Final answer:
The leftover product from the secondary line in a wastewater treatment plant is processed in the tertiary treatment phase, which involves extra filtration to remove impurities.
Step-by-step explanation:
In a wastewater treatment plant, the leftover product from the secondary line that doesn't fit in the main line is processed in the tertiary treatment phase. This phase involves extra filtration to remove any remaining impurities or contaminants from the wastewater. After the tertiary treatment, the treated wastewater can be safely released back into the environment or reused for non-potable purposes.