Context: Solar energy may be used in a water stabilisation pond to treat waste water without chemicals or electricity. A further environmental advantage is that algae grow in such ponds and consume carbon dioxide in photosynthesis, although algae may produce toxic chemicals that make the water unusable.

Question: What is a possible negative effect of algae in water stabilization ponds?
Answer: toxic chemicals

Question: For what reason would solar energy be used in a water stabilisation pond?
Answer: to treat waste water without chemicals or electricity

Question: What is a reason why the water from a water stabilisation pond may be unusable?
Answer: algae may produce toxic chemicals