Explain why it is more difficult to produce drinking water from waste water than from water in lakes

Drinking water can be produced from waste water and from lakes but the difference is in the difficulty of producing water which we can consume.

Drinking water should be clean and toxic free so that if does not affect the human body. Waste water carry more toxic matter in comparison to lake water that's why, producing drinking water from waste water takes more time, expensive and difficult.

So, producing drinking water from waste water is more difficult as it require the removal of more toxic matter than water in lakes.

i believe the answer is three water molecules