Running very cold water through you RO/DI will severely cut down on your production rate. Optimum temperature for input water is around 70 Fahrenheit. Only problem with that is if you aren't careful and the water gets to hot you can destroy your RO membrane. Usually simpler to just turn on the cold water and be worry free, but in the winter months production can be painfully slow.
I know some people run their incoming water line through a bucket of warm water to heat the incoming water to improve production, might be something to consider, especially with the amount of water you are making.
OK, lets start from the beginning.
What system did you purchase, what does it contain as far as prefilters and carbons and in what micron ranges, what brand and model membrane is it and how is the DI configured as in is it a full size 10" canister style that is refillable or is it a horizontal on top either refillable or a throw away?
All these things make a difference
Next what are your incoming water pressure, water temperature and tap waterTDS readings?
What is your waste ratio?
Everything above has a direct bearing on how well a RO or RO/DI system works and how long the membrane and DI resin last.
In the beginning when you first set a system up you should do several important things and in the correct order.
1. If the filters are all preinstalled, remove the carbon(s) and disconnect the 1/4" line which leads from the carbon up to the RO membrane housing at the membrane end. Hook the unit up to a cold water supply and flush the prefilter for several minutes to remove any antimicrobial coatings, glues and binders so they do not foul the carbon block.
2. Install the carbon block, hopefully its a good unit and only contains the one necessary carbon block, multiples are unnecessary and can actually lead to problems. Again turn the water on and flush both the prefilter and carbon block to remove any fines or carbon dust so it does not fould the RO membranes surface.
3. If you have multiple carbon blocks add the second one if you wish and do the flush process again. If you have granular carbon at this point I suggest throwing it as far as you can as granular media has no place in front of a RO membrane since it grinds itself to dust and ends up ruining your membrane. If you have a GAC filter we need to talk further.
4. Now hook the line back up to the RO membrane and disconnect the line to the DI filter or remove the DI resin or cartrige and flush 3 to 5 gallons out of the RO membrane. This will be a slow process as you have already noticed, every a full 75 GPD under perfrct conditions is only about 3 gallons per hours at best.
Watch your TDS readings while flushing the membrane, you should see it drop to a low somewhere around 96-98% less than the tap water TDS, you are fully flushed at this point.
5. Now install the DI cartridge or resin or hook the line back up and do a final rinse up of the resin, again watching the TDS meter until it drops to 0 TDS. ANY RO/DI should be capable of 0 TDS, the mark of a good system is how long it remains at 0 TDS on each DI refill.
6. Check your waste ratio. Using a measuring cup and clock or watch, time how long it takes to make equal amounts of treated water and waste water. The waste ratio should be very close to 4:1 or 4 times as much brine as permeate or treated water.
Many if not most vendors neglect to tell you how to properly flush a system for use and the result is shortened filter life and lower qater quality. When you talk rejection rate or removal efficiency we are talking about the TDS of the tap water and the RO only water, not RO/DI TDS.
Water pressure and water temperature have the greatest effects on RO GPD and even water quality. Dow Fiilmtec tests their membranes at 50 psi and 77 degrees F., that does not mean these are the ideal parameters its just a way to set a standard. In fact higher pressures will produce not only more water but higher quality water as membranes work better under higher transmembrane pressures, I run my personal RO/DI at between 90 and 100 psi with a booster pump which allows me to get 99.35% rejection rate with RO only and closer to 120 GPD out of a 75 GPD Dow membrane. Water temperature also has an effect and truthfully colder water treats better than warmer water and again will give you lower TDS since it is more dense aor less viscous so fewer contaminant particles pass through the membarne fabric. The disadvantage of colder water is it reduces the GPD but for most of us with a 75 GPD system we can still make more than we need in a day anyway. Heating the water in a bucket is extremely expensive since heaters cannot keep up with the water flowing thru the tubing. Trying to blend or temper hot andcold is also a very poor idea since it is the number one way to permanently damage a RO membrane. Remember being in the shower when someone flushed the toilet or started the washing machine? You got your tush scalded and thats the same thing that melts a RO membrane. Membranes cannot tolerate anything over 113 degrees F and thats not really all that hot, don't take that chance.
Lets start out with your particulars, if you can answer the first several questions I had I'll bet we can solve your problem.
ebay purchase, seems like they are all the same with a company logo stuck to them before sale. the filters on the other hand are as follows: 5 micron sediment filter, 5 micron carbon filter, 1 micron carbon filter, ro membrane is dow filmtec 75gpd, the di is the 10 inch canister with color change resin. flow rate seems to be right on from what i see in the tank, just seemed slow to the ro/di newbie. pressure and temperature are unkown right now. it is cold and right off the main line from the meter post water softener. tap water tds was 340. tds meter is the fish doctors, not mine so it may be off. hope this helps.
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