Environmental Engineering a Design of Efficient Surface Aerators for Waste Water Treatment Aeration is the most important treatment process employed in all water and wastewater treatment plants PDF document - DocSlides

Environmental Engineering a Design of Efficient Surface Aerators for Waste Water Treatment Aeration is the most important treatment process employed in all water and wastewater treatment plants PDF document - DocSlides

2014-12-15 321K 321 0 0


In the biological treatment of wastewaters aeration is essential to raise the dissolved oxygen DO level to allow aerobic bacteria to work and reduce the pollution load BOD of the waste and thus to improve water quality The rate of oxygen transfer in ID: 24166

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Presentations text content in Environmental Engineering a Design of Efficient Surface Aerators for Waste Water Treatment Aeration is the most important treatment process employed in all water and wastewater treatment plants

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Environmental Engineering a) Design of Efficient Surface Aerators for Waste Water Treatment Aeration is the most important treatment process employed in all water and wastewater treatment plants. In the biological treatment of wastewaters, aeration is essential to raise the dissolved oxygen (DO) level to allow aerobic bacteria to work and reduce the pollution load (BOD) of the waste, and thus to improve water quality. The rate of oxygen transfer into the water body depends on the intensity of turbulence created in the water body. The intensity of turbulence and mixing in turn depends on three types of variables: dynamic, geometric and physical. The dynamic variable is the speed of rotation of the rotor. The geometric variables are: size, shape and number of blades attached to the rotor; diameter and immersion depth of rotor; water depth in the tank; and the size and shape of the aeration tank. The physical variables are density, viscosity and temperature of water in the tank. Investigations have been carried out to understand the performance of surface aerators with a view to develop an efficient aeration system along with the simulation criterion for oxygen transfer under different geometric, dynamic and physical pa rameters, by employing hydraulic principles. Design of one of the recently develo ped surface aerato rs is patented. b) Modeling Contaminant Transport in Ground Water and Vadoze Zone (i) Modeling Unsaturated flows Based on the Richardís equation and ADE equation, flow and transport model was developed for prediction of moisture and contaminant movement in the variably saturated zone. The flow model was built around finite difference approach, while the transport model was built using the finite volume approach with the TVD scheme. An operator split approach was used in which the dispersive part was solved using the standard implicit formulation while the advective part was solved using finite volume explicit scheme with TVD. Field measurements were conducted in the Moolehole forested watershed (Figure 45) for validation of the models developed. The measurements of moisture were made in the watershed for one rainy season at different test pits. The measurements in the soil horizon containing red and black soils were made usin g the Neutron probe. The dry period data was collated during summer. The soils characteristic data were obtained by the use of the soils texture data. Also the local calibration was done for fitting the field data with the simulated data. Investigations showed a reasonable ma tch between the field data and the model generated results. This indicates the successful application of Richardís equation for a field situation. Analysis is made for the entire hydrological year containing monsoon and non- monsoon periods. A study was undertaken to compare the non intrusive geophysical electrical resistivity methods, the neutron probe measurements a nd simulation model results to arrive at relationships among these for various test pi ts in the Moolehole watershed comprising of different types of soil horizons. In all thes e, the moisture content measurements using different techniques were compared. The preliminary results showed a good correlation between the geophysical methods and the neutron probe measurements for red soil horizons, while the same was not found to be satisfactory for black soils.
