5 Toxicity, Synergism, and Antagonism in Anaerobic Waste Treatment Processes IRWIN JAY KUGELMAN
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American-Standard, Inc., New Brunswick, N. J. K. K. CHIN Singapore Polytechnic, Singapore 2
Antagonism, synergism, and complexing reactions must be considered in reporting data on toxicity and stimulation in anaerobic waste treatment systems. Low concentrations (< 1 mg/liter) of all heavy metals except iron are extremely toxic, but higher concentrations can be tolerated if sufficient sulfide is present to act as a precipitant. Short chain volatile acids up to 6000 mg/liter have no adverse effect on methane formers. While pH control with alkaline substances can prevent catastrophic failure during digestion, the fundamental cause of imbalance must be corrected for a permanent cure. Light metal cations offer the most difficulty in designing and operating these systems because they can occur in both toxic and stimulatory ranges. An initial attempt to categorize quantitatively toxicity, stimulation, etc. on an absolute basis is presented.
A dequate knowledge of toxicity phenomena is crucial to the proper design and operation of a biological waste treatment system. This is especially true for the anaerobic waste treatment process because the key group of bacteria in the process, "the methane bacteria," is much more sensitive to environmental conditions than other groups of bacteria. Although this process has significant advantages over other methods of waste treatment, its use has been retarded because of a lack of underPresent address: Advanced Waste Treatment Program, Environmental Protection Administration, R. A. Taft Sanitary Engineering Center, 4676 Columbia Parkway, Cincinnati, Ohio 45226. 1
55 Pohland; Anaerobic Biological Treatment Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
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ANAEROBIC BIOLOGICAL TREATMENT PROCESSES
standing of toxicity phenomena. There are many reports in the sanitary engineering literature which detail the failure or inefficient operation of this process. In many situations, it is apparent that a toxicity situation existed but was not considered in the failure mode analysis. More frequently, toxicity was suspected, but the wrong substance was indicated as the toxic factor (1,2,3). During the last 10 years, much fundamental information has been discovered concerning anaerobic waste treatment. With judicious use of this information, anaerobic waste treatment systems can be designed and operated in a less empirical and more efficient manner than previously. Included in these fundamentalfindingsare basic data on toxicity. This paper presents a review of the available information on toxicity in anaerobic waste treatment. The data have been analyzed and are presented from the viewpoint of the waste treatment engineer. This mode of presentation was chosen to facilitate the utilization of the data presented and thus expand the applicability of the anaerobic waste treatment process. Fundamental Considerations The basic information which the sanitary engineer requires is a quantitative evaluation of toxicity under any set of environmental conditions. As discussed below, it is difficult to arrive at such an evaluation. The magnitude of the toxicity observed or reported is a function of several factors including concentration, antagonism, synergism, complex formation, and acclimation. Each of these is discussed in turn. Concentration. It is usual to classify qualitatively a substance as either toxic or stimulatory. For example, heavy metals are considered to be toxic, while glucose is classified as a food or growth stimulant. As Figure 1 illustrates, such a view is somewhat simplistic in that the effect of any substance on the metabolism of an organism is concentration dependent. As the concentration of any metabolite increases, the metabolic rate first increases, enters a constant zone, and eventually declines. At very low and very high concentrations the metabolic rate is negligible. Thus speaking absolutely, a substance is never toxic, but its degree of metabolic stimulation is concentration dependent. In general, when a substance is in the concentration range below its peak range, it is considered to be in its stimulatory range; when the concentration is above its peak range, the substance is considered to be in its toxic range. These definitions must be termed "common usage" because the metabolic rates obtained in the stimulatory region are not higher than those obtained in the toxic region. What is actually being defined is the trend of the metabolic rate with concentration increase.
Pohland; Anaerobic Biological Treatment Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
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