ENVIRONMENTAL ENGINEERING SCIENCE Volume 33, Number 4, 2016 ª Mary Ann Liebert, Inc. DOI: 10.1089/ees.2015.0369
Coupling Technique for Deep Removal of Manganese and Iron from Potable Water Lei Chen,1 Junjie Zhang,1 and Xilai Zheng2,* 1
College of Environmental Science and Engineering, Qilu University of Technology, Jinan, China. Key Laboratory of Marine Environmental Science and Ecology, Ministry of Education of China, Ocean University of China, Qingdao, China.
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Received: September 16, 2015
Accepted in revised form: January 23, 2016
Abstract
Although many treatment methods have been employed to remove soluble Fe(II) and Mn(II) from water, for some smaller water treatment plants or under emergent circumstances such as sharp increase of iron and manganese concentrations in source water during summer, traditional physicochemical removal methods are more flexible and effective. In this article, a coupling technique of chlorine dioxide (ClO2) preoxidation and manganese sand filtration for iron and manganese removal from potable water have been proposed. Fe(II) and Mn(II) removal efficiencies of different filter materials (natural manganese sand, quartz sand, and fiber bundle filter (FBF) material) were compared. Influences of ClO2 dosage, liquid flow rate, particle size of filter material, and thickness of filter bed on removal efficiency were also examined. In addition, performance of ClO2 preoxidation–manganese sand filter technique was investigated under the optimum operating conditions to remove Fe(II) and Mn(II). Results indicated that natural manganese sand filter showed higher capability of removing Fe(II) and Mn(II) than quartz sand filter and FBF. It was found that manganese removal efficiencies increased with higher dosage of preoxidant, lower liquid flow rate, smaller sand particle diameter, and thicker filter bed. When initial concentrations of Mn(II) and Fe(II) in feed water were 1.5 and 1.0 mg/L, respectively, ClO2 dosage was 0.99 mg/L, liquid flow rate was 282.6 mL/min, sand particle diameter was 0.6–1.2 mm, and filter bed thickness was 60 cm, the process showed good performance of Fe(II) and Mn(II) removal with both removal rates more than 95%. Key words: ClO2; iron removal; manganese removal; manganese sand filter; preoxidation Introduction
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ron and manganese are two important life elements for human beings. However, excessive intake of iron and manganese is very harmful to human body health. Besides, superfluous manganese in source water also brings enormous loss to manufacturing production (i.e., textile and papermaking industry). As a result, there are strict limits for the amounts of iron and manganese in source water, which are 0.3 and 0.1 mg/L (State Environmental Protection Administration of China, 2002), respectively, in China. Up to now, many treatment methods have been developed and employed to remove soluble ferrous iron (Fe(II)) and dissolved Mn(II) from water, mainly including chemical oxidation technology (Graveland and Heertjes, 1975; Wong, 1984), biological treatment technology (Vandenabeele et al., 1992; Tekerlekopoulou and Vayenas, 2008; Qin et al., 2009), sand filtration (He et al., 2010; Tiwari et al., 2011), membrane filtration (Kabsch-Korbutowicz and Winnicki, 1996;
*Corresponding author: Key Laboratory of Marine Environmental Science and Ecology, Ministry of Education of China, Ocean University of China, Qingdao 266100, China. Phone: +86 532 66781759; Fax: +86 532 66781759. E-mail:
[email protected]
Molinari et al., 2001; Teng et al., 2001; Lastra et al., 2004), and various combinations of them (Choo et al., 2005; Liu et al., 2009; Lin et al., 2012). Needless to say, each removal technology more or less has its own advantages and disadvantages. During recent years, researchers have focused on the technology of biological treatment and membrane filtration to remove Fe(II) and Mn(II) from potable water due to their high removal efficiency. Nevertheless, for some smaller water treatment plants or under emergent circumstances such as sharp increase of iron and manganese concentrations in source water during summer, the traditional physicochemical removal methods are more flexible and effective. Iron can be oxidized at low redox conditions (400 mV) for neutral pH (Mouchet, 1992). Thus, abiotic homogenous manganese oxidation by oxygen is very slow at pH values below 9, not to mention in most drinking waters (pH 6–8) (Stumm and Morgan, 1996; Katsoyiannis and Zouboulis, 2004). The conventional oxidants used for iron and manganese oxidation in water treatment are chlorine, chlorine dioxide (ClO2), hypochlorite, ozone, or potassium permanganate. ClO2 is used mainly as preoxidant and disinfectant in water treatment (Long et al., 1999). He et al. (2009) reported that ClO2
