Drinking Water Treatment

New Membrane Technology

Bingzhi Dong, Tian Li, Huaqiang Chu, Huan He, Shumin Zhu, Junxia Liu, Bingzhi Dong, Tian Li, Huaqiang Chu, Huan He, Shumin Zhu, Junxia Liu

• Publisher: De Gruyter
• Year: 2021
• ISBN:
  9783110593150
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería química y bioquímica

This book provides an up-to-date overview on the membrane technology for the drinking water treatment. The applications of PVDF-TiO2 nanowire hybrid ultrafiltration membrane, nanofiltration membrane, forward osmosis membrane, etc. in water treatment are discussed in detail. With abundant practical examples, the book is an essential reference for scientists, students and engineers in municipal engineering, environmental engineering, chemical engineering, environmental chemistry and material science.

Chapter 1 Classification and Characteristics of Organic Matter in Surface Water

Huan He

College of Environmental Science and Engineering, Tongji University, Shanghai, P. R. China

Shumin Zhu

College of Civil Engineering, Hunan University, Changsha, P. R. China

Zheng Liu

The Institute of Seawater Desalination and Multipurpose Utilization, Tianjin, P. R. China

Organic matter has a great influence on water treatment processes, especially the membrane process. There are many kinds of organic substances in natural water with different properties. They are usually categorized based on their properties, i.e. the molecular weight, hydrophobicity/hydrophilicity and fluorescence spectrum of organic matter. Organic matters with large size can be intercepted by membranes. The molecular weight of the organic matter can be used to determine the size of the organic matter and their distribution range, thereby providing a basis for the selection of the membrane and the corresponding process. Therefore, understanding the molecular weight of organic matter is an indispensable task in the membrane study. Membrane entrapment of organic matter not only depends on the size of the pores, but also its interaction with organic matter. The intensity of this interaction is closely related to the hydrophilicity and fluorescence characteristics of organic matter.

1.1 Principles and methods for determination of molecular weight of organic matter

1.1.1 Ultrafiltration membrane filtration method and gel chromatography

1.1.1.1 Principle and method of measurement

There are two main methods for determining the molecular weight distribution of organic matter, which are Gel Permeation Chromatography (GPC) and Ultrafiltration (UF) membrane filtration. In GPC method, a column is filled with a porous gel with a certain pore size distribution as a solid phase. When water flows through the gel, the organic matter with larger molecular weight in the water enters the effluent fast through the gel column because it cannot enter the gel, and the organic matter with smaller molecular weight enters the porous gel. The longer the molecular weight of the organic substance moves in the gel, the longer it takes to pass through the column. In this way, organic substances of different molecular weights pass through the gel at different time, appear in the effluent with different retention time, and achieve the purpose of separating organic substances of different molecular weights.

In the ultrafiltration membrane filtration method, an ultrafiltration membrane with known molecular weight cut-off is placed in a stirred cell with agitation and provided with the driving force from pure nitrogen for separation. The organic matter with molecular weight smaller than the molecular weight cut-off by the membrane in the water can pass through the membrane and appear in the effluent, and the organic matter with molecular weight larger than the molecular weight cut-off can be trapped by the membrane. The molecular weight distribution of organic matter can be obtained by separating the water sample with a series of different ultrafiltration membranes with known molecular weight cut-off (Figure 1.1).

Figure 1.1: Determination of molecular weight distribution by ultrafiltration membrane filtration method.

In the GPC method, some organic matter in the water can repel the ions with the gel, and pass through the gel column faster, resulting in a higher molecular weight measured; and some organic substances can adsorb the gel or induce the electrostatic interaction with the gel to hinder the movement, resulting in a smaller molecular weight measured. Moreover, pretreatment of the water sample by evaporation or freezing is required before the GPC measurement, which may change the size of the dissolved organic matter in the water, thereby affecting the analysis results. One of the advantages of GPC method is that the molecular weight distribution obtained is continuous (Figure 1.2). The UF membrane filtration method results are affected by the pore size distribution of the selected membranes, applied pressure, temperature of the water sample, pH, ionic strength, size and shape of the dissolved organic matter, and the properties of the membranes. The advantage of the UF membrane filtration method is that the analytical equipment and method adopted are simple, and a large amount of separated water samples can be obtained for further analysis. The molecular weight distribution obtained by the UF membrane filtration method is discontinuous.

Amy compared the GPC method with the UF membrane filtration method. The results showed that for the same water sample, different molecular weight distributions were exhibited in the two methods [1]. The molecular weight distribution measured with the GPC method was higher than that with the UF membrane filtration method. Since both methods are carried out using substances of given molecular weight, the molecular weight obtained is only Apparent Molecular Weight.

In recent years, in order to obtain the accurate relative molecular weight distribution of organic matter with the gel chromatography method, a variety of detectors are used to connect online. For example, Kawasaki et al. adopted a new high-performance gel chromatograph coupled with a UV detector and an NDIR total organic carbon analyzer to obtain a linear correlation between UV absorbance and NDIR total organic carbon concentration for relative molecular weight (R² > 0.99) [2]. In addition, to better detect the ultraviolet response of humus in water and prevent the interference such as nitrate, the UV wavelength is set at 260 nm. The improved HPLC-UV-TOC system can well determine the molecular weight distribution of organic matte...