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Membrane filtration vs UV and chemical disinfection for water treatment ('Membrane vs...' series, 5)

This video is the fifth in our 'Membrane versus..' series, in which membrane processes are compared with the conventional alternatives. This one looks at membrane technology as a disinfection process, comparing it with chemical disinfection and UV irradiation. Disinfection is the significant reduction of disease-forming microorganisms (or pathogens), normally by several orders of magnitude. This is to be distinguished from sterilisation, which is the elimination of all microorganisms. The extent of disinfection is often expressed as the log removal value, or LRV: the log of the ratio of the feed to treated water concentration. So, for example, if there are between ten thousand and a hundred million more pathogens in the feed than in the treated water, as is often the case, then the LRV would be between 4 and 8. There are three recognised ways of achieving disinfection or removal of pathogenic microorganisms, or pathogens: • Attacking the cell wall with chemical oxidants • Stopping the pathogen from reproducing by attacking the DNA • Sieving them out. The first of these is most widely achieved using chlorine, with ozone and chloramines also used and which act similarly. UV irradiation acts by penetrating the cell wall and breaking the bonds of the organic bases of the cell's DNA. Membranes simply physically remove the microorganisms - along with anything else larger than the membrane pores. The effectiveness of all these methods is impaired to some extent by other water constituents. Oxidising chemicals like chlorine react with oxidisable species in the water, specifically organic matter, which exerts an additional disinfectant demand (i.e. requires a higher dose of chemical than that needed for disinfection alone). This not only increases disinfectant dose required, it also increases the tendency to form undesirable disinfection byproducts, or DBPs. Some of these DBPs formed from chlorination (notably trihalomethanes and trihalo acetic acids) are genotoxic; they can cause cancer at high concentrations and/or long exposures. In the case of UV disinfection, materials in the water which absorb UV light impair light transmittance and so reduce the UV dose. Also, minerals like ferric oxide and other compounds can build up on the surface of the UV lamp over the course of time - and this fouling layer also reduces transmittance. Most modern UV systems have a mechanism for periodically wiping the UV lamp to keep it clean. For membranes, there’s no similar reduction of disinfection effectiveness by other components in the water, provided they don’t actually damage the membrane or module. However, fouling of the membrane surface - or else plugging of its pores - by particulate and colloidal matter reduces the membrane permeability. This increases the membrane cleaning frequency and lowers the net flux, which has cost implications. To sum up, all disinfection processes are subject to constraints on their effectiveness and/or economic viability imposed by other contaminants in the water. For oxidative chemical disinfectants, and chlorine in particular, organic matter in the water generates undesirable halogenated DBPs. Suspended materials, as well as sparingly soluble minerals, reduce the effectiveness or productivity of UV irradiation and membrane filtration through fouling. Of the processes considered, only chlorination and chloramination provide a residual disinfection: remaining in the water for several hours after dosing. Chloramine, formed from the combination of chlorine with ammonia, has a lower DBP formation tendency than chlorine, but is generally a less powerful disinfectant. Lastly, there are question-marks over determining the LRV when the pathogen is undetectable in the effluent - but that’s another story. ----------------------------------- Other videos in our 'Membrane versus..' series: 1. Metrics for assessing water and wastewater treatment technology – Introduction:    • Metrics for assessing water and waste...   2. MBRs vs CAS - Membrane bioreactor versus conventional activated sludge technology for wastewater treatment:    • Membrane bioreactor (MBR) v Conventio...   3. Membranes vs. conventional clarification #1 - Principles of membrane filtration versus media filtration and sedimentation for water treatment:    • Membrane filtration vs conventional c...   4. Membranes vs. conventional clarification #2 - Membrane filtration versus media filtration and sedimentation for water treatment – footprint:    • Membrane filtration vs conventional c...   6. Reverse osmosis (RO) vs evaporation for desalination:    • Reverse osmosis vs evaporation for de...   7. MABRs vs MBBRs - membrane-aerated bioreactor versus moving bed bioreactor technologies for wastewater treatment:    • MABRs vs MBBRs: membrane-aerated bior...