Methodology to evaluate the performance of the in-situ treatment of groundwater containing high iron concentration DAVID IOAN1, ACHIM CAMELIA1, GIRBACIU ALINA1, CHEBUTIU ADRIAN2 1 University “ Politehnica “ Timisoara, George Enescu, Nr. 1A, 300020 Timisoara Romania
[email protected] 2 UCM Resita, StrGlobului, Nr. 1, 320053 Resita, Romania
[email protected]
Abstract Conventionally, the ground water containing high iron and manganese concentration is treated in surface plants with physical chemical and biological processes. An alternative way is the in situ treatment when water enriched with oxygen is injected cyclic into the underground where an oxidation reactor appear and the iron(II) is oxidized. The paper present a methodology to evaluate the expected efficiency of the in-situ treatment of ground water containing high iron concentration using mathematical modeling for the transport processes and minimal field or laboratory measured data. Keyword: in situ treatment in underground, ground water with high iron concentration, dissolved iron, oxidation zone, underground treatment reactor
1. INTRODUCTION The treatment of ground water with high iron concentrations directly in underground called “in-situ” method has several advantages in opposite to the classical over ground treatment plants. The most important advantage is that, the classical over ground treatment plant will be missing, and also it is eliminate the environmental pollution which appears during the periodically washing of the filter. This treatment process is based on the cyclical infiltration of oxygen-enriched water in the aquifer directly through the recovery wells (infiltration phase). The infiltrated water in the first step of a cycle displaces the natural ground water with high iron concentration. At the same time, adsorbed iron on sand granules in the aquifer in the vicinity of the well is oxidized to hard soluble compounds e.g. iron hydroxide (oxidation zone). During the recovery phase which is the second phase of a cycle the former dislocated ground water with high iron concentration moves back towards the well and passes through the oxidation zone. The dissolved iron will be held back through adsorption on the oxidation products around the sand particle of the aquifer. The oxygen and iron adsorption capacities will increase through several cycles of infiltration and recovery and a zone called ‘underground treatment reactor’ in the vicinity of the well is formed. The ratio between the quantity of treated water collected during recovery and the quantity of oxygen-enriched water used for infiltration determines the performance of the in-situ treatment called performance or efficiency coefficient (EC) which should always be greater than 2. This coefficient strongly depends from the aquifer and transport parameter and from the iron concentration. Presently doesn’t exist a forecast possibility of this coefficient. The main goal of this paper is to elaborate a forecast method for estimation of the performance coefficient.
2. ESTIMATION OF THE EFFICIENCY COEFFICIENT The in situ proceed is structured in three fazes. The infiltration phase, consist in infiltration of aerate water in aquifer, through infiltration well. The water enriched with oxygen, infiltrated in aquifer, dislocates the natural underground water which contains high quantities of iron, from wells zone. Oxidation phase, which is know like rest phase consist in evolution of oxidation processes of Fe 2 and Mn 2 . The iron which was adsorbed by sand granules will be oxidize and will form some hard soluble compounds, for example iron hydroxide. Which mean, that will be form a treatment space. Collect phase consist in putting in function of well like an underground tapping. In this phase, take place the movement of water in the direction of well, which is obligate to pass through oxidation space. The ratio between the quantity of water which is collected and water which is infiltrated will determinate the efficiency of underground treatment method (efficiency coefficient): V EC = C VI
1)
This coefficient depends of aquifer nature and raw water and can be between 2 and 12. The third phases can be realized using one well or two coupled wells which work alternatively as recovery and infiltration well respectively. The equation (1) allows to determinate the efficiency coefficient by measuring of the infiltration and discharge volume flows. A reasonable application is the determination of the efficiency coefficient by analyzing the natural composition of the aquifer and its accompanying flow and transport processes. In this way, the efficiency can be estimated even before a treatment plant is built. The governing equation which describe the dissolved iron and oxygen transport processes during infiltration and discharge is expressed as: ∂c 1 ∂s = ∇ ⋅ D ⋅ ∇c − v a ⋅ ∇c − ∂t n e ∂t
