Iron Removal in Low Salinity Water

Abstract

A reverse osmosis (RO) membrane followed by a packed tower aerator and rapid sand filter are used by an agricultural company in Emmen to treat brackish groundwater to supply water for growing crops and cleaning purposes. The iron removal by the treatment plant is sufficient most of the time throughout the year. However, the farmer periodically (once per year) reported yellowish treated water, indicating insufficient iron removal. Nevertheless, it is still not clear what is the cause of the insufficiency of the iron removal. This thesis aimed to investigate the iron removal in the water treatment plant in Emmen and propose a solution to improve the iron removal. The iron removal of the treatment plant was investigated through a set of batch iron oxidation, flocculation, and filtration experiments. The pH of the water was the main parameter that influenced the oxidation of Fe(II). It was found that within the retention time that was available in the treatment plant’s tower aerator and the rapid sand filter (20 minutes), the Fe(II) was not fully oxidized and flocculated. At pH of the permeate in the range of 6 – 7, only <11.5% of the initial Fe(II) was oxidized in 20 minutes. Increasing the pH to 8 accelerated the oxidation of Fe(II), and the Fe(II) was completely oxidized within 30 minutes.Although removal of iron that is dominated by floc filtration was not achieved, the treatment plant also removed the iron through adsorption on iron hydroxide deposit in the packed tower aerator and sand particle. However, the Fe(II) adsorption capacity of adsorbents was low at low pH. The regeneration of the adsorption capacity was achieved through oxidation of the adsorbed Fe(II) which is also influenced by pH. COMSOL model showed that without regeneration, the adsorption capacity of new sand and iron oxide-coated sand was exhausted after 24 hours and 230 hours, respectively.When the concentration of CO2 in the permeate is in equilibrium with air, the pH of the permeate should be in the range of 7.9 – 8. However, the maximum pH of the permeate was 7 after aeration because the contact time of the tower aerator was not sufficient to completely strip the CO2. The tower aerator was also clogged by iron deposits that reduces the airflow and causes short-circuiting that decrease the CO2 stripping efficiency over time.Installation of a bubble column reactor was proposed to improve the CO2 stripping and increase the pH of the permeate. The Phreeqc model showed that the CO2 could be stripped until approx. 1 mg/L within 4 minutes, and the pH also increased to approx. 7.9. Moreover, the bubble column reactor will provide additional retention time for Fe(II) oxidation, and approx. 20 – 30% of the initial Fe(II) concentration can be oxidized within 4 minutes. The bubble column was considered preferable compared to the packed tower aerator because it was not susceptible to clogging by iron deposits and requires lower maintenance.