Membrane filtration uses filters with pores smaller than 0.2 micrometers to remove particles like giardia and cryptosporidium from drinking water. It is effective for tertiary water treatment and reuse or discharge into rivers. However, it does not remove dissolved substances. There are several types of membrane filtration that separate particles of different sizes using pores and pressure. Solar water disinfection (SODIS) uses sunlight to make contaminated water safe to drink. It involves placing water in clear PET plastic bottles and exposing them to full sunlight for 6 hours, utilizing UV radiation and increased temperature to inactivate pathogens. SODIS is recommended by the WHO and widely used in developing countries as a free and effective household water

1. Membrane Filtration and SODIS
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ManeshP. Malla (nec)
Membrane filtration
Membrane filters are widely used for filtering both drinking water and sewage. For drinking
water, membrane filters can remove virtually all particles larger than 0.2 ᵤ m
including giardia and cryptosporidium. Membrane filters are an effective form of tertiary
treatment when it is desired to reuse the water for industry, for limited domestic purposes, or
before discharging the water into a river that is used by towns further downstream. They are
widely used in industry, particularly for beverage preparation (including bottled water). However
no filtration can remove substances that are actually dissolved in the water such
as phosphorus, nitrates and heavy metal ions.
Processes:
Reverse Osmosis:
Small solute particles to be separated.
Molecular weight < 100
Pore size: 2 – 10 A0
Pressure: > 25 atm.
Permeation is main transport mechanism
Example: Filtration of salt solution
Ultrafiltration:
Molecular weight of particles: 103 - 105
Pore size: 20 – 1000 A0
Pressure: 6 – 8 atm.
Transport Mechanism: Convection (main) +
diffusion
Example: Filtration of protein, Red blood cells,
polymers, etc.
Nanofiltration:
Particles to be separated with Molecular
weight: 200 – 1000
Pore size: 5 – 20 A0
Pressure: 15 – 25 atm.
Particle retention of salts.
Example: Filtration of dyes, small molecular
weight organics, etc.
Microfiltration:
Molecular weight > 1 lakh
Pore size: more than 1000 A0
Pressure: 2 – 4 atm.
Example: Filtration of clay solution, latex,
paint, etc.
Solar water disinfection is a type of portable water purification that uses solar energy to make
biologically-contaminated (e.g. bacteria, viruses, protozoa and worms) water safe to drink. Water
contaminated with non-biological agents such as toxic chemicals or heavy metals require
additional steps to make the water safe to drink.
There are three primary subsets of solar water disinfection:
1. Electric. Solar disinfection using the effects of electricity generated
by photovoltaic panels (solar PV).
2. Heat. Solar thermal water disinfection.
3. UV. Solar ultraviolet water disinfection.

2. Membrane Filtration and SODIS
2
ManeshP. Malla (nec)
Solar disinfection using the effects of electricity generated by photovoltaic typically uses an
electrical current to deliver electrolytic processes which disinfect water, for example by
generating oxidative free radicals which kill pathogens by damaging their chemical structure. A
second approach uses stored solar electricity from a battery, and operates at night or at low light
levels to power an ultraviolet lamp to perform secondary solar ultraviolet water disinfection.
Solar thermal water disinfection uses heat from the sun to heat water to 70C-100C for a short
period of time. A number of approaches exist here. Solar heat collectors can have lenses in front
of them, or use reflectors. They may also use varying levels of insulation or glazing. In addition,
some solar thermal water disinfection processes are batch-based, while others (through-flow
solar thermal disinfection) operate almost continuously while the sun shines. Water heated to
temperatures below 100C is generally referred to as Pasteurized water.
High energy ultraviolet radiation from the sun can also be used to kill pathogens in water.
The SODIS method uses a combination of UV light and increased temperature (solar thermal)
for disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective
method for decentralized water treatment, usually applied at the household level and is
recommended by the World Health Organization as a viable method for household water
treatment and safe storage. SODIS is already applied in numerous developing countries.
Educational pamphlets on the method are available in many languages,[2] each equivalent to the
English-language version.
Principle of SODIS
Exposure to sunlight has been shown to deactivate diarrhea-causing organisms in
polluted drinking water. Three effects of solar radiation are believed to contribute to the
inactivation of pathogenic organisms:
 UV-A interferes directly with the metabolism and destroys cell structures of bacteria.
 UV-A (wavelength 320–400 nm) reacts with oxygen dissolved in the water and produces
highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) that are
believed to also damage pathogens.
 Cumulative solar energy (including the infrared radiation component) heats the water. If the
water temperatures rises above 50 °C (122 °F), the disinfection process is three times faster.
At a water temperature of about 30 °C (86 °F), a threshold solar irradiance of at least
500 W/m2 (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose
contains energy of 555 Wh/m2 in the range of UV-A and violet light, 350–450 nm,
corresponding to about 6 hours of mid-latitude (European) midday summer sunshine.

3. Membrane Filtration and SODIS
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ManeshP. Malla (nec)
At water temperatures higher than 45 °C (113 °F), synergistic effects of UV radiation and
temperature further enhance the disinfection efficiency.