for. The five-day time period is also convenient since it usually takes the slow growing nitrifiers in the seed culture longer than five days to achieve significant numbers to impact the oxygen consumption noticeably. Nevertheless, Standard Methods published in 1998. recommend the use of a nitrification inhibitor during the BOD test so that only oxygen uptake associated with the oxidation of organic compounds (i.e. carbonaceous BOD or CBOD) is measured during the test. This inhibitor, allylthiourea, is not thought to significantly affect the growth of the heterotrophic organisms (those organisms that use the carbonaceous BOD as a carbon and energy source).
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