What’s Really in Tap Water & Optimizing Reverse Osmosis RO/DI Filtration | 52SE

Categories: Reverse Osmosis, 52 Weeks of Reefing - Second Edition

Last updated: Jan 25, 2024

Optimizing your RO/DI system can help you save money in filter costs, produce water faster, and even save you time with less frequent maintenance. How do I optimize my RO/DI system you ask?

The answer starts with an understanding of what's in your tap water, why you need an RO/DI system to remove it, and finally how each stage of your reverse osmosis system works to produce pure 0 TDS water for your aquarium.

Why Do I Need an RO/DI System?

The easiest question to answer - an RO/DI system will remove harmful pollutants from your tap water making it safe to use in your aquarium. Additionally, you need to produce 0 TDS freshwater to safely mix artificial seawater at home.

0 TDS: 0 total dissolved solids; pure water with no measurable impurities.
TDS Meter: A device that measures total dissolved solids
Artificial seawater: Saltwater for your aquarium made using 0 TDS fresh RO/DI water and dry synthetic salt mix.

Synthetic sea salt mix is formulated to be mixed with pure 0 TDS water and accurately reflect the water chemistry found in the ocean. If you start with tap water that has impurities and/or elements, the resulting water chemistry of the saltwater would not be balanced.

For example, if your tap water contains 200 ppm of Calcium and your salt mix contains 400 ppm of calcium, the resulting saltwater would contain 600 ppm of calcium which is far outside the ideal range for an aquarium.

Tap Water Contaminants

Common Tap Water Pollutants

Disinfectants
Elevated levels of major elements
Nutrients
Harmful levels of minor & trace elements

Some of these pollutants can be immediately harmful to your fish and corals such as chloramines (disinfectant). There are also a variety of more sneaky pollutants that are not immediately harmful (phosphate) but can slowly build up over time and eventually reach harmful levels and have adverse effects in your aquarium.

Not all tap water sources are the same which means the impurities can vary significantly from source to source. There are also a variety of different tap water "treatments" that are commonly used to make tap water safe for consumption by humans. These very same treatments that make the water safe for humans can make the water toxic to your aquarium pets.

Knowing what is in your tap water is important for using an RO/DI system efficiently.

Is your tap water coming from a well or municipal water supply?
Do you have a water softener or is the water softened at a water treatment plant?
Are chloramines used in your area?
What is the TDS of your water right out of the tap?

If you don't have these answers, you can start by contacting the organization that manages your municipal water supply to request an analysis of the tap water. Keep in mind the quality of the water and the various treatments they use might change seasonally and/or over time so an annual inquiry is recommended.

5Stage RO/DI System

How Do I Know Which RO/DI Filter To Choose?

This is exactly where our exploration begins because choosing an RO/DI filter is not always straightforward. In a nutshell, a basic 4-stage RO/DI system will create 0 TDS water when applied to just about any source of tap water (excluding very extreme circumstances) but the rate at which the filter cartridges become exhausted and how fast the water is produced can vary significantly based on the quality of the source water and water pressure feeding the RO/DI system.

Insufficient water pressure results in slow water production and less efficient membrane performance.
The type and amount of impurities directly affect how long your filter cartridges last and negotiate the optimal filter configuration

The good news is that RO/DI systems are very modular and can easily be expanded and or changed to meet the demands of the situation. All of our Bulk Reef Supply RO/DI filters are built using industry-standard sizing which means you can easily mix and match the filter cartridges and even reconfigure your RO/DI system by adding additional stages.

Each of the filter stages of an RO/DI system has a specific purpose and understanding how each of these stages works together to filter your tap water will help you make the best possible choice. You will gain insight into what is in your tap water and how that may affect your aquarium. You will also be armed with the knowledge to maintain your RO/DI system efficiently and adapt to any changes in your tap water quality.

When to change out RO/DI Filters

Stages of Reverse Osmosis & When To Change Them:

Sediment - change out when the pressure starts to drop
Carbon - change every 12 months
RO Membrane - change every 3 years
DI - Deionization - change when the color change is 1" from the top

How a Reverse Osmosis Deionization (RO/DI) System Works

Learn how water flows through an RO/DI system with this easy-to-follow diagram.

Reverse Osmosis Membrane

This is the heart of the RO/DI system where a majority of the contaminants are removed and separated from your product water. RO membranes are rated using GPD - Gallons Per Day which is the amount of purified water the membrane can produce in a 24-hour timeframe under ideal conditions. These are the most expensive filters in your RO system but are also the longest lasting; a 2-3 year lifespan is typical.

As a general rule, we recommend the DOW 75 GPD RO Membranes because they offer the highest rejection rates (up to 99%) which means they can remove 99% of the impurities from your tap water under ideal conditions. There are faster 100 and 150 GPD membrane options as well as the dual membrane (200 GPD) "Water Saver" options which will produce water faster but come at the cost of a lower rejection rate under certain conditions.

