Editing Microfiltration (section)

==Process design variables==

* Operation and control of all processes in the system
* Materials of construction
* Equipment and instrumentation ([[controller (control theory)|controller]]s, [[sensors]]) and their cost.

===Fundamental design heuristics===

A few important design heuristics and their assessment are discussed below:

* When treating raw contaminated fluids, hard sharp materials can wear and tear the porous cavities in the micro-filter, rendering it ineffective. Liquids must be subjected to pre-treatment before passage through the micro-filter.<ref>Water Treatment Solutions. 1998, Lenntech, accessed 27 September 2013 < http://www.lenntech.com/microfiltration.htm</ref>  This may be achieved by a variation of macro separation processes such as [[Mechanical screening|screening]], or granular media filtration.
* When undertaking cleaning regimes the membrane must not dry out once it has been contacted by the process stream.<ref>Cheryan, M 1998, ''Fouling and Cleaning''. 2nd edn. ''Ultrafiltration and Microfiltration Handbook'', CRC Press, Florida p. 237-278</ref>  Thorough water rinsing of the membrane modules, pipelines, pumps and other unit connections should be carried out until the end water appears clean.
* Microfiltration modules are typically set to operate at pressures of 100 to 400 kPa.<ref>Baker, R 2012, ''Microfiltration, in Membrane Technology and Applications'', 3rd edn, John Wiley & Sons Ltd, California p. 303-324</ref> Such pressures allow removal of materials such as sand, slits and clays, and also bacteria and protozoa.
* When the membrane modules are being used for the first time, i.e. during plant start-up, conditions need to be well devised. Generally a slow-start is required when the feed is introduced into the modules, since even slight perturbations above the critical flux will result in irreversible fouling.<ref>Cheryan, M 1998, ''Fouling and Cleaning.'' 2nd ed. ''Ultrafiltration and Microfiltration Handbook'', CRC Press, Florida p 237-278</ref>

Like any other membranes, microfiltration membranes are prone to fouling. ''(See Figure 4 below)'' It is therefore necessary that regular maintenance be carried out to prolong the life of the membrane module.

* Routine '[[backwashing]]', is used to achieve this. Depending on the specific application of the membrane, backwashing is carried out in short durations (typically 3 to 180 s) and in moderately frequent intervals (5 min to several hours). Turbulent flow conditions with Reynolds numbers greater than 2100, ideally between 3000 - 5000 should be used.<ref>Cheryan, M 1998, ''Fouling and Cleaning. in Ultrafiltration and Microfiltration Handbook'' 2nd edn., CRC Press, Florida, p. 237-278</ref>  This should not however be confused with 'backflushing', a more rigorous and thorough cleaning technique, commonly practiced in cases of particulate and colloidal fouling.
* When major cleaning is needed to remove [[Entrainment (engineering)|entrained]] particles, a CIP (Clean In Place) technique is used.<ref>Baker, R 2012, ''Microfiltration, in Membrane Technology and Applications'', 3rd edn, John Wiley & Sons Ltd, California. pp. 303–324</ref>  Cleaning agents/[[detergent]]s, such as [[sodium hypochlorite]], [[citric acid]], [[caustic soda]] or even special enzymes are typically used for this purpose. The concentration of these chemicals is dependent on the type of the membrane (its sensitivity to strong chemicals), but also the type of matter (e.g. scaling due to the presence of calcium ions) to be removed.
* Another method to increase the lifespan of the membrane may be feasible to design two microfiltration membranes in [[wikt:Special:Search/series|series]]. The first filter would be used for pre-treatment of the liquid passing through the membrane, where larger particles and deposits are captured on the cartridge. The second filter would act as an extra "check" for particles which are able to pass through the first membrane as well as provide screening for particles on the lower spectrum of the range.<ref>Baker, R 2000, Microfiltration, in Membrane Technology and Applications,  John Wiley & Sons Ltd, California. p. 280</ref>

===Design economics===

The cost to design and manufacture a membrane per unit of area are about 20% less compared to the early 1990s and in a general sense are constantly declining.<ref>Mullenberg 2009, 'Microfiltration: How Does it compare, Water and wastes digest, web log post, December 28, 2000, accessed 3 October 2013,<http://www.wwdmag.com/desalination/microfiltration-how-does-it-compare.></ref>  Microfiltration membranes are more advantageous in comparison to conventional systems. Microfiltration systems do not require expensive extraneous equipment such as flocculates, addition of chemicals, flash mixers, settling and filter basins.<ref>Layson A, 2003, Microfiltration – Current Know-how and Future Directions, IMSTEC, accessed 1 October 2013 {{cite web |url=http://www.ceic.unsw.edu.au/centers/membrane/imstec03/content/papers/MFUF/imstec152.pdf |title=Archived copy |access-date=2013-10-15 |url-status=dead |archive-url=https://web.archive.org/web/20131015111520/http://www.ceic.unsw.edu.au/centers/membrane/imstec03/content/papers/MFUF/imstec152.pdf |archive-date=2013-10-15 }}> University of New South Wales. p6</ref>  However the cost of replacement of capital equipment costs (membrane cartridge filters etc.) might still be relatively high as the equipment may be manufactured specific to the application. Using the design heuristics and general plant design principles (mentioned above), the membrane life-span can be increased to reduce these costs.