2)
Where: c – Concentration of dissolved substance [kg/m3]; s – Concentration of adsorbed substance; D – Dispersion [m2/s]; v a – flow velocity [m/s]; n e – effective porosity Modeling the sorption term as a linear one, 3) s (....) = H (....) ⋅ c(....) the solution of this equation allows the description of the efficiency coefficient as a term of the general form: ~ 4) EC = f (G Fe , GO 2 , QI , t I , L z ) Where G Fe = 1 +
H Fe ne
;
GO 2 = 1+
H O2 ne
, are the retardation coefficient of the iron and of
the oxygen respectively which represent the effects of the sorption. A first estimation can be done by using the simplified form of the efficiency coefficient. G −1 EC ~ = Fe GO 2
5)
The case of the two dimension radial symmetric flow system it was presented in (David 2005) which is based on the radial symmetric movement and minimum of field dates. To evaluate the expected efficiency of the in-situ treatment for complex groundwater recovery plants it will be use the mathematical modeling for the transport processes and minimal field or laboratory measured data. In the figure 1 is presented the unidimensional laboratory model used to determine the necessary parameter to estimate the sorption coefficients G Fe , G O2 which allow the
estimation of the efficiency coefficient EC. This laboratory model plant represent an alternative which can eliminate the field measurements.
Figure 1. Unidimensional experimental model used to estimate the efficiency coefficient. To can estimate the efficiency coefficient it will be analyze a soil sampling and will be introduce in experimental model to determinate the effective porosity and also dispersion. The porosity can be calculated with the following equation: ne =
Q I ⋅ t 50 L⋅ A
6)
The dispersion will be determinate using the results from the porosity determination and also helping by the next equation: D=v⋅L⋅
(t 50 − t 24 )2 t 24 − t 50
7)
Where: t 50 – time when the concentration will decrease at 50 %, from the maximum concentration; t 24 – time when the concentration will decrease at 24 %, from the maximum concentration. The oxygen sorption will be determinate in the infiltration phase and the iron sorption in the collecting phase by measuring the oxygen and iron concentration in different points of prelevation. GO 2 = t 50 ⋅
QI A ⋅ ne ⋅ L
8)
G Fe = t 50 ⋅
QE A ⋅ ne ⋅ L
9)
Where: G O2 – Oxygen sorption; Q I – Infiltration discharge [m3/s]; A – Surface of the cross section [m2]; n e – Effective porosity; L – Length of the experimental model [m]
G Fe – Iron sorption; Q C – Collected discharge [m3/s]; Having the values for the iron and oxygen adsorption can be estimate the efficiency coefficient using the simplify form equation 5. 4. CONCLUSION The advantages of this in situ underground treatment water are the lower costs, high efficiency and also it is avoid the environment pollutions. The paper introduces a methodology to pre-estimate the expected efficiency of a simple in situ underground water treatment system on the basin of minimal measured field data. All the general equations used in pollutants transport and also, physical and mathematical modeling can be adapted for underground water treatment. 5. REFERENCES David I., Wissenschaftliche Begleitung der In- Situ- Untersuchunger zur subterrestrischen Aufbereitung des Grundwassers des Brunnenfeldes Altmanns in Niederosterreich, Technische Universitat Darmstadt (2003) David I., In-situ Treatment of Groundwater Containing high Iron Concentration. Proceedings of the 30 th Annual Congress of the American Romanian Academy of Arts and Science, ISBN 9975-75-313-2, 2005 David I., Grundwasserhydraulik. Strömungs- und Transportvorgange, Vieweg Verlag Braunschweig Wiesbaden, 1998. Ralf M., Entwicklung von Planungs- und Anwendungskriterien fur die In-situ Aufbereitung eisen- und manganhaltinger Grundwasser. München, 1996. Olthoff. Die Enteisenung und Entmanganung von Grundwasser im Aquifer, Hannover, 1986. Meyerhoff, R. Entwicklung von Planungs- und Anwendungskriterien für die In-Situ-Aufbereitung eisenund manganhaltiger Grundwässer. Stuttgarter Berichte zur Siedlungswasserwirtschaft, Bd.139, R. Oldenbourg, München, (1996)
