Anoxic transformations

Wright JR, Bouwer EJ. 1986. Anoxic transformations of halogenated aliphatics. In Young RA,ed. 

There is — in addition to the use of nitrate for control of anaerobic conditions in sewers — a potential for anoxic treatment in terms of removal of organic matter. The anoxic treatment is an alternative to aerobic treatment. An advantage is that the addition of nitrate is simple compared with the injection of oxygen. However, a NUR value that is of the same order of magnitude as the actual OUR value—compared in units of electrons transferred—is crucial to obtain a relatively high removal rate of organic matter. For this and other reasons it is important to compare aerobic and anoxic transformations (cf. Example 5.5). 

Example 5.5 Comparison between the stoichiometry of aerobic and anoxic transformations... 

To compare the stoichiometry of aerobic and anoxic transformations, the electron acceptors 02 and NO3 must be considered by using electrons as a common and suitable unit for redox processes. The basis for this comparison is shown in Examples 2.3 and 2.4 ... 

Denitrification in activated sludge systems is well known and well described. However, only a few investigations of anoxic transformations relevant for sewer systems have been performed. 

Although the present experience on the variability of anoxic transformations is limited, the rate of removal of organic matter for energy purposes corresponds under nonlimited conditions to about 1-2 gN03-N nr Ir1. This value is generally lower than the corresponding rate (the OUR value) under aerobic conditions. 

Poulsen (1997) investigated the anoxic transformations of wastewater in biofilms originating from a biofilter and found maximum NUR values of 0.025-0.055 gN03-N nr2 h-1 at 20°C. The change from 0-order to 1/2-order kinetics was found to be about 3 gN03-N m-3. Aesoey et al. (1997) found an NUR from a 1-2 mm thick sewer biofilm to be 0.15-0.18 gN03-N m-2 h-1 at... 

Abdul-Talib, S., T. H vitved-Jacobsen, J. Vollertsen, and Z. Ujang (2001), Anoxic transformations of wastewater organic matter in sewers — process kinetics, model concept and wastewater treatment potential, Proceedings from the 2nd International Conference on Interactions between Sewers, Treatment Plants and Receiving Waters in Urban Areas (INTERURBAII), Lisbon, Portugal, February 19-22, 2001, pp. 53-60. 

Poulsen, B.K. (1997), Anoxi sk omsal n I ng a organisk stof i biofiltre (Anoxic transformations of organic matter in biofilters), MSc thesis, Department of Environmental Technology, Technical University of Denmark (in Danish), p. 56. 

The integrated aerobic-anaerobic WATS model has changed this situation. As an example, it is possible to use the model in a gravity sewer with changing aerobic and anaerobic conditions. As previously stressed, a number of in-sewer processes still need to be dealt with. Examples are the anoxic transformations and the processes related to the extended sulfur cycle, particularly, the oxidation of sulfide and the emission of hydrogen sulfide into the sewer atmosphere, including its further oxidation at the sewer walls. Combined use of empirical and conceptual models is still needed. 

Bouwer, E. J., et al, Anoxic Transformations of Trace Halogenated Aliphatics, in Toxic and Hazardous Wastes, G. D. Boardman (Ed.), Proc. 18th Mid-Atl Ind. Waste Conf., Technomic Publishing Co., Lancaster, PA, 1986. 

Peijnenburg WJGM, MJ t Hart, HA den Hollander, D van de Meent, HH Verboom, NL Wolfe (1992) QSARs for predicting reductive transformation constants of halogenated aromatic hydrocarbons in anoxic sediment systems. Environ Toxicol Chem 11 301-314. 

Environmental Fate. Hydrogen sulfide is known to easily evaporate into the air (EPA 1993 Layton and Cederwall 1986 Leahey and Schroeder 1986), although its solubility in water may also cause it to persist in unperturbed, anoxic sediments. Additional information on the transport, transformation, and persistence of the compound in soils and groundwater, particularly at hazardous waste sites, would be useful in identifying the most important routes of human exposure to hydrogen sulfide. 

Baughman GL, Weber E (1994) Transformation of dyes and related compounds in anoxic sediment Kinetics and products. J Environ Sci Technol 28 267-276... 

As indicated in Figure 3.4, the covalent bond, i.e., two common shared electrons, between two carbon atoms in the complex molecule is cleaved when initiated by the exoenzymes. The highly reactive intermediates that are formed react and produce new and stable bonds resulting in two new molecules that may undergo further hydrolysis. Hydrolysis is, thus, an important initial step in the transformation of complex organic matter present in a form that cannot directly be used at substrate. Hydrolysis is a process that—with different reaction rates — proceeds under aerobic, anoxic and anaerobic conditions. It is important to note that hydrolysis takes place without use of an electron acceptor. 

