Lecture 11 Water pollution control by membrane based technologies

Transcription

1 Lecture 11 Water pollution control by membrane based technologies

2 MEMBRANE Membrane can be described as a thin layer of material that is capable of separating materials as a function of their physical and chemical properties when a driving force is applied across the membranes. Physically membrane could be solid or liquid. In membrane separation processes, the influent to the membrane module is known as the feed stream (also known as the feed water), the liquid that passes through the semipermeable membrane is known as permeate (also known as the product stream or permeating stream) and the liquid containing the retained constituents is known as the concentrate also known as retained phase. MEMBRANE PROCESS CLASSIFICATION Membrane processes can be classified in a number of different ways [1]: The type of material from which the membrane is made The nature of the driving force The separation mechanism The nominal size of the separation achieved Table General characteristics of membrane processes [2] Membrane process Driving force Method of separation Operating structure Typical operatin Permeate descriptio Range of application (pore size) g range, µm n Microfiltration Hydrostatic Sieving mechanism Macropores (>50 nm) dissolved solutes Sterile filtration clarification Ultrafiltration Hydrostatic Sieving mechanism Mesopores (2-50 nm) molecules Separation of macromolecu lar solutions Nanofiltration Hydrostatic Sieving mechanism + solution/diffu sion Micropores (<2 nm) very molecules, ionic Removal of molecules,

3 solutes harness, viruses Reverse Hydrostatic Solution Dense (< Separation of osmosis diffusion nm) salts and mechanism + molecules microsolutes exclusion from solutions Dialysis Concentrati Diffusion in Mesopores - Separation of on gradient convection (2-50 nm) ionic salts and free layer solutes microsolutes from macromolecu lar solutions Electrodialysis Electrical Electrical Micropores - Desalting of potential charge of (<2 nm) ionic solution gradient particle and size Table Advantages & disadvantages of membrane technologies [1, 3, 4]. Advantages Disadvantages Microfiltration and ultrafiltration Can reduce the amount of treatment Uses more electricity; high- chemicals systems can be energy-intensive Smaller space requirements May need pretreatment to prevent (footprint); membrane equipment fouling; pretreatment facilities requires 50 to 80 percent less space increase space needs and overall costs than conventional plants Reduced labour requirements; can be May require residuals handling and automated easily disposal of concentrate New membrane design allows use of Require replacement of membranes

4 lower s; system cost may be competitive with conventional wastewater-treatment processes Remove protozoan cysts, oocysts, and helminth ova; may also remove limited amounts of bacteria and viruses Reverse osmosis Can remove dissolved constituents Can disinfect treated water Can remove NDMA and other related organic compounds Can remove natural organic matter (a disinfection by-product precursor) and inorganic matter about every 3 to 5 years Scale formation can be a serious problem. Scale-forming potential difficult to predict without field testing Flux rate (the rate of feedwater flow through the membrane) gradually declines over time. Recovery rates may be considerably less than 100 percent Lack of a reliable low-cost method of monitoring performance Works best on ground water or low solids surface water or pretreated wastewater effluent Lack of a reliable low-cost method of monitoring performance May require residuals handling and disposal of concentrate Expensive compared to conventional treatment MEMBRANE MATERIALS & CONFIGURATIONS Membranes can be made from a number of different organic and inorganic materials. The membranes used for wastewater treatment are typically organic. The principle types of membranes used include polypropylene, cellulose acetate, aromatic polyamides, and thinfilm composite (TFC).

5 Membranes used for the treatment of water and wastewater typically consist of a thin skin having a thickness of about 0.20 to 0.25 µm supported by a more porous structure of about 100 µm in thickness. Term module is used to describe a complete unit comprised of the membranes, the support structure for the membranes, the feed inlet and outlet permeate and retentate ports, and an overall support structure. The principle types of membrane modules used for wastewater treatment are 1) tubular, 2) spiral wound, 3) hollw fibre,4) flat. Table Comparison of different membrane configurations [5] Membrane geometry Suspended solids tolerance Control of fouling Cleaning easiness Packing density Cost for unit of volume Tubular Good Excellent Excellent Lowmedium Mediumhigh Spiral-wound Low Limited Medium High Low Hollow fibre (external feed) Scant (good) Scant (good) Scant (good) Excellent High (low) Flat Medium Good Medium Medium Medium-low MEMBRANE FOULING Membranes can be seen as sieves retaining part of the feed. As a consequence, deposits of the retained material will accumulate at the feed side of the membrane. In time this might hamper the selectivity and productivity of the separation process. This process is called fouling. koros et al gave the definition of fouling as The process resulting in loss of performance of a membrane due to deposition of suspended or dissolved substances on its external surfaces, at its pore openings, or within its pores. Membrane fouling is an important consideration in the

6 design and operation of membrane systems as it affects pretreatment needs, cleaning requirements, operating conditions, cost, and performance [6]. Three approaches are used to control membrane fouling: 1) Pretreatment of the feed water: pretreatment is used to reduce the TSS and bacterial content of the feed water 2) Membrane backflushing: to eliminate the accumulated material from the membrane surface with water and/or air. 3) Chemical cleaning of the membranes: Chemical treatment is used to remove constituents that are not removed during conventional backwashing. Chemical precipitates can be removed by altering the chemistry of the feed water and by chemical treatment. REFERENCES [1] Medaware, Development of Tools and Guidelines for the Promotion of the Sustainable Urban Wastewater Treatment and Reuse in the Agricultural Production in the Mediterranean Countries. Task 4: Urban Wastewater Treatment Technologies, Part I. European Commission: Euro-Mediterranean Partnership, ME8/AIDCO/2001/0515/59341-P033, December ( [2] Environmental Engineers Hand Book, CRC Press LLC, 2000 Corporate Blvd., N.W., Boca Raton, FL [3] EPRI Community Environmental Centre. Membrane technologies for water and wastewater treatment, [4] Metcalf & Eddy, Wastewater Engineering, 4 th edition. [5] Bottino, A., Capannelli, G., Comite, A., Ferrari, F., Firpo, R., Venzano, SMembrane technologies for water treatment and agroindustrial sectors. Comptes rendus Chimie, 2009, 12 (8), [6] Pandey, S. R., Jegatheesan, V., Baskaran, K., Shu, L. Fouling in reverse osmosis (RO) membrane in water recovery from secondary effluent: a review. Reviews in Environmental Science and Bio/Technology, 2012, 11(2),