Water+Treatment

= ﻿ Water Treatment= IB Syllabus required for this topic: 6.1 List the primary pollutants found in waste water and identify their sources. 6.2 Outline the primary, secondary and tertiary stages of waste water treatment, and state the substance that is removed during each stage. 6.3 Evaluate the process to obtain fresh water from sea water using multistage distillation anf reverse osmosis.

Importance of Water
All known living organisms depend on water for survival, include human! Human bodies us made of 60-78% water. Water is needed for our organs to work properly. However, only 3% of the Earth's water is fresh water and only 0.3% of fresh water is surface fresh water which is available for human to consume.

Waste Water Pollutants
Waste water is any water that has been affected in quality by anthropogenic influences. There are several anthropogenic contaminants that can be found in water supply which enter by different ways. They have different effects on human health and the quality of the water supply.

In order to make water safe for human consumption it first must be treated. There are three steps to the treatment process, primary, secondary and tertiary, which involve filtration, sedimentation, flocculation and percipitation processes.

Primary Treatment
Primary treatment is the stage in which filtration, sedimentation and flocculation take place. About 60% of the solid materials and 1/3 of the oxygen-demanding waste (such as organic materials and dangerous bacteria) have been removed in this stage. 1. Waste water is run through a series of filters and screens to remove large solid before any chemical treatment of water takes place. 2. Water passes through a sand or grit chamber where close control of the flow rate allows for sand, gravel, stones, and small grit to settle. 3. The wastes are disposed of in local landfills or incinerated. 4. The water is sent to sedimentation tanks where suspended solid settel out as sludge. 5. Flocculation is initiated in to speed up the sedimentation process,. 6. Fats, oils and grease are skimmed off the top of the water. 7. The effluent is sent for secondary treatment or discharged into main water supply.

**Flocculation**
Flocculation involves adding a mixture of calcium hydroxide and aluminium sulfate to the waste water. The combination results in the percipitation of solids particles of aluminium hydroxide that when formed carry with them the smaller suspended dirt particles, which then sink to the bottom of the sedimentation tank as sludge. Smaller suspended particles are coagulated into larger particles that can settle in the sedimentation tanks more readily. 3Ca(OH) ﻿2 (aq) + 2Al ﻿2 (SO ﻿4 ) 3 → 2Al(OH) 3 (s) + 3CaSO ﻿4 (aq)

Secondary Treatment
Secondary treatment involves a biological treatment process in which dissolved organic matter is removed by the activated sludge process. The process involves the use of bacteria and aerobic conditions to degrade the organic matter. About 90% of the oxygen-demanding wastes are removed. 1. The waste water enters aeration tanks where oxygen is mixed with the water in order to create ideal conditions for aerobic bacteria to grow and flourish. 2. The bacteria digest the organic matter. 3. Any untreated oragnic matter again settles out as sludge in a second sedimentation or clarifier tank. 4. The water is returned to aeration tank and the process is repeated until all the organic matter has been removed. 5. The effluent is discharged to be disinfected with either ozone or chlorine, or sent for tertiary treatment.

Tertiary Treatment
Tertiary treatment is used to remove heavy metals, phosphates and nitrates. These pollutants are removed through different chemical or biological processes. In the removal of phosphates (mainly from detergents and soaps), insoluble aluminium phosphate, iron(III) phosphate and calcium phosphate are proudced. PO 4 3- (aq) + Al 3+ (aq) → AlPO 4 (s)

PO 4 3- (aq) + Fe 3+ (aq) → FePO 4 (s)

2PO 4 3- (aq) + 3Ca 2+ (aq) → Ca 3 (PO 4 ) 2 (s)

During the removal of arsenic from waste water, it is percipitated out by the addition of iron(III) salts. During this process arsenic is removed as iron(III) arsenate precipitates. Thoses heavy metal pollutants are can be removed through similar precipitation reaction.

Nitrates are harder to remove from water as the compounds are soluble. Hence, percipitation cannot be used while ion-exchange column or anaerobic denitrifying bacteria is more suitable. 1. Ino-exchange column involve exchanging like-charged ions in large column filled with resin. 2. Hydroxide ions (OH - ) in resin will be exchanged with nitrates ions (NO 3-﻿ ).
 * Ion-exchange Column**

3.Nitrate are trapped on the ion-exchange resin and less harmful hydroxide passes through and out of column with the effluent.

Nitratea are removed from the water by the addition of anaerobic denitrifying bacteria that reduce the nitrate ion into nitrogen gas. This process is cheaper. Sedimentation Flocculation || Activated sluge process || Percipitation Ion exchange || Oxygen-demanding waste || Dissolved organic matter || Heavy metals Phosphates Nitrates ||
 * Anaerobic Denitrifying Bacteria**
 * ~ Table 6.3 Comparison of Primary, Secondary and Tertiary Water Treatment ||
 * || Primary || Secondary || Tertiary ||
 * Process used || Filtration
 * Pollutant removed || Solid material

Distillation
Distillation is a method of separating mixtures based on differences of the boiling point between different elements. The Principle of separation by distillation is possible because compunds have different boiling points. People used this process to obtain fresh water from sea water. 1. Cold salt water enters the distillation chamber and passes into a heating coil. 2. The heated salt water enters a separate chamber with a pressure lower than atmospheric pressure. 3. The lower pressure causes the heated salt water to boilign rapidly and be coverted to steam. (Flashing process) 4. Most of the freshwater is removed and a briny solutionl that has a high concentration of salt is left behind. 5. The steam that is formed during the flashing proces condenses on the cooler water coil and fresh distilled water is produced.



Reverse Osmosis
Osmosis is a natural phenomenon which is defined as the tendency of a solvent to pass through a semipermeable membrane from an area of low solute concentration to an area of high solute concentration until the concentrations on both sides of the membrane are equal. Water desalination uses reverse osmosis which is opposite of natural osmosis. 1. A high pressure is applied to the side of the membrane where salt water is located. 2. The high pressure forces water (the solvent) through the membrane, leaving salt (the solute) befind. Pressures of 6000-8000 kPa are used depending on the salinity values of the salt water. Major problem: Finding a menbrane that can withstand these high pressure, which is 600-800 times atnospheric pressure. Solution: A membrane made from cellulose ethanoatem aromatic polyamides or thin-film polymer composites, which is extremely strong.