Basically, a concept for microbial transformations in sewer networks should cover soluble and particulate components and relevant processes in the water phase, in the biofilm and in the sewer sediments. In addition, mass transfer between these phases and an air-water transfer of oxygen should be taken into account (Figures 1.3 and 5.2). Although only the aerobic microbial activity will be focused on in the concept presented in this chapter, anoxic and anaerobic processes should be considered possible extensions (cf. Chapter 6). 

It is important to notice that Equation (5.13) is valid for the relation between oxygen and nitrate as electron acceptors in aerobic and anoxic processes, respectively. These processes thereby relate to a corresponding transformation of organic matter as electron donor. In addition to the transformation of organic matter in the redox processes, organic matter is also used for the production of heterotrophic biomass. 

Prediction of the nitrate removal rate under anoxic conditions in a sewer can be done by a simple empirical approach. The following equation including transformations in the water phase and the biofilm of a sewer pipe may be applied under substrate nonlimited conditions ... 

The microbial activity of wastewater under anoxic conditions is lower compared with aerobic conditions (Abdul-Talib et al., 2001). This is important to consider, because a low nitrate uptake rate (NUR) compared with the oxygen uptake rate (OUR) in units of electron equivalents means a reduced transformation rate of the most biodegradable fractions of the organic matter. As mentioned under the point on injection of air, this may have implications in terms of treatment. Furthermore, a relatively low NUR value also has operational advantages because of a reduced demand for nitrate to suppress sulfide formation. 

Extension of the WATS model to integrate further dry-weather processes is considered important. Examples of such extensions are the description of the wastewater quality and nitrite/nitrate transformations under anoxic conditions and the emission of hydrogen sulfide into the sewer atmosphere followed by its transformation (oxidation) at the sewer walls. 

Transformation of Fe(II,III) at an oxic-anoxic boundary in the water or sediment column (Modified from Davison, 1983)... 

Further transformations of N take place at the oxic interfaces between the soil and floodwater and root and soil where NH4+ diffusing in from the neighbouring anoxic soil may be nitrified to NOs. Subsequently, NOs diffusing out into the anoxic soil may be denitrified to N2. This process results in significant losses of N from wet soils but its importance in submerged soils is unclear (Section 5.3). 

Biological. Under aerobic conditions or in experimental systems containing mixed cultures, hexachloroethane was reported to degrade to tetrachloroethane (Vogel et al, 1987). In an uninhibited anoxic-sediment water suspension, hexachloroethane degraded to tetrachloroethylene. The reported half-life for this transformation was 19.7 min (Jafvert and Wolfe, 1987). When hexachloroethane (5 and 10 mg/L) was statically incubated in the dark at 25 °C with yeast extract and settled domestic wastewater inoculum for 7 d, 100% biodegradation with rapid adaptation was observed (Tabak et al, 1981). 

In anoxic hypolimnion samples collected from Lower Mystic Lake, MA, hexachloroethane was abiotically transformed into tetrachloroethylene via reductive elimination and to pentachloro-ethane via hydrogenolysis. Tetrachloroethylene accounted for 70% of hexachloroethane in unaltered lake water and 62% in filter-sterilized water after 10 d. Trichloroethylene and pent-achloroethane accounted for <1 and 2% in unaltered lake water and filter-sterilized water, respectively. Disappearance rate constants for hexachloroethane were 0.33/d for unaltered water and 0.26/d for filter-sterilized water. At least 80% of the hexachloroethane disappearance in unaltered water was abiotic in origin due to the reactions with naturally occurring aqueous polysulfides, H2S and (Miller et al, 1998a). 

Cozzarelli, I.M., Eganhouse, R.P., and Baedecker, M.G. Transformation of monoaromatic hydrocarbons to organic acids in anoxic groundwater environment, Environ. Geol. Water Sci., 16(2) 135-141, 1990. 

Subsurface environments under anoxic conditions may contain high levels of Fe(II) on the solid phase or dissolved within immobile pore water or groundwater. The role of Fe(II) species in reductive transformation reactions of organic and inorganic contaminants in the subsurface was reviewed by Haderlein and Pecher (1988). A major finding of current studies is that Fe(II) associated with solid phases is much more reactive than Fe(II) present in dissolved forms (e.g., Erbs et al. 1999 Hwang and Batchelor 2000). 

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