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Stochastic modeling studies were undertaken to determine effective constitutive relationships for field scale unsaturated flow under steady gravity drainage conditions. The block scale heterogeneity was assumed to be represented by a single realization, generated by using a three dimensional turning band random generator proposed in literature. The studies were performed for isotropic field and anisotropic field with perfectly and imperfectly layered stratifications. The results matched well with the stochastic theory results in low tension ranges and diverged with increase in tension. The studies indicated that the conductivity shows moisture dependent anisotropy, and the pressure field shows suction dependent variance (Figure 46). Two-dimensional transient transport simulations were performed using the steady state flow simulation results. The movement of the solute transport was studied for both isotropic and anisotropic two-dimensional random fields. For anisotropic field, transport simulations in perpendicular and parallel to bedding were performed. The solute plume was characterized by analyzing its spatial moments, for each case. Macro-dispersivities were calculated on the basis of the spatial moments, for all the cases considered. The effective flow properties generated using Gardnerís model is applicable to only low tension ranges. In this part, the soil parameters of the Las Cruces Trench site were considered, and upscaling was performed for high tension ranges as large as 25 m. Van Genuchten, Mualem constitutive relationships were used in the numerical model of flow. Both conductivity and soil moisture retention curv es were considered as stochastic processes. a)
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b) c) A8 A3 A1 A4 A2 1.0 2.0 3.0 m 5.0 10.0 15.0 20.0 25.0 m Top Soil Red Soil Clay Cutans Red soil with grey matrix Black Soil Black soil with Calcite Sandy weathering level Auger Holes Rock Line Sandy Horizon Tensionics Figure 45: Toposequence for Moolehole watershed, South India.
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Figure 46: Simulated and Measured moisture for location A3 (shown in Figure 45c) after a) 100 and b) 138 days (ii) Modeling Multiphase flows System parameters of a porous media, in which multiphase flow takes place, play an important role in the determination of the extent of the plume movement. This was necessitated by the proper remediation strategy to be implemented to clean up the contaminated site due to nonaqueous phase li quid (NAPL) contamination which requires field determination of the system parameters. In this direction inverse models were built based on the earlier developed simulation models . In the inverse model, based on the field observations and also on the simulation model results, a least square based Gauss Newton (GNSAT) and Genetic Algorithms (GA) models were developed to estimate the system parameters such as the intrinsic permeability, grain size distribution index. Also the models were extended to the case of source strength and location identification. This will help in proper identification of the source location and also its strength so that the proper estimation of the NAPL plume migration can be made (Fi gures 4.47 and 4.48). Diff erent cases such as even determined, overdetermined were considered which reflect the availability and use of data. In the models studied, different signals (m easurements) such as pressure and saturation measurements and their combination were considered. Also comparison of both GNSAT and GA based models were made for different types of problems. The error bands for the parameters estimated were also analyzed. It wa s found that the saturation signals give better estimation of the parameters. Also the power of the stochasticapproach based GA models are realized when more number of parameters are to be estimated.
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Figure 47: Aperture distribution for a rough walled fracture plane Figure 48: DNAPL migration in the 2D variable aperture field c) Fuzzy-Stochastic Modeling for Water Quality Control in Streams Fuzzy multiobjective models were developed for water quality control in a river system receiving municipal and industrial effluents. Co nflicting objectives of the dischargers and the pollution control agencies were modeled with fuzzy membership functions. The objectives were treated as imprecisely defined fuzzy goa ls. Given an appropriate pollutant transport model, the fuzzy multiobjective model was formulated to obtain decisions on optimal fraction removal levels for point sources of pollu tant loads. The model is extended, using the theory of fuzzy probabilities, to include the random nature of the streamflows. In a single model, uncertainties due to both fuzziness and randomness were simultaneously addressed. The fuzzy optimization model was formulated to obtain decisions on fraction removal levels for the effluents, for a known streamflow probability distribution. The model results may be used to derive the probability distributions of th e water quality indicators (e.g., DO deficit) at a check point. An implicit stochastic fuzzy optimization model was developed for obtaining
