Water treatment is an important process to make the water eliminate contamination in water. It made the water become desirable and could be used in different ways.

Water treatment is crucial to protect the planet. When the contaminants are removed, waste water could be properly treated and dumped without harming the ecosystem. Hence, toxins would not spread through the food chain.

Besides, water treatment could ensure the supply of water. With the rising demand of people, water treatment increases the supply and the quality of water. Water could be reused and back to the water cycle. To sum up, water treatment restores the water supply and protects the health of people effectively.

Water Treatment – types

Sewage treatment

Before dumping waste water to the sea and the environment. Sewage treatment is important to remove contaminants to protect the environment.

Drinking water treatment

The safety of drinking water is important to secure public health. However, only 0.4% of water in the world is safe for human consumption. Hence, drinking water treatment is important. Mostly, water could be collected from wells, rivers, lakes and glaciers.

Desalination

97% of water on the earth is seawater. Desalination is important to turn seawater into clean water which is safe to consume.

Water Treatment – steps

Stage 1: Preliminary treatment (screening)

large solids like paper, leaves and debris in the water would first be removed by passing through mechanical bar screens. Screening is important to prevent the damage of the equipment. Afterwards, Coarse particles would then be removed by grit channels.

Stage 2： Aeration

In the aeration tank, air circulated, mixed and dissolved into the water. Then, materials like dissolved gasses, metals and organic compounds could be removed through oxidation.

Types of aeration

Types of aeration	Principle	Advantages	Disadvantages
Fine bubble diffusers	Introduces air into water via very fine bubbles. While fine bubbles promote the transfer of oxygen to water, maximizing air-water contact in the process.	Efficient Aeration and oxygen transferHigh energy efficiency	Weaker mixingA more complex installation and cleaning process
Coarse Bubble Diffusers	Coarse bubble diffusers produce a larger diameter of bubble (3-50mm) compared to fine bubble diffusers (0-3mm) to displace, churn and mix the wastewater effectively.	Stronger mixingHigher durabilityLow maintenance	Less efficient aeration and oxygen transferLow energy efficiency
Surface Aeration	Splashing surface aerators create more surface area for gas exchange, which can add larger amounts of oxygen to the water.	Perform good in shallow waterEasier installation	The maintenance requirements are highLimited mixing performances as it may have trouble in low temperature (winter)

Afterwards, microorganisms are also used to decompose pollutants into harmless inorganic compounds. The pumping of air also activates the growth of microorganisms. Which accelerates the process of decomposition.

Examples of microorganisms used:

– Bacteria: Aerobic bacteria remove organic nutrients. For example, nitrates. Denitrifying bacteria could convert nitrogen in the water into nitrate.

– Protozoa: Remove and digests dispersed bacteria and suspended particles

– Metazoans: They dominate longer age systems including lagoons

– Filamentous bacteria: bulking sludge (poor settling & turbid effluent)

– Algae and fungi: They are useful for pH changes & commonly used in older sludge

Stage 3: Coagulation and flocculation

Coagulation: a process of removing fine particles like sand, silt, clay or even oil. Coagulant with positive electric charge was added to neutralise the charge of fine particles. After entering the rapid fix tank, fine particles become destabilized and begin to clump together to form flocs.

Particles sized 10-100μm are generally considered “turbidity”. Particles smaller than 10μm are “colloidal particles”, Both particles are always treated with coagulation as it is expensive to remove particles using only mechanical water treatment like filtration.

Following chemical are commonly used:

Aluminium Sulfate (Al2(SO4)3): aluminium hydroxide floc will be formed as the result of the reaction between the acidic coagulant and the natural alkalinity of the water
Aluminium hydroxide (Al (OH)2)
Ferric chloride (FeCl3): common iron salt to archive water coagulation
Organic polymers: Their large size and charge characteristics could promote destabilization through bridging and charge neutralization.

Flocculation: It occurs when the water enters the flocculation basin. With the decline of mixing speed, clumps attach together to form larger and heavier larger clumps.

Stage 4: Sedimentation

In the sedimentation tanks, solid and flocs suspended in the bottom of the tank as it is heavier than the water. Afterwards, accumulated solids form sludge and could be removed. Sedimentation has been practiced as a way to treat water, using small vessels, larger basins, cisterns, and storage tanks. Removal rates can exceed 90% for protozoa and helminths, but are inconsistent for viruses and bacteria (Sobsey, 2002). However, Sedimentation is not effective against dissolved chemicals, and fine clay particles. The whole process can be repeated two or three times to make turbid water much clearer.

Particles over 100μm like gravel and sands are generally considered “settleable solids” and their settling time in a one-meter water is 1-10 seconds.

