"Class A certificate." Water systems of all types and capacities.
"Class B certificate." Water systems serving an average of not more than five million gallons per day.
"Class C certificate." Water systems serving an average of not more than one million gallons per day.
"Class D certificate." Water systems serving an average of not more than one hundred thousand gallons per day.
"Class E certificate." This certificate covers the distribution portion of water systems only.
63 P.S. § 1006