I use a vinegar solution to rinse the pump off before use. Rinse, well, then it should be good. This removes any nasties that may have grown inside.
To dissassemble the D5, you simply have to remove the large ring on the outside by unscrewing it, and separate the top from the base of the pump, noting the O-ring. Then inside, you will see the impellar. This is held in with magnet and can be removed by pulling out on it.
When reassembling, make sure the O-ring is in place before tightening the outer tightening ring.