What is meant by retentate in membrane filtration processes?

Retentate refers to the portion of feedwater that is retained by the membrane during a filtration process. In membrane filtration, the feedwater is subjected to pressure which allows certain components to pass through the membrane while others are retained. The retentate consists of the larger particles, colloids, and some dissolved substances that are unable to permeate the membrane due to its selective permeability characteristics.

In this context, it's important to understand that the membrane acts as a barrier that separates materials based on size and/or charge. The components that do not pass through the membrane accumulate on the feed side and are termed retentate. This plays a critical role in processes that aim to concentrate desired materials or remove contaminants from water.

The other choices describe different aspects of the wastewater treatment process: treated water and waste streams, but they do not define the term "retentate," which specifically identifies what is held back by the membrane during filtration.