Denitrification and phosphate

Sewage treatment has both ecological and economical importance. Ammonium and phosphate ions are the primary inorganic contaminants in municipal wastewater and these ionic compounds are responsible for the phenomenon of eutrophication. In common sewage treatment, ammonium is removed by biological denitrification and phosphates are precipitated with the help of aluminum and iron salts [56], However, the use of low-cost natural zeolites has been found promising for... 

Figure 15. Future complex waste treatment systems for denitrification and phosphate removal...

The first quantitative estimates of denitrification in the eastern tropical North Pacific were made by Codispoti and Richards (1976). Codispoti and Richards used apparent oxygen utilization, AOU, and phosphate data to stoichiometricaUy extrapolate back to the nitrate concentration present when a given water mass was previously at the surface. In this way they were able to develop a relationship between pNOj and sigma-f for waters of the ETNP-ODZ, from which they calculated nitrate deficit as outlined in Eq. (6.4). 

Nitrogen supplies are obtained from the atmosphere. The latter forms an inexhaustible reservoir because of a relatively short natural cycle of the element involving continuous bacterial processes of fixation, nitrification and denitrification and so forth (Chapter 11.5). The eventual depletion of readily available phosphate rock supplies, on the other hand, seems not unlikely. 

Fig. 2 Longitudinal changes in nutrient concentrations below the effluent input of a WWTP without tertiary treatment in La Tordera Stream. Values are the average ( SEM) of monthly measurements done over a year (see more details in [47]). In the left panel, note the net decline of ammonium concentration with concomitant net increases in nitrate concentration, suggesting a potential hot spot for nitriflcation. However, in the latest meters downstream, dissolved inorganic nitrogen (DIN) tends to decrease, which indicates net lost of DIN possibly due to denitrification. The right panel shows net changes in phosphate and dissolved organic carbon (DOC) concentrations. While phosphate does not exhibit any clear trend on an annual basis, DOC seems to decline similarly to DIN, which supports the relative dominance of denitrification...

Kepro [Kemira process] A process for recovering valuable products from municipal sewage sludge. It makes four products crude iron phosphate, a biofuel, water treatment chemicals, and a carbon source for denitrification in the sewage plant. Developed by Kemira Chemicals in the 1990s and first installed in Helsingboig, Sweden... 

Because phosphate is released during remineralization with no decrease in O2, the A02/AP0 produced via denitrification should be lower than that predicted by the aerobic respiration of Redfield-Richards planktonic detritus. To reach the suboxic conditions required for denitrification requires the aerobic respiration of a considerable amount of POM and, hence, release of phosphate. Thus, A02/AP0 ratios less than 138 are most likely to be found in waters with high phosphate concentrations. The prevalence of denitrification in deep waters is suggested by their low (14.7) average N-to-P ratio (Figure 8.3). Areas where the OMZ are pronoimced, such as coastal upwelling areas, have particularly low N-to-P ratios as shown in Figure 10.7. 

At this site in the eastern tropical North Pacific, denitrification is responsible fiar the midwater loss of nitrate and production of nitrite. The size of the secondary nitrite maximum is dependent on the relative rates of its production from NO3 and its loss via dissimilatory reduction to N2. The amount of nitrate lost to denitrification is shown as the difference between the measured nitrate and the calculated nitrate. The latter was estimated by multiplying the observed phosphate concentrations by the average nitrate-to-phosphate ratio in the three deepest samples (11.9 1.6pmolN/L). Note that the zone of denitrification is restricted to mid-depths, i.e., the depths of the OMZ at this site. 

Here N is the deviation from the amount of IN predicted stoichiometricaUy from phosphate and the world-ocean N P regression relationship. Negative values of N are interpreted to show net denitrification whereas positive values show net... 

Denitrification occurs in environments that are deficient in O2. An increase in the ocean s nitrate content will drive higher export production (neglecting, for the moment, the possibility of phosphate limitation). When this increased export production is oxidized in the ocean interior, it will drive more extensive O2 deficiency. This will lead to a higher global rate of denitrification, which, in turn, will lower the ocean s nitrate content. Thus, the sensitivity of denitrification to the O2 content of the ocean interior generates a hypothetical negative feedback that may, like the N2 fixation-based feedback, work to stabilize the nitrate content of the ocean (ToggweUer and Carson, 1995). 

For the southern North Sea, Hydes etal. (1999) report an average N P ratio at the end of winter (March) of 10 with a corresponding nitrate concentration of 12 pM. During the spring bloom, the N P ratio rose to 25 and then fell back to below 3 in the summer. This represents an initial limitation of growth by phosphate due to the high ratio of N P in waters where the dominant source of nutrients is river water. The ratio then fell as production continued using phosphate which was recycled more quickly than nitrate, and nitrate was lost due to denitrification. 

Nitrogen fixation, denitrification, soil weathering, phosphate fixation, clay mineral degradation, and potassium and transition metal fixation are problems for which the reaction rates are usually as, or more, important than equilibrium. Most soil chemical applications of kinetics have been in soil microbiology and soil biochemistry, where the lack of equilibrium is more obvious. The use of kinetics in inorganic soil chemistry will undoubtedly broaden in the future. It can even be argued that kinetics is basic to thermodynamics, because equilibrium is the condition where opposing reaction rates are equal. 

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