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Listen to part of a lecture in an environmental science class. The professor is discussing the role of wetlands in water purification. "Today, I want to talk about wetlands, which are often called 'nature's kidneys.' This nickname comes from their remarkable ability to filter pollutants from water. When runoff from farms or cities enters a wetland, the slow-moving water allows sediments to settle. Plants like cattails and bulrushes absorb excess nutrients, such as nitrogen and phosphorus, which would otherwise cause harmful algal blooms in lakes and rivers. Moreover, beneficial bacteria in the wetland soil break down organic pollutants and some toxic chemicals. However, these processes are not unlimited. If a wetland becomes overloaded with contaminants, it can lose its filtering capacity and become a source of pollution itself. This is why protecting and restoring wetlands is critical for maintaining clean water supplies. But interestingly, scientists are now designing 'constructed wetlands' to treat wastewater in a more sustainable way. These artificial systems mimic natural wetlands and are increasingly used in rural areas where traditional sewage treatment is too expensive." Now, get ready to answer the question.