Depending on your needs you may find tangential-flow ultrafiltration works well; we routinely use a 30kd ultrafiltration membrane, and can to purify phage with near 100 % efficiency from 200L of seawater. However, the efficiency can depend on the medium, and the specific conditions. More details can be found at: http://aem.asm.org/content/57/3/721.long
If you want to extract almost pure phages from sediments I woud suggest to use 10% beef extract (Difco; 1 ml/g sediment) followed by 1 h shaking at room temperature and 20 min centrifugation at 3000 g. The supernatant contains the extracted phages. There exist some protocols for enrichment: e.g. ISO 10705-1:1995 for F-specific ribonucleic acid bacteriophages
PEG (polyethelyene glycol) precipitation in the cold is the standard means of phage isolation. The precipitate can then be dialyzed into appropriate buffer. This can then be run in a CsCl2 equilibrium density gradient to generate high titer ultra pure phage.
This correct reference for the PEG and CsCl2 procedures should have been our prior 1978 article, now attached,
Determination of capsid size by satellite bacteriophage P4,