Adsorptive iron removal from groundwater

Iron is commonly present in groundwater worldwide. The presence of iron in the water supply is not harmful to human health, however it is undesirable. Bad taste, discoloration, staining, deposition in the distribution system leading to aftergrowth, and incidences of high turbidity are some of the aesthetic and operational problems associated with iron in water supplies. Iron removal from groundwater is, therefore, a major concern for water supply companies using groundwater sources. The WHO recommended guideline value of iron in drinking water is 0.3 mg/l and the EC directive has set a parametric value of 0.2 mg/1. In the Netherlands, the guideline value of iron in drinking water is≤0.05mg/1 and several Dutch water supply companies are aiming at iron concentrations below 0.03 mg/1 to minimise distribution network maintenance costs. Of the different methods available to control iron in water supplies (oxidation-filtration, ion exchange, lime softening, sub-surface iron removal, and sequestration), aeration followed by rapid sand filtration is the most commonly used.

 

Different mechanisms (physical, chemical, and biological) may contribute to the removal of iron in filters and the dominant mechanism depends on water quality and process conditions applied. Fig. 8. 1 summarises the different mechanisms of iron removal in filters and the steps involved. Under anoxic conditions, adsorption is the only mechanism of iron removal from groundwater. In the presence of oxygen, iron removal can take place via three different mechanisms, namely i) oxidation-floe formation (floe filtration), ii) biological oxidation, and iii) adsorption-oxidation (adsorptive filtration). Under the commonly applied treatment conditions in iron removal plants, the oxidation-floe formation mechanism is commonly believed to be dominant. The adsorptionoxidation mechanism (adsorptive iron removal), however, has several potential advantages over the oxidation-floe formation mechanism, namely longer filter run, shorter filter ripening time, and less backwash water use and sludge production.