**1. Introduction**

#### **1.1. Membrane biofouling**

Pressure-driven membrane processes can be used to filter a wide range of small materials, ranging from monovalent ions and dissolved organic matter to biological substances. They have become very popular for treating sea and waste water. However, they face the problem

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of fouling on a continuous basis. Fouling is the unwanted accumulation of substances on the membrane surface. There are five types of fouling including scaling (by divalent ions), heavy metal fouling, organic fouling, colloidal fouling, and biofouling [1, 2]. Among these fouling types, biofouling is the most severe since it is a dynamic process and is also the most confronted one, and can contribute as much as 45% of the total fouling [1].

porous membrane is usually calculated by Hagen-Poseuille equation where the pores are

<sup>8</sup> \_\_\_ Δ*P*

\_\_\_\_\_ *Am*

*<sup>τ</sup>* <sup>=</sup> (<sup>2</sup> <sup>−</sup> *<sup>ε</sup>*)<sup>2</sup> \_\_\_\_\_ *<sup>ε</sup>* (3)

Matin et al. Provided a list of typical bacteria species that can cause biofilm formation on the membrane surface as well as a reduction in flux decline and salt rejection due to the formation of biofilm on the membrane surface [25]. They observed that, without bacterial adhesion, the membrane was able to reject (R) 98.2% salt. The rejection was decreased by 4.6% because of

Biofilm is a complex structure due to the viscoelastic nature of EPS that can lead to the formation of memory effect in a material [10, 26]. Rheological measurement of the biofouling layer on the membrane surface is required to understand the EPS nature. Patsios et al. [27] performed some rheological measurements of the biofouling layer on the membrane. They obtained nonlinear behavior of shear stress and strain of the EPS. They claimed that EPS shows more elastic nature than viscous on the membrane surface. The storage modulus Gˊ, the elastic part, was higher than the loss modulus Gˊˊ that is the viscous component. [25].

In wastewater treatment, microfiltration membranes with the pore sizes lying between 0.1 and 10 μm are used to remove bacteria. Membranes are usually part of an opaque setup, where only the input and the output can be measured. Advancements in micro-/nano-technologies, for example, microfluidic devices can be employed to study membrane processes at the porescale. An example of this is the use of microfluidic-based membrane mimics, which are being used to explore a wide variety of membrane related issues, including biofouling. An essential advantage of microfluidic membrane mimics in studying biofouling is that they make realtime microscopy of biofouling possible. **Figure 1** shows a basic schematic difference between membrane filtration mode and microfluidic approach. The pillars are shown in **Figure 1b** are solid in structures and usually made of polydimethylsiloxane (PDMS). The gap between the

pillars is considered as the pore. The coverslip is used to seal the device.

Where, Δ*x* is the membrane thickness, Δ*P* is pressure difference across the membrane, *η* is the viscosity, *τ* is tortuosity, *r* is the radius of the pore, and *ε* is the porosity of the membrane.

<sup>Δ</sup>*<sup>x</sup>* (1)

Microfluidic Membrane Filtration Systems to Study Biofouling

http://dx.doi.org/10.5772/intechopen.75006

is the number of pores. Tortuosity is defined by:

(2)

295

assumed to have the same radius,

Porosity can be calculated by,

*Am* is the membrane surface area and *nP*

the biofouling on the membrane.

**2.2. Microfluidic approach in biofouling study**

*J* = *<sup>ε</sup> <sup>r</sup>* <sup>2</sup> \_\_\_

*<sup>ε</sup>* <sup>=</sup> *nP <sup>π</sup><sup>r</sup>* <sup>2</sup>

Biofouling due to biofilms (matrix-encapsulated bacterial colonies) and colloidal materials act as the main components of membrane fouling [2]. Moreover, biofilms have a significant impact on the membranes used for different types of water filtration such as brackish and seawater. Once a cell is attached to the membrane surface, it decreases membrane permeability by forming a gel layer [3]. Biological substances always remain in the membrane. Even if 99.9% of these materials are removed by pre-treatment, the remaining 0.1% can grow exponentially by using biodegradable substances in the feed (waste) water [4].
