A feasibility study of in-situ iron removal in the atlantis primary aquifer, Western Cape province, South Africa

[摘要] English: The sustainability of many wellfield is threatened due to elevated iron (Fe2+) and manganese (MN2+) concentrations in groundwater. Their presence causes aesthetic and potability problems but the greatest concern is production borehole clogging. Physicochemical processes and biological activities cause clogging by oxidation of these ions at the borehole and aquifer interface resulting in lowered borehole yields, increasing operations and maintenance costs. South African research has focused on the remediation of clogging problems, but knowledge is needed for preventative measures in controlling the source of the problems (i.e Fe2+ and Mn2+). The in-situ iron removal(ISIR) method is a technique involving periodic injection of oxygenated water into the aquifer. It has successfully been applied overseas for decades and more recently in Egypt in reducing the need for ex-situ removalof Fe2+ and Mn2+. Long-term applications maintain borehole yield by reducing Fe2+ movement towards the borehole and spreading the oxidation processes over a large surface areain the aquifer. The precipitates stabilize into crystalline oxides, inhibiting reductive dissolution and Fe2+ and Mn2+ mobilization.The study site was the Atlantis Aquifer, where clogging has reduced the wellfield capacity by 60%, necessitating surface water augmentation. The aims of investigating ISIR applicability in a South African context were: •Feasibility investigation at the Atlantis Aquifer;•Design a site-specific ISIR prototype and methodology;•Assess Fe2+ and Mn2+ removal effectiveness of the prototype.In-situ iron removal literature suggested there was a high probability of success for applying the technique at this locality. The high water table necessitated injection into surrounding well-points, rather than directly in the test borehole (G30966). The presence of organic compound and silica-rich groundwater also required the novel use of ozone. The World Health Organisation (WHO) drinking water standards guided the desired threshold, i.e. Fe < 0.3mg/l and Mn < 0.1mg/l. The study area showed nativeFe and Mn cocentrations of 0.5 mg/l and 0.2mg/l, respectively. A mobile ISIR prototype using aerated water from G30966 supplied ozonated groundwater with a high dissolved oxygen content. Initial test saw injection in a well-point 10 m from G30966 with abstraction being shut down. Entrained gases in the ozonated water prevented sufficiently high injection rates being reaches. These gases were subsequently removed using a degas column, resulting in a reasonable injection rate. Due to the previous test not showing removal, a third technique was applied with injection into a well-point 4 m from G30966. The iron and manganese concentrations remained higher than the known baselines, suggesting that intermittent pumping dislodged deposits from G30966 contributing to spurious high iron and manganese levels. A fourth methodology was tested with abstraction during injection. This resulted in iron and manganese concentrations within the baseline concentrations and desired removal of iron removal below the WHO standard. Manganese removal was not as effective because the Mn2+ oxidation takes longer compared to Fe2+ and requires a higher ph.This study operated at a smaller scale than international case studies but showed that iron and manganese removal is achievable at much lower injection parameters (i.e. <2 m3/h and <10m3/day). The use of ozone was very effective in increasing DO concentrations, better than aeration investigations and comparable to using oxygen gas. The test also showed that DO can be increased in the subsurface with successive injections over 4 to 5 days, elevating the DO between 3 to 9 times above the baseline of 0.4mg/l in aquifer.

[发布机构] University of the Free State