What percentage of suspended solids is removed during preliminary treatment?

In the context of preliminary treatment processes in wastewater treatment, it's important to understand the role of this stage. Preliminary treatment typically includes the removal of large solids and debris, which may include grit, rags, and other large particles, primarily through screening and sedimentation.

The correct answer indicates that during preliminary treatment, approximately 0% of suspended solids that are more finely divided, such as those that might remain in the water after the removal of larger materials, are removed. This is characteristic of the preliminary treatment stage because it primarily focuses on getting rid of large particulate matter and does not address the finer solids, which are removed in later stages of treatment.

Thus, while some larger particles and floatables might be removed, significant quantities of suspended solids in the form of colloidal and fine particulate matter typically remain and are targeted for removal during subsequent treatment phases, such as primary and secondary treatment. The primary goal of preliminary treatment is to protect downstream equipment and facilitate more efficient treatment in stages that follow.