Water Treatment

Water Sources and Water Treatment Drinking water should be essentially free of disease-causing microbes, but often this is not the case. –A large proportion of the world’s population drinks microbially contaminated water, especially in developing countries Using the best possible source of water for potable water supply and protecting it from microbial and chemical contamination is the goal –In many places an adequate supply of pristine water or water that can be protected from contamination is not available The burden of providing microbially safe drinking water supplies from contaminated natural waters rests upon water treatment processes –The efficiency of removal or inactivation of enteric microbes and other pathogenic microbes in specific water treatment processes has been determined for some microbes but not others. – The ability of water treatment processes and systems to reduce waterborne disease has been determined in epidemiological studies

1. Pre-treatment: controls algae and arrests any biological growth 2. Aeration: along with pre-chlorination, removes dissolved iron and manganese 3. Coagulation: prepares for flocculation 4. Sedimentation: for solids separation, that is, removal of suspended solids trapped in the floc 5. Filtration: removing particles from water 6. Disinfection: for killing bacteria The water treatment process generally uses the steps below:

Pre-treatment 1.Pumping and containment: water must be pumped from its source or directed into pipes or holding tanks. 2.Screening: remove large debris such as sticks, leaves, rubbish and other large particles 3.Storage: water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. 4.Pre-conditioning: water rich in hardness salts is treated with sodium carbonate to precipitate calcium carbonate out 5.Pre-chlorination: water is chlorinated to minimize the growth of fouling organisms on the pipe- work and tanks

Aeration In the process of aeration, water first goes to aeration tanks where large quantities of air are injected into and bubble through the water. Aeration can reduce tastes and odors and can oxidize soluble iron. Aeration Tank

Chemical Coagulation-Flocculation Removes suspended particulate and colloidal substances from water, including microorganisms. Coagulation: colloidal destabilization Typically, add alum (aluminum sulfate) or ferric chloride or sulfate to the water with rapid mixing and controlled pH conditions Insoluble aluminum or ferric hydroxide and aluminum or iron hydroxo complexes form These complexes entrap and adsorb suspended particulate and colloidal material.

Coagulation Aluminum sulfate and other chemicals are added to the water, which cause small particles to clump together (coagulate), forming floc. These floc attract dirt particles, making them eventually heavy enough to sink to the bottom of the water storage tank. Some of the floc begins to settle during this stage.

Sedimentation Gravity and time are all that is needed to remove the floc from the water. The water and floc flow into a sedimentation basin. These settling basins are typically twelve to eighteen feet deep and hold the water for two to four hours at a forward velocity of 0.5 to 3 feet per minute.As the water sits there, the heavy floc settle to the bottom, where they remain until removal.

Filtration Water passes through layers of gravel, sand and perhaps charcoal, which serve to filter out any remaining particles. Suspended materials (such as floc, microorganisms, algae, silt, iron, and manganese) are filtered out when water passes through beds of granular material, usually composed of layers of sand, gravel, coal, garnet, or related substances.

Filtration Processes Used Rapid filtration –used in United States –fast filtration rates through media (sand or anthracite) –backwashing needed Slow sand filtration –common in United Kingdom and Europe –slow filtration rates through media (sand and gravel) –removal of biological layer needed –higher removal rates for all microorganisms

Granular Media Filtration Used to remove suspended particles (turbidity) incl. microbes. Historically, two types of granular media filters: – Slow sand filters: uniform bed of sand; – low flow rate <0.1 GPM/ft2 – biological process: 1-2 cm “slime” layer (schmutzdecke) – Rapid sand filters: 1, 2 or 3 layers of sand/other media; – >1 GPM/ft2 – physical-chemical process; depth filtration Diatomaceous earth filters – fossilized skeletons of diatoms (crystalline silicate); powdery deposit; few 10s of micrometers; porous

Slow Sand Filters Less widely used for large US municipal water supplies Effective; widely used in Europe; small water supplies; developing countries Filter through a 3 ‑ to 5 ‑ foot deep bed of unstratified sand flow rate ~0.05 gallons per minute per square foot. Biological growth develops in the upper surface of the sand is primarily responsible for particle and microbe removal. Effective without pretreatment of the water by coagulation ‑ flocculation Periodically clean by removing, cleaning and replacing the upper few inches of biologically active sand

Microbial Reductions by Slow Sand Filtration Effective in removing enteric microbes from water. Virus removals >99% in lab models of slow sand filters. – Up to 4 log 10 ; no infectious viruses recovered from filter effluents Field studies: – naturally occurring enteric viruses removals 97 to >99.8 percent; average 98% overall; Comparable removals of E. coli bacteria. – Virus removals=99 ‑ 99.9%; – high bacteria removals (UK study) Parasite removals: Giardia lamblia cysts effectively removed – Expected removals almost 99%

Roughing Filter Used in developing countries inexpensive low maintenance local materials Remove large solids Remove microbes 1-2 log 10 bacterial reduction 90% turbidity reduction

Adsorbers and Filter-Adsorbers Adsorbers: Granular activated carbon adsorption –remove dissolved organics –poor retention of pathogens, esp. viruses –biologically active; develops a biofilm –can shed microbes into water Filter-adsorbers Sand plus granular activated carbon –reduces particles and organics –biologically active

Fluoridation & Disinfection In most places, Fluoride (F - ) is added to water to reduce tooth decay. During disinfection, disease- causing organisms are destroyed or disabled. Chlorine (Cl 2 ) is the most common disinfectant used in the United States because it is practical, effective and economical. Ozone is used in some places as well. A small amount of chlorine is added to kill any bacteria or microorganisms that may be in the water.

Disinfection: A Key Barrier Against Microbes in Water Free chlorine still the most commonly used disinfectant Maintaining disinfectant residual during treated water storage and distribution is essential. –A problem for O 3 and ClO 2, which do not remain in water for very long. –A secondary disinfectant must be used to provide a stable residual UV radiation is a promising disinfectant because it inactivates Cryptosporidium at low doses –UV may have to be used with a chemical disinfectant to protect the water with a residual through distribution and storage

Storage Finished water (the term water treatment professionals use) is stored in holding tanks, in order to serve homes and businesses in the community.

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