Through the design of more intelligent process control systems and efficient plant designs some general tips to reduce [[operating costs]] are listed below<ref>Layson A, 2003, Microfiltration – Current Know-how and Future Directions, IMSTEC, accessed 1 October 2013 <{{cite web |url=http://www.ceic.unsw.edu.au/centers/membrane/imstec03/content/papers/MFUF/imstec152.pdf |title=Archived copy |access-date=2013-10-15 |url-status=dead |archive-url=https://web.archive.org/web/20131015111520/http://www.ceic.unsw.edu.au/centers/membrane/imstec03/content/papers/MFUF/imstec152.pdf |archive-date=2013-10-15 }}> University of New South Wales. p6</ref>

* Running plants at reduced fluxes or pressures at low load periods (winter)
* Taking plant systems off-line for short periods when the feed conditions are extreme.
* A short shutdown period (approximately 1 hour) during the first flush of a river after rainfall (in water treatment applications) to reduce cleaning costs in the initial period.
* The use of more cost effective cleaning chemicals where suitable (sulphuric acid instead of citric/ phosphoric acids.)
* The use of a flexible control design system. Operators are able to manipulate variables and setpoints to achieve maximum cost savings.

Table 1 (below) expresses an indicative guide of membrane filtration capital and operating costs per unit of flow.

{| class="wikitable"
|-
! Parameter !! Amount !! Amount !! Amount !! Amount !! Amount
|-
| Design Flow (mg/d)|| 0.01|| 0.1 || 1.0|| 10 || 100
|-
| Average Flow (mg/d) || 0.005 || 0.03 || 0.35|| 4.4 || 50
|-
| Capital Cost ($/gal) || $18.00 || $4.30 || $1.60 || $1.10 || $0.85
|-
| Annual Operating and Managing Costs ($/kgal) || $4.25 || $1.10 || $0.60 || $0.30 || $0.25
|}

Table 1 Approximate Costing of Membrane Filtration per unit of flow<ref>Microfiltration/Ultrafiltration, 2009, Water Research Foundation, accessed 26 September 2013; <{{cite web |url=http://www.simultaneouscompliancetool.org/SCToolSmall/jsp/modules/welcome/documents/TECH7.pdf |title=Archived copy |access-date=2013-10-15 |url-status=dead |archive-url=https://web.archive.org/web/20140309015714/http://www.simultaneouscompliancetool.org/SCToolSmall/jsp/modules/welcome/documents/TECH7.pdf |archive-date=2014-03-09 }}></ref>

Note:

* ''Capital Costs are based on dollars per gallon of the treatment plant capacity''
* Design flow is measured in millions of gallons per day.
* Membrane Costs only (No Pre-Treatment or Post-Treatment equipment considered in this table)
* Operating and Annual costs, are based on dollars per thousand gallons treated.
* All prices are in US dollars current of 2009, and is not adjusted for inflation.''

===Process equipment===

====Membrane materials====

The materials which constitute the membranes used in microfiltration systems may be either organic or inorganic depending upon the contaminants that are desired to be removed, or the type of application.

* Organic membranes are made using a diverse range of polymers including [[cellulose acetate]] (CA), [[polysulfone]], [[polyvinylidene fluoride]], [[polyethersulfone]] and [[polyamide]]. These are most commonly used due to their flexibility, and chemical properties.<ref name="Perry, RH 2007"/>
* Inorganic membranes are usually composed of [[sintered]] metal or porous [[alumina]]. They are able to be designed in various shapes, with a range of average pore sizes and permeability.<ref name="Perry, RH 2007"/>

====Membrane structures====

General Membrane structures for microfiltration include

* [[Screen filter]]s (Particles and matter which are of the same size or larger than the screen openings are retained by the process and are collected on the surface of the screen)
* [[Depth filter]]s (Matter and particles are embedded within the constrictions within the filter media, the filter surface contains larger particles, smaller particles are captured in a narrower and deeper section of the filter media.)

