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I was deeply involved in 'filtration engineering' for well over 35 years working in ultra-pure filtration and 'molecular separation' with 'filters'.

First off, the SeaGull housing is VERY well made and equates to the quality of ultra-pure or 'pharmaceutical' filtration. It is easily cleanable, its stainless steel, its polished to 'mirror bright' surface finish, all the internal welds are ground flat and polished ..... all cost a lot to accomplish; plus, there are costs (margins) for 'distribution', 'vendors' and suppliers.
Second, the SeaGull filter cartridges are of a proprietary configuration - no other competitors filter will FIT in the SeaGull housing. Its a very good system, albeit very expensive. The typical cost of a 5 or 10" polished stainless pharma grade filter housing is $1000.

The common filter configuration in 'the industry' is in multiples of 10 inch (or 9.75") long filter cartridges of either 2.75 or 2.5" in diameter. There are 'plastic' housings ($80-90 ea.) that are suitable for 'water filtration' using COMMON (industrial) filters @ $15-20 each. There are various configurations to seal the cartridge to the housing - O-rings, flat gaskets, knife-edge seal, etc. The housing configuration of how the cartridge 'seals' to the housing determines which type of cartridge used. 10" (9.75") long X 2.75 or 2.5" diameter cartridges are the industry standard size ... and there are 'hundreds' of such manufacturers.
Flat gasket or 'knife edge seal' cartridges are 'ok' for boat systems. You can use a 9-3/4" length cartridge in a 10" housing ... just use 2 'extra' flat gaskets on the end of the filter cartridge.

There is a SIMPLER and FAR LESS EXPENSIVE, more 'efficacious' and SAFER way to do this !!!!!! Its called HYGIENE .... the periodic use of soap and water to CLEAN out the tank, removing ALL water filters from inside the boat, and only FILTERING THE WATER AT THE DOCK HOSE before it gets into the boat, and periodically add a wee bit of 'clorox' (4 oz. per 100 gallons of water) !!!!! Treat your 'boat water' as you do with your household dishes/water glasses etc.

If your tank water stinks or is 'funky' you HAVE a contaminated tank or some of the 'plastics' used are INCOMPATIBLE for use with POTABLE WATER.

Issues:
• Remove and replace any 'hose' that has become 'cloudy' or 'yellowish' - the 'plastic is breaking down' and you'll be drinking 'fragments' of these polymers.

• Water tank - install 'clean out' ports so you can get inside the tank with a long handled brush and scrub the tank walls anytime you can reach in and 'feel' any 'slime' on the tank walls.

• Tank VENT .... replace the current vent hose with new and install a hydrophobic (PTFE), 0,2µM (absolute 100% rated) 'capsule' filter of at least 0.5 sq. ft. on the end of the hose ... (about $100 from a pharmaceutical filtration supplier). This will prevent the aspiration of bacteria, mold, fungi when the tank is drawn down - possibly the CHIEF entry point for 'microorganisms'. Mount the capsule filter 'high' in the inside of the boat so that it never 'gets wet'.

• FILTER THE WATER BEFORE IT GETS INTO THE BOAT !!!!!!!!!
Removable filters for attachment to 'delivery end' of DOCKSIDE water hose - use ONLY water hose RATED for POTABLE WATER.
1. Carbon pre-filter at 5µM
2. final filter 1.2µM CERTIFIED FOR THE 99.99% REMOVAL OF OOCYSTS
Home Depot or Lowes filters are OK (Cheap) but 'final filter' MUST be CERTIFIED FOR 99.99% removal of oocysts. Filtration for oocysts is VERY important as they are 'chlorine resistant', are still present in 'some' municipal USA water systems ... are 'pathogenic'. do websearch for Giardia Lamdia and Cryptosporidium - VERY 'NASTY'.
The filters (and system 'cleanliness') will keep 'funky taste', most common organisms, particulate from ENTERING the clean tank/system.
Dry out the filters when finished filling the tank.
Wash the boat's water 'fill port' with clorox solution before opening it.
When using a carbon 'pre-filter', fill the tank SLOWLY because it 'takes time' for 'contaminates' to adsorb to the carbon. Fill SLOWLY when using carbon filters.
(REMOVE ALL 'INSIDE THE BOAT' FILTERS)

