Environmental Engineering Reference
In-Depth Information
of 100 tonnes was supplied. The oxidation of the sediment matrix and nitri-
fication rates were increased slowly during this time and a reliable homog-
enization of dissolved oxygen distribution of 5-50 mg/L were achieved.
Groundwater redox potential was increased from approximately—200 mV
to +500 mV after 3 years.
In situ gas storage monitoring was used to optimize the performance of
the BIOXWAND. The mean gas saturation of 2%-4% was found to be an
appropriate range for effective operation (Figure 10.13), however this was just
the range of residual gas saturation in the sediment. The amount of time
required for complete dissolution and consumption of such oxygen gas was
estimated to be equal to that it required exchanging 1.5-2.0 pore volumes of
groundwater. Local saturations of up to 17% were detected in some coarse
sandy layers. This was linked to a localized reduction in hydraulic conduc-
tivity from 4 × 10
−3
to 1 × 10
−3
m/s. In this case, a hydraulic self-regulation
and homogenization of the groundwater flow occurred. High groundwa-
ter fluxes in the coarser sections were decreased by preferential gas stor-
age, whereas low fluxes in the finer-grained sections were increased with
an increase in the local hydraulic gradients. Monitoring and control of the
hydraulic flow homogenization in gas PRBs at the field scale are subjects of
research, as they are important factors in the cost-effective operation of PRBs
and their increasing acceptance from the point of view of the authorities.
It is reported in the literature that the supply of oxygen gas causes pyrite
oxidation. Dissolved ferrous iron is predominantly precipitated as ferric iron
hydroxides. Mass and volume balances for the in situ iron removal and field
observations indicated that there was no significant risk of long-term pore
clogging to groundwater flow or gas storage. Iron hydroxides precipitated
mainly in the low-pore diameter regions (Figure 10.14).
Gas saturation data from TDR
Gas saturation estimation from balancing
5
4.5
4
3.5
Gas storage dynamics
Formation
Dissolution
3
2.5
2
1.5
1
12345
Injection cycles
67 89
FIGURE 10.13
Gas storage control during BIOXWAND operation: changes of residual gas saturation (left) and
local TDR gas sensing results (right).
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