Drinking Water Filtration--The Short Version

rev. 6-21-2014 

(The LONG version was published in Practical Sailor in 4 parts in Spring 2015.)

The Problem. Marina water is basically untrustworthy. Either the well is brackish of the hoses are contaminated with algae growth. Either way, the water contains dirt and too much sulfur for pleasant consumption. In the midst of researching a 3-part article on water treatment and filtration for sailors I decided to apply what I'd learned to a simple up grade for Shoal Survivor. I'll explain a little--if you want the really long version,subscribe to Practical Sailor.

The Solution. The way I see it, there are 4 parts to the clean water quest:

Step 1. Filter the incoming water. Skip this step, even with potable water, and you will build solids in your tank. Pipe scale will shed, harmless bio-films in distribution piping will shed, and dockside hoses always grow algae. Build up solids on the tank floor, and disinfectants and the residual chlorine in the tap water can't reach the bugs underneath. Given time, biomass is food for bugs, the tank will be come gross. I've even seen lines plugged. My choice? First, run the water for a few minutes to run the junk out of the hose, perhaps rinsing off the dodger; it needs it anyway. After that, I like the Camco RV filters which simply screw on the end of the hose, $10 from Walmart. The marine store stuff is the same for 2-3 times the price. I put a 4-inch PVC pipe extension on the outlet so that it stands easily in the fill pipe.

What about the fact that the carbon filter will remove the beneficial chlorine residual? Opinions vary, but ...

• We've used these for 10 years and never built any biomass in the tank. If the tank stays clean and the carbon removes most organic material, there isn't much for the bugs to eat.
• The chlorine residual is not going to last more than 24 hours, and finally...

 ... Testing (mine and others) confirms that hose end filters only remove about 10-20 % of the chlorine anyway. The flow rates are simply too great to give the residence time needed for effective removal. Honest, it's a marketing hoax, but we're conditioned to buy carbon. A brief review of the certification documents will show that they are only required to remove 25% of the chlorine to claim "removal,", and the test was run at a greatly reduced flow rate. Get some aquarium test tapes and run your own tests. You'll be stunned.

A much better solution is a 1 micron Baja water filter, featured in Practical Sailor. My invention, you're going to love it. Perfect for tap water, rainwater, and jerry can water.

(And don't EVER put glycol in your tank to winterize. It will ferment, it will smell, and there is more than enough food to clog lines. Empty the tank and dry it. the glycol goes ONLY in the pipes. In the pipes it must be installed at sufficient concentration to serve as a biostat (pH >9, and >25% glycol). If the winterizing solution pH is less than 9, add a touch of borax to bring it up. the result will be an effective sterilizing solution and little or not need for spring cleaning.

Step 2. Disinfect the tank.  About 1 teaspoon per 50 gallons is enough to refresh the residual for good water. Better yet, try Aqua Mega Tabs; really convenient and because they are based on dichlorisocyanurinate, the chlorine residual lasts weeks instead of days, keeping the thank fresh. Tip: use 1/4 to 1/2 the recommended dose, as they are rather conservative. Use aquarium test tapes to test for residual (0.1-1.0 ppm).

Bugs that crawled into the venter filter in a single season.

Step 3. Secure the tank vent. Plumbing codes required a 16-mesh screen and a down turn protected from dust, but many boat builders just leave an open pipe--PDQ did. If you look closely, every so often there will be stink bug doing the back stroke or the bones of some no-see-ums on the floor, decomposing. And while your about it, make certain that seawater can't find its way in. Secure the vent.



Step 4. Filter the water prior to the tap. Even if you have followed all of the forgoing, something could have gotten past and the bleach will dissipate in a few days.

Just how big are microbes we are trying to exclude?

Organisms

Size range (microns)

Example (size in mcrons)

Cysts (NSF 53 filters                                     4-14                                     Cryptosporimium  (4-8)

Prokaryotes

Bacterium: typical rod

1.0-0.5 x 1.0-10

Pseudomonas aeruginosa (1.5 x 0.5)

Bacterium: typical sphere

1.0

Bacillus megaterium (7.6 x 2.4)

Eukaryotes

Fungi: filamentous Red Tide

8-15 x 4-8 5-75

Mucor hiemalis (8 diameter) (disolved toxins may be present in red tide)

Fungi: yeast cell

Saccharomyces cerevisiae (29-49.1 μm3)

Alga

28-32 x 8-12

Chlamydomonas

Viruses (NSF P231 filters)

Virus

0.015 x 0.3

Poliovirus (0.03 x 0.03)

Tobacco mosaic (0.02 x 0.3)

While cysts, fungi, algae and some bacterium can be filtered out with 0.5 micron mechanical filters, viruses require either adsorption mechanisms which are hard to define and fade as the cartridge is spent, or membrane filtration (RO through ultra filter). Several carbon block filters have been upgraded with a membrane on the inside, achieving NSF P231 approval (microbial barrier, tested with real bugs to block everything).

