Anaerobic cycle

Anaerobic stages of SBR studies have been shown efficient with color removal rates mostly higher than 70% [3, 18-20], Meanwhile, COD removal efficiency of anaerobic phase of SBR was found to depend on dyestuff type, amount of initial COD concentration, anaerobic cycle time, etc. Nevertheless, there are also reports about no efficient COD removal in anaerobic cycle of SBR [21, 22],... 

Though cycle time plays an important role in the SBR for the decolorization process, not many reports are found in the literature. The long retention times are often applied in the anaerobic phase of the reactor studies, such as 18 and 21 h. In several studies, it was reported that there is a positive correlation between the anaerobic cycle time and the color removal [30, 31]. Indeed, in combined anaerobic-aerobic SBRs, since bacteria shifted from aerobic to anaerobic conditions, or vice versa, anaerobic azo reductase enzyme can be adversely affected by aerobic conditions, which is essential for aromatic amine removal, thereby resulting in insufficient color removal rate. To investigate the effect of cycle time on biodegradation of azo dyes, inar et al. [20] operated SBR in three different total cycle times (48-, 24- and 12-h), fed with a synthetic textile wastewater. The results indicated that with a... 

Two remaining factors need to be considered in addressing the ultimate fate of arsenic within the sedimentary basins of Southeast Asia. First, arsenic, while cycled in the surface soils, is partially leached into deeper portions of the profile (i.e.. As is not conserved during the anaerobic cycle). Second, these basms are active and contmue to receive sediment loads, leading to continual burial of arsenic deeper in the soil profile. Once deeper in the soil, the cycling of iron, and hence the fate of arsenic, is altered. Dampened fluctuation in anaerobic-aerobic... 

In the anoxic atmosphere the closed anaerobic cycle of nitrogen would have consisted of the following thermodynamically permissible reactions ... 

In anaerobic environments, provided that light can penetrate, distinctive microorganisms are able to maintain an almost completely closed anaerobic cycle of matter (Fig. 4.18). 

Alternating aerobic-anaerobic cycles in wetland soils promote N2O emissions to the atmosphere, whereas under continuously flooded conditions most of the N2O formed will be reduced to N2. Like methane, appreciable amounts of N2 and N2O produced from denitrification remain entrapped in wetland soils and sediments (Lindau and DeLaune, 1991). A low partial oxygen pressure during the denitrification process enhances the formation of N2O. The critical redox potential for N2O production in soil is between +100 and +300 mV (Figure 16.7). 

Upon binding of the substrate, a fatty acid radical is formed after the abstraction of a hydrogen atom from the bisallylic methylene group, and concomitantly Fe(III) is reduced to Fe(II) (step b). In the absence of dioxygen, i.e. in the anaerobic cycle, the radical dissociates from the enzyme (step f) and yields a variety of products, part of which are derived from the alkoxy radical formed in step a, via free radical reactions (Scheme 3) [26,36]. 

A reversed, reductive TCA cycle would require energy input to drive it. What might have been the thermodynamic driving force for such a cycle Wachtershanser hypothesizes that the anaerobic reaction of FeS and H9S to form insoluble FeS9 (pyrite, also known as fool s gold) in the prebiotic milieu could have been the driving reaction ... 

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

It is also often taken for granted that many of the Earth s subsystems are exposed to free oxygen (O2), leading to a range of one-way reactions of reduced materials (such as organic carbon or metal sulfides) to an oxidized form. As pointed out many times in earlier chapters, the oxidation-reduction status of the planet is the consequence of the dynamic interactions of biogeochemical cycles. As is the case with the acid-base balances, there is considerable sensitivity to perturbations of "redox" conditions, sometimes dramatically as in the case of bodies of water that suddenly become anaerobic because of eutrophication. Another extreme... 

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