Water is essential for all living being on Earth to survive. While made up of 60~78% of water, a person cannot go very long without access to clean water. However, only 0.3% of Earth’s water is theoretically readily drinkable to human beings. Unfortunately, due to rapid industrialization and chemistry advancements, large quantities of waste water emissions have polluted much of this readily drinkable proportion of fresh water. As a result, nearly 2 out of every ten people 10 have no access to safe drinking water. Thousands and millions, mostly children, die every year infected by disease transmitted through unhygienic drinking water.
DrinkingPollutedWater
DrinkingPollutedWater

Picture 1

Therefore, water treatment, a process used to modify water into desirable quality for uses such as drinking, industrial processing or environment protecting, is very important.

The first step, also the most important step, in water treatment is to identify the pollutants because specific process can only be used to treat a certain kind of pollution. Below are some examples of primary pollutants, pollution emitted directly from a source found in water


Organic pollution
Organic pollution refers to the contaminants in an environment that is biodegradable by microorganisms.This category includes all decaying organic matters such as vegetables, organisms and sediment matters. If these decaying organic matters enter the water body in large quantities, it could boost microorganism growth which uses a lot of oxygen. This could lead to a depletion of oxygen in the water body and kill aquatic organisms. As the aquatic organisms die, they create more organic matter for microorganisms to feed on. This cycle eventually leads to the breakdown of the local water body ecosystem. The microorganisms filled water would also be unsafe to drink and possibly cause serious illness or disease if drank.


Dioxin pollution
Dioxin is one of the more severe pollution that causes great consequences to human society. It is also a kind of organic pollution but different in a sense that it is persistent. It can be released into the atmosphere when household and medical wastes are incinerated or when a forest fire or volcanic eruption occurs. They are then brought down to the surface and flow into water bodies with rainfall. In fishes and birds, dioxin exposure could cause effects on embryo developments that reduces hatchability and increases mortality. In human beings, dioxin exposure could cause developmental and reproductive abnormality and also effect the immune system together with carcinogenicity. y


Inorganic pollution
This category includes chemicals that do not belong in nature in large quantities and high concentration such as heavy metals or other toxic chemicals. Heavy metals including Copper, Lead, Cadmium, Nickel, Zinc, Arsenic, and Mercury can be leaked into the water source through the emissions of factories, incinerators, and many other various industrial processes. Heavy metal polluted water can cause serious effects on human beings if drank and can even be lethal if ingested in large quantities or high concentration.


Agricultural pollution
Pesticides and fertilizers can run off from farmlands after rain or irrigation. Pesticides can be very useful in agriculture but if it is leaked into water bodies it could poison the organisms and disrupt the ecosystem. If ingested by human beings, it could cause serious issues. Fertilizers, also very useful in agricultural, contains Nitrates that if leaked into water bodies, can cause algae boom that can devastate the ecosystem as they will use much of the oxygen available which leads to death of other organisms.


Household pollution
Water is used to conduct many things every day. It can be used to flush toilets, wash cloths and shower. These daily activities all contribute to the household sewage that is transported away from the community to dispose. This contamination of water eventually flows back to the city. Water used to flush toilets that is released into water bodies can cause many waterborne diseases. Detergents from the water used to wash cloths contain high phosphates and can cause health issues if drank by human beings.


Plastic pollution
Plastic pollution has rapidly become a big concern because of its slow decomposing rate and popularity in the society. If thrown into rivers or other water bodies, plastic could cause fish to choke and destroy some natural habitat if in great quantity such that it piles up. Toxic chemicals are released if incinerated and can pollute water bodies as it comes down with precipitation and cause health issues to human beings if drank.





Water treatment
Water treatment is a very delicate process because all steps of the process must be monitored carefully to ensure the water quality is up to the desired standard. Water treatment is also a very complicated process because various chemical and physical processes are required to be able to remove multifarious substances that are not desirable. These processes can be classified into three categories as primary, secondary and tertiary treatment.


Primary Treatment
Primary treatment is the first stage of water treatment and aims to remove most of the solid materials from the water. Waste water is first processed through a series of filters to remove large solid materials such as plastics, wood, metal, and some other organic matter. Afterwards, the water is passed through a grit chamber, very similar to a sedimentation tank, that allows heavier inorganic matters to settle and lighter organic matters to move on by closely controlled flow rate. The water then flows to the sedimentation tanks for other remaining solid to be removed. Water is pumped into the sedimentation tank where there is little or no water flow at all so that the heavier solids can settle to the bottom as a result of gravity without any disturb.

external image T210_1_024i.jpg
Picture 2 – Sedimentation tank



To speed up the sedimentation process, flocculation is used. Flocculation is a process in which smaller particles are combined together to form larger particles, which are heavier, so that they will settle to the bottom of the sedimentation tank faster. Flocculation is carried out by adding a mixture of Calcium Hydroxide and Aluminum Sulfate.
3Ca(OH)2(aq)+2Al2(SO4)3à2Al(OH)3(s)+3CaSO4(aq)

In the last step of primary treatment, liquids with a lower density than water that floats above such as oil and grease are skimmed off the top of the water. The effluent is then sent for secondary treatment.












Secondary treatment
Secondary treatment continues to process the effluent, with most of the inorganic matters removed, from the primary treatment and aims to remove most of the organic matters. Secondary treatment includes a few sub processes which include biofiltration, activated slude process/oxidation ponds.


