(a) pollutants
(b) sewage
(c) sludge
(d) oils and fats
Sewage is wastewater released by homes, industries, hospitals, offices and other users. It contains dissolved and suspended impurities, organic waste, disease-causing microorganisms and chemicals. Discharging untreated sewage into rivers or seas pollutes water, harms aquatic life and spreads diseases.
Oils and fats can harden and block drains and pipes. They also reduce the action of helpful microbes in wastewater treatment, so they should not be released into drains.
Wastewater first passes through bar screens that remove large objects. It then goes to a grit and sand removal tank where sand and grit settle. In a large settling tank, solids settle as sludge and oils or grease are skimmed off. Air is then passed through the clarified water to help aerobic bacteria grow and consume organic waste. The water is finally disinfected before it is released or reused.
Sludge is the solid waste that settles at the bottom of the settling tank during wastewater treatment. It is removed with a scraper and transferred to a separate tank, where anaerobic bacteria decompose it. Biogas is produced and the dried sludge can be used as manure.
Untreated human excreta contains disease-causing microorganisms and can contaminate soil and water. It may spread diseases such as cholera, typhoid, dysentery and hepatitis, so it must be treated safely.
Chlorine and ozone are used to disinfect water.
Bar screens remove large objects such as rags, sticks, cans, plastic packets and napkins from wastewater. This protects later treatment equipment and makes the rest of the cleaning process easier.
Poor sanitation allows human excreta and wastewater to contaminate water, soil and surroundings. This spreads disease-causing microorganisms. Good sanitation safely collects, treats and disposes of wastes, reducing diseases.
An active citizen should not throw solid waste, oils or chemicals into drains; should use toilets; should keep surroundings clean; should report open drains or broken sewer lines; and should encourage safe wastewater disposal and hygiene in the community.
Ozone is used for disinfecting water and the ozone layer absorbs harmful ultraviolet rays. Oxygen, not ozone, is essential for breathing, and ozone is not about 3% of air.
(ii) b and c