Sufficient source water pressure is extremely important for an RO membrane to work properly:

75 and 100 GPD membranes require a minimum input water pressure of 50 PSI to achieve optimal rejection rates.
150 GPD & Dual Water Saver Membranes require a minimum of 65 PSI input water pressure to achieve optimal rejection rates.
Dual membrane water-saving systems are NOT recommended for areas with 300+ TDS in the source water.
RO membranes generally last 24-36 months when the sediment and carbon prefilters are maintained properly.
Booster Pump Kit: An accessory kit that increases input water pressure when household water pressure is insufficient.

A lower rejection rate results in more pollutants passing through the RO membrane which ultimately depletes the Deionization resin much faster. This is why shooting for the best possible rejection rate out of your membrane is the primary goal.

Sediment Filter

This is the first line of defense in your RO/DI system and is the least expensive filter in your system. Sediment filters are designed to physically remove particles of dirt, rust, and debris from the tap water. This prevents these particles from clogging up downstream filters and maintains optimal water pressure going into the membrane. When your sediment filter is clogged, it reduces pressure to all downstream filters ultimately reducing their longevity and performance.

Change your sediment filter when the system water pressure starts to drop indicating the sediment filter is clogged.
If your water contains fine sediments, use lower-micron sediment filters.
If your water contains a high amount of sediment (pretty rare), using two sediment filters may be the most economical way to improve system performance.
Micron(μ): Unit of measure equal to 1/1000th of a millimeter (1 micron (1μ) = 1/1000 mm). This is used to describe the pore size of your sediment filter; smaller micron filters have a tighter knit and, therefore, a smaller pore size allowing them to catch finer sediments.

In addition to monitoring system water pressure, keep an eye on your carbon block filter. While it is normal for the sediment filter to become discolored as it catches dirt and debris, your carbon block filters should not discolor. If the carbon block filters are clogging prematurely and/or becoming discolored, swap out your sediment filter more often and consider changing the micron size.

Carbon Block

A carbon block filter will remove various chemicals and compounds that could damage your RO membrane or are otherwise difficult for the membrane to remove. This includes disinfectants, pesticides, herbicides, and volatile organic compounds. It is generally recommended to run dual universal carbon black filters to ensure these harmful chemicals do not damage your membrane or make their way into your aquarium.

Not all carbon block filters are created equal and this is important because of the widespread use of chlorine and chloramines to disinfect tap water. Chloramines are becoming more and more widely used by municipalities because it is stable and more effective when compared to chlorine alone. As it pertains to your carbon filters, the less expensive standard carbon block filters will not suffice if chloramines are found in your tap water.

Standard Carbon Block: Can only remove chlorine; should be swapped every 6 months
Universal Carbon Block: Removes both chlorine and chloramines; may last up to 12 months
Free & Total Chlorine Test Strips allow you to check for the presence of chloramines in your tap water

Using some simple Chlorine Test Strips can help you determine whether chloramines are in your tap water and, therefore, which carbon block filter is best. Keep in mind the presence of chloramines may be seasonal and could change at any given time should your water treatment center decide to use it. Additionally, Universal Carbon Block filters last longer so a Universal Carbon Block really is the safest option for a majority of tank owners regardless.

No matter what kind of carbon block filter you decide to use, it's never going to be 100% effective in terms of removing contaminants. That means some amount (albeit small) of chemicals will pass through the carbon filter. Using a series of two carbon block filters will significantly reduce (or eliminate) the small amount of contaminants that can pass through and reach your membrane and DI filters. This will ultimately save you both time and money in the long run because you will get a much longer life expectancy out of your RO membrane and DI resin.

DI Resin - Deionization

This is the final stage of filtration in your tap water filtration system that contains positively and negatively charged ion resins that are intended to catch and remove ANY pollutants that may be left. DI resin is the second most expensive filter to maintain which is exactly why you want to configure your RO/DI system to give you the best possible results before the water hits the DI stage. Water exiting the DI resin filter should always measure 0 TDS.

For most of us, a single mixed bed DI resin filter is all that is needed to keep 0 TDS water on hand. That said, there are a couple of different approaches you can take with your DI resin to optimize the rate of exhaustion and be extra careful in terms of letting harmful contaminants pass through.

Single Stage Mixed-Bed DI Resin: Most common approach and works well as long as you stay on top of filter changes.
Dual Stage Mixed-Bed DI Resin: Adds a layer of protection if you tend to be forgetful when it comes to filter changes or simply have a high water demand (larger tanks).
Three Stage Triple DI Saver: Allows you to specifically target the contaminants that are elevated in your water and only change out the type of resin that depletes the fastest. 1st position contains Cation DI Resin, 2nd position contains Anion DI Resin, and the 3rd position contains Pro Series Mixed Bed DI Resin.

Always swap your color-changing DI resin when the color change is approximately 1" away from the top of the media. If you wait until the DI resin is completely exhausted (100% color changed) the resin will begin to purge the captured contaminants back into your product water.

Tags: tap water, ro/di, reverse osmosis, 52se, 52 weeks of reefing second edition