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Nanofiltration + RO to make reject water disposable

I am a student of IIT Kharagpur doing a project on water purification.

We are using a RO purifier system to make the water potable and safe for drinking. But since the reject water is having more than 1.25 to 1.5 times TDS as that of the raw water, we think that it will be unethical to dispose off the reject water as such.

We are using medium-scale water purifying systems which produce product water at 500 litres per hour rate.

I have planned to put in a nanofiltration stage just before reverse osmosis. This will help reduce the TDS of the reject water as well as help increase the lifespan of the RO membrane.

My question is:  Should I use the nanofiltration membrane before the RO stage, is it economically viable?

And what are the other ways to reduce the TDS of the reject water, especially in these medium-scale water purifying systems? 



1. Dear PrattBhatt As you are a

Dear PrattBhatt

As you are a strudent at an IIT, we look forward you telling us, where should one place nanofiltration, before RO or after RO, to reduce TDS. Do it and find out. In doing so, you may also stumble upon answers to following questions.

1: If nanofiltration can reduce TDS, then why do you need RO at all ?

2: If nanofiltration reduces TDS, then how are the removed solids disposed off ? Does that too involve wasting some water ? How much ?

Do it and tell us what you find.

Chetan Pandit

2. other innovative ways to use the wastewater

Thanks a lot Chetan for your reply.

We are currently working on trying this thing.

But, apart from this, I would still like to know about innovative ways to use the RO reject water, maybe like using it for flushing in a toilet, or maybe car-washing.

My point is even if we are not able to reduce the TDS of the wastewater, we should atleast find ways to use it as many times as possible before disposal.

3. What to do with RO reject water

Dear Prattbhat,

For a start,it would help if you said what is the level of TDS in your feed water to the RO.A complete test report of the water would help.

You have mentioned that you are using a 500 lit/hr RO plant.What is the reject flow rate of this RO? If the test report of your feed water was given, I could indicate to you roughly what would be the TDS in the reject.Yes,if the reject TDS is high,disposing of it would be a problem because all pollution control norms now in force would not permit TDS levels of RO reject.

Normally, Nano filtration is used to treat the RO reject water to remove hardness in it before it is fed to an evaporator(this is to ensure a zero discharge situation.A nano filtration system will not reduce TDS.It will prolong the life of your RO membranes,but if you consider the cost of Nano filtration,you will find it is far cheaper to change RO membranes once in 2 to 3 years than using a Nano filtration.Please also remember that you also have waste/reject water from a Nano filtration system.What do you propose to do with this?

I repeat once again,if you give me the complete test report of your raw water, I could tell you whether the experiment you are doing is worthwhile or not.




4. details of the water we are dealing with

These are some of the details regarding the water we are dealing with:


TDS of feed water = 1500-2000 ppm

Flow rate of feed = 1300 LPH

Flow rate of product water = 500 LPH

Flow rate of reject = 800 LPH


Content (in PPM)

Calcium  = 125

Magnesium = 117

Sodium = 301

Potassium = 7.8

Carbonate = NIL

Bi-carbonate = 354

Sulphate = 371

Chloride = 560

Fluoride = 0.90

Nitrate = 5.30

Total Hardness = 800

Total Alkalinity = 265

Iron = 2.10

Total Colifoms = NIL

5. Reducing the TDS of water

Dear Prattbhat,

Thanks for details of the feed water to the RO. The RO give you a recovery of 38.5% of the feed flow while reject is 61.5% of feed flow,which according to me is wasteful.I expect the TDS of the reject to be in the range of 2000 to 3000ppm and this limits the options of re-using it.

To drastically curtail losses due to waste, it would be worth considering a Nano filtration plant with an RO to treat the reject from the first RO to help recover a part of the first reject and then use the second reject  to feed an evaporator to most of  this reject.The Nano filtration would reduce the heavy scaling in the evaporator which would limit its output.

I have proposed such systems in the past for locations where waste discharge rules were very stiff and installing a Zero discharge system was mandatory.

Unfortunately, small RO capacities like 500lits/hour limit the recovery and result in large reject volumes.If the flow rate was in the region of 1.5 m3/hr or more,it would be possible to design an RO with upto 85% recovery.




6. Dear Mr. Bhatt, There is no

Dear Mr. Bhatt,

There is no way to reduce the TDS in any way. What you can do is installing another RO system (second stage) for the reject water and for that matter a third stage as well. This would considerably reduce the extent of the reject water, and increase the TDS in it. However the number of stages that you can go to would depend upon the TDS values. The process only alters the concentration but not the total solids.

With regards,

Keshav Agarwal
Technical Consultant
Biotech Services
Noida, UP

7. other innovative ways to use the wastewater ??

Thank you all for your replies.

Evaporation is not a economically feasible solution, when we are trying to purify water at such a small scale.

Recycling of the reject water can only be done upto a certain level, but then the TDS of the reject water increases even more.

Is it possible to find ways to utilise the wastewater (RO reject, 1500-3000 ppm), for growing algae in a tank. 

