In 1990, the York W.W.T.P. installed 5 shallow bed sand filters which removed any remaining particulate matter after secondary treatment. These filters were capable of treating a maximum peak flow of 42 mgd. Effluent leaving the Sand Filters typically had suspended solids concentrations of less than 5 mg/l.

In 2000, the sand filters were modified with a more efficient under drain and backwash system. The maximum peak flow through these units is now 55 mgd. Cost of this renovation was $3,000,000 less than adding additional filters to handle the new peak flows.