membrane filtration

  • rapidly growing filed in water treatment
  • different kinds of membranes
  • how they change water quality
  • applications for household water treatment

size exclusion function

  • membrane pores
  • cake filtration

electrostatic effects

  • charge exclusion
  • adsorption

biological activity

  • affects removal of pathogens and dissolved compounds after filtration

types of membranes

  • are defined based on pore size

  • membranes with smaller pores exclude more particles, even dissolved substances

    • but will require more pressure to force water through the membrane

pressure units

  • atmospheric pressure at sea level is 100 kPa - 1 Bar
  • 1 millibar == pressure by 1 cm of water
  • 1 Bar = 10 vertical meters of water
  • 1 Bar = 15 psi

cloth

  • most clothes have an effective pore size of around a 100 - 150 microns
    • the diameter of a hair strand
    • bacteria, viruses and most protozoans can easily pass through
  • however, some pathogens are attached to larger particles
    • could be removed by cloth including insect hosts
  • the guinea larvae worm is transmitted by copepods
    • copepods are 1mm in size
    • are easily removed by cloth filtration
  • cloth folded over 4-8 eight times can increase the effective pore size up to 20 microns
  • cholera in part is also transmitted by copepods
  • cloth filtration also reduces cholera transmission by up to 50%

sand, silt and clay

  • gravel and boulders > 1 mm (100 microns)
  • sand: 63 - 100 microns
  • silt: 8 - 63 microns
  • clay: 2 - 4 microns

micro-filtration membranes

  • pore size: 0.1 - 5 microns
  • need little pressure to force water through
  • less than 1 Bar pressure
  • flux: several hundred L/hr of filtration from square meter of membrane
  • removes protozoa, like cryptosporidium and giardia

  • sometimes removes bacteria like e.coli and shigella

  • viruses and chemicals would not be retained except the fraction retained on larger particles or absorbed onto the membrane’s surface

ultra filtration membranes

  • pore size: 10 - 100 nm (1000 nm = 1 micron)
  • completely filter viruses and larger protein molecules
  • require 1 Bar to several bars of pressure to get water through
  • flux: depends on operating pressure, ≈ 100 L/hr

ultra filtration membranes

  • pore size: 1 - 10 nm
  • completely exclude all pathogens including viruses
    • also molecules in the size of 200 - 1000 daltons
    • divalent metal cations (Mg) that cause harness are also removed due to electrostatic effect
    • so these filters also used for water softening
  • operating filters can also go 10 - 15 bars

reverse osmosis

  • tightest membranes
  • removes any kind of ions
  • used for water softening
  • operating pressure: up to 80 Bar

summary

membrane-filtration-comparison

fig: comparison chart for membrane filtration

membrane configuration

  • goal:
    • maximize the exposure surface area of the membrane
    • maximize water quantity that can be processed at a go for a given space

flat sheet:

  • sheets of filter material
  • mounted on racks
  • wrapped around a spacer in a spiral roll

hollow fibers or tubes

  • water is sent inside and collected outside or vice versa

dead-end

  • water put on top of membrane and pushed down

cross-flow

  • water put on passes laterally across the surface on the membrane
  • some water permeating through membrane to clean side

membrane filtration challenges

membrane fouling

  • a clean new membrane will process a lot of water with relatively little pressure
  • with time particles build at the filter interface
  • a cake layer forms especially if the membrane operates in a dead end mode

membrane internal

  • particles can lodge in pores
  • dissolved compounds in water can restrict pore size
  • flux reduces over time, more pressure required to obtain filtered water

solution

  • regular backwashing
    • water is passed in reverse direction to remove the cake layers
    • and some of the particles filling the holes
  • high velocity lateral cleaning
    • pass water at high velocity across the filter interface
    • to scour the cake layer and particles stuck
  • these two methods remove reversible fraction of fouling
    • but some irreversible fouling that still remains
  • some irreversible fouling can be removed by cleaning the membrane with
    • solvents
    • with acids or bases
    • depending on the fouling compounds in the filter

application to HWTS

  • many examples of household application of membrane filtration
    • in high income and middle income countries
    • reverse osmosis or nano filtration systems could be installed in the kitchen
    • requires electricity and an additional pump to generate pressure
  • increasing use of ultra-filtration and micro-filtration membranes
    • use less pressure to treat the water
    • can be applied without electricity in some cases
    • in low income and middle income settings
    • best known example
    • lifestraw family produced by vestergaard fransen

example products

  • lifestraw 1.0
    • ultra filtration scheme
    • 20 nm pore size: protozoa, bacteria, viruses
    • hollow fibre, dead-end mode
    • manual backwash
    • 9 L/hr
    • 18000 L - filter life
    • halogen compartment - holds chlorine tablets to minimize membrane fouling
    • since 2005
      • for emergencies - natural disaster situation
      • western kenya - 880000 distributes for household use
  • lifestraw 2.0
    • similar membrane to 1.0
    • 80 micron prefilter
    • two large reservoirs
      • 5 L dirty water reservoir
      • 5 L filtered water reservoir
    • 30000 L filter lifetime
  • Del Agua Health Porgramme - Rwanda
    • lifestraw 2.0 + improved cook stove
    • improve air pollution
    • 600000 households

other examples of filters

considerations for membrane systems

advantages

  • absolute barrier to particles
    • protozoa
    • bacteria
    • viruses
  • simple operation
  • no change to taste of water
  • turbidity reduction

challenges

  • looser membranes have little effect on chemicals
  • need for backwashing, cleaning
  • no protection against recontamination
  • some models require electricity, high pressure
  • supply chains for initial purchase, replacement parts, and service