Sedimentation and Filtration Flashcards

What is 'filter-to-waste' and when is it typically employed?

Answer: Routing initial filter effluent to waste until turbidity meets standards after backwash

Filter-to-waste routes the first portion of filtrate (during the ripening period after backwash) to waste rather than the clearwell, preventing poor-quality water from entering distribution.

Slow sand filtration differs from rapid sand filtration primarily because slow sand relies on:

Answer: A biologically active layer called the schmutzdecke for removal

Slow sand filters develop a biologically active layer (schmutzdecke) on the surface that physically strains particles and biologically degrades organics, requiring no coagulation pretreatment.

What does a rising head loss curve that suddenly flattens during a filter run most likely indicate?

Answer: A broken underdrain lateral allowing short-circuit flow

A sudden flattening of a rising head loss curve can indicate a broken underdrain or lateral, which creates a short-circuit path that bypasses the clogged media and artificially reduces measured head loss.

Which backwash strategy introduces air at a low rate before water to improve media cleaning?

Answer: Air scour followed by water backwash

Air scour followed by water backwash first introduces air to aggressively scrub the media grains, then uses water alone to flush detached solids out of the filter.

In pressure filtration systems, an advantage over gravity filtration is:

Answer: Ability to operate at higher hydraulic loading rates without head loss concerns

Pressure filters use pressurized vessels that overcome head loss without relying on gravity head alone, allowing higher flow rates that would cause unacceptable head loss in an open gravity filter.

A sedimentation basin's detention time is calculated by dividing basin volume by:

Answer: The volumetric flow rate through the basin

Detention time (hours) = basin volume (gallons) ÷ flow rate (gph), representing the theoretical time water spends in the basin for settling to occur.

What is the primary risk of operating a sedimentation basin at a surface overflow rate that is too high?

Answer: Particles with settling velocities below the overflow rate will carry over to the filter

When the surface overflow rate exceeds the settling velocity of particles, those particles are carried upward with the flow and pass over the effluent weir rather than settling to the floor.