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optimal waste load allocations to a river system. Random variation in streamflow, air temperature, re-aeration rates and industrial and municipal effluents was addressed through a Monte-Carlo simulation. A new concept of fuzzy risk was introduced as a performance measure for water quality control in streams. The models developed have been extended to integrate the QUAL2E water quality simulation model and genetic algorithms into fuzzy optimization. A first order uncertainty analysis of the resulting optimal solutions was carried out to provide solutions at various reliability le vels and to identify key locations in the river system at which hydrologic uncertainty plays a significant role. In a recent study, stochastic dynamic programming has been used to addres s seasonal variations of streamflows through transition probabilities for specifying steady state fraction removal policies. The procedures and models developed are demonstrated with the case study of the Tunga Bhadra river system in Karnataka. d) Water Distribution Network A GIS based approach was used to develop an urban water distribution model. The developed model was based on the popular EPANET model. The main emphasis has been to address the issue of equitable distribution of wa ter to the entire city based on the GIS data gathered for the city. The approach was applied to Bangalore city water distribution network consisting of several reservoirs located at various elevations spread across the city with three sources of supply. Based on the field experime nts and also on the supply policy, model was developed to address the issue of intermittent supply in a service area. Studies were conducted to understand the change in quality of a parcel of water as it travels along the network. Several models were developed based on steady state and also unsteady state conditions. For steady state models, the pr inciple of complete mixing at a node was used. While in the case of unsteady models, an advection equation was solved with the velocity coming from the hydraulic models. Several numerical schemes such as Eulerian, Lagragian methods were used in model building. To handle the case of reactive transport, reaction equations based on kinetic reactions were considered. Several kinetic reaction equations were derived based on the order of reactions. Through the studies, the role of wall reaction vis-a-vis overall reaction parameter was clearly brought out. The Model was also built to handle the biological reactions in the network based on attachment/detachment concept. All the models developed were applied on field problems for its demonstration of chlorine attenuation and the results were found to be very good. Identification of reaction parameter, type and order of reaction was done using inverse modeling. It was possible to identify the type of reaction and also the wall and bulk reaction parameters by using the inverse modeling tech niques. For the case tested both the Gauss- Newton technique and also the Genetic Algorithm technique performed equally well. However, the Genetic algorithm technique was fo und suitable for cases where a large number of parameters are to be estimated. The problem of equitable supply among various reservoirs can also be handled through control of valves and pumps. Several models ar e being developed based on partial integrator differentiator (PID) controller, linear quadrati c controller, dynamic inversion etc for better control of the system. With these controls the supply to various reservoirs can be controlled so that an equitable supply can be ensured at a fast rate. Also these controls can be the basis for feed back mechanism whereby a supervisory control or an automatic control mechanism
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can be evolved. Such control models can lead to optimal control systems with leakage / energy minimization and equitable supply among reservoirs e) Electrokinetic Decontamination of Soils Soils are often contaminated with heavy metals such as lead, zinc, iron. Arsenic, cobalt, nickel etc due to industrial waste disposal and many pre-industrial operations. Removal of heavy metals by conventional methods is an extremely difficult task. Electro kinetic method is being used to decontaminate soil from these metals (Figure 49). The effectiveness of the method for different soils as well as the cond itions, under which the efficiency is optimum, needs to be checked by laboratory electrokinetic experiments. For this task it is necessary to check the concentration of heavy metals from the leachate. Estimation of these metals by conventional approaches is cumbersome. The Atomic Absorption Spectrometer was used to estimate the concentration of different heavy metals accurately and conveniently. This method is particularly useful for estimation of a given metals from a large number of samples. Several research and post-graduate students have used this facility for their dissertation works. Figure 49: Removal of metal ions from fine grained soils by electrokinetics f) Re-Use of Hazardous Wastes in Civil Engineering Applications The four main characteristics of hazardous wastes are ignitability, corrosivity, reactivity and toxicity. Recycling of hazardous wastes appear s in top of the hierarchy of priorities in hazardous waste management. The environmental and technological issues involved in reuse of phosphogypsum sludge (from fertilizer indust ry) and lead bearing gypsum sludge (from lead-acid battery industry) in civil engineerin g applications. The potential of these waste materials to release toxic metal ions before and after chemical stabilization along with mechanical properties relevant to their use as building materials are examined (Figure 50).
