Water purification and pathogen control

Lecture 10

Prepared bySamira fattah

Assis. Lec.College of health sciences-HMU

Sources of potable water

Sources where water may be obtained include:

• Ground sources such as groundwater, springs and aquifers.

• Precipitation which includes rain, snow, etc.

• Surface water such as rivers, streams, glaciers.

Why do we need to purify water?

• There are many impurities in the raw water that can harm human health if the water is ingested without treatment. These impurities can be grouped into three categories:

• Physical: materials that do not dissolve in water and make the water appear "dirty".

• Chemical: substances dissolved in the water from both natural and man-made

processes.

• Biological: viruses, bacteria, algae, and other small living organisms.

• Drinking water should be essentially free of disease-causing microbes, but 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 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 ability of water treatment processes and systems to reduce waterborne disease has been determined in epidemiological studies

Water treatment methods

Purification of Water on large scale

• The method of treatment to be employed depends upon nature of raw water and desired standards of quality.

• Components of water purification system - Storage - Filtration - Disinfection

Storage

• Water is drawn out from the source and impounded in natural or artifical reservoirs.

• Optimum period of storage : 10 – 14 days

Advantages• Storage provides a reserve of water from which further

polution is excluded.

• By mere storage about 90% of suspended impurities settle down in 24 hours by gravity, the water becomes clear allowing penetration of light.

• Aerobic bacteria oxidize organic matter present in the water with the aid of dissolved oxygen. Free ammonia and nitrates drop.

• Pathogenic organisms eventually die out. Decrease in bacterial count by 90% within 5 – 7 days.

Water Storage and Microbial Reductions

• Microbe levels reduced over time by natural antimicrobial processes and microbial death/die-off.

• Human enteric viruses in surface water reduced 400-1,000-fold when stored 6 7 months .‑

• Indicator bacteria reductions were less extensive, probably due to recontamination by waterfowl.

• Protozoan cyst reductions by storage were 1.6 for Cryptosporidium and 1.9 for Giardia after about 5 months.

• If water is stored for long periods there is likelihood of development of algae which imparts foul odor and color to water.

Filtration •Filtration is the most widely used method in developing countries.

•One of these filtration methods is the sand filter.

•As the name implies, sand filters filter water through sand and gravel, thus mimicking nature as groundwater percolates through the ground.

• There are two types of sand filters:• a rapid sand filter • a slow sand filter.

• Sand filters require some time to mature, usually 10-20 days before the filtered water is safe to drink.

Microbial Reductions by Slow Sand Filtration

Effective in removing enteric microbes from water. • high bacteria removals ― removal of E. coli bacteria.• Virus removals >99%. – naturally occurring enteric viruses removals• 97 to >99.8 percent; average 98% overall;

• Parasite removals: Giardia lamblia cysts effectively removed.

Disinfection

Factors affecting disinfection effectiveness• Time of exsposure• Concentration of the disinfectant• Concentration of organisms• Nature of the disinfectant• Nature of the organisms to be inactivated

Chlorination• Kills pathogenic bacteria, no effect on spores and certain

viruses.

• destroys some taste and odor producing constituents.

• Also controls growth of algae and slime organisms .

• Principles of Chlorination

• Contact period of at least one hour.

• Minimum recommended concentration of free chlorine : 0.5mg/L for one hour

Super chlorination • method of choice for highly polluted waters• high dose of chlorine is added• after 20 minutes of contact, dechlorination is done with sodium sulphate / sodium thiosulphate to reduce

the taste of excess chlorine

Other disinfectants• Ozonation• UV irradiation

Ozone

• Strong oxidizing agent• Strong Virucidal• unlike chlorine products, can

deactivate resistant micro-organisms.

UV Rays

• The Ultraviolet energy attacks the genetic core of the microorganism and rearranges the DNA /RNA eliminating the microorganism's ability to function and reproduce.

• The process is simple but effective, destroying 99.99 percent of harmful microorganisms without adding chemicals to the water.

Purification of water on small scale

1-Boiling

Rolling boil for 10 min.Kills all bacteria,spores,cysts,ova.

2-Chemical disinfection -Chlorine solution -Chlorine tablets -Potassium permanganate -Iodine

3-Filtration a- Ceramic filters Water pass through micro-pores placed inside the water

container. Bacteria: Unable to pass through the pores, Virus: Can pass through the pores.

b-katadyn filter:

katadyn: activated form of silver, filter coated with silver catalyst, certain metals in very small doses act as powerful germicide.

Disinfection of wells

• Wells are main source of water supply in rural areas

• Need arises to disinfect them during epidemics of cholera, gastroenteritis etc.

• Most effective method is by bleaching powder or solution.

Bottled Water Purification

- There are several sources to find water:• protected underground springs• wells • municipal supplies.

- The next step is to filter the water through multi-barrier sources which could included :

• source protections• source monitoring• ultraviolet light• distillation• ozonation Water bottlers may use one or more of those processes.

• Water bottles are not very strictly required to monitor or disinfect for parasites.

• This is mainly because the source the water is bottled from, it is unlikely to harbor parasites or contain these dangerous elements.