Raw water is water found in the environment that has not beentreated and does not have any of its minerals, ions, particles, bacteria, or parasites removed. Raw water includesrainwater,ground water, water from infiltration wells, and water from bodies likelakes andrivers.
Raw water is generally unsafe for human consumption due to the presence of contaminants. A major health problem in somedeveloping countries is use of raw water for drinking and cooking.[1]
Without treatment, raw water can be used forirrigation,construction, or cleaning purposes.[2] Farmers use it for watering theircrops and give it tolivestock to drink, storing it in man-made lakes or reservoirs for long periods of time. Construction industries can use raw water for makingcement or for damping down unsealed roads to prevent dust rising. Raw water can also be used for flushing toilets and washing cars, as well as any other purposes that do not require it to be consumed by humans. Water in this form is considered raw, as opposed to water which has beentreated before consumption, such asdrinking water or water which has been used in an industrial process, such aswaste water.
Raw water flushing is a method ofwater conservation where raw water is used for flush toilets.
The composition of raw water is naturally variable, but commonly contains one or more of the following significant contaminants, in the form of dissolved ions, particles and living organisms:[3]
Other, less common, contaminants of raw water include:
These contaminants can be harmful to humans if they drink raw water containing them, or if the water is not treated properly before consumption. The contaminants can also have an effect on the local ecosystem. For example, endocrine disrupting chemicals can have harmful effects on fish populations living in naturallakes andrivers.[7] These growing health concerns have led to the development of guidelines, such HACCP (Hazard Analysis and Critical Control Points), for the production of safe drinking water from wastewater and raw water, and research intosustainable water supply alternatives.[8][9]
Raw water can be used for many purposes, such ascooling water, water forrinsing andchemical production,purified water, anddrinking water.[10] However, due to the possiblecontaminants, raw water must be treated before it is allowed for human consumption or industrial use.[11] There are several steps involved in the treatment of raw water, and different methods in which it can take place.
Reverse osmosis has been used for the production of demineralised water for over 30 years.[10] In reverse osmosis, water is transported across amembrane under high pressure, leaving a product that consists mainly of water and aconcentrate containing most other components such asminerals and unwantedresidues.[12] Many contaminants, includingiron,manganese,ammonium, traces ofpesticides andmedicines, organic micro pollutants, andradioactive particles can be removed with reverse osmosis alone.[4][12] This efficiency has made reverse osmosis the primary method ofwater purification, often being used in combination with other methods, as a final treatment. A downside to reverse osmosis is that the removing of most minerals from water can have negative effects on its taste.[12] Minerals are sometimes added to drinking water that was already filtered to improve its taste.
The conventional treatment method for water purification is a complex, multistage process that was used for many years.[10] It generally consists of five primary steps.[11] First, raw water is adjusted foralkalinity andpH with the addition of hydratedlime andcarbon dioxide. Second, particulate matter is congregated withaluminum sulphate and othercoagulants, such aspolymers, which the water flows in a cascade that mixes the chemicals and raw water with the coagulants. Third, the water is slowly mixed inclarifiers where larger particles settle down to the bottom and are periodically removed (sedimentation). Fourth, water is directed from the clarifiers to the filters (e.g.anthracite andsand filter) to entrap any smaller particles that survived the clarification process. Finally,sodium hydroxide is added to adjust the final pH/alkalinity,sodium hypochlorite fordisinfection andfluoride forfluoridation. This process is often used as a pre-treatment method while another processes, such as reverse osmosis, is used for the final treatment.[10]A disadvantage of this method is its use of chemicals, such asozone,flocculents,hydrogen peroxide, lime, andchlorine for the filtration process.[10] These chemicals could be dangerous if used incorrectly or if they remained in the water after complete treatment. This requires special precautions, and each step of the process has to be controlled to achieve an overall optimal performance.[10] The need for a complex control system for conventional treatment can result in it being financially costly.[10] This has led to the development of alternative pre-treatment and treatment methods for raw water.
Ultrafiltration is amembrane filtration process and provides an alternative to conventional pre-treatment.[10] In this method water is only pre-filtered with a commonscreen filter before being filtered at high pressure through a membrane, separating the water from contaminants within it. Ultrafiltration can be used on its own for water purification or as a pre-treatment for reverse osmosis. Its advantages over conventional pre-treatment include: very highwater quality independent of the contaminants in the initial raw water, a plant that is simpler in design and more flexible, which makes it easier to automate, lower use ofchemicals, and final product free ofviruses andmicroorganisms.
Thebiofilm membrane filtration method is used to remove harmful chemical by-products, such as NH4+-N, totalnitrogen, anddissolved organic matter, that could form from disinfection processes.[13] Biofilm Pre-treatment is a necessary step for water treatment in many parts of world due to its low operation cost and effectiveness atpollutant removal. In this process microorganisms remove contaminants in raw and wastewater that are harmful to humans, but nutrients to them.[13] Bio-diatomite dynamicmembrane reactor filtration combines dynamic membrane technology forwastewater treatment, and microbial colonies that formdiatomite particles topurify water.[14] These methods are under development inChina as part of addressing the country'spollution problems.
Turbidity is how murky orhazy water seems due tosuspendedparticles. The more suspended particles, the higher the turbidity. Turbidity is used to visually measure water quality, being most common in unfiltered raw water.[15]High amounts of turbidity in raw water can occur due to:[15]
During high turbidity peaks,pathogens are more commonly found in raw water. This can contribute to the spread ofillnesses.[16] When turbidity increases, reported hospital visits for the elderly (65+) with gastrointestinal illnesses also increase. Even places with water filtration systems that meet standards can have anendemic ofgastrointestinal illness, orwaterborneinfectious diseases. Those hospitalized represent a small percentage of totalmorbidity caused by these illnesses.[9]
Controlling the land surrounding raw water reservoirs and other sources is key to reducing turbidity. Areas of high sediment resuspension and erosion need attending regularly, and screens and other devices are needed to catch suspended particles. Screens that trap particles anddebris must be cleaned consistently. Flushing out water pumps should be done when demand is lowest. This gives time for the water to settle, and decrease turbidity before is it needed in high amounts. "Turbidity reduction is best achieved when the water is run through a series of chemical and physical treatment methods before reaching the filter".[17]
Around 2017, untreated spring water, described as "naturally probiotic" raw water, became a commercial product distributed through health food shops, but experts have raised public health concerns about the possible occurrence of harmful bacteria, viruses, and parasites such asGiardia. The products are associated with afad diet of "raw waterfasting",[18][19] reportedly finding purchase among the "health-conscious elite" ofSilicon Valley, including figures such as theraw vegan founder ofJuicero Doug Evens.[20][21] One doctor compared the movement'srejection of established science to theanti-vaccination movement.[22] One raw water business in the United States has even claimed that methods used by bottled and spring water producers couldchange the molecular structure of water.[23][24]
The fad was mocked onThe Daily Show in April 2018 in a segment that included an interview withMarion Nestle.[25][26]
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