What is wastewater in Reverse Osmosis (RO) Filtration systems and how can you reduce it?

What is wastewater in Reverse Osmosis systems and how to reduce it?

Reverse osmosis water filters have become a familiar household product these days. It has been possible as they can serve you pure and clean water for a long time.

Only you have to replace the cartridges from time to time. Despite all these, the RO filters also have a bad reputation as well.

  • It revolves around the fact that the RO purifiers waste a lot of supply-water. The claim is natural when you observe the working mechanism of these filters.

Today, in this article, we’ll see what RO wastewater is and how we can reduce it to increase the system efficiency.

So, let’s begin.

How do reverse osmosis water filters work?

RO purifiers use reverse osmosis to keep out dissolved molecules from the water. It happens with the usage of a semipermeable membrane.

The RO membrane has a property that only allows the water molecules to pass through it. As a result, any other particles, soluble or insoluble, get blocked.

So, you see, this property of the membrane plays a vital role in getting rid of the impurities from water. 

We all know that our supply-water carries some portion of impurities. It goes through the initial screening in the sediment and activated carbon filters.

Due to this, most of the organic and inorganic substances get filtered out. Finally, it enters the RO chamber.

  • Here, incoming tap water pressure forces the water molecules to move across the membrane. But, the remaining dissolved particles can’t follow through. 

As a result, you get pure and clean drinking water at the output without most of the pollutants.

So, you see, after reverse osmosis, the filter produces two types of water.

  • Product or purified water
  • RO-reject water

Now, let’s clarify those.

What is product water?

The RO product water is the purified water that we get at the output of the reverse osmosis membrane. Sometimes, we also call it the Permeate water. 

As you can realize, the permeate water mostly consists of pure water molecules. The impurity level in it will be significantly low.

  • You probably have heard the word, TDS or total dissolved solids. Most RO purifiers produce permeate water having a TDS value of 20-50 ppm.
  • Meanwhile, the tap water, in general cases, comes with a TDS level of 500-1000 ppm. So, you see, the RO purifiers remove most of the pollutants from the water.

Then, the permeate water proceeds to the reserve tank. The internal mechanism of the tank ensures you get water fast when you open the faucet.

What is RO-reject water?

RO-reject water is the water that will have a very high TDS value and goes to the drain leaving the RO chamber. That’s why it is also known as wastewater or drain water.

The purpose of the RO membrane is to reduce the TDS of your drinking water. As a result, the soluble particles in the water need to be filtered out.

The RO-reject water does this job. It would absorb most of the dissolved molecules from the pure water. As a result, its TDS level goes higher.

  • And, finally, we dump it in the drain. On this note, another parameter is the TDS rejection rate. It signifies the percentage of TDS that doesn’t make it into purified water.

For example, assume your tap water and the product water have a TDS value of 1000 ppm and 20 ppm, respectively. That means the RO rejection rate of the filter is 98%.

So, let’s explain it with some data analysis. Suppose your supply-water has a TDS level of 1000 ppm.

Also, your RO system has a drain and pure water ratio of 3:1. That means it would waste 3 gallons of water to produce 1 gallon of purified water.

Now, we’ve compiled a table showing you how the TDS rejection rate of the RO membrane influences the TDS level of the permeate water and the RO-reject water.

TDS Rejection Rate

Drain to Product Water Ratio

TDS level of tap water

TDS level of permeate water

TDS level of RO-reject water

80%

3:1

1000 ppm

200 ppm

1267 ppm

90%

100 ppm

1300 ppm

95%

50 ppm

1317 ppm

98%

20 ppm

1327 ppm

So, you see, in the above example, the RO-reject water has a TDS value of 1300 ppm on average. As a result, it is not suitable for drinking according to WHO standards.

Why do the RO purifiers produce wastewater?

From the principle of reverse osmosis, the RO purifiers get rid of the dissolved solids to produce pure drinking water. Now, the solids need to go to the drain.

