RO Question

so i got a killer deal on a nice RO unit i couldnt pass...my first RO unit...anyway, the seller knew the membrane needs replacing...I hooked it up and everything is fine...the only thing that comes out right now is "waste" water...is this typical of a unit with a spent membrane? the only thing that comes out the "good" tube is like a drop every 3 secs...im just wondering if the membrane is def. spent like the seller said, or if there could be the chance of another problem??? any ideas?

 

yes this is normal. a properly operating ro machine has a 9-1, 10-1 waste to product ratio. that is normal. if the membrane needs replaced, then it will be slower. input pressure (supposed to 60 normally) and temp play a big roll too.

 

ok, thanks for the speedy reply!! i know it takes a TFC membrane so ill be going to my LFS sometime this week to price one.

 

cool, you should also check out a di canister. it would bring your water down close to zero or atleast <1 ppm

 

what do you think my tds? would be if i didnt have one?

 

lots of different factors come into play. only way to know would be to buy a tds meter. i prefer the handheld models.

 

Lets get this straight. A properly functioning RO membrane has a 4:1 waste ratio, for every 1 gallon of good you produce 4 gallons of waste, no where near 9 or 10:1.
In order to know if its functioning properly you need to do several things and you will need TDS meter, a pressure gauge preferreably an inline type, a thermometer to check water temp, a clock or watch and a measuring cup.

First you need to know your tap water pressure, your pressure available at the membrane and your water temperature. These will tell you what you can expect to produce in GPD. You also need to know which brand and model or size membrane you have, they are not all the same. You also need to know the temperature. Dow Filmtec RO membranes are designed to produce their rated flow at 50 psi and 77 degrees F water temp. Colder water or lower pressure reduces the GPD and higher pressure or warmer water raises the output. GE Water or Applied membrane require 65 psi to produce their rated flows.

Next you need to test the TDS of both the tap water and the RO effluent or treated water. You should be seeing somewhere between a 90 and 98% reduction in TDS between the tap and RO, so say tap water TDS is 200 you should be seeing around 4 to 20, prefferably the lesser with a 75 GPD or less membrane. Most membranes with the exception of the Dow Filmtec 100 GPD which is the absolute worst possible choice should give you 96-98% reduction in TDS and the Dow 100 only 90% reduction since its not really an RO membrane at all but a nano filter not approved for drinking water in the USA.

Next you need to time and measure both the good and waste flows for the same amount of time and determine what the waste ratio is. If its working right you should get only 4 times as much waste as good in the same amount of time. Try 3 minutes to start as see what you get. If its higher than 4:1 it could be due to cold water or low pressure or both. If its less than 4:1 you may need an adjustable or capillary tube flow restrictor to get it right.

Signs of a bad membrane are slow or no good water output and/or elevated TDS or both if everything else is in order. Any of the conditions above can affect output and quality though so you need to know all the above in order to troubleshoot correctly.

 

I just learned more about RO filters than I learned reading my RO manual. Thanks Desertrat.

 

No problem, glad to help out. You do this long enough and you pick up a lot!