Sump pump in my basement has no lid. Does the dehumidifier suck up the sump pump's water?

The sump pump should be pumping water into the waste line somewhere above it. If you have a dehumidifier that allows you to buy pass the collection tank and run vinyl tubing directly into the sump you wouldn't need to ever empty it. Besides the obvious advantage it will also keep the air in your basement drier as it never shuts off the way it does now when the collection tank becomes full. As for the lid, it certainly would keep any water in the sump from evaporating and entering the air just to be removed again.

Yes-your de-humidifier will suck any moisture in the air, but make sure your doors and windows are shut, otherwise you are de-humidifying way too much area. Just cover your sump with almost anything like plastic, but drain into the nearest sink or even through a small hole through a window. Do NOT use any fans.

Sumps put a lot of humidity into the air...you want to cap it off...what I found was an old plastic x-mas tree stand like this...http://dm61q01mhxuli.cloudfront.net/images/a137/im... and it fit like a glove...I had to notch out where the pipe come out of sump using a hack saw...its also ez to remove if you need to get in the sump...