What substance is NOT used for neutralizing acid waste?

The correct response is based on the understanding of how different substances interact with acids in wastewater treatment processes. Hydrochloric acid is actually an acid itself, and its purpose in wastewater treatment is not to neutralize acid waste, but rather to adjust pH downwards when necessary. In contrast, substances like soda ash, lime slurry, and caustic soda are bases that are commonly used for neutralizing acid waste by raising the pH.

Soda ash (sodium carbonate) reacts with acids to form carbon dioxide and a salt, effectively neutralizing the acid. Lime slurry (calcium hydroxide) functions similarly by reacting with acids to form calcium salts and water. Caustic soda (sodium hydroxide) is also a strong base that neutralizes acids effectively, producing water and a salt.

Thus, because hydrochloric acid is an acid and not a neutralizing agent, it is not appropriate for neutralizing acidic waste, making it the correct choice in this scenario.