Its not that easy unfortunately. The proper way to plumb two membranes is to run the waste from the first one into the second one so you filter the brine with the second one in line. This is not promoted as nor should it be considerd a low waste system as the membranes still need flushing at near 4:1 waste ratio or they will fail prematurely. It also requires at least 65 psi to operate efficiently, this makes up for the pressure drop through the first membrane. You can also accomplish this with a booster pump.
Several of the newbies in the RO/DI market are misrepresenting their products as low waste when in point of fact they have not done their homework and they are prone to failure and the resulting customer complaints. Talk with vendors such as Spectrapure, Air Water & Ice, PurelyH2o and others who have been doing this for years and you will find none of them will sell you a system as you describe, they have a reputation in the industry and rely on customer satisfaction. The simple fact is membranes must be continously flushed in order to function and to last. They have all tried what you are describing many many years ago and gave up on it, it does not last long term, even with low TDS water such as yours with a TDS of 40 ppm (not ppt).
There is no zero waste system on the market nor is it possible. Watts Premier does sell a system they promote as zero waste but it still flushes the membrane same as al the others, the difference is it uses a booster pump to overcome the pressure and dumps the brine or waste back into your hot water heater. not a very good idea and it has limitations like only about 15-25 GPD maximum and shortened hot water heater and plumbing fixture life.
Spectrapure sells a ultra low waste system unlike any other that uses stored DI water to flush the membrane at timed intervals and at shutdown so it stores the membrane in DI water when it is off line. This is the only proven low waste system and can easily operate at 0.66:1 waste ratio or less than 1:1 waste in good water conditions like yours but no one has a zero waste system.
DI has a finite capacity for TDS, its usually something like 3000 to 6000 total TDS per 16 oz of resin. If you are wasting .75 gallon for every gallon you make with an incoming TDS of 40 you are seeing a waste TDS somewhere around 100 ppm. If this is the case a full 20 oz vertical DI would last you somewhere between 37 and 75 gallons under perfect conditions. DI replacements would get very expensive like that not to mention replacing the membarnes every year or sooner since they will be fouled and plugged with the low waste ratio. Take a look at Spectrapures dual membrane MaxCap system and how it is plumbed and note it still has a recommended 4:1 waste ratio. This is needed to make the membranes and DI last. Its not unheard of to get 1000+ gallons per DI refill and 10 years out of a single RO membarne with a MaxCap, I do over 1000 gallons per DI refill with mine and thats in Phoenix with a tap water TDS of over 800 most times. My RO/DI water costs me less than a nickel per gallon figuring in the wate rand sewer rates during the highest sumer rate months and the cost of replacement filters and resins, its not worth messing with in my mind.