That’s why the RO purifier uses a portion of the supply-water that carries the dissolved particles to the drain. 

  • As you’ve seen from the previous table, the TDS level in the drain water exceeds 1200 ppm on average. And, this is an indication of water, with poor taste.

That’s why we can’t use the RO-reject water for drinking purposes. As a result, the RO purifiers naturally divert this poor-quality water to the drain.

So, you see, the RO purifiers need to produce wastewater to flush the dissolved particles and serve you pure and clean drinking water.

How much water gets wasted?

In general, it takes almost 4 gallons of tap water to produce 1 gallon of purified water from the RO filters. Therefore, you can see, on average, 75% of the supply-water goes to waste.

But, this stat doesn’t show the total picture. The amount of wastewater differs from one product to the other. It depends on the filter’s age and other parameters as well.

As the filter ages, the RO membrane wears out and loses efficiency. As a result, more water goes to the drain. Also, if your installation isn’t accurate, it might cause more wastewater.

Now, let’s look at some of the best RO water filters in the market and how much supply-water they waste.

  • Most APEC RO purifiers would comply with the average stat. That means those require 3 gallons of tap water to produce 1 gallon of pure water.
  • The scenario is similar in most Express water RO systems as well. Here, the drain water to the product water ratio is equal to 3:1.
  • But, compared to those, the Home Master TMAFC ERP RO water filter is more efficient. Here, only 50% of the water goes to the drain.

So, if you’re looking for a RO purifier that wastes less supply-water, you must check out the Home Master TMAFC ERP RO water purifier. Check out our review here.

Where can you use the wastewater?

The RO-reject water contains a high TDS level. So, it doesn’t make proper drinking water. Due to high TDS, the water contains Calcium, Magnesium, and other minerals.

So, we can’t use it for drinking, but can we use it somewhere else? The answer is yes. Now, let’s see where you can use RO-reject water.

Using the water for gardening

As the RO-reject water contains lots of mineral contents, you can easily use it in the garden or lawn.

  • The plants and grass will absorb the Calcium, Magnesium, and other nutrients readily from the RO-reject water. 

That’s one way you can use the drain water from your RO purifier.

Rinsing your cookeries

  • Although the RO-reject water contains a lot of dissolved solids, it is pure and clean. That’s because there are pre-filters before the RO membrane, which takes out most of the pollutants.

That’s why you can use the RO-reject water to clean your dishes and cookeries. Generally, we use a healthy portion of our supply-water for this purpose.

So, if you use the RO-reject water to rinse the bottles, cookeries, and other equipment, it can save a lot of water wastage.

Washing the vegetables and fruits

Another usage of the RO-reject water can be in washing the vegetables, fruits, or cleaning the meat or fishes.

As we’ve already mentioned, the drain water from the RO purifiers contains a lot of dissolved solids, but it doesn’t mean all of those are pollutants.

Therefore, you can easily wash your vegetables or fruits with it to rinse the dust off its surface.

Cleaning the house

Many of us like to use water to mop the floor. Also, we use water to clean the dust and rust of any equipment.

You can use RO-reject water for this purpose. The water only contains minerals that will behave like regular water.

That’s how you can put the RO-reject water to better use.

Using in the washroom

Another usage of the RO-reject water can be in the washroom. As we use water for various purposes each day there, the RO-reject water can be a great source.

Using it to take a bath, wash your clothes, or flushing the toilet will make sure a significant amount of your supply-water gets used well.

How can you collect the RO-reject water?

You can easily use a bucket or a small tank to collect and store the RO-reject water for later usage.

So, let’s explain it further.  If you already know what’s inside a RO purifier, you also know that two tubes leave the reverse osmosis chamber. You can check those out in our article here.

One takes the permeate water to the storage tank, while the other one carries the RO-reject water to the drain.

  • So, instead of connecting the drain tube to the drain line, you can keep it in your intended storage device, which may be a tank or a bucket.

Therefore, when the reverse osmosis membrane works, the RO-reject water will travel through the tube into the bucket.

