Iron and manganese removal from water sources is important for drinking and both domestic and industrial uses. The formation of MnO2, even at a concentration of 0.2 mg/L of manganese, causes the formation of black sludge in the inner walls of the pipe. According to the US Environmental Protection Agency (EPA) and European Union (EU) regulations, the allowed value for manganese is 0.05 mg/L . The presence of dissolved, colloidal, and particulate iron and manganese in water varies greatly depending on the ambient pH and the amount of dissolved oxygen. The presence of organic matter and various anions in the environment are important factors that determine the type of iron and manganese oxide formed by aeration and its conversion over time [2,3,4]. Iron oxide minerals have a high specific surface area (>100 m2/g). Similarly, manganese oxide flocs have a large surface area. Therefore, they are effective adsorbents for many dissolved ions, molecules, and gases.
Various technologies are enriched and used in iron and manganese removal. Ion exchange, biological trickling filter, reverse osmosis, nanofiltration, and aeration are some of these methods . In studies on the treatment of iron and manganese with aeration, it is stated that the reaction accelerated with the addition of Fe(OH)3; in other words, Fe(OH)3 flocs had a catalytic effect on Fe2+ oxidation . O’Connor claimed that in the majority of iron and manganese removal facilities in the USA, aeration, retention tank/settling, and filtration are widely applied. They explained that Fe(OH)3 flock has a very high capacity to adsorb Fe2+, and this is explained by iron removal in contact filters as well as inside the filters, where the filter medium is covered with Fe(OH)3. They also stated that an aging process is required in the filter for the flocs that will replace the precipitate . Takai pointed out that among many iron oxides, only γ-FeOOH is an effective catalyst . Andersen et al. revealed that catalysts play an important role in the oxidation of iron and manganese . They explained this by increasing the efficiency of multiple treatment plants after the formation of oxidized iron and manganese in an aeration or filter medium. The study of Coughlin and Matsui handled higher initial Mn2+ concentrations and revealed the catalytic effect of manganese oxides formed by the aeration on Mn2+ oxidation . They also stated that manganese oxides catalyze the removal of Mn2+ by aeration and the increase in removal cannot be explained only by adhering to the manganese oxide surface. Sung investigated the effect of iron oxides on the removal of Mn2+ by aeration . Accordingly, it is determined that iron oxide is a catalyst as effective as manganese oxide. Davies and Morgan stated that Mn2+ oxidation is faster in the presence of goethite (α-FeOOH) than in the presence of lepidocrocite (γ-FeOOH) or silicon oxide . Tüfekci and Sarikaya observed that the catalytic effect of Fe3+ increased up to 600 mg/L and beyond this value, Fe3+ did not have a significant catalytic effect on the oxidation of Fe2+ . In addition, it is observed that the catalytic effect increased up to three days with the aging of Fe(OH)3 sludge. It has been expressed that the increase of the catalytic effect with the aging of the sludge may cause the reaction of Fe2+ radicals to be accelerated in the reaction of Fe2+ with oxygen, and that any of the different structural forms of iron oxides can be effective in this reaction rate. Similar catalytic effects were observed up to 700 mg/L concentration for MnO2 for the oxidation rate of Mn2+ . In the study by Ormancı et al., it is pointed out that MnO2 accelerated the Mn2+ oxidation up to 800 mg/L and that there is no significant effect beyond this value . In the study of Gunes Durak et. al., it is stated that the catalytic effect increased up to four days with the aging of MnO2 sludge . It is emphasized in the study conducted by Celik, although the removal of Mn2+ with aeration is quite slow at pH = 8.5, Mn2+ removal efficiency increased significantly if Fe(OH)3 and/or MnO2 is added . Similar results were obtained by Ormanci and Turkoglu [18,19]. In the study conducted by Cheng, it is found that when dissolved oxygen is sufficient, iron and manganese are completely removed from the solution . When dissolved oxygen is below 3 mg/L, only iron is removed, while manganese remained in solution. Various aeration systems are used in four different plans by Štembal et al. . Dissolved oxygen ranges from 8-17 mg/L values. Groundwater iron concentrations used in the study were 0.98-2.45 mg/L. After treatment, the iron is reduced to a standard value of 0.3 mg/L in the filter at a depth of 0.8 m (Table 1).