How are nominal and absolute filtration performance defined? There is no agreed upon standard, but generally speaking, nominal filtration requires 70-98% retention and absolute filtration requires >98% retention of inert test particles.

I could install UV sterilizers (too much power) or an RO system (they waste about 4 gallons for every gallon produced and they leak bacteria anyway), but I'm not nearly that paranoid. I've had no filter on the boat for many years, and I drank out of some real mud puddles in my backpacking days without ill effect.

I could install the Seagull filter($600-$1000 for a filter--they are completely out of touch with the market) or the equally effective Microguard by Pentek ($129 complete), or better yet, the Pentek DVG-50 (NSF P 231 certified, $131 complete), which are based upon less expensive standardized parts. Both are well tested and rated to remove cysts, bacteria, and viruses. The Seagull filter just costs more. But US coastal sailors don't need either.

My choice? In the US, water treatment and on-boat chlorination are very effective on bacteria and viruses. However, some protezoa and cysts can sneak by, resisting chlorine treatment. These are all we really need to remove at the tap. I like carbon block filters, available in 0.5 micron nominal filtration (Pentek  Slimline or G3 housing with 2x10 CBC-10 or Floplus 10) . They will easily stop of protozoa and cysts (NSF 53 certified filters are rated for this), >98% of the bacteria, and >90% of the viruses. True, viruses are smaller than 0.5 microns, but many are attracted to the carbon surface by electrostatic forces, just as other chemicals are adsorbed. Many bacteria and viruses are attached to larger particles and are removed in that way. Although removals vary according the referenced data source, by the combined mechanisms the reduction is still material. WHO has documented 98.5% reduction of bacteria and 88% reduction or viruses by simple sand filters--a depth filter with similar mechanisms and much larger pore size**.  Granted, it may only take one organism to infect you--I'd worry about that traveling in some areas--but not in the US. This little miracle is only $25 including the housing and replacement cartridges are available through many sources. The time to install? About 15 minutes.

We later switched to a 10-inch filter to get more flow--the Pentek CBC-5 in the photo was OK for drinking, but slow when filling pots and washing dishes. We switched to the Pentek Floplus-10, which is NSF 53 certified for cyst removal and does not restrict flow. The longer housing hangs closer to the floor, but only about 1-inch is required for removal, which can be done without spilling a drop.

(A note to PDQ Altair owners: that big grey carbon-reinforced beam crossing the forward end of the salon bench is the mast compression beam. Do not drill holes in the flange.)

 This universal housing in the photo is from Pentek (158203) was $11, the 0.5 micron carbon block cartridge (2 x 5 standard, Pentek CBC-5) was $10, and the fittings were left behind in a bag by the previous owner. It does reduce the flow at 30-40 psi (standard for boats) from a thunderous 4 gpm that splashed all over, to a somewhat slow 0.7 gpm, about like the refrigerator dispenser at home. It would certainly be suitable for smaller boats.

I later upgraded to a Pentek G3 2x10 housing ($9) and a Floplus 10 cartridge ($16), with about 4 times the flow; all I can use without splashing. I did not filter water for showers; pre-filtered and chlorinated US tap water is more than adequate. Do place any filter on the pressure side of the pump; the pump will cavitate trying to draw through a carbon block or any other fine filter media. I've had the same cartridge for 2 years, allowing it to dry out each winter in the off season and then re-installing. I'm very pleased with it.

The Result. When finished we did a blind taste test with 5 sailors along the dock; not one could distinguish the tap water from Deer Park bottled water (based upon mineral content analysis, the two waters are very similar). For the coastal US sailor, we have improved the taste as far as possible, and reduced the biological risk by at least 3 orders of magnitude. I feel I've reached beyond the point of diminishing returns clear to the point of no return.

I'm sure I've save the price of bottled water, and the reduction in hauling and trash has been wonderful.

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The full version can be found in "Keeping a Cruising Boat for Pennies."