TDS Creep...thoughts on prevention/avoidance...

A customer wrote us-
I'm hoping you can give me a moment of your time in answering a few questions. This stems from a recent blog discussion but knowing your expertise, I thought I would try and get your input via email. The discussion involved the debate as to the amount of impurities passed through a RO unit every time it is started via an auto shut off valve. It's been suggested that it would be better to utilize a large vat for water storage made with a single run rather than a small reservoir and many cycles of on/off. The reef tank pertinent questions I have are:
Q. 1. The amount of impurities passed at the start of the RO great enough to cause harm to a reef tank?

Yes there could be, but it will depend on how the system is installed.

First, it is true that there is osmotic flow through the RO membrane while the system is off that will allow high levels of contaminants to pass into the product water upon start-up. The first few ounces of product water will be contaminated by up to 50% of the tap water contaminant level.

There are two main solutions to this situation that will help correct the loss of purity due to on/off cycling. Minimizing the number of cycles per day is an important part of the solution.

A. Our LLC-M-115 and the LLC-S-115 liquid level control kits were created to correct this problem. Unlike a float valve mounted in a sump or aquarium for direct evaporation replacement, the LLC-M and LLC-S level controls limit the number of ON / OFF cycles to only a few times per day. This minimizes the total volume of contaminants introduced into the aquarium. If evaporated water is being replaced directly, one of these two LLC kits should be used to limit the cycling and lower the total volume of contaminants. The small volume of contaminant introduced into the product water a few times per day will be diluted into a much larger volume of water.

B. Adding a DI system after the RO is the second part to solving the containment problem. RO water even at its best is still contaminated. There are very few areas in the US where an RO membrane alone is sufficient for dropping the Silica, Phosphate, and Nitrate levels low enough to eliminate algae and diatom growth. (Some of our salt water customers are successful with RO only but the vast majority must add DI to eliminate algae growth and there are some locations that require RO and an Ultimate DI (4 DI stages) in order to remove the contaminants. To be entirely successful, silica levels must be reduced from the PPM range to just a few PPB and that requires the use of a DI system.
Note: Float valves mounted in sumps or aquariums do not work well due to the constant cycling, erratic backpressure and resultant contamination problem. (We would never recommend a float in a sump directly after an RO system, and definitely not after a bladder tank).

This discussion brings to mind another problem and a major misconception. A few years ago one of the companies in the Aquarium industry introduced what they touted as a “ High Silica†removal membrane. This has mislead many aquarium hobbyists into thinking that they are purchasing something “special†that is exclusive to that particular brand. This is a lie that has permeated the minds of the public and the public has accepted it nearly without question. Using the term “ High Silica Removal “ to describe any particular membrane is extremely misleading to say the least. Misleading the public in this way gives the entire industry a bad name. It is very irritating when this type of deceptive practice is used to manipulate people.

High silica rejection is directly related to the “percentage of rejection†of the RO membrane. Many years ago we determined that the only way to guarantee the rejection rate of a membrane (thus the silica rejection) is to test the membranes (every single one of them). It is very rare to find any two membranes with exactly the same rejection characteristics and it is not unusual for us to reject entire batches of membranes due to low rejection characteristics. This process is extremely time consuming for us and irritating to the membrane manufacturers but there is no other way to guarantee membrane rejection rates. Our quest for consistency and quality has raised the bar that manufacturers must meet in order to match our requirements and it has benefited the entire industry in the process.

The bottom line is that every single membrane must be tested, and the lower quality membranes rejected. This is what we do to all of our deluxe MEM-S membranes. It is the only real guarantee a customer can depend on if he truly wants to achieve “ High Silica Removal “.

Water pressure is another important aspect to consider when attempting to achieve the highest percentage of rejection possible from an RO membrane. The higher the water pressure on the input side of the membrane the better the rejection rate of the membrane and purity of the product water. Best results are obtained with no output water pressure, such as provided by a pressurized storage tank.

2. Does the storage of large quantities of purified water promotes negative effects within (i.e.. - algae, bacteria or other tank detrimental substances)?

Our recommendation would be to use the water produced in bulk within a week or two and to sterilize the container periodically in order to minimize contamination issues.

3. Do you recommend one style of make up water systems over the other and why?

The LLC-M and LLC-S are correctly designed for this purpose. The details supplied above should have answered most of this question.

Charles Gibbs
SpectraPure, Inc
Technical Support