Anaerobic waste treatment, toxicity

Toxicity, Synergism, and Antagonism in Anaerobic Waste Treatment Processes... 

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 fundamental findings are 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. 

A review of the literature on anaerobic waste treatment indicates considerable variation in the toxicity reported for most substances. In some cases, the range is over more than one order of magnitude. The major reason for these variations is the complexity of the phenomenon of toxicity. Unfortunately, in most studies of toxicity in anaerobic waste treatment little or no attention is paid to the effect of antagonism, synergism, acclimation, and complexing reactions. Indeed, in very few studies are the chemical characteristics of the control defined adequately. 

Much published toxicity data for anaerobic waste treatment are the product of studies in which toxicity was not to be studied Rather, toxicity developed during a study of some other phenomenon, and the toxicity data were reported, sometimes as an afterthought, with the other data. 

Although there are good reasons why much of the data on toxicity in anaerobic systems arjg suspect, such data have not and cannot be interpreted properly. At best, most of these data are useless at worst, they are misleading. Recently, studies of toxicity in anaerobic waste treatment were conducted utilizing experimental procedures which circumvent the inadequacies discussed above. Emphasis is placed on the data from these studies in the following sections of this paper. 

Table I. Reported Values of Toxic Concentrations of Heavy Metals in Anaerobic Waste Treatment...

The most important complex-type reaction for controlling toxicity in anaerobic waste treatment is the precipitation of heavy metals by sulfides. This was noted by Barth et al. (7) and Masselh and Masselli (8). The reason for the preeminence of sulfides is the extreme insolubility of heavy metal sulfides. The solubility product of heavy metal sulfides ranges from 3.7 X 10" for FeS to 8.5 X 10" for CuS (9). Thus, in the presence of sulfides, the heavy metal concentration is expected to be virtually zero in an anaerobic waste treatment unit. 

Therefore, a prime objective in the operation of anaerobic waste treatment processes is maintenance of a proper pH range. The signal that trouble is imminent is a sudden rise in the volatile acids. One group of investigators has indicated that the proper action to take when the volatile acids rise suddenly is to add alkaline substances to maintain the buffer capacity 12, 13, 14), A second group led by Buswell 15) and Schlenz 16) contends that this is detrimental because the volatile acids themselves are toxic to methane bacteria but not to acid formers at concentrations of about 2000 mg/liter. The use of alkaline materials thus only stimulates acid production leading to even greater toxicity. This latter... 

Until a decade ago, the available experimental data could be used to support either point of view. The major reason was that carefully controlled studies had not been conducted to prove or disprove either contention. At that time, McCarty and McKinney (17,18) published the results of their work in which they investigated the toxicity of acetic acid to methane bacteria. Acetic acid is the most prevalent acid produced during anaerobic waste treatment (19). 

The studies of McCarty and co-workers have shown clearly that volatile acids are not toxic to methane bacteria at concentrations that would occur in stuck or sour digesters. On the contrary, evidence has been elucidated which indicates propionate retards the acid formers. Thus, the use of alkaline substances to maintain an adequate buffer capacity in an anaerobic waste treatment unit is a valid procedure. A word of caution is necessary pH control is not a universal palliative. Its only advantage is to prevent a bad situation from getting out of hand. The basic cause of the digester biochemical imbalance must be discovered and rectified. Unless this is done, pH control is worthless in the long run. In addition, care must be exercised in selecting an alkaline material that will not produce a toxic reaction. 

Many other groups of substances have been reported to exert a toxic effect on anaerobic waste treatment systems. However, the experimental data for most of these are quite meager. In this section some of the available data on these substances are given. 

Much of the published data on toxicity in anaerobic waste treatment systems is erroneous and misleading because of inadequate experimental techniques. 

A great deal of additional work is required to provide the design engineer with the information he needs to deal rationally with toxicity problems in anaerobic waste treatment. 

In order to prevent SO2 emission, H2S has to be removed from gas streams prior to combustion. Apart from environmental reasons, removal of H2S from waste gas streams is also required for health reasons (H2S is a toxic gas, lethal at concentrations exceeding 600 ppm) and to prevent corrosion of equipment. Gases that can contain H2S and need treatment are, for instance, natural gas, syngas and biogas (formed in anaerobic wastewater treatment). 

Metal reclamation of sediments uses many of the same approaches as for soils, except that sediment access is often more difficult. Once removed from the bottom of a lake or river, sediments can be treated and replaced, or landfilled in a hazardous waste containment site. The actual removal of sediments involves dredging. This can pose serious problems since dredging includes the excavation of sediments from benthic anaerobic conditions to more atmospheric oxidizing conditions. This can result in increased solubilization of metals, along with increased bioavailability (see Section 10.3) and potential toxicity, and increased risk of contaminant spreading (Moore, Ficklin Johns, 1988 Jorgensen, 1989 Moore, 1994). There are ongoing discussions as to whether it is more detrimental to remove sediments, whether for treatment or removal, or simply to leave them in place. 

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