Cara's Pool School — de filter

If it's a D.E. filter, ensure that the grid manifold and assembly were properly reinstalled, inspect the grids more closely for possible tears, confirm there are no structural cracks or breaks throughout the internals, and inspect the backwash valve for possible o-ring or structural flaws that could allow the debris to unexpectedly return to the pool. If it's a sand filter, inspect the backwash valve in the same manner; and, if nothing seems visibly suspect, there may be a need to replace the sand inside the filter and/or broken internal components.