In this fashion, you can easily collect the RO-reject water or the drain water or the RO wastewater and use it for various purposes.

How can you reduce the RO wastewater?

Now, let’s take a look at some of the ways how you can reduce the amount of rejected water from your RO system.

Using an auto shut off valve with a flow restrictor

The auto shut off valve stays after the RO membrane. It is one of the built-in protection systems against wasting more supply-water.

The valve with the flow restrictor regulates the water flow in the permeate tube and the drain tube. With its hydraulic system, it determines whether your storage tank is full or not.

As soon as it senses that the tank is full, it will automatically shut off the drain tube. And, the water flow inside the system goes down to zero.

Now, when you open the faucet, some water gets out of the tank. The valve senses it and immediately opens the RO-reject water tubing.

As a result, reverse osmosis resumes again until the tank fills up completely. 

  • If your RO system doesn’t have an auto shut off valve, it will not stop the RO chamber even when the reserve tank is filled with water. 

As there is no way of containing the permeate water, all the supply-water goes to drain. So, you see why the shut off valve is vital if you want to reduce RO-reject water from your system.

Using a permeate pump

Another excellent way to reduce RO-reject water is to use a permeate pump in parallel with your RO system.

In this regard, a permeate pump is a hydraulic pump that diverts the energy from the RO-reject water to the permeate water.

As a result, the purified water quicks enters into the tank. It also increases the capacity of the tank. 

So, you see, as more and more water gets purified, the ratio of drain and product water goes down as well. The quick discharge allows more tap water to go through the membrane.

Also, if you’re curious to learn more about permeate pumps and how those work, you should visit our recent article here.

Using a booster pump

A booster pump can also increase the efficiency of your household RO purifier by providing enough pressure at the membrane.

Here, a booster pump is an electric pump that gives a boost to the incoming water at the RO chamber by improving the pressure and the flow rate.

By now, you already know the semipermeable membrane relies on water pressure to purify. The higher the pressure, the higher the purification.

As a result, more and more water can come out as product water, and consequently, less water will go to the drain. In this fashion, the booster pump can reduce RO-reject water.

Recycling the wastewater

  • One other way that you can follow to reduce the RO wastewater is to recycle it again to produce pure water.

In this case, you can use a RO booster pump to create enough pressure at the RO membrane so that the drain water can get pure.

But, there is one drawback. As the RO-reject water holds more dissolved solids, it will surely harm your reverse osmosis membrane.

  • As a result, you might need to replace it far more often than not. That’s why this is not a well-recognized way to reduce RO-reject water.

Concluding remarks

Throughout this article, we tried to address one of the primary concerns of all the RO purifier owners out there, why my filter is wasting water.

We showed you what the RO-reject water is and how it gets produced. You have also seen it is possible to use the drain water for purposes other than drinking.

Finally, we showed you some traditional ways to reduce the wastewater from your RO filtration system.

So, what do you think? Have we got the answer to your question? Then, don’t forget to share this piece of content with your friends.

Also, if you still have some questions left, feel free to leave those in the comment section. 

2 thoughts on “What is wastewater in Reverse Osmosis systems and how to reduce it?”

  1. I have water softened water going into my ro unit.the water softened water is about 1700ppm tds because my well water has huge amounts of caco3 and manganese.My waste water from the ro is also 1700 tds.I have been channeling it into a small lined pool for the deer to drink.Is this too much tds for them to handle?The plants around the pool are thriving,so i was hoping it would be alright for wildlife.

    1. Thank you sir for asking. According to WHO, the TDS level in drinking water should be within 500 ppm. For wildlife, especially plants, it can be around 1000-1100 ppm.

      Therefore, you might like to think again about the pool if its source is only RO waste water. But, if you’re 100% sure that only the Calcium and Manganese are reason behind high TDS, then it’s perfectly fine. Both these minerals are good for the plants and wildlife as well.

      Also,if it rains at your location more often, then the rain water will mix with pool water and reduce the TDS.

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