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Figure 50: Un-soaked and soaked behaviour of calcined gypsum sludge g) Characterization of Bentonites as Buffer Material for Deep Geological Repository Final disposal of vitrified high level radioactive waste and spent nuclear fuel in Deep Geological Repository (DGR) is under consider ation worldwide. Such repositories, in general, are based on multi barrier system concept to ensure long term confinement and isolation of these wastes from environment. It typically comprises the natural geological barrier provided by the repository host rock and its surrounding and an engineered barrier system (EBS) consisting of man-made, engineered materials placed within a repository, including the waste form, waste canisters, buffer materials, backfill, plugs and seals (Figure 51). In most of the deep geological repos itory concepts worldwide especially those considering granites & gneisses as their host rock, the bentonite clays are under consideration for use as buffer and backfills. Comprehensive study is undertaken to evaluate the geotechnical properties of Indian bentonite clay s for their suitability as buffer material in deep geological repository for high-level nuclear wastes. The bentonite samples are characterized for index properties, compac tion, hydraulic conductivity and swelling characteristics. Strong correlation was obs erved between ESP (exchangeable sodium percentage) and liquid limit/swell potential of tested specimens. Relatively less well-defined trends emerged between ESP and swell pressure/hydraulic conductivity. Additionally, the microstructural processes that control the macr o behaviour of the buffer material on changes in chemical environment are being examined. Figure 51: Use of Bentonite as buffer ma terial in deep geological repositories
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h) Defluoridation of Ground Water High levels of fluoride in drinking water lead to dental and skeletal fluorosis. The fluorosis problem is severe in India as almost 80% of the rural population depends on untreated groundwater for potable water supplies. To address this problem, IISc has developed a new defluoridation technique, which relies on magnesium oxide based precipitation- sedimentation-filtration technique to reduce fluoride concentrations in water to permissible levels (< 1.5 ppm as per Indian Standards). Fluoride ions are removed by magnesium oxide through chemisorption mechanism (Figure 52). The method does not involve any recharge process and thus avoids generation of corrosive and toxic wastes. A simple to use Domestic Defluoridation Unit (DDU) is developed to trea t 20 litres per day of fluoride-contaminated water. Figure 52: Domestic Defluoridation Unit The device comprises two units, each of 20 litres capacity. The upper unit which is equipped with a manually operated geared mechanical stirring device serves as a mixing-cum- sedimentation unit, while the lower unit serves as treated water-collection unit. Fifteen litres of fluoride-contaminated water is poured in the upper unit. Calcium hydroxide + magnesium oxide mix is added to fluoride-contaminated water and manually stirred for five minutes using the stirring device. The suspension is allo wed to stand for 16 h, at the end of which fluoride-bearing sludge settles at the bottom of th e container. The clear water is decanted into lower collection unit through flexible connecting pipe fitted with a fine filter to trap any escaping sludge particles. Water-soluble sodium bisulfate is dissolved in the lower collection unit and the water is ready for use. Sludge pro duced by the method can be disposed in an environment friendly manner by consuming it in the manufacture of sludge admixed stabilized mud blocks. The cost of treating 1 litre of fluoride-contaminated water (having fluoride concentrations from 2 to 5 ppm) by this method is 7 paisa/litre. The cost of the DDU is approximately Rs 2000 per unit. The method has also been scaled up to treat fluoride-contaminated water at community level (500Ė2000 litres per day). Field trials of this method at individual household levels are undertaken in Yellodu a nd Chakavellu villages (Kolar district, Karnataka). Magnesium oxide + calcium hydroxide packets containing specified dosages of
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chemicals to treat 15/100 liters of raw water and bottles containing specified volumes of 7.5 % calcium chloride solution and 5 % sodium bisu lfate solution to treat 15/100 liters of water on a daily basis are supplied to selected household s in these villages. At both villages, this method is being successfully implemented. i) Natural Coagulants for Water and Wastewater Treatment There is a need to reduce the usage of expensive and dangerous chemicals in water and wastewater treatment. In this regard, research on the use of Indu seeds (natural coagulants), for water and wastewater treatment is under progress at the University of Sherbrooke, Canada, as a collaborative research project. j) Pollution Induced Heave in Soils Due to interaction with sulphuric acid, considerab le increase is observed in the swell of black cotton soil under nominal surcharge. This has been attributed to the formation of new minerals such as mereiterite [K Fe(SO (H O) ] and alunogen [Al(H O) (SO ).3(H O) ] by alteration of kalsilite and amesite upon intera ction with sulphuric acid. It was also seen that swell in red earth increased with increase in the concentration