There are 2 types of plastic pipe approved for acid waste piping: _______ and _______.

The correct answer identifies polypropylene and polyethylene as the two types of plastic pipe approved for acid waste piping. Polypropylene is highly resistant to a wide range of acids and bases, making it suitable for handling corrosive substances typically found in industrial and laboratory settings. Similarly, polyethylene exhibits excellent chemical resistance, which is critical for transporting acidic waste safely without risking leaks or degradation of the pipe material.

Understanding the context of waste management and plumbing, it's essential to choose materials that can withstand harsh chemicals. The other materials mentioned, such as PVC and ABS, while strong in various applications, are not as well-suited for continuous exposure to acidic environments. This distinction is crucial in plumbing practices where the integrity of the piping system is paramount to prevent environmental contamination.