Stage 5: Filtration

Filtration separates solids from the water through gravitational force-rapid gravity filtration, or pressure-pressure filtration. After sedimentation, unremovable solids and flocs pass through sand and the gravel beds. Filters have different pore sizes which could remove different kinds of particles.

Types of filtration

Types of filtration	Types
Membrane filtration: filtrate water that passes through a permeable membrane. performed under low pressure, where water is pushed through the small pores of the filter.	Microfiltration: Pore size around 0.01 micron, remove bacteria and larger particles.Ultrafiltration: Pore size around 0.01 micron, remove smaller particles, bacteria, protozoa and some viruses.Nanofiltration: Pore size around 0.001 micron, remove microbes and natural organic matters and mineralsReverses Osmosis: Pore size around 0.0001 Micron, water pass through could remove all organic molecules and viruses
Mechanical filtration (mainly use in water treatment plants): physical capture and absorption of pollutants	Slow sand filtration: Low filtration rate (0.1-0.3 m/h, or lower). Remove protozoa pathogensRapid gravity: water flow through the media (usually sand or other inert material) through gravity pressure filtration: water flow through the media (usually sand or other inert material) through pressureStrainers: most have greater than 25-micron removal ratings. Straining part might be made of metal or other inert material e.g., plastic, cotton or a ceramic. Precoat filters: precoat layer might be created with loose fibers or powdersUpflow filters: backwashing of the filter media, downward filtration. Allows the capacity of the media to collect and store solids to be exploited betterBag filters: wastewater goes into the bag, where the solid particles from the water are caught, allowing only clean water to flow through the bag’s pores to the other side.Cartridge filters: trap particles and even chemicals through the filtration processSelf-Cleaning filters: cannot be shut down for cleaning purposes. utilize backwashing or mechanical processes for the purposes of removing debris.Carbon(activated/catalytic): char creates a porous material that binds to certain toxins and impurities thus pulling them out of the water flowing through the system.
Distillation	Through heating, water evaporates. Evaporated water is then collected. Distillation could remove many contaminants from drinking water, including bacteria, inorganic and many organic compounds.

Stage 6: Disinfection

Disinfection is the process that eliminates many or all pathogenic microorganisms, except spores and inanimate objects. Chemical and physical media are added to the water during the process. Which could protect people against water-borne diseases such as cholera and diarrhea.

For chemical disinfection of water, the following disinfectants can be used:

Chlorine (Cl2) and Chlorine Dioxide (ClO2): Chlorine is the most common disinfectant used to kill microorganisms in the water.
Ozone(O3): Ozone is commonly used in wastewater treatment together with chlorine.
Bromine (Br2), Iodine(I) (Halogens): Bromine is a common alternative disinfectant for swimming pools, spas and cooling tower water, but not for municipal drinking-water.
Bromine chloride (BrCl)
Copper (Cu2+), Silver (Ag+) (metals)
Kaliumpermanganate (KMnO4)
Fenols and Alcohols
Soaps and detergents
Kwartair ammonium salts
Hydrogen peroxide
Several types of acids and bases

For physical disinfection of water, the following disinfectants can be used:

Ultraviolet light (UV light)
Electronic radiation
Gamma rays
Sounds
Heat (boiling, pasteurization)

Chemical Disinfection vs Physical Disinfection

	Chemical Disinfection	Physical Disinfection
Advantages	Cheaper, especially for halogens Easy to use and most of them are effective in low concentration	Eliminate the production of disinfection by product (DBPs)Does not alter the quality of water
Disadvantages	Create disinfection by- product (DBPs) which is toxic (e.g., Chlorine)Corrosive and unstable (acids and ozone)	high energy requirements as it may require electricity (especially heat)

Stage 7: Supplementary treatment, Water Transportation and Storage

Supplementary treatment: Different water treatment systems would have different standards. Supplementary treatment may be needed in specific water treatment plants.

Methods and purposes of supplementary treatment

Methods	Purposes
Fluoridation	Drinking water: The adjustment of fluoride ions in the optimal level could decrease the rate of dental cavities.
pH correction	lime could be added to adjust the pH level of the water. This could stabilize the water so corrosion could be minimised.

Water Transport: water is transported through built infrastructure such as pipes, canals or in containers. Sometimes, Pipes and containers might be corroded after a long period of time. So, it is important to check the infrastructure to prevent the contamination of water.

Water Storage: When treated drinking water is not carefully protected, it can become recontaminated with pathogens. Hence, it is important to identify containers currently used for water collection, transport, and storage in the community. These existing containers might already be safe or could easily be modified to be safe storage containers.

CONCLUSION

To conclude, this report goes through the importance, types and steps of water tests and treatment.

Regular Water tests and effective water treatment infrastructure are very important to ensure the safety use of water and public health.

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