Effect of potassium permanganate dosing position on the performance of coagulation/ultrafiltration combined process

doi:10.1016/j.cjche.2017.03.037

Chinese Journal of Chemical Engineering

Volume 26, Issue 1, January 2018, Pages 89-95

https://doi.org/10.1016/j.cjche.2017.03.037 Get rights and content

Abstract

The effects of potassium permanganate (KMnO₄) dosing position on the natural organic matter (NOM) removal as well as membrane fouling were investigated in the coagulation/ultrafiltration combined process. KMnO₄ oxidation altered the NOM characteristics in terms of hydrophobicity and molecular weight, and destroyed humic substances originated from terraneous organisms in raw water. The optimal KMnO₄ dosage was 0.5 mg·L^− 1 in the peroxidation enhanced coagulation process with respect to the dissolved organic carbon (DOC) removal. When KMnO₄ was dosed into both upstream and downstream of coagulation, namely in the proposed two-position dosing mode, coagulation and KMnO₄ oxidation worked individually on the apparent DOC removal. However, compared to the KMnO₄ addition prior to or after coagulation, the two-position dosing mode dramatically alleviated membrane fouling and reduced fouling irreversibility. This was attributed to the change of NOM characteristics as a result of KMnO₄ addition prior to coagulation and the presence of MnO₂ on membrane surface as a result of KMnO₄ addition prior to ultrafiltration. This work may provide useful information for the application of KMnO₄ oxidation in the coagulation/ultrafiltration combined system.

Introduction

Drinking water safety is a worldwide public health concern. Conventional drinking water treatment process, which comprises coagulation, filtration, and disinfection in series, is very effective for turbidity removal and pathogen inactivation [1], [2]. It has been commonly applied around the world for quite a long time, and guaranteed the drinking water safety for the majority [3].

Natural organic matter (NOM) is ubiquitous in surface water and introduces undelightful color and odor problems. NOM includes the micro-organisms that are the source of infection and hence require disinfection agents to be added. Moreover, NOM is also believed to be the precursor for disinfection by-products (DBPs) of chlorination [4], [5], which are generally associated with potential causes for cancer and reproductive/developmental effects [6]. Therefore, NOM removal before disinfection is highly desired. Unfortunately, conventional coagulation is not effective in terms of NOM removal, especially for dissolved organic carbons (DOC). Enhanced coagulation, which achieves high NOM removal by lowering solution pH or increasing coagulant doses, is proposed by the United States Environmental Protection Agency [7]. However, accompanying with the improvement on NOM removal, the production of sludge is increased and the operation system is complicated.

Oxidation is a strong technique for organic pollutant degradation [8], [9], [10]. It is increasingly used in drinking water treatment to deal with the conflict between stringent regulations and poor surface water quality. Besides the strong mineralization capability, oxidation can also change the properties of NOM in water. Ferrate (VI) oxidation cleaves humus into small hydrophilic fractions at pH 7, and a high dose of ferrate (VI) (20 mg·L^− 1) preferentially oxidized organic matters with molecular weight (MW) less than 1000 [11]. Ozonation destroys organic molecules with MW higher than 3000 and produced smaller ones [12]. Compared with ozone and chlorine, potassium permanganate (KMnO₄) is a mild oxidant [13] and can markedly avoid the production of hazardous by-products. The application of KMnO₄ in drinking water treatment has received great attention nowadays [14], [15], [16]. Besides the oxidative degradation, NOM is also removed by adsorbing onto the in-situ produced MnO₂ nanoparticles as a result of KMnO₄ reduction [17]. It was reported that KMnO₄ oxidation successfully removed DOC and reduced UV₂₅₄ from sand filter effluent due to the breakdown of strong hydrophobic organic matters, and the removal of neutral organic matters was improved due to the adsorption of intermediate MnO₂ [18]. In addition, KMnO₄ oxidation can enhance the coagulation performance. MnO₂ nanoparticles produced by KMnO₄ reduction serves as coagulant seeds [19], and both the growth ability of flocs and the re-growth ability of broken flocs are consequently strengthened [14].

