What Should the TDS of RO Water Be in PPM? Find Out Here!
Written by: Alexandra Uta // Last Updated: Jul 14, 2023
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Reverse osmosis filtration is highly effective at reducing total dissolved solids (TDS) in water.
Due to the ultra-fine RO membrane, nearly all impurities down to 0.0001 microns in size will be removed from feed water.
Let’s explore how TDS readings factor into reverse osmosis, what the TDS level of RO water should be ideally, and how you can use this figure to determine when to change your RO membrane.
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Key Takeaways
- The TDS of RO water should typically be around 10-50 ppm. This is considered very low TDS water and will have nearly all dissolved solids removed.
- There is no need to aim for zero TDS when it comes to RO water, unless you want to remove one or more specific and potentially harmful contaminants.
What Should the TDS of RO Water Be in PPM?
So, what should the TDS of RO water be?
Generally, RO water should have a TDS level of between 10 and 50 parts per million (ppm). However, this range is an estimate, as several factors will affect the TDS level of your filtered water.
First and foremost, the TDS level of your feed water will determine the final TDS of your RO water. Feed water with high TDS will inevitably lead to higher TDS RO water – even if the filter system is working perfectly.
Other factors that influence the TDS of RO water are the condition of the membrane and pre-filters, feed water pressure, water temperature, and drain line or flow restrictor problems.
Ideal RO Drinking Water TDS
RO systems typically reduce TDS levels to below 50 ppm, sometimes below 25 ppm. While this is an exceptionally low level, when it comes to drinking water some people find this imparts a flat taste. In fact, the ideal TDS range in drinking water for flavor and taste is about 150 ppm.
An easy fix to this issue is adding a remineralization filter stage to your RO system. These filters add back healthy minerals like magnesium, calcium, and potassium after they’re removed from your water by the earlier filter stages. TDS and mineral content increases and give your RO water a more rounded taste.
Ideal RO Water TDS for Brewing Coffee
When it comes to brewing the perfect coffee, low TDS levels are preferred. Boiler-based steam ovens and espresso machines require water of about 50 ppm, while coffee brewers can operate with TDS levels between 100 and 200 ppm.
This makes reverse osmosis systems an ideal choice for cafes and the food service industry as a whole.
Bottom line: If you want to make premium café-quality coffee at home, then installing an RO system is the way to go.
Isn’t 0 TDS in Reverse Osmosis Water the Best?
There is no need to aim for a zero TDS level when it comes to reverse osmosis water – unless you want to get rid of a specific and potentially harmful contaminant that contributes to overall TDS.
But if you’re simply looking to reduce high TDS level feed water for aesthetic means, then RO filtration will be more than enough, even if you don’t end up with 0 water TDS.
Drinking Water TDS Levels Classified
| TDS in ppm | Taste |
|---|---|
| Less Than 300 | Excellent |
| 300 to 600 | Good |
| 600 to 900 | Fair |
| 900 to 1200 | Poor |
| Greater Than 1200 | Unacceptable |
What Are Total Dissolved Solids Exactly?
Total dissolved solids refer to the concentration of the combined dissolved substances in water. TDS includes both organic and non-organic matter and it includes both harmful contaminants and healthy impurities.
Non-organic matter can be metals like lead, iron, and mercury, as well as salts and minerals. Organic matter could be pesticides and certain water disinfectants among other stuff.
It’s important to keep in mind that TDS levels only give you a reading of the concentration of dissolved solids and don’t provide any information on their content. This means that high TDS water doesn’t necessarily indicate poor quality, it can simply indicate water with high mineral content.
Effects of High and Low TDS in Water
High TDS water is defined as water with more than 500 ppm and water with more than 1,000 ppm is deemed too high for human consumption. Water with high TDS will have a bitter or salty taste and a cloudy appearance that can be unpalatable and unappealing.
Also, high TDS water often has high levels of dissolved minerals like calcium and magnesium, which can leave scale deposits in your water fixtures, pipes, and home appliances. This will shorten the lifespan of your plumbing system, water heater, dishwasher, and washing machine.
Once again, it’s important to note that high TDS does not necessarily indicate poor drinking water quality. Water with high TDS from mineral content is not harmful to your health and can be a beneficial source of essential calcium, magnesium, potassium, etc.
Low TDS water is often defined as water with TDS levels below 50 ppm. RO water would fall into this category.
Low TDS water has no documented negative health effects, but some people find its taste a bit flat. Adding a remineralization filter stage is always a possibility if you want to increase mineral content of your RO water and improve its taste.
How Reverse Osmosis Removes TDS
Reverse osmosis primarily removes TDS through the semi-permeable RO membrane, but also through the activated carbon and sediment pre-filters.
At the membrane stage, water is pushed through the diaphragm under high pressure. Most impurities in the water are too large to diffuse through the tiny membrane pores and remain on the feed water side. The purified water gets stored, while the rejected impurities are flushed away in a wastewater stream.
Help, My RO Water TDS Is Too High!
Several issues can cause your RO water TDS level to be higher than normal. Let’s take a look at these issues in detail.
- Feed Water TDS: Very high TDS feed water is the most important factor which leads to higher TDS in RO water. Even if your RO system is functioning perfectly, high TDS feed water can only be reduced so much.
- Feed Water Pressure: If your feed water pressure is too low, the RO membrane will not function efficiently, which will lead to a higher TDS RO water. The ideal TDS level is generally 60 psi, with 40 psi being the bare minimum.
- Feed Water Temperature: High feed water temperature causes the pores in the RO membrane to expand, allowing more dissolved particles to pass through.
- RO Membrane Condition: An old or damaged membrane will remove fewer dissolved solids than a new one in good condition.
- TDS Creep: TDS creep refers to the tendency of an RO membrane’s internal pressure to equalize if unused for a long period. If you haven’t used your filter system in some time, make sure to discard the first tank when measuring TDS.
- New Membrane or Pre-Filters: When you install a new membrane or pre-filter it’s important to discard the first tank or two before measuring TDS. The membrane must be hydrated before it can work effectively, while carbon pre-filters must be flushed to remove carbon dust.
- Faulty Flow Restrictor or Clogged Drain Line: If your system’s drain line is clogged, or the flow restrictor is malfunctioning, the reverse osmosis system will not filter properly and the TDS level in the output water may be elevated.
Further Reading:
How to Measure and Calculate TDS Rejection to Know When to Change an RO Membrane
TDS or salt rejection rate is a useful way to calculate how efficiently your RO membrane is removing dissolved solids. It can be used to determine if your RO membrane is still working or if it needs replacement.
To calculate your TDS rejection rate:
- Measure the TDS of your unfiltered feed water.
- Measure the TDS of your filtered RO water.
- Plug those figures into the formula below.
Rejection rate % = (TDS of Unfiltered water – TDS of RO water) / Unfiltered Water TDS x 100
If you have any questions about acceptable TDS of RO water please don’t hesitate to leave a comment below!
About the Author Alexandra Uta
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