====Membrane modules====
[[File:Cutaway of a microfiltration module with hollow fiber membranes at a NEWater plant.jpg|thumb|Cutaway of a microfiltration module with [[hollow fiber membrane]]s]]

;Plate and frame (flat sheet)
Membrane modules for dead-end flow microfiltration are mainly plate-and-frame configurations. They possess a flat and thin-film composite sheet where the plate is asymmetric. A thin selective skin is supported on a thicker layer that has larger pores. These systems are compact and possess a sturdy design, Compared to cross-flow filtration, plate and frame configurations possess a reduced capital expenditure; however the operating costs will be higher. The uses of plate and frame modules are most applicable for smaller and simpler scale applications (laboratory) which filter dilute solutions.<ref name="Seadler 2006, p.503">Seadler, J & Henley, E 2006, ''Separation Process Principles'', 2nd Edn,  John Wiley & Sons Inc. New Jersey p.503</ref>

;Spiral-wound
This particular design is used for cross-flow filtration. The design involves a [[pleated]] membrane which is folded around a [[perforation|perforated]] permeate core, akin to a spiral, that is usually placed within a pressure vessel. This particular design is preferred when the solutions handled is heavily concentrated and in conditions of high temperatures and extreme [[pH]]. This particular configuration is generally used in more large scale industrial applications of microfiltration.<ref name="Seadler 2006, p.503"/>

;Hollow fiber
This design involves bundling several hundred to several thousand [[hollow fiber membrane]]s in a tube filter housing. Feed water is delivered into the membrane module. It passes through from the outside surface of the hollow fibers and the filtered water exits through the center of the fibers. With the flux rate in excess of 75 gallon per square foot per day, this design can be used for large scale facilities.<ref>{{cite book |title=Water treatment. |date=2003 |publisher=American Water Works Association |location=Denver, CO |isbn=9781583212301 |pages=441–444 |edition=3rd |url=https://books.google.com/books?id=WO6A_4JAdVsC&pg=PA441 |access-date=14 November 2021}}</ref>

===Fundamental design equations===

As separation is achieved by sieving, the principal mechanism of transfer for microfiltration through micro porous membranes is bulk flow.<ref>Seadler, J & Henley, E 2006, ''Separation Process Principles'', 2nd Edn,  John Wiley & Sons Inc. New Jersey p.540-542</ref>

Generally, due to the small diameter of the pores the flow within the process is laminar ([[Reynolds Number]]  < 2100) The flow velocity of the fluid moving through the pores can thus be determined (by [[Hagen–Poiseuille equation|Hagen-Poiseuille]]'s equation), the simplest of which assuming a [[parabola|parabolic]] [[boundary layer|velocity profile]].

:<math> v = \frac{D^2*\Delta P}{32*\mu *L} </math>

'''Transmembrane Pressure''' (TMP)<ref>Cheryan, M 1998, ''Fouling and Cleaning. in Ultrafiltration and Microfiltration Handbook'' 2nd edn., CRC Press, Florida, 645.</ref>

The transmembrane pressure (TMP) is defined as the mean of the applied pressure from the feed to the concentrate side of the membrane subtracted by the pressure of the permeate. This is applied to dead-end filtration mainly and is indicative of whether a system is fouled sufficiently to warrant replacement.

:<math> v = \frac{P_F + P_C}{2} - P_P </math>

Where

* <math>P_f</math> is the pressure on the Feed Side
* <math>P_c</math> is the pressure of the Concentrate
* <math>P_p</math> is the pressure of the Permeate

'''Permeate Flux'''<ref>Ghosh, R, 2006, ''Principles of Bioseparations Engineering'', Word Scientific Publishing Co.Pte.Ltd, Toh Tuck Link, p.233</ref>

The permeate flux in microfiltration is given by the following relation, based on [[Darcy's Law]]

:<math> J_v = \frac{1}{A_M}*\frac{dV}{dt} = \frac{\Delta P}{\mu *(R_u + R_c)}</math>

Where
* <math>R_u</math> = Permeate membrane flow resistance (<math>m-1</math>)
* <math>R_c</math> = Permeate cake resistance (<math>m-1</math>)
* μ = Permeate viscosity (kg m-1 s-1)
* ∆P = Pressure Drop between the cake and membrane

The cake resistance is given by:

:<math> R_c= r*\frac{V_S}{A_m} </math>

Where

* r = Specific cake resistance (m-2)
* Vs = Volume of cake (m3)
* AM = Area of membrane (m2)

For micron sized particles the Specific Cake Resistance is roughly.<ref>Ghosh, R, 2006,''Principles of Bioseparations Engineering'', Word Scientific Publishing Co.Pte.Ltd, Toh Tuck Link, p.234</ref>

:<math> r= \frac{180*(1-\epsilon)}{\epsilon^3*d_s^2 } </math>

Where
* ε = Porosity of cake (unitless)
* d_s = Mean particle diameter (m)

'''Rigorous design equations'''<ref>Polyakov, Yu,  Maksimov, D & Polyakov, V, 1998 'On the Design of Microfilters' ''Theoretical Foundations of Chemical Engineering'', Vol. 33, No. 1, 1999, pp. 64–71.</ref>

To give a better indication regarding the exact determination of the extent of the cake formation, one-dimensional quantitative models have been formulated to determine factors such as

* Complete Blocking (Pores with an initial radius less than the radius of the pore)
* Standard Blocking
* Sublayer Formation
* Cake Formation

See External Links for further details