Biofiltration
Biofiltration is the process in which intermittent sand filter, contact filters or trickling filters are employed for finer filtration of inorganic materials that were not filtered out in primary treatment. In additional, some organic contaminants can also be removed by the microorganisms that settle on the filter. Among these filters, the trickling filters are the most efficient and commonly used. The trickling filter is basically a bed of plastic, broken rock, gravel, clinker or slag that are ideally packed uniformly to provide gaps sufficient for water molecules to pass but insufficient for larger inorganic molecules to pass. The effluent is passed through the bed for finer removal of suspended solids that might be passed on during primary treatment. These filters can filter out some organic contaminant through the nitrification process. For example, hazardous chemical ammonia can be converted to harmless nitrogen gas. The filters must first be aerated because microorganisms require oxygen to operate. The chemical process which microorganisms break down ammonia into Nitrogen gas is as following
NH4+ + 02 + CO2 à biomass + NO3- + H2O + H+
Biological Filters
Biological Filters

Picture 3 – Biofiltration bed








Aeration/Activated sludge process
To remove organic matter, the activated sludge process is employed. Before the activated sludge process can begin, the tank must be first pumped full of high purity oxygen which can stimulate microorganism growth. The activated sludge process is a process which is based on the aeration of wastewater and fast biological growth.
external image 201043233141147.png
Picture 4 – Activated sludge process outline


Bacteria, fungi, protozoa, and rotifers are all commonly used as the biological component of the activated sludge process. In addition, some Metazoa, such as Nematode worms, may also be present to decompose some of the organic matters. Conditions are monitored to maximize microorganism growth to fasten the decomposition of organic matter. This process is repeated many times until all organic matter has all been decomposed. The water is then flowed to a secondary clarifier where flocculation and sedimentation to remove the decomposed organic matter as sludge. The water is then to be disinfected by either ozone or chlorine to kill the bacteria and sent off to tertiary treatment.

Oxidation ponds
Oxidation ponds are usually employed in countries with hotter climate. The effluents process through biofiltration are passed through a lagoon and maintained there for 2 to 3 weeks. The organic contaminants will undergo bacterial decomposition and carbon dioxide, ammonia and nitrates are released and used by the algal community. Organic sludge settles at the bottom of the pond and releases methane. The sludge is removed and the effluent proceeds to tertiary treatment with most of the organic matters removed.




Tertiary treatment
Tertiary treatment continues to process the effluent from the secondary treatment by using several different chemical or biological processes aiming to remove heavy metals, Phosphate and Nitrates from the effluent.

Below is an overview diagram of the tertiary treatment.


Waste Water Treatment Heavy Metal Removal System
Waste Water Treatment Heavy Metal Removal System

Picture 5 – Tertiary treatment process outline


First, the effluent flows into the storage tank where the pH level is adjusted by adding acid or base to desired level for the chemical reactions that will occur later. Then the effluent flows through a few tank where a different kind of chemical precipitation process occurs in each. A precipitation process is a process by which hazardous heavy metals can be removed by substituting with less dangerous elements.

Below are some examples of chemical precipitation

PO43-(aq)+Al3+(aq)àAlPO4(s)

PO43-(aq)+Fe3+(aq)àFePO4(s)

After the precipitation tanks, the effluent moves on to the ionic removal tanks where water soluble chemicals such as Nitrates are removed. The tank either consists of Ion-exchange column or anaerobic denitrifying bacteria. A ion-exchange column contains resins with hydroxide ions (OH-) that can be exchanged with Nitrate ions which are then trapped on the Ion-exchanged resin. A anaerobic denitrifying bacteria are bacteria that can decompose Nitrate and reduce it into Nitrogen gas, which is harmless and easily separated from water, and effectively remove Nitrate from the effluent.



As the demand for clean water increases rapidly every day, fresh water is not enough to support the population’s need. Therefore, some countries have been desalinating seawater to provide adequate and safe water for their citizens.

Photograph of the Jebel Ali desalination plant in Dubai, courtesy of Lahmeyer International.
Photograph of the Jebel Ali desalination plant in Dubai, courtesy of Lahmeyer International.
Picture 6 – a Desalination station in Dubai




Desalination
Desalination is the process which removes salt from sea water thus making it usable or drinkable by human beings. The process is possible because compounds have different boiling points so they can be separated by the use of distillation. Seawater enters the distillation chamber and passes into a heating coil that then enters another chamber with a low pressure. The water is flashed, boils rapidly and converts into steam, due to the lower pressure. Steam condenses and forms fresh distilled water which the salt content is left behind in the heating coil.


Animation of a steam distillation system showing how saline water is heated, the evaporative is cooled and condenses into freshwater.
Animation of a steam distillation system showing how saline water is heated, the evaporative is cooled and condenses into freshwater.
Picture 7 – a diagram and outline of the distillation process



Another way of desalination is carried out by the use of reverse Osmosis. Osmosis is a natural phenomenon that is defined as the tendency of solvents to pass through a semi permeable membrane from a less concentrated area to a more concentrated area until the concentration on both sides are equal. Reverse osmosis forces this flow of solvents from less concentrated area to more concentrated area by applying great pressure to the side of the membrane where salt water is located. The high pressure forces salt water to pass through the membrane and leaves behind salt. Fresh distilled water can then be collected from the other side of the membrane.


reverse osmosis
reverse osmosis
Picture 8 - Reverse Osmosis



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