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Water Treatment

Sedimentation, Flocculation, Filtration, Chlorination, Fluoridation, pH adjustment


Water is pumped into the bottom of tanks and rises up towards the surface very slowly to allow particles to settle at the bottom of the tank.


Very small suspended solids in the water are removed by being made to form larger particles by adding flocculating agents (aluminium sulfate) to the water.


The water from the top of the settlement tanks is passed through large beds of graded sand and gravel to remove any remaining suspended solids.


Chlorine is added to the water in order to sterilise it (to get rid of any harmful micro-organisms).


Fluorine is added to water because it has been shown that the presence of fluoride ions in water helps to reduce dental decay by strengthening the enamel of teeth.

pH adjustment

This is raised by adding calcium hydroxide (lime) or lowered using dilute sulfuric acid or carbon dioxide to reach a value of 7-8.


Effect on the solubility of gases in water with an increase in temperature

Fish die

What happens when so much organic waste is present that it reduces the dissolved oxygen level in the water


The release of substances into the environment that damage the environment

Biochemical oxygen demand

The amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20 °C in the dark for five days


A high B.O.D. indicates that this is the condition of the water


The enrichment of water with nutrients such as phosphates and nitrates, which leads to excessive growth of algae.


Nutrients in fertilisers which pollute waterways when the run-off of from farmland leaks into rivers or lakes

Heavy metals

Metals such as lead, mercury and cadmium which are cumulative poisons and can cause water pollution


Term used to describe the material delivered to waste treatment plants of towns and cities

Primary Treatment

Physical Process involving screening and settlement. Removes about one half of the suspended solids and removes about one third of the B.O.D. in the sewage.

Secondary Treatment

Biological Process which reduces the levels of suspended and dissolved organic materials by bacterial breakdown using the Activated Sludge Method. Removes about 95% of the B.O.D. of the original sewage.

Tertiary Treatment

Chemical Process which removes phosphorus compounds (by precipitation with aluminium sulfate) and nitrogen compounds (by biological nitrification) from sewage

Winkler's reagent

The mixture of concentrated solutions of manganese(II) sulfate, MnSO4, and alkaline potassium iodide, a mixture of NaOH and KI, used in the B.O.D. experiment

White precipitate

What forms when the Mn2+ ions and the OH- ions from the alkali react together in the B.O.D. experiment

Brown precipitate

What forms when the white precipitate reacts with the dissolved oxygen in the water in the B.O.D. experiment


Substance formed when a small quantity of concentrated sulfuric acid is added to the bottle in the B.O.D. experiment causing the iodide ions (from the alkaline KI) to react with the manganese(III) hydroxide

Sodium thiosulfate

Substance against which the iodine solution is titrated in the B.O.D. experiment


Ratio of moles of S2O32- to moles of O2 which is used to calculate the dissolved oxygen in the water from the amount of iodine liberated in the B.O.D. experiment

Atomic Absorption Spectrometry

An analytical technique that makes use of the wavelengths of light specifically absorbed by an element to measure the concentration of elements.

Uses of AAS

Environmental analysis (finding out the levels of heavy metals in water), Clinical analysis (analysing metals in biological fluids like blood and urine), Pharmaceuticals and Mining.


A technique whereby white light is passed through a coloured solution and the colour of the solution is then compared with the colour of solutions of known concentrations of that substance

Uses of Colorimetry

Analysing water for presence of chlorine, lead, nitrates and phosphates

Potassium iodide

Chemical used in the experiment to determine the amount of chlorine in bleach as chlorine readily oxidises to iodine which has sufficient colour to be analysed using a colorimeter

Ethanoic acid

Used in the experiment to determine the amount of chlorine in bleach as acidic conditions are necessary to ensure complete reaction of chlorine with potassium iodide.

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