bout to get a used RO/DI have a few questions.

im about to get a used 6 stage RO/DI filter from a good friend. when all the filters are new on this unit, how long approx till i have to replace them? we have well water here and use softener salt to help with it. im just wondering how many times a year i will have to replace the filters or if it will last more than a year.

also, how much are TDS meters to add onto a filter?

any basic knowledge on RO/DI filters would also be great.

 

Be very careful when buying a used RO/DI system. Unless you know the owner, the usage and history of the RO/DI system and have a TDS meter on hand to test its efficiency look at it as buying the housings and fittings only.
Chances are great it needs a complete overhaul including prefilter, carbon(s), RO membrane and DI resin at a minimum. Add the costs of those up and you already stated it did not have a TDS meter which is mandatory along with a pressure gauge you are approaching if not exceeding the $145 cost of a brand new reef quality RO/DI system.

It not to say there are not deals to be had but it is rare to find a used reef quality system functioning properly. You mention "six stage" so right off that tells me it is probably an ebay quality system to begin with and probably has small inefficient horizontal DI holow tubes and probably two high micron low quality carbon filters. Worth about $80-$90 retail brand new. It probably also includes a 90% efficient 100 GPD nano filter membrane and not a 96-98% efficient 75 GPD RO membrane. Good reef quality RO/DI systems have four stages, a 1 micron or less prefilter, a single 1 micron or less carbon block, a 75 GPD RO membrane and a single full size vertical DI canister and refillable cartridge. They also come standard with a TDS meter and inline pressure gauge.
Ask questions about the system, what brand, what membrane, what micron ranges on the filters, how old is it, how often were the filters replaced and was it disinfected at the time? Do they own a TDS meter they are not selling and what have the TDS readings been historically, thats tap water, RO only water and RO/DI water as you need to know all three numbers.

Compare the cost of what you are looking at along with the cost of new replacement filters ($8-$13 for a prefilter, $10-$15 for a single carbon block, $30-$45 for a new 98% efficient RO membrane and $15-$20 for new DI resin, $20-$40 for a handheld or dual inline TDS meter with the handheld being the better choice and $15 or so for an inline pressure gauge) to a new reef quality, non ebay system, for between $145 and $199 with everything mentioned above from many 3Reef Sponsors and reef specific sites. Some new systems carry a written guarantee and most I would recommend have free technical support.

The system you are contemplating may be a good deal but thats not the norm in my experience, you almost always end up spending much more to bring it up to reef quality and you still have a low end used system.

All RO/DI systems should have the prefilter and carbon block replaced every 6 months like clockwork and the system should be disinfected at that time, regardless of manufacturer or how much water you make.

 

I did not know that about disinfecting the system. How do you do that exactly? Wipe everything down with bleach?

 

Every vendor should have a method they publish.
For me I shut off the water supply, unscrew and remove the filters from the prefilter and carbon housings, add no more than two tablespoons of regular unscented bleach in the first canister and screw the empty canisters back on. Now very important, disconnect the 1/4" line that goes from the carbon block up to the RO membrane at the membrane housing end and stick this end in a bucket or sink, membranes cannot take any bleach or it damages them. Slowly turn the water on until the cansiters are full and water starts to run out the disconnected tube. Shut the water off for 3-5 minutes then turn it back on and flush until all scent of bleach is gone. Unscrew the empty canisters, dump the water out and install only the prefilter in the first housing at this time. Screw the other empty cansiter housings back on and flush the prefilter out the dsisconnected line for several minutes to remove any traces of glues or binders and antimicrobial treatments. Shut the water off and install the carbon block, if you have more than one do them one at a time. Flush the carbon(s) same as the prefilter to remove any fines or dust. Finally reinstall the disconnected line and you are ready to make water again.

 

the guy is on here that is selling it to me. hes never given or sold me bad stuff so i trust him. ill see if he can answer these questions for me though. thanks.