Chapter 2 Membrane Processes for Water Production

Transcription

1 Chapter 2 Membrane Processes for Water Production

2 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

3 Some Membrane Processes and Driving Forces

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

5 Flux range and trans-membrane pressure in pressure driven membranes

6 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 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 -

7 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

8 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.

9 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.

10 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.

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

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

15 Osmotic Pressure as a function of solution concentrations

16 FO

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19 Applications of FO in the fields of water, energy and life science. JMS, 396, 1-21, 2012

20 The potential benefits of FO used in water treatment. Shuaifei Zhao, Linda Zou, Chuyang Y. Tang, Dennis Mulcahy Journal of Membrane Science, Volume 396, 2012, 1-21

21 Internal concentration polarization (ICP) and external concentration polarization (ECP) through an asymmetric FO membrane. ICP occurs within the membrane support layer, and ECP exists at the surface of the membrane active layer.

22 dilutive ICP and concentrative ICP across an asymmetric FO membrane.

23 ICP is one of the most important phenomena in osmotically driven membrane processes. It has been recognized that the water flux decline in FO is predominantly caused by ICP. The earliest FO studies found that ICP could reduce the water flux by more than 80%. Two types of ICP, namely dilutive ICP and concentrative ICP can occur within the membrane support layer depending on the membrane orientation as illustrated in the previous figure. When the draw solution is placed against the membrane support layer, dilutive ICP will occur within the membrane support layer as water permeates across the membrane from the feed solution to the draw solution. In the alternative membrane orientation (i.e. the feed solution facing the membrane support layer), concentrative ICP occurs as the solute in the feed solution accumulates within the membrane support layer. More critically, because ICP occurs within the support layer, it cannot be mitigated by altering hydrodynamic conditions such as increasing the flow rate or turbulence.

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26 Schematic drawing of a PRO (Pressure Retarded Osmosis) power plant.

27 Schematic drawing of a PRO power plant. The principle of power generation by PRO is illustrated in the previous figure. When concentrated seawater and diluted fresh water (i.e. river water) are separated by a semipermeable membrane, water will diffuse from the feed side into the draw solution side (i.e. seawater side) that is pressurized. The pressurized and diluted seawater is then split into two streams: one going through a hydroturbine to generate power by depressurizing the diluted seawater, and the other one passing through a pressure exchanger to assist in pressuring the seawater and thus maintaining the circulation.

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

29 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.

30 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

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

32 5. Osmotic distillation

33 Other Options for High Retention MBRs W/W H 2 O Membrane Distillation Bioreactor (MDBR) M Driving force : waste or solar heat W/W S A Q H Forward Osmosis Bioreactor (FOMBR) M X H 2 O S A Q H 33

34 Membrane Distillation Bioreactor (MDBR) ~ To achieve reasonable fluxes need to operate with raised temperature ( say > 50 C). This requires available low grade heat and thermophilic bacteria. ~ Concentration factor for retained solutes = SRT/HRT, typically 10 to 30 x. Salt tolerant bacteria required. 34

35 MDBR Performance MD (55C) + thermophilic/halotolerant biomass MDBR: Flux and organic removal efficiency Permeate TOC (mg/l), Flux (LMH) Membrane wetting Time (days) 100.0% 99.5% 99.0% 98.5% 98.0% 97.5% 97.0% 96.5% 96.0% 95.5% 95.0% Organic removal efficiency Permeate ToC Flux Overall organic removal efficiency Without membrane wetting, organic removal efficiency of MDBR remains high (> 99.5%). Goh Shuwen et al, Desalination 323 (2013) 35

36 Summary Technical challenges for MDBR Controlling salt level. Note CF = SRT/HRT Optimising thermophilic biomass. Membranes to limit fouling and/or wetting. Improving flux (hydrodynamics/module optm). Energy efficiency. 36

37 Potential Benefits of MDBR Organics retention time > HRT Recalcitrants more opportunity to be degraded. Permeate quality > MBR ~ RO quality. Primary energy < 1 kwh/m 3 (< MBR+RO). Footprint and capital cost < MBR + RO (?). 37

38 6. Pervaporation - In the pervaporation process, typically a heated liquid phase mixture containing at least two components, for example, A & B, is fed to the membrane that has a higher permeation flux, for at least one of the components in the feed mixture. For example, the membrane in the illustration below has a higher flux for B. - The term pervaporation was first introduced by Kober in a study published in 1917 in JACS.

39 6. Pervaporation - The majority of component B and a small fraction of component A permeate (pervaporate) the membrane in the vapor phase, resulting in the cooling of the feed mixture. This is due to the phase change associated with pervaporation across the membrane. - The feed mixture is typically reheated to increase the driving force, before it is directed to the next membrane module. Feed: A-B Mixture (Liquid Phase) Retentate: A-Rich (Liquid Phase-Cooler) Permeate: B-Rich (Vapor Phase)

40 6. Pervaporation - The driving force for permeation of the components is the difference in partial pressure between the feed side and the permeate side of the membrane. - Therefore, the permeate side of the membrane is maintained under vacuum. - In order to maximize the driving force for separation: 1) The feed should be heated to the highest temperature compatible with the membrane module. 2) The permeate should be cooled to the lowest possible temperature in order to maintain a deep vacuum.

41 6. Pervaporation There are three kinds of pervaporation membranes: 1) Hydrophilic Membranes Dehydration of organic-water mixtures using hydrophilic zeolite membranes. There are many organic solvents that form azeotropes with water. Using these hydrophilic membranes pervaporation can break the azeotropes in the solvents. 2) Hydrophobic Membranes Extraction of organic solvents or volatile organic compounds (VOCs) from water. 3) Organophilic Membranes Extraction of organic solvents from organic solvents. The membranes are designed to attract certain organic molecules to the membrane surface, but reject other types of molecules.