What type of solids cannot be removed from effluent filtration systems like sand filters?

Dissolved solids are substances that are present in a solution at the molecular or ionic level and do not settle out or can pass through filtration media used in effluent filtration systems such as sand filters. Unlike suspended solids, which can be physical particles that may be removed through mechanical filtration, dissolved solids are too small and often too uniformly distributed within the body of water, making their removal through typical filtration systems unfeasible.

Colloidal solids are complex as well; these particles are larger than dissolved solids but smaller than suspended solids, which makes them harder to filter out using simpler methods. Organic solids might include biodegradable materials and can often be removed via sedimentation or filtration to some extent, depending on their size and structure. Hence, dissolved solids stand out as the type of solid that cannot be effectively removed by standard effluent filtration methods.