of sulphuric acid. The observed swell was attributed to the breakag e of hydrogen bonding and the formation of rozenite [FeSO .4H O] in the soil due to interaction with sulphuric acid (Figure 53). Also, the swell in the presence of acid sulphate wa s initiated after considerable delay since the mineralogical changes commence and progress gradually, affecting the conventional hyperbolic nature of swell time relationships (Figure 54). Studies conducted on illite and soils containing di fferent percentages of montmorillonite with different fluids revealed that montmorillonite alkali reactions would not produce significant mineralogical changes where as illite is dissociated into smectite with the formation of potassium silicate formed by the interaction of released potassium with soluble silica. This confirms that the ultimate products of rectorite with alkali solutions would be smectite and compounds of potassium. While the swelling of montmorillonite generally decreases with increase in the alkali concentration, they increa sed in illite. However, in both the minerals the swelling occurs only in one phase. Thus the second phase of swelling that has been observed in rectorite is due to delayed swelling of montmorillonite that has been released by the attack of alkali on rectorite (Figure 55). Figure 53: Effect of sulphuic acid on swell behavoiur of red soil Figure 54: SEM pictures of BC soil before and after treating with 4N sulphuric
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Figure 55: Photomicrograph of consolidated BC soils The effect of lime on the volume change beha viour of alkali contaminated non-swelling soil has been explained based on the formation of swel ling minerals of zeolite the actual form of which vary with the concentration and formati on of cementitious compounds in the presence of lime. X-ray diffraction data has indicated that zeolite of type Na 3.6 Al 3.6 Si 12.4 32.14 O is formed at higher concentration of alka li in the presence of lime slowly than Na (AlSiO (OH) .4H O which is formed in the presence of higher concentration of alkali only in the soil. It is observed that fly ash, when included in soil by 25 -50%, effectively controls the alkali induced swelling in so ils by preventing the formation of new zeolite minerals. k) Role of Osmotic Suction in Volume Change Behaviour of Expansive Clays Pore fluid osmotic suction is related to the dissolved salt content in soil water (soil water salinity) and increases with pore water salinity. Exposure of clay soils to chemical solutions (example landfill leachate, brine pond soluti ons) induces osmotic suction difference between soil water and the chemical reservoir. Osmotic suction difference between soil water and the chemical reservoir is dissipated through diffusion of cations and water molecules. The impact of osmotic suction dissipation on the swell and compression behaviour of compacted expansive clays is examined by monitoring the diffusion of metallic ions with time. Experimental results have illustrated that under certain conditions dissipation of osmotic suction induces much larger volume changes than dissipation of matric suction. l) Geochemistry of fluoride rich groundwater Groundwater is a significant water resource in India for domestic, irrigation and industrial needs. By far the most serious natural groundw ater-quality problem in India, in terms of public health, derives from high fluoride, arse nic and iron concentrations. Hydrogeochemical investigation of fluoride contaminated groundwater samples from Kolar and Tumkur Distrcits in Karnataka are undertaken to understand the quality and potability of groundwater from the study area, the level of fluoride contamination, the origin and geochemical mechanisms driving the fluoride enrichment. Majority of the groundwater samples did not meet the potable water criteria as they containe d excess (> 1.5 mg/L) fluoride, dissolved salts (> 500 mg/L) and total hardness (75 mg/L to 924 mg/L). Hydrogeochemical facies of the
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groundwater samples suggest that rock weathering and evaporation-crystallization control the groundwater composition in the study area with 50 to 67 % of samples belonging to the Ca- HCO type and the remaining falling into the mixed Ca-Na-HCO or Ca-Mg-Cl type. The saturation index values indicated that the groundwater in the study area is oversaturated with respect to calcite and under-saturated with respect to fluorite. The deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess fluoride concentration. m) Modeling of solute transport in heterogeneous porous media A common problem encountered these days is that of groundwater pollution. For instance, a contaminant source, on or near the ground surface, creates a plume that reaches groundwater, which spreads further due to natural or for ced water flow. Modeling of the phenomenon is needed in order to predict the development of the plume. The modeling of reactive solute transport equations in groundwater system requires focusing attention on (i) numerical aspects, (ii) spatial variability of flow and transport parameters (hydraulic conductivity, porosity, dispersivity, diffusion coefficient, sorption and decay coefficient), (iii) dual- porosity medium and (iv) presence multi-component reactions. Modeling studies have been performed dealing with the above aspects and the important features are highlighted below. (i) SFEM for flow and solute transport Studies have been performed