A series resistor may waste power, but not as much as running it directly on 220V.
220V direct connection:
Rp = effective resistance of pump
Power used = 220^2 / Rp = 48400 / Rp
220V connection with resistor:
Rp = effective resistance of pump
Rs = series resistance = Rp
Power used = 220^2 / (Rp + Rs) = 48400 / 2Rp = 24200 / Rp
The series resistor cuts the total power IN HALF. The direct connection to 220 V is clearly the biggest power waster.
Of course, a transformer will cut the wasted power even more. But transformers cost a LOT more than resistors. That extra cost also represents a waste of power and/or resources. Care to do a "break even" calculation?
And if it goes up in smoke then there is the replacement cost for the overall device. And possibly for whatever else catches fire.
I would NOT run it directly on 220 Volts. Either of the options is clearly a better one than that.
BTW, just why is a step down transformer such a bad long term solution? Other than cost, of course.
Good suggestion, I will measure power consumption at 120V and 240V, however using a resister to reduce the voltage from 240 to 120 is a huge waste of power, better to use a step-down transformer, but that is an additional $$ and it's not a good long term solution.