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and KMnO4 both showed effective Mn(II) removal capabilities, with removal rates more than 96%. Zhang et al. (2003) compared the Mn(II) removal rates of ClO2 and Cl2 under the same weight dosage and found that the former was 40% higher than the latter. Furthermore, the application of manganese removal by ozone is limited due to its complex operational process and high cost. After Fe(II) and Mn(II) are oxidized to their higher oxides, they can be removed by either clarification or sand filtration, or both. Manganese sand is made of natural manganese ores and widely used as water treatment filter media in China due to its high mechanical strength, active surface property, large specific surface area, and porosity (Wu et al., 2011). However, some scholars pointed out that manganese could not be removed by solely adopting manganese sand filters (He et al., 2009; Qin et al., 2009). Consequently, it is better to use the manganese sand filter in conjunction with preoxidation or other treatment technologies. There are a few published studies concerning the coupling technique of ClO2 preoxidation and manganese sand filtration (He et al., 2009; Li, 2011). However, most of them focused on the removal of high Mn concentrations (>5 mg/L) of groundwater and the application of this coupling technique in potable water or raw water with low Mn concentrations is few. Furthermore, the addition of ClO2 can promote the ‘‘Mn sand self-catalytic oxidization’’ process, which was proposed by Li and Liu (1987), thereby improving Mn and Fe removal rate. Therefore, it is important to study the reasonable operating parameters of the coupling technique on the basis of ‘‘Mn sand self-catalytic oxidization’’ theory to save chemical cost. Wangjuan Resevoir locates in Jimo, China, which is an important source of freshwater for local urban water supply, agricultural irrigation, and aquaculture. During recent years, the dramatical increase of iron and manganese concentrations in reservoir water during summer brings great challenges for the local water treatment plant. Our multiple investigation results showed that the maximum values of iron and manganese concentrations of the reservoir were 0.75 and 1.28 mg/ L, respectively. Through the currently employed treatment process, KMnO4 is used as the preoxidant to oxidize Fe(II) and Mn(II), followed with fibrous bundle media filtration. However, the treatment performance of iron and manganese is unsatisfactory and the effluent concentration of Mn(II) is more than 0.1 mg/L. Therefore, the aim of this work was to study Fe(II) and Mn(II) removal from potable water using ClO2 preoxidation– manganese sand filter technique and solve the key water quality problem of Wangjuan Reservoir. The Fe(II) and Mn(II) removal efficiencies of different filter materials (natural manganese sand, quartz sand, and fiber bundle filter (FBF) material) were compared. The influences of ClO2 dosage, liquid flow rate, particle size of filter material, and thickness of filter bed on Fe(II) and Mn(II) removal efficiency were also examined. Finally, experiments were conducted to investigate Fe(II) and Mn(II) removal performance of ClO2 preoxidation–manganese sand filter technique under the optimum operating conditions. Materials and Methods Reagents and materials
Feed water was a mixture of solutions of iron (in the form of FeSO4$7H2O) and manganese (in the form of MnSO4$H2O)
CHEN ET AL.
Table 1.
Main Characteristics of Quartz Sand and Natural Manganese Sand
Characteristics
Quartz sand
Manganese sand
SiO2 content (%) MnO2 content (%) Porosity (%) Specific surface area (m2/g) Silt-carrying capacity (%) Density (g/cm3) Bulk density (g/cm3) Solubility in hydrochloric acid (%) Attrition rate (%)
‡93.6 — 45 4.21 0.64 2.66 1.65 0.2