to analyze the proba bilistic behavior of solute concentration in three dimensional (3-D) heterogeneous porous media. The popular and simple Monte Carlo Simulation method (MCSM) is computationally exhaustive when a few thousands of realizations are required especially for higher degrees of medium heterogeneities along with higher spaceĖtime grid resolutions. Perturba tion based stochastic finite element method (SFEM), is an alternative method for performing probabilistic analysis of concentration. The perturbation based SFEM is developed and in vestigated for linear and nonlinear transport problems. In this study the governing medium properties viz. hydraulic conductivity, dispersivity, molecular diffusion, porosity, sorpti on coefficient and decay coefficient are considered to vary randomly in space. The performance of SFEM is compared with Monte Carlo Simulation method (MCSM) for both 1-D and 3-D problems. Further, the randomness in the source or boundary conditions can also be a major source of uncertainty in the concentration field while analyzing the solute transport problems. Investigations are carried out to assess the combined effect of randomness of system parameters and the source conditions on 3-D problems. (ii) Modeling biodegradation reactions with finite volume method A globally second order accurate multi-dimensional finite volume model was developed to solve the coupled transport (acceptor, substr ate and biomass) with dual Monod reaction kinetics. This model is ideally suited to track the sharp fronts that develop in a typical bioremediation scenario and is capable of ha ndling all ranges of pore scale dispersivities normally encountered in the field. The coupled problem is split into a transport problem and a reaction problem, and solved independently and sequentially. The advection part of the transport equation is solved using an explicit finite volume method whereas a fully implicit finite difference method based model solves th e dispersion part. Studies are conducted in a porous medium exhibiting physical and/or chemical heterogeneity. The heterogeneous porous medium is generated using HYDROGEN mo del and MCSM is used for the solution.
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When pore scale dispersion and molecular diffusion are absent, the removal due to biodegradation is found to be slightly lower than that for the corresponding homogeneous case, even though some mixing is generated du e to physical heterogeneity (variations in hydraulic conductivity). The temporal variation of biodegradation rate is assessed for various cases of physically and chemically heterogene ous (variations sorption coefficient) media. Studies conducted using a moving biomass model produces higher removal rate in the initial stages, but the long-term biodegradation rate is found to be the same as in the case with the assumption of a stationary biomass. (iii) Modeling of reactive solute transport in a single fracture The movement and mixing of solutes in fractur ed media is of particular interest in an environmental context because of the possibility of very rapid and extensive movement of contaminants through fractures, cracks, or fiss ures in otherwise low permeability rock. In such scenarios, there is a need to analyze the dispersion affected by matrix diffusion. Most radionuclides that could be stored in the repos itory sites are subject to sorption on crystalline rocks and hence there is a need for analyzing the behavior of the dispersion resulting from a single fracture with matrix diffusion in the presence of linear and nonlinear equilibrium sorption and first order decay cases. The behavi or of the solute velocity and effective macrodispersivity of solute front in the fracture for the transport in a single fracture is analysed for linear and nonlinear equilibrium sorption in the presence of rock matrix diffusion using numerical modeling and spatial moment analysis. (iv) Modeling of multi-component reactive transport in groundwater Geochemical modeling requires robust and efficient multi-component solute transport models for describing the geochemical reactions arising from the water-rock interactions. An oscillatory free solution of transport eq uations under advective dominated cases is required while combining with geochemical models such as MINTEQA2 when using an operator-split approach. A multidimension al numerical model based on finite volume method for the multi-componen t equilibrium reactive transport (HYDROS) is developed, which combines the multi-component transport model with equilibrium speciation module MINTEQA2. A second-order accurate implicit tim e stepping finite volume method based on TVD scheme is used here for solving the ad vection equations. The model is benchmarked with the exact solutions available in the literature for advection-dominated cases with equilibrium chemical reactions of sorption, ion-exchange, and precipitation/dissolution. Further, the model performance is evaluated and compared with the MINTOX simulations used to model the metal mobility in aquifers impacted by acid mine tailings discharge. HYDROS is then applied to the case study of the transfer of transition metals with organic colloids in the swamp groundwater of the experi mental Nsimi watershed, representative of the humid tropical ecosystem of the South Cameroon Plateau. The model is extended to include combination of nonlinear kinetic and equilibrium reactions.

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