Impact of ionic composition of groundwater on oxidative iron precipitation

In the Netherlands, approximately 60% of drinking water is obtained from (generally anaerobic) groundwater. This requires aeration followed by rapid sand ltration (RSF) to remove iron, manganese, arsenic and ammonium. The mechanisms responsible for their removal or the clogging of RSFs and breakthrough of colloidal iron or manganese oxides have not been fully elucidated in previous studies. In this work, factors affecting iron precipitation have been studied in aerated, continuously stirred bench scale jar experiments to simulate the supernatant layer of submerged sand filters. Time series data of filtered iron concentration and precipitate size have been collected in experiments with synthetic groundwater with and without P, Si, HCO3 and Ca at neutral pH. We observed that precipitate growth is not influenced by different HCO3 concentrations but is reduced drastically when natural organic matter (NOM) is present in water and, to lesser extent, Si as well. The addition of P appears to hamper precipitate growth to some extent, but requires more research to fully understand the implications. We also observed that addition of Ca improved the growth of Fe precipitates in the presence of Si and especially NOM. These results have great significance for improving Fe removal efficiency of groundwater treatment plants in The Netherlands and abroad.