You can set it up any way you want, but most common is the "occasional" (weekly, bi weekly, monthly) use rather than "constant" use...
Make up a tub of NSW...mix and heat... then pump old out and pump new in...
More effective if you pump old out first then add new, (but then you have water level issues) or can pump new in at same rate as pumping old out, (but then you have "dilution" issues)
Either way beats hauling buckets around and dumping and siphoning...