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Innovative Solutions for Water Scarcity: Reverse Osmosis Reject Water Recycling

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Benefit of RO Reject Water Recycling

Water is a fundamental element that has been present on Earth since the time of dinosaurs. It is a finite resource that cannot be created or destroyed, only continually recycled through the planet's natural processes. This emphasizes the importance of conserving and protecting water sources to ensure their availability for future generations.

Water recycling and roadblocks

Water recycling is a vital practice that contributes to water conservation and sustainable living. However, there are still some individuals who reject the idea of water recycling due to various reasons. It is important to address and debunk these misconceptions in order to promote the widespread adoption of water recycling.

One common reason for rejecting water recycling is the belief that recycled water is not safe for consumption or use. This misconception stems from a lack of awareness about the different types of wastewaters and their probable uses. Wastewater is divided into different categories like black water, greywater, sewage, industrial effluent, etc. Whereas recycled water reuse can be categorized into different segments like car washing, gardening, floor cleaning, in cooling towers, or depending on the water quality for laundry and other general-purpose uses.

Another reason for rejecting water recycling is the misconception that it is a costly and burdensome process. While initial infrastructure investments may be required to establish a water recycling system, the long-term benefits, such as reduced water and energy costs, can outweigh the initial costs.

The reverse osmosis system and reject water

Although we cannot use wastewater directly for general purposes, yet we have a major source of water, which, because of misconceptions, we consider as wastewater otherwise, which is actually an asset for all of us.

We are talking about reject water generated from our drinking or potable reverse osmosis systems. These types of systems we generally use in our home, offices, communities, and industrial setups for our drinking purposes and which are primarily used for reducing TDS (total dissolved solids) only, and in this process these systems generate a lot of reject water.

There are many other systems, like ultrafiltration and ultraviolet, available to make water bacteria- and infection-causing viruses-free. These systems make water safe for drinking while not reducing the TDS of water.

The reverse osmosis system generates permeate, the low TDS water, which we use for our potable use, and concentrates, the high TDS water, which we generally dispose of in gutters. This reject water, although good in nature, cannot be used directly because of its high TDS concentration.

The quantum of reject water

Under ordinary conditions (as per IS 1172:1993), the minimum domestic water demand for a town with a full flushing system should be taken at 200 liters/day/capita, out of which 5 liters/day/capita are for drinking water and 5 liters/day/capita are for cooking purposes, so a total of 10 liters/day/capita is required for potable use. The RO membrane used for drinking water has a recovery of 15% to 20%, which means to generate low TDS potable water of 40 liters for a family of 4 persons, we need 200 liters of water, out of which 160 liters of high TDS water generate, which is generally discarded as reject water.

If we explore it for a 6-story, 24-flat residential complex, the quantum of reverse osmosis reject water comes out to be around 3,840 liters, which is 20% of the total water consumption of 19,200 liters (200 liters/day/capita x 4 people per flat x 24 nos. of flats).

Quantum of reject water vs. rainwater harvesting

Rainwater recharge is an important method for increasing groundwater levels and replenishing aquifers. By capturing and redirecting rainwater into the ground, we can help ensure a sustainable water supply for future requirements. This practice not only helps to reduce the demand for municipal water supply but also helps to alleviate pressure on existing water sources. Rainwater recharge is an effective way to conserve water and promote environmental sustainability.

Now consider the same scenario of a 6-storey, 24-flat residential complex with a moderately sized flat with a corporate area of around 1250 sq. feet (~116 sq. meters) per flat. Considering 4 flats per floor, the terrace of this building will be around 1250 x 4 = 5000 sq. feet (~464 sq. meter).

If we consider an average rainfall of 42 inches (~106 cm) per year and if we capture all of the rainwater that drops on the building terrace, the total quantity of rainwater available per year for recharging will be 464 sq. meter x 106 cm / 100 = 492 cubic meters, or 4,92,000 liters.

The per-year quantity of reverse osmosis reject water as per the earlier considered example will be around 3,840 liters per day x 365 = 14,01,600 liters, which is almost 3 times more than the rainwater available for recharging purposes.

