Nitrification modeling

While most authors have used the finite-difference method, the finite element method has also been used—e.g., a two-dimensional finite element model incorporating shrinkable subdomains was used to de.scribe interroot competition to simulate the uptake of N from the rhizosphere (36). It included a nitrification submodel and found good agreement between ob.served and predicted uptake by onion on a range of soil types. However, while a different method of solution was used, the assumptions and the equations solved were still based on the Barber-Cushman model. 

The activated sludge process for meat processing wastewater treatment was reported by Annachhatre and Bhamidimarri.37 When a model reactor was operated at an HRT of 5-15 h, a COD removal of more than 85% was achieved. The COD loading for the above performance was 3.2kg COD/(m3d). An SRT of 13 d resulted in almost complete nitrification. El-Gohary et al.30 reported that the activated sludge process reduced the BOD and COD of a potato-chips factory wastewater by 86% and 84%, respectively. The organic loading rate and HRT were reported as 8.9 kg BOD/ (m3d) and 6h, respectively. 

PROFILE is a biogeochemical model developed specially to calculate the influence of acid depositions on soil as a part of an ecosystem. The sets of chemical and biogeochemical reactions implemented in this model are (1) soil solution equilibrium, (2) mineral weathering, (3) nitrification and (4) nutrient uptake. Other biogeochemical processes affect soil chemistry via boundary conditions. However, there are many important physical soil processes and site conditions such as convective transport of solutes through the soil profile, the almost total absence of radial water flux (down through the soil profile) in mountain soils, the absence of radial runoff from the profile in soils with permafrost, etc., which are not implemented in the model and have to be taken into account in other ways. 

Nitrification. The nitrogen reactions in the PROFILE model are very simple, since only nitrification and uptake are included explicitly. 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

In this example, a fluidised biofilm sand bed reactor for nitrification, as investigated by Tanaka et al. (1981), is modelled as three tanks-in-series with a recycle loop (Fig. 1). With continuous operation, ammonium ion is fed to the reactor, and the products nitrite and nitrate exit in the effluent. The bed expands in volume because of the constant circulation flow of liquid upwards through the bed. Oxygen is supplied external to the bed in a well-mixed gas-liquid absorber. 

ASMl- A basic model dealing with nitrification and denitrification in wastewater,... 

Klaver A.L. and R.A. Matthews (1994). Effects of ox)detracychne on nitrification in a model aquatic system. Aquaculture 123 237-242. 

Denitrification takes place in anoxic environments where nitrate and nitrite act as electron acceptors (oxidizers) and nitrification reactions then reverse NOf => NO2 => NO. The processes of denitrification (H2) on land are important channels for nitrogen to get into the atmosphere. The intensity of these processes depends on temperature, humidity, pollution of soils with poisonous chemicals, and pH. The quantitative and functional characteristics of these dependences have been well studied. The global model need only take into account temperature and humidity ... 

This model assumes irreversible first-order kinetics for nitrification, denitrification, mineralization, immobilization, and plant uptake. It takes the following form, in which sinks and sources are aggregated ... 

Shaffer et al. (1977) Dutt et al. (1972) model extended to tile-drained croplands and incorporated into a large irrigation return flow model to handle nitrogen as well as dissolved mineral constituents. Model allows user to select the degree of sophistication in simulation or to bypass certain subroutines. Zero-order denitrification and transition-state nitrification added. Model verified for salinity but not nitrogen. 

Tillotson et al. (1980) Nitrification and urea hydrolysis by first-order kinetics NH3 volatilization by first-order kinetics from (NH4)2C03 formed from urea hydrolysis NH sorption by linear partition model NH and NOy plant uptake involving diffusion to roots. 

Davidson et al. (1978a) also used first-order kinetics for nitrogen transformations, but they considered that some of the transformation rate coefficients were dependent on several factors including environmental ones The rate for nitrification was empirically adjusted for water suction. Overall, most of the nitrogen models assume first-order kinetics. Some of them also consider the effects of temperature on rate coefficients. 

Jensen, K., Sloth, N.P., Rysgaard-Petersen, N., Rysgaard, S., and Revsbech, N.P. (1994) Estimation of nitrification and denitrification from microprofiles of oxygen and nitrate in model sediments. Appl. Environ. Microbiol. 60, 2094—2100. 

Even if rate measurements in sediments are made using whole core incubations, e.g., when the inhibitor is a gas, it is still difficult to obtain a depth distribution of the rate (usually, an areal rate is obtained). A sophisticated measurement and model based system that avoids direct rate measurements has been used to overcome this problem. Microelectrodes, which have very high vertical resolution, are used to measure the fine scale distribution of oxygen and NOs" in freshwater sediments. By assuming that the observed vertical gradients represent a steady state condition, reaction-diffusion models can then be used to estimate the rates of nitrification, denitrification and aerobic respiration and to compute the location of the rate processes in relation to the chemical profiles (e.g., Binnerup et ai, 1992 Jensen et ai, 1994 Meyer et ai, 2001 Rysgaard et ai, 1994). Recent advances and details of the microelectrode approach can be found in the Chapter by Joye and Anderson (this volume). 

Ward et al., 1989b). A large role for in situ nitrification to supply the phytoplankton demand is also supported by biogeochemical models... 

Gilbert, F., AUer, R. C., and Hulth, S. (2003). The influence of macrofaunal burrow spacing and diffusive scaling on sedimentary nitrification and denitrification An experimental simulation and model approach. Joumfl/ of Marine Research 61, 101—125. 

Martin, A. P., and Pondaven, P. (2006). New primary production and nitrification in the western subtropical North Atlantic A modeling study. Global Biogeochem. Cycles 20, GB4014, doi 10.1029/ 2005GB002608. 

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