I think the best methods are: 1) Filtration. You can use a membrane with small pore size to filtrate out the water and get concentrated phage solution. Check this: ULtra-15; w/ULtracel-10 membrane; 10,000 NMWL; Dia. x L: 29.6 x 72mm; 24/Pk. Cat. #: UFC9-010-24.  2) Ultracentrifugation: Use a high-speed centrifuge for concentrating the phage (20,000 g). 

You can use CsCl gradient centrifugation and dialysis (concentrating phages using PEG precipitation prior to applying on gradient would be advisable).

I fully support the proposal of Dr. Chiqian Zhang.

But I suggest using centrifugation at 50,000 g for 2 hours.

But before centrifugation advise treate phage suspension with DNase and RNase at a concentration of 30 micrograms per milliliter for 1 hour at room temperature or 37 degrees Celsius.

The essential point is gently resuspende the sediment. For this purpose it should be resuspended with a small magnet at low speed magnetic stirring in a small volume of the required buffer. The suspension can be diluted.