Presentation Information
[O02-P03]Distribution of Oxidation-Reduction Potential, Fine-Grained Material, Iron-Oxidizing Bacteria in Groundwater reflecting Injection Wells of Open-loop Ground-source Heat Pumps: an example of Gifu City, Central Japan★Invited Papers
Nadia Zahra Binti Hayazi1, *Tomoyuki Ohtani1 (1.Department of Civil Engineering, Gifu University)
Keywords:
Ground-source Heat Pumps,Oxidation-Reduction Potential,Fine-Grained Material,Iron-Oxidizing Bacteria
Open-loop ground-source heat pumps are expected to be installed to net-zero energy buildings to help realize a carbon-neutral society. This is because they utilize renewable energy use, can use groundwater with an annually stable temperature as a heat source when appropriately designed, and can reduce the initial installation cost. These systems exchange heat between pumped groundwater and heat pumps, and subsequently reinject the water into the aquifer through injection wells. Clogging in the injection wells is known to reduce injection capacity, and major causes include the presence of fine-grained material, chemical reactions, microbial growth, and bubble formation (Lee, 2013). However, previous studies on the spatial distribution of fine-grained material, bacteria, and the oxidization-reduction potential (ORP) of groundwater are limited, and thus these factors are not fully considered in designing open-loop systems. In this study, these parameters were measured in Cifu City, Central Japan, to clarify their relationships and the influence of well clogging.
Twenty-two wells with a 30 m length were studied. ORP and Modified Fouling Index (MFI) of pumped groundwater were measured onsite. For quantifying iron-oxidizing bacteria, groundwater samples were filtered through a membrane, placed on Winogradsky's medium (Beimeng et al., 2015), incubated at 28 °C for three days, and colony-forming units were counted.
The pumped groundwater shows pH ranging from 5.9 to 7.2, electrical conductivity from 98 to 290 μS/cm, and ORP from -160 to 314 mV. ORP exhibits wide variety, with values mainly > 100 mV, -100 to 0 mV, and -200 to -100 mV in the northern, southern, and eastern area. MFI values were < 1.0 s/L2 in the northern area and > 2.0 s/L2 in some well of southern area. Iron-oxidizing bacteria ranged from 0.01 to 330 CFU/mL, with higher values detected in the southwest part of the northern area.
Based on ORP values, MFI tend to be higher at ORP < 160 mV. Iron-oxidizing bacteria exceeded 5 CFU/mL at ORP <200 mV and exceeded >50 CFU/mL at ORP between 160 to 200 mV. Higher MFI suggests that dissolved iron in groundwater under such redox condition precipitates due to oxidation when the water is pumped. The higher abundance of iron-oxidizing bacteria indicates that these microorganisms prefer microaerobic conditions. For open-loop systems, even when iron concentration in groundwater are high, it may still be possible to operate the loop system without clogging if oxidation is sufficiently controlled before reduction occurs, thereby preventing the formation of iron oxides and the activity of iron-oxidizing bacteria.
Twenty-two wells with a 30 m length were studied. ORP and Modified Fouling Index (MFI) of pumped groundwater were measured onsite. For quantifying iron-oxidizing bacteria, groundwater samples were filtered through a membrane, placed on Winogradsky's medium (Beimeng et al., 2015), incubated at 28 °C for three days, and colony-forming units were counted.
The pumped groundwater shows pH ranging from 5.9 to 7.2, electrical conductivity from 98 to 290 μS/cm, and ORP from -160 to 314 mV. ORP exhibits wide variety, with values mainly > 100 mV, -100 to 0 mV, and -200 to -100 mV in the northern, southern, and eastern area. MFI values were < 1.0 s/L2 in the northern area and > 2.0 s/L2 in some well of southern area. Iron-oxidizing bacteria ranged from 0.01 to 330 CFU/mL, with higher values detected in the southwest part of the northern area.
Based on ORP values, MFI tend to be higher at ORP < 160 mV. Iron-oxidizing bacteria exceeded 5 CFU/mL at ORP <200 mV and exceeded >50 CFU/mL at ORP between 160 to 200 mV. Higher MFI suggests that dissolved iron in groundwater under such redox condition precipitates due to oxidation when the water is pumped. The higher abundance of iron-oxidizing bacteria indicates that these microorganisms prefer microaerobic conditions. For open-loop systems, even when iron concentration in groundwater are high, it may still be possible to operate the loop system without clogging if oxidation is sufficiently controlled before reduction occurs, thereby preventing the formation of iron oxides and the activity of iron-oxidizing bacteria.