Mitigation of groundwater iron-induced clogging by low-cost bioadsorbent in open loop geothermal heat pump systems

Green energy production from natural resources can reduce emissions of greenhouse gases and pollutants from burning of fossil fuels in power plants. Recently, groundwater geothermal energy (GGE) is harnessed by deploying closed- and open-loop heat systems. In open-loop geothermal heat pump systems (OLGHPS), groundwater is reinjected into aquifer after harnessing GGE. Nevertheless, OLGHPS face noxious clogging issue because of elusive chemistry (corrosion or precipitation) of chemical species, principally of iron (Fe), in pipes and aquifers during reinjection process via oxidation reactions. Plethora of filtering materials are available for removal of ions, but these are quite expensive and environmentally unsafe. More recently, low-cost, eco-friendly, green filtering materials gain much interest. These materials can remove ions from groundwater that can minimize clogging in heat exchange systems, injection wells, and aquifer. In the present study, three filtering materials, i.e., wooden charcoal (biomaterial), yamazuna fine sand, and volcanic ash, were tested to estimate their Fe removal capacity. In upward flow mode with minimum oxygen-water contact, serial column (each with 6 ports) experiments were conducted under constant pressure head and constant velocity conditions. Columns were connected to well water having dissolved Fe concentration of 10.85 mg L-1. Sampling was done at the well, column inlets, column's six sampling ports and column outlets, and samples were analyzed for Fe by atomic absorption spectroscopy. Related tested parameters include pH, EC, temperature, turbidity, porosity, particle diameter, and dissolved oxygen. Volcanic ash showed less Fe removal, while sand filter showed substantial reduction in velocity. Biomaterial (wooden charcoal) displayed higher Fe adsorption capacity compared to other materials that can be ascribed to its surface chemistry and functional groups. Under different flow rates, maximum Fe content of 3.5 g Fe kg(-1) dry charcoal was obtained. By considering a safety factor and influence of groundwater composition, it is possible to design a biomaterial-based iron filter system to minimize Fe-induced chemical clogging in OLGHPS which is an eco-friendly, green energy source.