When I set up the RO/DI unit for my aquariums, I had difficulty to understand how the whole things work and had to ask a lot of questions to the vendor and on some aquarium forums. So I hope the following will give you some details on how I setup my unit and how it may be setup in some other ways. Some of them are from my own experience and many of them from reading on the web.

Okay, here at first someone would ask what is RO and DI?

Osmosis, according to the dictionary, is movement of a solvent through a semi-permeable membrane into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane. The normal flow of solvent is from the dilute solution to the concentrated solution.

This osmosis process can be understood by the following:

First separate a container into two equal sections by a semi-permeable membrane. On one side pour some distilled water and on the other side with equal volume of salt water. Leave it like that and over time, the level of the distilled water will decrease while the level of salt water will increase. So this process is called osmosis.

So it is very easy to understand Reverse Osmosis (RO) is the reverse of the above process. In order to achieve this, we need to apply pressure on the salt water side to force the water pass through the membrane. In actual application, we substitute the salt water with tap water. Most of the impurities from the tap water will be blocked by the membrane leaving a desirable purified water at the other end of the membrane. In the process a lot of water (80~90%) is wasted to drain which actually helps keep the membrane clean by flushing water over the incoming side of the membrane to remove particulates and dirt.

But reefers as us are still not satisfied with the result as the membrane can only reject 95~98% of ionic and organic impurities. The produced "clean" water still contains a little phosphate, silica, nitrate, etc. So another canister with de-ionizer (DI) resin is added at the final stage of the unit to further purify the water. The resin cartridge contains positive and negative charged polymers that interact with the contaminant to remove them. Thus 99% of pure water can be achieved.

DI water has no taste so it is not recommended for drinking. However, the Aqua-safe unit I bought has a carbon canister filter (6th stage) for adding taste and odour. In the absence of the 6th stage filter, you can bypass the DI and connect the output from the membrane for drinking water.

The following is the whole 6-stage 100GPD RO/DI unit I have:

The following is a setup example:

Although the unit is rated at 100 GPD, if you have about 40 gallons output per day you should be satisfied.

From the diagram, the bottom 3 filters are pre-filters. The first stage is a 5 micron sediment filter. The second stage is a Granular Activated Carbon (GAC) filter, and the third stage is a 5 micron Carbon Block (CB, sediment and carbon combined) filter. Pre-filters are necessary to filter out a lot of impurities including chlorine which will damage the membrane. Thin Film Composite (TFC) membrane is commonly used these days and they can be damaged by chlorinated water.

From the diagram, just on top of the 3 pre-filters is the membrane. On top of the membrane is the DI canister. Then the last stage is the GAC filter for taste and odour.

Now come the fun part: connecting the unit.

First you may want to know the PSI pressure and TDS reading of your water source.

You can buy a PSI meter and have it connected to the tap water source. My tap water source ranges from 32 to 46 PSI, depending on the time of the day. I actually connect the PSI meter between the last pre-filter and the membrane. The pre-filter will normally decrease the PSI by about 4, depending on how long you have used the pre-filters. If the meter reads 31 or less PSI just before the membrane, then the membrane won't work. You either choose another time to make the RO water or buy a RO booster pump and have it connected in-line to the water source. Watch out the boosted PSI is within your RO/DI unit's pressure limit.

In-line TDS Meter Handheld TDS Meter PSI Meter RO Booster Bump

The TDS reading of my tap water ranges from 150 to 200. I think a reading between 100 and 200 is the norm. Less TDS reading will give you more produced water and longer operating time to the filters.

Temperature of the water source also affects the efficiency of the membrane. After many readings, it seems that 78F is the best. But most tap water in North America is very cold and is between 50F and 60F.

I fill up a 2.5 gallon bucket with water and place a 150-watt heater in it and set the temperature to 80F. Then between the RO unit and the tap water source, I install a 25 feet rigid tubing and coil it and put it in the bucket hoping that it will warm up the water. It does the trick but it only brings up the water to about 68~72F, depending on how long I have turned on the RO unit. So my conclusion is that a 150-watt heater is not strong enough. You may need to put a 200W or even 250W heater in there to keep up the cold water being passed through the tubing all the time.

Some people also use other ways such as tying the water source rigid tubing to some hot/warm water pipe in your utility room. Others also use a water mixing valve which connects the hot and the cold water pipe. You can adjust the valve to get the desired 78F water.

There are few ways to connect the water source to your RO unit. I use a hose adapter and a T to connect it to share with the washer machine hose. No plumbing is required. Some use a faucet adapter to connect to the faucet, also no plumbing is required. Yet arguably the best way is using a piercing saddle to tap into copper pipes (no drilling is required) or brass pipe (drilling is required). But I am afraid this last method over time, is prone to corrosion failure.

Hose Adapter Faucet Adapter Piercing Saddle

As I mentioned before I connect my PSI meter between the last pre-filter and the membrane. This can monitor the water going in the membrane. If you connect yet another PSI meter before the first pre-filter, then you can even monitor how good or bad are your pre-filters doing.

I also have an in-line TDS meter which has two ports (input and output) for monitoring. Once I know the water source reading is between 150 and 200, then I don't think I need to connect the input port to the source. I rather connect it to the output of the membrane. This can somehow measure the performance of the pre-filters and the membrane as a whole. Then I connect the output port of the TDS meter to the DI output side. This is the RO/DI unit output and the reading should read 0 or at most 1. If not, it's time to change the filters and/or DI unit. When the time comes, I actually change all the 3 pre-filters, the DI unit, and the carbon taste filter at the same time. The membrane can usually last for 5 years and the vendor suggests me to change the whole RO/DI system rather than just replace the membrane which usually costs about 50% of the entire system. I agree because after 5 years, parts within the system somewhat may be clogged or worn out.

In order to maintain the membrane, the system come with a flow restrictor (flush unit). Every month or 2 weeks, flush the unit by turning the flow restrictor switch to the position inline to itself for about 15 to 30 minutes. Note that doing that with other valves turned on as if you are making RO water. The position shown below is the normal operating position.

Most people use the Rubbermaid Roughneck garbage can which is food safe to store the pure water. I use a Rubbermaid garbage can (which is rectangular in size and use the same material as the Roughneck) as the water reservoir.

I install a Kent float valve on the can in order to stop the water from producing when the can is full. Drill a hole about 6 to 8 inches from the top edge of the can for the float valve unit. This valve will stop the water from filling the can when it reaches the set level. However, it will not stop the RO/DI from continuing to drain water unless you have some auto shut off mechanism in place. Luckily, the Aqua-safe unit I bought has an auto shut-off valve. So problem solved. For those not so lucky, you may buy a Kent shut-off kit to install in it. I also bought one (not used) but I found that the instructions were specifically for Kent RO/DI units. You may find it a little bit confusing if your RO/DI system is not from Kent.

Kent Float Valve Auto Shut-off Valve Kent Shut-off Kit

I also drill a hole just above the Kent float valve and connect it with a rigid tubing. I place the other end of the tubing to the drain. This is an added safety measure to prevent from flooding in case the Kent float valve fails.

Other people use a high/low switch installed on the reservoir. The switch has solenoid to control the shut off of the system. Then a Kent float valve is installed slightly above the high/low switch for backup.

That's about it for the aquarium setup. Hope this helps.