No you cannot use a filter bag.
Lets do a little troubleshooting to see where your problem lies before you do anything.
1. What system do you have, what micron ratings are the filters and what membrane (brand and GPD) does it have? How old is the system and when were the prefilter and carbon last replaced and the system disinfected with bleach?
2. How is the DI system designed? Is it refillable or do you use throw away DI filters? Is it mounted vertically or horizontally and does it have more than one DI filter presently? How long since a DI change?
3. Now lets do some hands on troubleshooting. What is your water pressure at the tap? If your system has an inline pressure gauge, what is the pressure at the RO membrane? What is your water temperature? What is your waste ratio set at? You can use a measuring cup and clock or watch to time and measure the flows for one or two minutes to get a close estimate.
4. Assuming you have a handheld TDS meter, first test the tap water TDS using a squeaky clean clear glass drinking water glass with absolutely no water spots or soap residue on it. Fill the glass half way and stick the meter in it allowing the readings to stabilize. Record the TDS and triple rinse bothe the glass and meter in RO/DI or distilled water. Next test the RO only TDS before the DI filter, this may require disconnecting the DI or removing the cartridge if you do not have an RO bypass valve (nice thing to have and only about $5-$8). Again let the reading stabilize and record the results then triple rinse the galss and meter. Finally take a RO/DI TDS , record the results, do a final triple rinse and put the meter away capped andclean and store the glass upside down for next time.
Now lets answer the above questions in order.
1. The system you have is important since there are untold hundreds of systems on the market, some good some bad and the quality is all over the map. Some systems will never ever deliver 0 TDS water even on their best day period, others will last and last for years. The micron rating of replacement filters is critical sine their purpose is to protect the RO membrane. The lower the micron rating generally the better they do this. Absolute rated filters are much better than nominal rated filters but you wil pay more. Thye are worth the difference if you plan to keep your RO/DI system for years. Filters need to be replaced every 6 months like clockwork and the system needs to be disinfected at that time. Plugged or fouled prefilters cause a pressure drop to the membrane reducing not only its GPD output but also its final water quality. I use a prefilter of 1 micron or less always. My present filters are 0.2 micron pleated absolute rated prefilters that have 10x the surface area of a normal spun poly filter cartridge so they fillter much better, last longer and provide less pressure restriction.
Disinfection is a simple 5 minute process that only requires 2 or 3 tablespoons of regular unscented household bleach like Clorox. Unscrew the prefilter and carbon housings and remove the old filters. Add no more than 3 spoons of bleach to the prefilter housing . more is NOT better in this case, and screw the empty housings back in place. Now VERY IMPORTANT, disconnect the 1/4" line leading from the carbon up to the RO membrane at the membrane housing end and stick this line in the sink or a bucket. you may need to keep a spare foot or two of 1/4" line around to reach the sink or bucket for thsi step. Slowly turn the water on and allow the housings to fill until water starts to exit out the disconnected tube. Shut the watre off and allow it to sit for 3-5 minutes. Turn the water back on and flush it until all traces of chlrorine smell are gone. Remember household bleach is 30,000 to 50,000 parts per million in strength so a little bit goes a long way, more is not needed. The Centers for Disease Control (CDC) in Atlanta has determined that 0.5 parts per million will kill just about anything known to man in as little as 1 minute so don't overdo it.
Now lets install the new filters. First, leave the line disconnected to the membrane. Unscrew the first housing and add the prefilter by itself, no carbon, then flush to waste again for a minute or two to clean the binders and glue off the prefilter. Next add the carbon block, if you have two do them one at a time, and again flush the filter to waste to remove any carbon fines or dust. Finally reconnect the line up to the RO membrane and make some water. If the system is making wierd noises it may have trapped air which can be removed by holding the RO/DI system in your hands and rotating it around in different orientations to release any trapped air pockets.
2. DI requires contact time to work effectively. Good DI filters are mounted vertically and have the water enter from the bottom and exit the top so you get a bottom up flow and good treatment. Cartridges should be packed tightly with no empty space or voids. If yours is mounted horizontally on top move it to the side of the vertical canisters and mount it on the wall or a bracket with the RO line going in the bottom and RO/DI exiting the top, you will get better contact time with no channelling or short circuiting with a properly packed cartridge.
3. Water pressure and temperature have major effects not only on production rates (GPD) but on final water quality too. The greater the pressure the more efficient the membrane becomes. If your pressure is below 50 psi a booster pump might be a wise investment. The same membrane will pick up a couple of percent efficiency just by increasing the pressure from 50 to 80 or 90 psi. Water temperature is something we have little control over but I don't worry much about it because cold water actually makes lower TDS water since it is more dense and filters better. Warmer water will give you higher TDS. Dow Filmtec RO membranes are designed to work at 50 psi and 77 degrees F. GE and Applied are designed to work at 65 psi and 77 degrees F. This means a 100 GPD GE or Applied membrane is equal to a 75 GPD Dow Filmtec membrane! Lay the graphs over each other and you see all three of them producing 75 GPD at 50 psi, 90 GPD at 60 psi and 100 GPD at 65 psi. Almost identical so no advantage to one or the other. Just stay away from the Dow Filmtec 100 GPD at all costs, its not a RO membrane at all but only a 90% efficient nanofilter not intended for drinking water use in the USA.
Do not try to blend or temper hot and cold water together! This is the quickest way there is to ruin a membrane. Remember being in the shower when someone flushes the stool or starts the washing machine? You got your bum scalded, same thing happens to a membrane and it melts into mush. They cannot take any water over 110 degrees period. Live with colder water or get a booster pump to overcome it, remember colde rwater produces lower TDS even though it slows production. Just plan further ahead when making water.
4. Now lest look at the the TDS readings. Your RO membrane, assuming it is not a Dow Filmtec 100 GPD nanofilter and is either a GE, Applied or Dow 75 GPD or smaller, should be removing 96-98% of the incoming TDS. Since a TDS of 200 is pretty common lets use that number as an example. If the tap TDS is 200 you should be seeing a RO only TDS of between 4 and 8 maximum from a membrane in good working order. If it higher than that the membrane may be in need of replacement. TDS readings will NOT tell you when to replace the prefilter or carbon so stick to the 6 month rule on them. Thye remove large sediment (TSS not TDS) and chlorine not TDS (Total Dissolved Solids). TDS is ONLY for RO or RO/DI troubleshooting.
Higher TDS from the membrane wears out the DI prematurely so needs to be monitored. A very accurate rule of thumb says " For every 2% you increase the RO membranes efficiency you DOUBLE the life of your DI". This can be a huge cost savings over time since a couple of DI replacements can buy a new membrane.
Keep track of your tap water, RO only and RO/DI TDS and it will tell you what is in need of replacement as far as RO and DI goes. replace the prefilter and carbon every 6 months and use good replacements and your membrane can last for 3-5 years minimum and by using premium replacements its possible to get 10 years or more out of a membrane.
