Here's what worked for us. We had a Pur 40E on the previous boat and cruised 2 1/2 years, using the watermaker about 2 of them. We currently have a Pur 80E. We wanted and want to make water in silty conditions so added another pre-filter (Lowe's $30, NOT marine
$300 +) with a 5 micron filter in the second one.
After a lot of trial and error found that using our raw water pump
(auto cut-off like the fresh pressure water system) and a 2 Gal accumulator feeding the pre-filters to watermaker eliminated all air problems that seemed to be more from cavitation than anything else.
The feed water is teed off the engine
raw water intake, which is always under water.
We've made water offshore
underway, power and sail, and at anchor
. Even in our very silty bayous. Never in an industrial type area where there could be oil
on the water. Oil
and chlorine will quicly destroy the membrane. Of course the prefilters will load up a lot quicker in dirty water.
Speaking of prefilters, they can be simply cleaned and reused as long as they're in good shape. That is not collapsing. We changed the 20 micron once in the 2 years. We changed the 5's a few times because they're a bit harder to clean. We used the "string" type. Be sure NOT to use a paper filter.