BOD: the modern alchemy.
I'm referring, of course, to the biochemical oxygen demand, or BOD.
The BOD may be the best known and most fundamental test in the field of water pollution control. Ironically, the current importance of the test stems more from political considerations than technical ones, and in its unaltered form it bears an astonishing resemblance to alchemy.
Certainly, the BOD is one of the most diabolical procedures in the modern water quality laboratory. It was developed in 1868 by an engineer who, like Victor Frankenstein, apparently had good intentions and was simply ignorant of the monstrous nature of his creation. His idea was to determine how much pollution (which in those days usually meant sewage) was present in a stream by measuring how much oxygen the microorganisms in the water used in breaking down the wastes.
If the concept for the test appeared simple, its execution seemed even more so. All a person had to do was grab a sample of stream water, fill a couple of bottles with the sample, use one bottle to measure how much oxygen was present in the water initially, incubate the other bottle for a few days, and then measure how much oxygen was left in the second bottle. The difference between the two was the amount of oxygen consumed by all those busy little organisms living off the wastes during that period of time.
From the outset, therefore, the BOD was an empirical test, intended to yield useful results rather than to embody the niceties of any particular biological theory. As wastewater treatment developed, a means of determining wastewater strength was needed, and the BOD seemed to fit that need. Early practitioners assumed that the test told them how "strong" the wastes in a particular stream were at the time a sample was taken: the stronger the wastes, the more oxygen needed to digest it. From this developed the mistaken impression that the BOD measures the amount of organic material in a waste.
Unfortunately, the BOD doesn't measure the presence of a specific entity, but rather the uptake of one - oxygen. Although it can sometimes be usefully assumed that oxygen demand is proportional to the quantity of organic material present, this is not always the case, as can be shown in the generalized reaction equation for the oxidation of organic material: [Mathematical Expressions Omitted]
The oxygen demand, n, can be solved mathematically in terms of the other variables as: n = x + Y/4 - Z/2
Thus the oxygen demand is not only determined by the amount of organic material ([C.sub.x][H.sub.y][O.sub.z]) present, but also by its specific structure as defined by the variables x, y, and z.
Even for those willing to overlook the BOD/s theoretical weaknesses, a number of practical problems soon became glaringly apparent, especially when the wastes in a stream were so strong they overwhelmed the oxygen in the water before the end of the test, or when an organic waste other than sewage was involved and the microorganisms present were incapable of digesting it. So engineers and other pragmatic types spent the next hundred years "perfecting" the test by adding a variety of ingenious complications. Dilution of samples became necessary to compensate for the high oxygen uptake of most wastes, leading to lengthy, heated arguments over hat salts and buffers, if any, should be included in the dilution water. Where suitable microbial populations were lacking, these were added to the mixture in the form of a "seed," further complicating interpretation of test results. More recently, additional modifications have been introduced to suppress nitrogenous BOD, in order to limit the test to carbonaceous forms of oxygen demand.
An Inadequate Test?
All of these "improvements" have resulted in a test that drives laboratory personnel to distraction. A few years ago, "The Bench Sheet," a newsletter for water and wastewater analysts, surveyed its readers regarding the BOD. In releasing the results of that survey, the newsletter called the biochemical oxygen demand "the test that everybody loves to hate" and coined a new meaning for the acronym "BOD" by referring to it as the "bottle of dissatisfaction." Over half of the survey respondents held that the test was inadequate and not scientifically defensible as a measure of the oxygen demand of water and wastewater. An even greater number reported difficulties in running the test. In general, criticisms of the test cited its failure to measure industrial and toxic wastes, the difficulty of obtaining an adequate "seed," the lack of sufficient standards and controls, the presence of too many variables, poor precision and reproducibility, results that tend to be analyst- and laboratory- dependent, and the limitations imposed by the five-day lag time between sampling and conclusion of the test.
In all, the survey concluded that the BOD is defended by only a few and denounced vehemently by many.
In spite of these problems, by the end of the 1970s, politicians and legislators (acting on the advice of many of those in the profession who had helped develop the test) had incorporated the BOD into the basis of the country's water pollution control laws and engineering treatment standards without fully understanding what the test measured or how arbitrary its parameters really were. BOD values were considered fundamental for the proper design and operation of wastewater treatment facilities, as well as for monitoring effluent effects on receiving streams. The BOD had thus become a cornerstone of modern pollution control technology, despite the fact that most of the technicians employed to perform the test loathed it.
The fact remains that after more than a hundred years of use, the test has probably never once been performed correctly, is seldom if ever properly interpreted, and (because it takes five days to complete) rarely results in beneficial process adjustment for the wastewater treatment plants that perform it.
So much for the infallibility of modern science.
To be fair, some laboratory personnel do defend the test. It is, according to these brave souls, the best indicator yet developed of whatever it is the test actually measures, and therefore an important though admittedly flawed analytical technique. This lackluster defense tends to be among the more supportive, and beyond that most non-engineers are subject to diminishing enthusiasm. In the survey cited previously, even those laboratory analysts who supported the test did so reluctantly or with qualifications, referring to the test's uniqueness in at least approximating the natural environment and the processes which take place in a receiving stream. Furthermore, they emphasized the need to regard the BOD as a general indicator rather than as an absolute value.
