Chapter 3 Membrane Processes for Water Production

Chapter 3 Membrane Processes for Water Production

Application of Membrane Processes in Water Environment Fusion Tech Hydrology Molecular biology Surface Chem Nano particles Biofilm CFD Catalyst Space station Shower water Grey water Recreation Drinking water Industrial water Ecological water Ground water recharge

Some Membrane Processes and Driving Forces

1. Pressure driven membrane processes 0.1µm 0.01µm 0.001µm No pores

Flux range and trans-membrane pressure in pressure driven membranes

2. Rejection Mechanisms in Nanofiltration During the last few decades, the drinking water industry has become increasingly concerned about the occurrence of microorganic pollutants in the source waters for the drinking water supply. 1980 s : pesticides in surface waters. : Approach to tackle the problem 1) developing alternatives for the use of pesticides 2) implementing activated carbon filtration 3) installing NF/RO 1990 s : Endocrine disrupting compounds (EDCs) and pharmaceutically active compounds (PhACs) having negative effect on the hormonal system of human and animal life <example; Estradiol, NDMA (N-nitrosodimethylamine)> : The polar compound, NDMA can not be removed by activated carbon : New approach to tackle the problem 1) installing NF/RO -

2. Rejection Mechanisms in Nanofiltration - Three major solute membrane interactions affecting removal efficiency of organic pollutants in Nanofiltartion : 1) Steric Hindrance (Sieving effect) 2) Electrostatic repulsion 3) Hydrophobic-hydrophobic/ adsorptive interactions - These solute - membrane properties are determined by i) solute properties: molecular weight/ size, charge, hydrophobicity (expressed by low K ow Values) ii) membrane properties: molecular weight cutoff (MWCO)/pore size, surface charge (zeta-potential) hydrophobicity (contact angle) iii) Operating conditions : pressure, flux, recovery iv) feed water composition: ph, temperature, DOC, inorganic balance

Three major solute membrane interactions in NF 1) Steric Hindrance (Sieving effect) - It is mainly determined by the size of solute and the size of the membrane pores. - It generally leads to a typical S-shaped curve in function of the molecular weight. (rejection vs. molecular weight of solute). - Solutes with a molecular weight higher than the MWCO of membrane are well rejected. - Solutes with a molecular weight lower than the MWCO of membrane can easily permeate through the membrane.

Three major solute membrane interactions in NF 2) Electrostatic repulsion - In the presence of electrostatic repulsion in NF, the flux may be described by extended Nernst-Planck equation. According to this eqation, the flux of charged solutes (or ions) through a charged membrane governed by several factors: i) Convection ii) Diffusion iii) Donnan potential -The effect of Donnan potential is to repel the co-ion having same charge of the fixed charge in the membrane) from th emmebrane, and because of elctroneutrality requirements, the counter-ion having opposite in charge of the fixed charge in the emnbrane) is also rejected. - This equation predicts that the solute rejection is a function of feed concentration and charge of the ion, but the equation includes the effects of convective and diffusional fluxes.

Three major solute membrane interactions in NF 3) Hydrophobic-hydrophobic/ adsorptive interactions - These interactions are important factors in the rejection of uncharged organic molecules. - Log K ow ; logarithm of the octanol-water partition coefficient log K ow <1 hydrophilic, 1 < log K ow < 2 intermediate, log K ow > 2 hydrophobic - Hydrophobic solutes adsorb more to the membrane and are thus more easily dissolved in the membrane matrix. As a result, solution and consecutive diffusion of hydrophobic solutes in the membrane matrix leads to higher permeation. - Hydrophilic molecules are better rejected compared to hydrophobic molecules of similar molecular weight. It might be explained by hydration of hydrophil molecules. When a hydrophilic is hydrated, the effective molecular size might be larger compared to a less hydrated hydrophobic molecule of the same molecular size.

3. FO, PRO, and RO FO: Forward Osmosis PRO: Pressure-Retarded Osmosis RO: reverse Osomosis

Direction and magnitude of water flux as a function of applied pressure in FO, PRO, and RO

Osmotic Pressure as a function of solution concentrations

FO

4. Membrane Distillation Figure 46-4. Schematics of direct contact membrane distillation with a microporous hydrophobic membrane

4. Membrane Distillation Definition : A thermally driven evaporation process for separating volatile solvent (or solvents) from solution on one side of a nonwetted microporous membrane. Generally the evaporated solvent is condensed or removed on the other side of the membrane. Membrane: Polytetrafluoroethylene (PTFE) Polypropylene (PP) Polyvinylidenfluoride (PVDF) Mechanism : 1) Due to the difference in water vapor pressure, water vapor will diffuse from the hot solution/membrane interface to the cold solution/membrane interface where the water vapor will condense. 2) Two liquids on two sides of membrane may be at any pressure as long as the membrane pores are not wetted by them.

4. Membrane Distillation Advantages : 1) Device can be horizontal, eliminating the need for a costly structure to support heavy columns like distillation columns. 2) Hydrophobic membrane surface reduces the possibility of precipitation of sparingly soluble inorganic salts (e.g., scaling) 3) highly compact if hollow fiber module is used. Application : ethanol recovery, seawater processing

5. Osmotic distillation Figure. Osmotic distillation with a microporous hydrophobic membrane

5. Osmotic distillation