Tutorials for AMDTreat PHREEQ Module
AMDTreat contains two PHREEQ modules for modeling mine drainage treatment. Two tutorials are provided below to give users an overview of how the two modules can be used to estimate chemical consumption and effluent quality.
Un Aerated PHREEQ Module
Pre Aerated PHREEQ Module
Un Aerated PHREEQ Module Tutorial
1. Supply data for all data blue and red data fields in the water quality window- Defining the amounts of ferrous iron and inorganic carbon are required to run the PHREEQC module. AMDTreat offers two methods to define the amount of ferrous iron and inorganic carbon. The first method is to enter the results from a water analysis and is the preferred method. AMDTreat recognizes that many datasets lack a characterization of ferrous iron and inorganic carbon. So, the AMDTreat offers users the choice to estimate the amount of ferrous iron and inorganic carbon by clicking on the “Est.” checkbox next to each parameter.
Ferrous Iron Estimate – AMDTreat provides the option to estimate the amount of ferrous iron. The estimate is based on the user-defined total iron concentration and the pH entered into the water quality screen. The equation used in AMDTreat was derived by Dr. Chuck Cravotta. Dr. Cravotta used a mine drainage dataset, which contained over 100 samples, and developed an equation that relates the ferrous/ferric iron ratio to pH.
Total Inorganic Carbon Estimate (TIC) – The integrated PHREEQC module requires a TIC value before the model can be run. Users can collect a TIC sample and enter the laboratory results into the water quality screen, or users can have AMDTreat estimate TIC. AMDTreat offers two methods to estimate TIC, the alkalinity method and the atmospheric CO2 method.
Alkalinity method - For waters that contain alkalinity, AMDTreat will use the pH, Alkalinity, and temperature values entered into the water quality screen to compute the TIC concentration of the water. This method is preferred over the atmospheric CO2 method because it uses site-specific water quality information to calculate the TIC of the water. The accuracy of the TIC estimate is directly dependent on the accuracy of the pH, alkalinity, and temperature values. The AMDTreat team recommends using field-measured results for these three parameters.
Atmospheric CO2 method - This method can be used for low pH waters that lack alkalinity. AMDTreat assumes the mine water is in equilibrium with the atmospheric concentration of CO2. This method doesn’t use site-specific data and assumes that all waters, regardless of pH, contain the same about of total inorganic carbon. This method should not be used to have the PHREEQC module estimate the effect of CO2 species on treatment.
2. Select the Chemical Cost module located under the Annual Costs menu on the Costs Screen.
3. Select a treatment chemical and check the PHREEQ checkbox.
4. Select a treatment pH based on effluent water quality - As soon as the PHREEQ checkbox is checked, the PHREEQ module runs and a new screen is opened that displays the results.
The figure below shows the estimated effluent water quality for a variety of treatment pHs. The dissolved effluent concentrations are expressed in mg/L. The figure shows the starting iron concentration of 20 mg/L was reduced to 0.43 mg/L at a treatment pH of 7.0. In addition, a treatment pH of 7.0 reduced the aluminum concentration to .002 mg/L and increased the alkalinity to 70 mg/L as CaCO3.
The figure below shows the PHREEQ module estimate TDS will increase from 1339 to 1525 mg/L when the water is treated to a pH of 7.0 with hydrated lime. In addition to estimating the effluent quality, the figure below shows the PHREEC module also computes the saturation indices for various mineral phases. The figure below shows that Fe(OH)3 is controlling the predicted iron effluent quality and Boehmite is controlling the aluminum concentration.
5. Select "Accept" on the PHREEQ module screen - After selecting a treatment pH that achieves the targeted effluent quality, press “Accept” and the annual hydrated lime cost and consumption are displayed under the chemical costs sub totals. Also notice that the treatment pH is shown next to the hydrated lime routine. AMDTreat records and displays the treatment pH to help remind users the treatment pH the cost estimate reflects.
6. Select the Sludge module from the Costs Screen.
7. Sludge Volume Estimate using PHREEQ module– In addition to estimating the chemical requirements for a defined treatment pH, the PHREEQ module keeps track of precipitate mass to estimate annual sludge volume. In addition to aluminum, iron, and manganese precipitates, PHREEQ also considers any precipitation of magnesium, calcite, gypsum, and unreacted chemical in calculating the mass of sludge. Currently, the model is constrained to only allow calcite to precipitate if the saturation index exceeds 2.5. In addition, PHREEQ will use the mixing efficiency defined in the Chemical Cost module to calculate the amount of chemical that will be unreacted and contribute towards sludge volume. Users define the mixing efficiency based on professional knowledge about a treatment site. For example, users may choose to assign low mixing efficiencies to sites where hydrated lime is fed dry into a waste water stream and that immediately discharged to a settling pond with minimal mixing. In this case, both dry feeding hydrated lime (as opposed to making lime slurry) and inadequate mixing of lime and waste water will result in poor chemical utilization.
The figure below shows a mixing efficiency of 80% was used to estimate the total amount of chemical required for treatment and the amount of unreacted chemical that will contribute to sludge. In the case of 80% mixing efficiency, the theoretical chemical requirement is increased by 25% to estimate the total annual chemical requirement. In addition, 25% of the total chemical requirement will be unreacted and will contribute to the mass of sludge.
The figure below shows PHREEQ estimated that the mass of metal precipitates equate to .074 g of precipitate per liter of water treated. The amount of unreacted chemical is added to the PHREEQ sludge mass estimate and the user-defined values for percent solids and sludge density are applied to determine and estimated annual sludge volume.
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Pre Aerated PHREEQ Module Tutorial
1. Follow steps #1 and #2 for running an un aerated PHREEQ simulation. We will use the follow water quality in this example:
2. Determine the amount of pre aeration (CO2 removal) desired – A drop-down menu box is located in the top left-hand corner of the Chemical Cost Screen. The selected value of PCO2 will be used to equilibrate the water. PHREEQC will remove CO2 from the raw water until the water achieves equilibrium with the selected PCO2. If PCO2 of the starting water quality is less than the selected PCO2, PHREEQ will not add CO2 to the water to achieve equilibrium. This purpose of this module is to simulate pre aeration for CO2 and not CO2 addition.
In the figure below, PHREEQ will remove CO2 from the solution until a –Log PCO2 of 3.0 is achieved.
3. Check the PHREEC with aeration checkbox to run the model.
Unlike the unaerated module, the aerated model contains two preliminary steps before the water is titrated with the selected caustic chemical. First the water is brought into equilibrium with the selected –Log PCO2 value. The figure below show the water is now in equilibrium with the selected –Log PCO2 value.
The second important step the aeration module performs is the complete oxidation of ferrous iron and manganese. The water initially contained 40 mg/L of ferrous iron.
The figure below shows the 40 mg/L of ferrous iron as been oxidized to ferric iron. In addition, the 10 mg/L of Mn2+ has been oxidized to Mn4+. Recall, the pH of the initial solution was 6.0 and the alkalinity was 18 mg/L as CaCO3. The figure below shows the pH has dropped to 4.6 and the alkalinity is -6.1 mg/L as CaCO3 from the oxidation and hydrolysis of ferrous iron and manganese. After the CO2 removal and oxidation steps are completed, the model titrates the solution with hydrated lime.
4. Select a treatment pH – The model predicts that effluent standards can be achieved at a much lower pH when ferrous iron and manganese are oxidized before chemical addition. In this case, a treatment pH of 7.0 is selected.
5. Click the Accept button and an estimate of the annual treatment costs are displayed on the Chemical Cost Screen subtotals.
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