Geoscience Reference
In-Depth Information
Figure 7.3.
Technical design of the pilot plant.
Three tanks have been installed in this container ( Fig. 7.3 ) : the first tank (aeration tank, 500 L)
allows increasing or decreasing of the concentration of dissolved oxygen. Excess gas bubbles
are removed from the infiltrated water in the second tank (degassing tank, 500 L). The third
tank (buffer tank, 500 L) receives the part of the pumped water which is afterwards pumped to
the remediation facility. FeCl 2 solution can be added into the degassed water which is finally
infiltrated into the infiltration well. The infiltration pipe is equipped with a bypass for measuring
in situ water quality parameters such as pH, redox potential, dissolved oxygen (DO) and electric
conductivity (EC).
The two wells have a very short screen length of 4 m. Short filter screens are very important
since they reduce the dwell period of the Fe enriched water and therefore reduce the risk of Fe
incrustations. Additionally, the wells are equipped with two pipes so that no infiltration water
uses the pipes of the pumped water. The infiltration pipe reaches down to the filter screen. Thus,
infiltrated Fe enriched water has only a short, direct contact to the potentially oxygen enriched
water within the well and is delivered to the aquifer more directly. As a result of the short screen
length, low maximum pumping and infiltration rate of 6 m 3 h 1 had to be applied.
The schedule of the pilot plant is characterized by the repetition of similar cycles, which
generally differ in the pumping direction. One cycle is subdivided into four steps ( Fig. 7.4 ).
Step (1) - infiltrating Fe-enriched water: Water is pumped into the aeration tank from well 1
and stripped with nitrogen gas (N 2 ) to decrease the DO-concentration in the water. The water is
infiltrated into well 2 after amending the water with FeCl 2 -solution once the DO-concentration
drops below 0.5 mg L 1 . The applied FeCl 2 -solution has a Fe content of 118 g per liter FeCl 2 -
solution.
Step (2) - infiltrating water with low DO-concentration: Water is pumped from well 1 through
the aeration and the degassing tank and stripped with nitrogen in the aeration tank. During this
step, the groundwater with dissolved Fe is forced to flow from the well into the aquifer.
Step (3) - infiltrating water with a high DO-concentration: This step is similar to step 2 with
the difference that ambient air instead of nitrogen is dosed in the aeration tank. In this step
groundwater with high dissolved oxygen concentration is transported into the aquifer. DO reacts
with the dissolved Fe and therefore the Fe precipitates.
Step (4) - infiltrating untreated water: Water is pumped from well 1 through the aeration tank
as well as the degassing tank and directly infiltrated into well 2 without further treatment. This
step creates a buffer around well 2 in which water with a lower concentration of DO is stored
than infiltrated in step (3). Applying this method the risk of clogging within the well is reduced
because the pumping direction is switched on the next day and water is pumped from well 2 and
then again stripped with N 2 [step (1)].
 
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