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2.6: Water Purification

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  • Water purification further cleans rainwater, depending on the end-use and need for purity. Rainwater harvesting water, especially in systems that use a first-flush, can be used for many applications without purification. For example, watering ornamentals needs no first-flush or purification, and watering edible plants depends on context. However, potable water always requires some purification. Some typical examples of that purification (also known as treatment) include:

    • Canister filters most often use conventional sedimentation, ceramic, and activated carbon filters (Figure 2-31).
      • Pros: Easy to incorporate, precise.
      • Cons: More expensive.
    • Activated carbon filters use chemical adsorption to attract contaminants to its surface. Can be incorporated into a canister filter.
      • Pros: Removes volatile organic compounds, tastes, and odors. Removes chlorine. Works great as part of a treatment chain with other filters.
      • Cons: Not effective at removing viruses and bacteria.
    • Ceramic filters use a labyrinth of microscopic holes to filter out contaminants based on size.
      • Pros: Locally manufacturable. Cleanable with a brush.
      • Cons: Slow flow rates. Can clog easily. Usually not effective against viruses, especially if made locally.
    • Hollow fiber membrane filters use a semi-permeable barrier to filter out contaminants based on size. Often incorporated into a canister.
      • Pros: Small micron filters (0.1 micron) can effectively filter out bacteria, protozoa, or cysts, even smaller (0.02 micron) can also filter out viruses. Long life and relatively high flow rates compared to a ceramic filter.
      • Cons: Can clog easily when used with dirty (high sediment) water. Relatively expensive upfront cost. Bad tastes and odors are not removed like they are with activated carbon.
    • Slow-sand filters (Figure 2-31 and Figure 2-32) use layers of sand and gravel with a developed biological layer on top called a Schmutzdecke.
      • Pros: Inexpensive and accessible.
      • Cons: Slow; large area needed; less precise.
    • Bioremediation uses living organisms as treatment, such as mycoremediation with fungi or phytoremediation with living plants.
      • Pros: Resilient and regenerative.
      • Cons: Less precise and more sensitive.
    • SoDis (Solar water Disinfection) uses plastic bottles in sunlight to purify water using heat and UV.
      • Pros: Very inexpensive and accessible.
      • Cons: Only does disinfection, not purification. Needs the sun and low turbidity water (which is easy to achieve with rainwater).
    • Solar pasteurization uses the sun to bring water to a certain temperature, below boiling, for a certain amount of time.
      • Pros: Lower energy requirements than boiling.
      • Cons: Only does disinfection, not purification. Needs the sun and larger area. 6 Solar distillation uses the sun to hasten the evaporation of water and collects the condensation on a surface. Pros: Relatively inexpensive energy compared to other fuels. Produces cleaner water than some other methods, i.e. does purification. Cons: Large area needed; more expensive. Relies on the Sun.
    • Boiling uses energy, usually from wood or fossil fuels, to boil the water.
      • Pros: Accessible and common.
      • Cons: Expensive in energy and time. Only does disinfection, not purification.
    • Reverse Osmosis uses electrical energy to apply pressure and push water through a water-permeable membrane.
      • Pros: Can be used with brackish water. Produces reliably clean water (except when the filter is compromised physically or biologically).
      • Cons: Very expensive and sensitive. Energy-intensive. Must dispose of brine waste.
    • UV uses ultraviolet radiation, usually from an electrical lamp, to prevent microorganisms from reproducing by altering their DNA.
      • Pros: Very effective against viruses and bacteria, as long as the water is visually clear.
      • Cons: Expensive and energy-intensive. Does not work on non-biological contaminants.
    • Chlorination uses any one of a number of forms of chlorine to disinfect water by killing many pathogenic (disease-causing) organisms.
      • Pros: Very common for water disinfection and effective at treating many of the most prevalent forms of waterborne illness, e.g. cholera, typhoid, and dysentery.
      • Cons: Needs careful management as chlorine and its products can be very toxic. Needs a supply of chlorine.
    • Electrochlorination uses electric current through saltwater to produce sodium hypochlorite. This is just a special form of chlorination, but deserves special mention for the following pros that make it a strong candidate for village-scale potable water:
      • Pros: Common and effective for water disinfection and treating many of the most common forms of waterborne illness. Does not need a supply of chlorine. Produces chlorine at relatively safe levels.
      • Cons: Requires an electrical energy source. Requires mechanical and electrical expertise.



    It is possible and often desirable to use the above-listed methods of water purification in series for extra treatment. For example, systems for use as drinking water may use a slow-sand filter followed by UV treatment. The slow-sand filter cleans the water so that a relatively smaller UV lamp can be used for final purification of biological activity. Different treatment methods treat different pathogens. For example, UV is more effective against parasites whereas chlorine is more effective against viruses.