Final answer:
A. Toilets Gray water is untreated wastewater originating from domestic activities except for toilets, which is known as blackwater.
Step-by-step explanation:
Gray water is untreated wastewater that has not come into contact with waste from toilets. This type of wastewater, also called greywater, originates from domestic activities such as laundry, dishwashing, and showering. It is different from blackwater, which contains human waste and comes from toilets.
The distinction is essential for wastewater treatment and recycling, as gray water can be reused more readily for purposes like irrigation after suitable treatment.
Proper wastewater treatment prevents waterborne diseases and can turn this resource into a sustainable supply of water and nutrients for different applications.
Tertiary treatment at a water treatment plant ensures that most contaminants and pathogens are removed, making the water safe for reuse.
However, without treatment, wastewater can contribute to the pollution of rivers, lakes, and oceans, leading to environmental and health issues.
Gray water can be reused after treatment for purposes such as irrigation, while blackwater requires more extensive treatment to remove pathogens and contaminants.