• Shock sanitization and 'maintenance/stability' sanitization
1. Shock Sanitizing - After any cleaning or every 6 months without cleaning ......
40 ounces of 'clorox' per 100 gallons. Fully fill system, run spiggots until you can SMELL the clorox, let soak 2-3 hours. Then rinse and 'dump' the entire tank several times ... until you cannot smell any clorox odor from the spiggots.
2. Maintenance Sanitizing - approximately 4 oz. clorox per 100 gallons; or (better), just enough added 'clorox' so that the average WOMAN can 'just barely' smell the clorox at a spiggot. Check for that 'just barely smell' every week, add more if needed.

Reason for NOT having an onboard carbon filter - it will remove the 'maintenance dose' of clorox from the water, even if the water is not flowing. Without 'active' free chlorine available in (essentially stagnant) water, microorganisms that do pass through the dockside filtration or tank vent will 'thrive'. Maintenance sanitization is very important. (Municipal water systems typically 'dose' their water so that 1 parts per million of chlorine comes out the users spiggot.)

Other - carbon will be used by various microorganisms as their nutrient source in a 'stagnant' water system ... why FEED them which only promotes 'growth'? Do your 'deodorizing' and 'de-colorozing' with the 'dockside' carbon pre-filter!

• For long term storage of a boat (more than a few weeks) - EMPTY THE TANK. You wouldnt leave a picnic cooler filled with water for a month would you ?!!!!! Shock sanitize before refilling tank, etc. after very long 'lay-up'.

Do all the above and you'll have much less chance of 'getting the TROTS' ... not fun when on a boat.
 

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To answer your 'leaking' problem if a weld has cracked or 'burned through' (due to crevice corrosion) any experienced stainless welder can repair it.

Those filters that are CERTIFIED for oocyst removal will meet the 'minimum' ... must state on the package: CERTIFIED for 99.99% removal of oocysts.
 

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Virus 'removal' is an easy trap to fall into, a quite misleading statement but still consistent with FDA (NSF), etc. terminology.

Viral removal is a statistical reduction removal based on whats chiefly known as "(positive) Zeta Potential" of the filter, etc. material. Zeta Potential is primarily an 'electronic charge' adsorptive surface effect (similar to the adsorptive effect found in activated charcoal which statistically removes essentially negative charged particles using a highly positive charged surface of the filter, etc. material). Most materials using 'zeta potential' are positively charged (enhanced); but, can be applied with negative zeta depending on the species being considered for 'a statistical reduction'.
The important consideration is that zeta potential (positive or negative) is NOT an absolute 100% means of reduction; usually in practice not even close to an absolute reduction and usually in the range of 10E10 (10 to the minus 10 reduction), 10E7, 10E5, etc. etc. reduction per single pass through the device.

So, when using a zeta potential material or a filter material or membrane whose porosity is greater than the actual physical size of the viral particle, one will ALWAYS have residual viral, etc. particles in that 'downstream' fluid being processed.

Also, with NO commercial, etc. connection to General Ecology Corp (SeaGull), their website 'does' state/list bacterial / viral removal ("to EPA standard levels" ... which correlates to actual repeatable testing methods per FDA approved methodologies and (traceable) with independent certified VALIDATION). However, even with FDA or even well LESS regulated NSF methods, one cannot be sure/certain of 100% (sterile) removal unless that mfg. states or certifies 100% (absolute) removal ... or for bacterial and similar organism a stated/certified removal of 10E13 (log reduction of 10 to the 13th power per square cm of 'filter' material) a standard FDA requirement for 'sterility', ..... especially if such device being used is not properly sterilized/sanitized prior to usage).
One really has to be able to understand the complex regulatory 'jargon' of the filter manufacturers and FDA (the responsible controlling agency) for the removal of viruses, bacteria, cycts, etc. etc. etc. etc., otherwise its very easy (even for scientists and engineers who dont work in the specific (FDA regulated) area of endeavor) to become innocently trapped into using something that is not correct for the application and its outcome/output.

Rx: Beware of 'marketing' to the general consumer.
 
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