The numbers in examples are indicative only, and these numbers may be different in your scenario. You can check the results by using the below calculators with your actual numbers.

How can we safely recycle rejected water from a reverse osmosis system?

We can reuse the reverse osmosis reject water in two ways: first is direct use, and second is use after dilution. We will consider both options one by one.

Direct use of reject water

For drinking water, the input to the reverse osmosis system is the same water, which we generally use for our bathing, laundry, and other general purposes. In other words, the input water quality is good, and hence the high TDS reject water quality is also equally good, except for the high TDS concentration.

The restriction to use high TDS water for general purposes like bathing, laundry, and cleaning is not high TDS but the high hardness of the water, which creates nuisances like scaling, white spots, and scum buildup on utensils and walls. Hard water can create challenges when it comes to bathing and doing laundry. The high mineral content in hard water can lead to soap scum buildup on skin and hair, as well as leaving clothes feeling stiff and rough.

If you want to use reject water directly, it is essential to address its high hardness first. A water softener is the answer to eliminating this hardness from the water and making it reusable again. The Ultimate Buying Guide: How to Choose the Best Water Softener for Your Home can be helpful for you to understand the factors that you should consider while buying a water softener. Or if you are a member of a residential society, If you own a flat, you must read it would be a good starting point.

Use of reject water after dilution

The reverse osmosis system has a salt rejection rate of between 95 and 99%. It means if input water TDS is 500 ppm, reject water TDS will be between 619 and 624 ppm, and the TDS of permeate, the low TDS stream will be between 25 and 5 ppm, which is very low for drinking purposes. Reverse osmosis systems increase the TDS by mixing the input water and permeate water by using a mixing valve, which controls the proportion of mixing and resultant TDS. By varying the amount of mixing input water, the resultant TDS can be easily increased to any required level.

In a similar fashion, we can use reject water after dilution of high TDS. Considering our earlier 6-floor, 24-flat residential building scenario, where total water consumption is 19,200 liters per day and reject water quantity is 3,840 liters.

On dilution of this high TDS reject water of quantity of 3,840 liters with the remaining quantity of 19,200 – 3,840 = 15,360 liters of TDS of 500 ppm, the resultant TDS is marginally increased. The various data for a seven-day period is tabulated as follows:

Day Water consumption per day RO Reject Water in Liters Tentative RO Reject Water TDS Effective Water Qty with Recycled Water Effective TDS after adding Reject RO Water
0 19,200 liters3,840 liters 619 ppm19,200 liters NA
1 19,200 liters3,840 liters 642 ppm15,360 liters 524 ppm
2 19,200 liters3,840 liters 647 ppm15,360 liters 528 ppm
3 19,200 liters3,840 liters 649 ppm15,360 liters 529 ppm
4 19,200 liters3,840 liters 649 ppm15,360 liters 530 ppm
5 19,200 liters3,840 liters 649 ppm15,360 liters 530 ppm
6 19,200 liters3,840 liters 649 ppm15,360 liters 530 ppm
7 19,200 liters3,840 liters 649 ppm15,360 liters 530 ppm

We can easily observe in the above-tabulated data that the resultant TDS is increased after the mixing of water, which also affects the TDS of rejected water. But after a couple of days, both the resultant TDS of mixed water and rejected water stabilize at a point that is slightly higher than the initial TDS.

The important point to consider here is that while TDS is marginally increased by 6%, the saving of water is 20%, which is quite a lucrative quantity.

The numbers in this example are indicative only, and these numbers may be different in your scenario. You can check the results by using the below calculators with your actual numbers.

Conclusion

In conclusion, to promote its widespread adoption, it is important to address and debunk misconceptions about water recycling and reject water generated by drinking water reverse osmosis systems. By increasing awareness about the safety and cost-effectiveness of water recycling, we can encourage individuals to embrace this sustainable practice and contribute to the conservation of our precious water resources.

Right now, we are throwing away valuable water, which is rather an asset and which can be saved and used again. You can also contribute to recycling, the environment, and sustainability at the individual level by sharing this information with your peers, friends, neighbors, and relatives.

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