My thoughts are as follows: ...

My thoughts are as follows:

The Coulter counter looks at particle size which is important in designing your system. 

The Silt Density Index is a measurement peculiar to RO membrane manufacturers.  It is hard to replicate, but gives an indication of how fast the filter clogs per unit of time.   The membrane guarantees have a clause related to this parameter - hence the reason for it being measured.  If the seawater is being reinjected, the particle size is important as well as the concentration of particle (probably more important), you will want to remove particles that may cake up your well casing/screens.