Thanks to everyone who responded. I can’t say enough about this group!

Some of the suggestions spurred me on to further research, down many rabbit holes. I’d like to give a synopsis of what I learned, read on only if interested, obviously.

It was reassuring to find out that the 280 TDS is within a very reasonable range. I am going to clean the membranes, 2% citric acid solution for acid, and 2% sodium metasilicate for the alkaline. I’ll see if that buffs up the TDS at all.

Indeed, the World Health Organization, and many governmental agencies has standards for TDS in drinking water, with a spectrum of acceptability, generally over 1,000 being unacceptable, but less than 500 as desirable.

However, not all TDS are created equal. TDS is generally anions, cations, and organic molecules. They may occur due to leaching from the ground water table, or agricultural runoff. Where I live (northeastern US), many homes with wells have high TDS due to the wells being in limestone and other similar rock formations. The usual problem from high TDS here is “hard water”, due to excessive calcium and magnesium. This can cause buildup of deposits in plumbing, and poor washing with detergents and soaps.

In other parts of the world, high TDS may be due to agricultural runoff, and include phosphates and organic molecules.

For our purposes on a boat, TDS reflects residual anions and cations from the sea water. In this case, it is almost exclusively sodium and chloride.

So, high TDS not associated with water makers is usually calcium, magnesium (plus or minus organics); indeed, San Pellegrino water, a highly regarded drinking water from Italy, has 1,100 TDS (mostly calcium, magnesium, and sulfate), unacceptable by some standards, but tasty to many people.

High TDS on a boat with a RO water maker implies poorly functioning membranes, will be mostly sodium and chloride, and, for many people, becomes unpalatable above 400 TDS. There is not a USA EPA standard for sodium in drinking water, but the EPA does recommend not exceeding 250 mg per liter of chloride. This standard is predominantly an “aesthetic” one, and reflects the salty taste that a level of 250 or above will impart to the water. As per Dessalator’s specs, RO water with a TDS of 250 has 183 mg per liter of chloride. The conclusion that I would draw from this is a TDS of about 350 or above would exceed the EPA’s recommendation. Again, this is predominantly a taste concern, but there are some potential health effects. People with heart or kidney problems may not tolerate it well and retain fluid as the TDS rises.

Tom Peacock
SM #240 Aletes
Chesapeake Bay, USA