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Environmental Chemistry - E6
Questions on water treatment
17
Chemistry
11th Grade
11/05/2012

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Term
What is polluted water?
Definition
Water that is unsuitable for drinking, irrigation, industrial use or washing.
Term
What is sewage?
Definition
Water-carried wastes
Term
What are water pollutants in sewage?
Definition
Heavy metals, pesticides, dioxins, PBCs, organic matter, nitrates and phosphates
Term
What are the main sources of water pollution?
Definition
Batteries, detergents, industrial processes and agricultural processes
Term
What is the purpose of sewage treatment?
Definition
To remove disease-causing microorganisms, organic materials and toxic substances and to reduce the BOD.
Term
Describe primary sewage treatment.
Definition
Water is filtered and allowed to settle twice (can be sped up with aluminium sulphate), then Cl2 or O3 is used to kill bacteria.
Term
What is the problem with primary sewage treatment?
Definition
Not sufficient to improve the water quality to a safe level, barely removes BOD waste
Term
Describe secondary sewage treatment.
Definition
Like primary sewage treatment, but it uses an aeration tank where aerobic bacteria oxidise and break down organic matter (activated sludge process)
Term
What is the problem with primary and secondary sewage treatment?
Definition
They both do not remove dissolved inorganic substances such as nitrates, phosphates and heavy metal ions.
Term
Decribe tertiary sewage treatment.
Definition
Uses carbon beds, chemical precipitation and/or anaerobic biological processes.
Term
What is the carbon-bed method?
Definition
It uses activated carbon granules with high surface area to absorb organic chemicals and purify the water.
Term
What is chemical precipitation?
Definition
Uses H2S to convert heavy metals into their sulphides which can be easily removed: M + H2S -> MS + 2H. Phosphates are reduced with calcium ions.
Term
What are anaerobic biological processes?
Definition
Denitrification; denitrifying bacteria reduce nitrates to nitrogen gas.
Term
What is desalination?
Definition
The removal of salt from seawater.
Term
What is distillation?
Definition
The process of heating the salt water creating fresh water as vapour and a salt precipitate.
Term
What is osmosis?
Definition
The movement of water molecules from areas of high concentrations to those of lower concentration across a semipermeable membrane.
Term
How can osmosis be reversed?
Definition
By increasing the osmotic pressure in the area of low concentration (seawater), so more water molecules move to the fresh water.
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