What type of discharge method is prohibited for waste that may harm the drainage system?

The answer identifies direct discharge as the prohibited method for waste that may harm the drainage system. This is because direct discharge allows potentially harmful substances to enter the drainage system without any preliminary treatment or filtration. Such a practice can lead to issues like clogging, damage to pipes, or contamination of water sources.

In contrast, methods such as indirect discharge after treatment involve some processing to reduce the harmful effects of the waste. They often include treatment systems that neutralize or mitigate damage before being released into the drainage system. Similarly, discharging through a trap or an air gap adds a layer of protection, helping to prevent a backflow of contaminated waste into the system and allowing for the safe handling of harmful materials. Therefore, the key distinction about direct discharge is that it lacks any safeguards to protect the drainage system from hazardous wastes, making it a non-compliant practice in professional plumbing standards.