Physical removal of bacteria by straining occurs when______________________.

The physical removal of bacteria by straining primarily occurs when the wastewater is passing through a medium with smaller pore sizes relative to the bacteria. When the pores in the soil are large, they cannot effectively trap or filter out the bacteria, which are typically on the order of 0.5 to 5 micrometers in size. Consequently, large pores allow bacteria to pass through without significant interception.

Instead, it is the smaller pores in the soil that create the necessary conditions for straining to take place, as these pores restrict the movement of bacteria, leading to their physical removal from the wastewater through filtration. This mechanism is critical in maintaining the effectiveness of soil as a treatment medium in on-site sewage facilities, emphasizing the importance of the soil structure in wastewater management.

In context, when the soil is saturated, this could lead to liquefaction of the structure, diminishing its ability to strain effectively. If wastewater is highly concentrated or if bacteria form aggregates, these conditions may also affect treatment efficiency but do not directly relate to the straining process itself as well as the size of the soil pores does.