Negative head pressure in a filtration system typically indicates?

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Negative head on the suction side means the pressure there is below the surrounding atmospheric pressure. In a filtration system, this most often happens when air is in or entering the suction line. When air pockets are present, the pump can’t draw water effectively, so the inlet pressure drops and may appear as negative head on a gauge. This condition signals an air leak or air entrainment in the suction path and can lead to reduced flow or cavitation.

If the discharge side had higher pressure, you’d simply have a larger discharge head, not negative suction head. Positive head difference or normal operation would show nonnegative suction pressure.