The most important issue for iron and manganese removal in membrane systems is that the selected membrane is below the particle size of iron and manganese so that it can function to hold the particles. However, when compared with ceramic membranes, polymeric membranes provide up to 100% iron and manganese removal . The particle size of Fe2+ and Mn2+ in dissolved form is too small to be kept by microfiltration and ultrafiltration. Ion exchanger, ultrafiltration (UF) membrane is tried for Fe2+ and Mn2+ removal, but it is determined that more than 74% of the parts passed through the membrane. Again, nanofiltration and reverse osmosis membrane are tried for Fe2+ and Mn2+ removal, and it is determined that reverse osmosis membrane did not meet the standards, although it provides better Mn2+ removal. Therefore, an oxidation process is essential before the membrane. While this oxidation process can be just simple aeration for Fe2+ at natural water pH, it requires a stronger oxidant for Mn2+ removal. Strong oxidants such as chlorine derivatives, potassium permanganate or ozone should be applied as oxidants. Iron and manganese oxides formed by oxidation can also contribute to the removal of turbidity and other pollutants, as they contain other pollutant particles in the water. In the work of Choo et al., while iron oxide particles do not cause fouling in the membrane, ultrafiltration is not sufficient for oxidized manganese particles and caused significant fouling in the ultrafiltration membrane . The membrane used in the study is cellulose acetate and it has 100 kDa MWCO. In a study conducted by Kan et. al., microfiltration is applied following NaOCl oxidation for Fe2+ and Mn2+ removal . Oxidized metal ion particles are examined with a particle counter. In the study, manganese values are reduced below the standards after two weeks of application. According to the results, it is concluded that the iron and manganese oxide layer deposited on the membrane surface had an important role in manganese removal. The membrane used in the study is made of PTFE material whose surface has been treated with a hydrophilic polymer. In their studies where Yu et al. compared the fouling properties of the PVDF membrane coated and uncoated with MnO2 nanoparticles, they determined that the membrane coated with MnO2 is less fouled, while the uncoated membrane is exposed to both recyclable and irreversible fouling . According to the results of the work of Celik, iron oxides were found more effective than manganese oxides to remove Fe2+ and Mn2+ in both aeration and aerated-submerged membrane systems, and that significant iron and manganese removal efficiencies were obtained if both oxides were present in the solution . Iron oxides also provided significant iron, manganese, and TOC removal efficiencies. Based on this, she stated that iron oxides increase the lifetime of the membrane and that it can be recycled by recycling or chemical cleaning rather than irreversible fouling. As a result of the study, it has been determined that Fe(OH)3 increases Fe2+ and Mn2+ removal efficiency through adsorption/oxidation on the surface, and also that the flocs it produces grows beyond the membrane and cause an increase in membrane productivity. Similarly, it is stated that Fe(OH)3 caused a decrease in pressure increase, which is an indicator of membrane fouling and an increase in removal efficiency  (Table 1).
When Table 1 is examined, there is an increase in the removal efficiency when catalysts such as MnO2, FeO, α-FeOOH, and Fe(OH)3 are used for the aeration method in iron and manganese removal. Membrane filtration method enriched with oxidants also provides high removal efficiency in treatment.
In this study, Fe2+ and Mn2+ removal by aeration and the aerated-submerged membrane systems were investigated experimentally. MnO2 and Fe(OH)3 were used as catalysts in order to remove iron and manganese in the aeration method. The pH was adjusted as 6.5 for iron removal and 9.2 for manganese removal. According to these values, the effects of different doses of oxidants on the oxidation time were determined. In the ventilated submerged membrane method, Fe2+, Mn2+, and Fe2+-Mn2+ removal were investigated with plate-type polyethersulfone (PES) and hollow fiber polypropylene (PP) membranes. The flux performance and fouling resistance of the membranes were determined. Clean membranes and after the Fe2+, Mn2+, and Fe2+-Mn2+ experiments the contaminated membranes were characterized by FT/IR and SEM, and the effect of iron and manganese on membrane fouling was determined.