However, if the BOD is an analyst's nightmare, it is also a poet's delight, and my own support for it stems almost as much from esthetic reasons as practical ones. Sure it's difficult if not impossible to perform correctly, and sure its results are questionable, its significance uncertain. But few other laboratory procedures even attempt to deal with such an awe-inspiring variety of living organisms.
For five days (a number chosen because in England, where the test originated, the water in any stream reaches these within five days, after which no one worried about it) the water inside a BOD bottle seethes with successive waves of bacterial and protozoan species. Whole populations rise to preeminence for periods ranging from days to hours, founding microbial empires that dominate other species, only to fall back into relative obscurity as the ranks of their successors swell to replace them. Every form of interaction between organisms may appear, including parasitism, competition, predator-prey, and symbiosis. With an abundance of organic material (a form of pollution which is, after all, merely food to these microorganisms) and oxygen, certain kinds of organisms predominate. Later, as both food and oxygen supplies wane, other kinds of organisms which could not compete successfully at first become more numerous.
Within the confines of each BOD bottle there exists an entire ecosystem, a world constantly evolving as it struggles between the forces of life and death. The BOD test thus represents a microcosm of the larger world we inhabit, and if we understand or measure that microcosm with only limited success, perhaps that's just a further reflection of our incomplete understanding of the world outside. In the face of this, is it really so important if the test isn't scientifically defensible?
So engineers and poets find themselves curious bedfellows in their mutual support of the test, the former because they're still under the delusion that it's practical, the latter because they are in a position to appreciate its mysteries.
All of which makes the BOD test the perfect environmental counterpart to the ancient art of alchemy. As with true alchemical procedures, the BOD represents an arcane rite performed and interpreted by adepts who preserve its secrets through the use of strange jargon, obscure symbols, and mysterious apparatus. As with alchemy, the BOD has a history of acceptance amounting to blind faith among its adherents, while its detractors oppose it vehemently. As with alchemy, the BOD demands tedious effort and exacting skill in an attempt to duplicate the required technique, with little chance of doing so correctly. As with alchemy, the BOD's political importance (based primarily on misconceptions) far outweights its technical significance. And finally, as with alchemy, the BOD offers an approach to knowledge which, although deficient from a practical standpoint, is nevertheless rich philosophically.
Indeed, the philosophies behind the two subjects show amazing parallels. Each seeks to transform that which is base and corrupt (lead or mercury in the case of alchemy; sewage and other wastes in the case of the BOD) into something pure and pristine (gold or silver with alchemy; stable, non-polluting end-products with the BOD). Each pursues this purification by manipulating natural processes, particularly those of decay and regeneration. And each seeks to transmute its original matter into the new form through the influence of three factors: an appropriate "seed" (a mystical precursor of silver or gold in the case of alchemy; an inoculum of suitable microorganisms in the case of the BOD), the breath of life (spiritus or pneuma in alchemy; oxygen in the BOD), and the nurturing warmth of incubation (with the dung-bed of the alchemists having been replaced by mechanical incubators for the BOD).
Fortunately, the arsenal of pollution control technology isn't dependent on the BOD alone, and many plant operators and lab technicians perform the BOD as required, but use other tests to actually run their treatment plants
Seeking an Elixir
Perhaps, however, at least part of the difficulty surrounding the BOD is akin to another concept associated with alchemy: the desire to develop a single procedure, or elixir, capable of meeting all needs or curing all ills, regardless how arbitrarily these are determined. We have bestowed that questionable objective not only on the BOD, but also on those tests which have increasingly been suggested as possible replacements, particularly the COD and TOC. Yet it is important to remember that COD and TOC results do not provide a complete picture of a given situation any more than do BOD results. Rather, these results must by used in conjunction with other another. In this way, a descriptive profile on a waste can be accumulated which includes the specific concentrations of organics in the waste, the rate at which these organics will be decomposed, the oxygen required to do this, finally the portion of organic materials which will remain inert.
The problem with direct comparisons between BOD, COD, and TOC values is that the tests do not measure the same kinds of parameters. COD and TOC are true indicators of the chemical strength of a waste. The BOD, however, is an indicator of biological activity rather than a purely chemical response. It represents an attempt to simulate the natural decomposition of organic wastes through aerobic microbial action. It not only approximates the biological availability of a waste, but also provides some indication of the rate at which the waste will be utilized and the extent of its eventual degradation. Nonbiological analysis will not identify these aspects of a waste.
while it is true that biological systems are indefinite quantities and do not allow for refinement of measurement as do chemical systems, it is also true that this very complexity makes biological analysis imperative in environmental work. Biological response to a mixed and fluctuating waste cannot be adequately approximated by mere physical or chemical means. To achieve this, it is necessary to monitor the interaction between the waste and biological communities with which it will come into contact. The only test presently available in a standardized form and about which enough analytical data is known and which is capable of fulfilling this role is the BOD.
Divorcing the BOD of unreasonable expectations, therefore, might enable us to take this test out of the Dark Ages and finally grant it the role for which it is most properly suited: as a general indicator of the rate and extent of bioavailability of the organic materials present in a waste, rather than as an absolute, specific value.
Mr. Clark is the author of Alchemy Unlimited (published by Avon Books) and president of Laboratory Consultants, Albuquerque, New Mexico.
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|Title Annotation:||biochemical oxygen demand|
|Author:||Clark, Douglas W.|
|Date:||Jan 1, 1992|
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