Ultrafiltration (UF) has been widely used in drinking water treatment thanks to its high effluent quality, high efficiency, high integrity, and low additive addition [20], [21], [22]. However, membrane fouling, which reduces the production efficiency and increases the operation cost, remains the major problem for its application [23]. NOM, including polysaccharides, proteins, and humus, is recognized as the main reason for membrane fouling in drinking water treatment. Therefore, effective removal of NOM prior to UF is a pre-requisite to overcome the membrane fouling issue [24]. Recent studies indicated that pre-oxidation with KMnO₄ significantly alleviated the UF fouling, especially for the irreversible fouling which was not removed by hydraulic backwashing. The possible mechanism involved the decline in NOM concentration, the alternation of NOM to species with less fouling capability, and the coarse and loose cake layer structure adjusted by MnO₂ nanoparticles [25]. Besides, KMnO₄ pre-oxidation also increases the membrane hydrophilicity and narrows the membrane pore size, and consequently increases the long-term stability of UF [26], [27]. In the KMnO₄/coagulation hybrid process, cake layer with porous structure can be obtained, and the filtration resistance decreases as well as the fouling irreversibility [28]. The positive effect, however, is significantly dependent on both KMnO₄ and coagulant concentrations [15].

KMnO₄ pre-oxidation as well as its assistance to coagulation has been testified to be effective for UF fouling mitigation, but little information can be obtained on the combination of these two positive approaches. In this study, a novel mode of KMnO₄ dosing was developed based on the conventional coagulation/UF process, i.e., KMnO₄ was added into both upstream and downstream of coagulation at the same time. The KMnO₄ dosage proportion between these two positions was investigated for lake water treatment in terms of effluent quality and membrane fouling. The underlying mechanism was discussed based on the alteration of NOM characteristics and the fouling layer morphology. The results may provide some insight into the application of KMnO₄ oxidation for UF system.

Section snippets

Feed water and membrane module

Raw water was collected from an artificial lake in Tianjin Polytechnic University, China. Its properties are shown in Table 1.

Polyvinylidene fluoride (PVDF) hollow fiber membrane was provided by Tianjin Motimo Membrane Co., Ltd., China. The average membrane pore size was 0.02 μm. The internal and external diameters of membrane fiber were 0.7 mm and 1.2 mm, respectively. Submerged U-shape membrane module composed of 24 fibers was homemade with an effective area of 0.036 m². Before use, new membrane

NOM removal by KMnO₄ addition prior to coagulation

KMnO₄ pre-oxidation enhanced NOM removal in the coagulation/UF process significantly (Fig. 2). Compared to the control test without KMnO₄, DOC removal increased by around 100% at a KMnO₄ dosage of 0.25 mg·L^− 1. However, further increase in KMnO₄ dosage barely changed the NOM removal. KMnO₄ oxidation prior to coagulation also markedly decreased the SUVA value from 2.26 L·mg^− 1·m^− 1 to 1.60 L·mg^− 1·m^− 1. High SUVA value of raw lake water indicated the presence of organic molecules containing abundant

Conclusions

KMnO₄ oxidation was able to alter the characteristics of NOM in lake water. The hydrophobic organic matters, such as HoN and HoA, decreased after KMnO₄ addition, and NOM species with MW between 30000 and 100000 kDa also decreased. Compared to coagulation without KMnO₄, KMnO₄ addition markedly improved the DOC removal. Even though the dosing position, either in raw water prior to coagulation or in the supernatant after coagulation, did not seem to affect the DOC concentration in the UF permeate,

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^☆: Supported by the National Natural Science Foundation of China (51478314, 51638011), the National Key Project for Research and Development Program of China (2016YFC0400503), the Natural Science Foundation of Tianjin (14JCQNJC09000), and Science and Technology Research Projects of Colleges and Universities of Hebei Province (QN2015122).

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ArticleEffect of potassium permanganate dosing position on the performance of coagulation/ultrafiltration combined process☆

Abstract

Introduction

Section snippets

Feed water and membrane module

NOM removal by KMnO4 addition prior to coagulation

Conclusions

Water Res.

Adv. Colloid Interf. Sci.

Water Sci. Technol.

Chemosphere

J. Hazard. Mater.

Chemosphere

Water Res.

Desalination

Sep. Purif. Technol.

Chem. Eng. J.

Desalination

Desalination

Desalination

Desalination

J. Membr. Sci.

Water Res.

Water Sci. Technol.

Mar. Chem.

Sep. Purif. Technol.

Desalination

Article
Effect of potassium permanganate dosing position on the performance of coagulation/ultrafiltration combined process☆

NOM removal by KMnO₄ addition prior to coagulation