We can use the algae species such as Schizochytrium sp. which have a high lipid content, and can also grow in high salinity levels of up to 15%.

Are there any other "revenue-generating" ways to utilise the RO reject?

8. Dear Friend, Unless you tell

Dear Friend,
Unless you tell me details of the water analysis it would not be possible for me to tell you if it is econoimially viable to have nano filtration before RO. However, it is very much beneficial for the RO to have nano filtration prior to it technically. In commercial reality, we do not do it unless it is for a wastewater recycling plant.
Taral Kumar
Executive Director
Akar Impex Pvt. Ltd.
Noida, Uttar Pradesh

9. details of the water we are dealing with

Dear Mr. Taral,

I have provided the details of the water we are using in an earlier comment:


If you want some other data regarding the water, we would be glad to provide that too.

10. Revenue generating methods are still under research

Dear Prabhatt,

As of now there are no esstablished revenue generating measures out of RO concentrate. As the TDS in the concentrate is very high  it is possible to extract commercially valuble salts like NaCl, KCl e.t.c., but the concentrate also contains anti scalent chemicals and chemical backwash liquids which complicates the salt recovery process and render them expensive. 




Project Officer

Arghyam, Bangalore

11. Universities in India are

Universities in India are often accused of teaching "what" to think, rather than "how" to think. Therefore, in my first response I had tried to set him on a direction for thinking. But that did not work. PrattBhat’s reply dated 31-May is strange, and takes the discussion off the mark.

His original question was “ And what are the other ways to reduce the TDS of the reject water”. But why do you want to reduce the TDS of reject water ? The objective should be to increase the TDS of reject water. A higher concentration of TDS in reject water means less reject water and more product water. Keshav Agarwal has already said the same thing in his reply, but he wrote it in short and therefore it seems the meaning was lost to PrattBhat. Now, PrattBhat’s reply of 31-May. My comments are in purple font.

Evaporation is not a economically feasible solution, when we are trying to purify water at such a small scale.

If the objective is to purify water then evaporation not a solution at all, economical or otherwise. Evaporation is a solution for managing rejects, and not for purifying water. The water evaporated, say in solar evaporation ponds, is lost. If you want to capture the evaporated water and condense it and put it to use, the word is “distillation”. That is an entirely different process. Since you are a S&T student, we expect you to be precise in your use of terminology.

Recycling of the reject water can only be done upto a certain level, but then the TDS of the reject water increases even more.

Of course. What else did you expect ? A certain quantity of salt has entered the system, and it has to get out. It can’t just vanish in thin air. More the water you manage to extract as product water, lesser the quantity of reject water, and hence higher the TDS concentration in it.

Is it possible to find ways to utilise the wastewater (RO reject, 1500-3000 ppm), for growing algae in a tank. 

But why do you want to do that ? To "make use of" reject water. Right ? Now, the "use" can be real, or contrived. If you have a use for the said algae, then the use of reject water is "real". But if you have no use for the algae, then growing it just because it can grow in reject watrer, is a contrived use. How will you dispose of this salt rich algae ? As they say – if no one liked the vegetable stew you made today, you better throw it away as such. Trying to make vegetable soup out of it tomorrow, which also no one eats, only adds to the waste.

Chetan Pandit

12. Universities??

Dear Mr. Pandit,

With due respect, I would like to say that the project I am doing is on an individual basis, and has nothing to do with my university. I understand that you feel that I am a bit confused, but thats how the problem case is.

I am trying to find an “affordable” solution to drinking water problems in rural areas in India. This is not at all related to my stream of study, yet the teaching I have received (at my university) has encouraged and motivated me enough to look into the problem, try to go into all the aspects regarding the technology and its sustainable implementation in rural areas.

I am designing a 5-stage RO purifier unit which gives a product water flow rate of 500 lph and reject water flow rate 0f 800 lph. This “optimises” the cost of maintenance and operations of the unit. And I am also looking into “other ways” of “utilising” the reject water.

As I had no background in water purifying or related technologies, it took me a while to figure out that the Reject water issue is not a disposal issue, but it is an issue of utilising it in the best way possible before disposing it off. I did all the research into the technology and the industry in only a month’s time.

Coming back to the “university teaching in India” topic, its easy for us to complain that it doesn’t promote this and it doesn’t promote that. But I don’t agree with this notion. The teaching is absolutely fine. Its just that there needs to be more encouragement to the students to pursue “whatever they want”, such as (in this case) more applied research into topics that will really help solve some of the world’s biggest crises.

And I appreciate the efforts of our Government to promote this, through various programmes such as TePP by DSIR, and setting up of incubation centres across different universities etc. I also applaud the Government’s effort in establishing more IITs and IIMs, and other institutes for higher studies.

I hope this answers your concerns.

Thank You.

13. Need details on non-traditional TDS reduction systems


I want to buy a TDS reduction system to change the water from 800 ppm to 150 ppm. The need is only for washing and bathing. But I don't have any interest in traditional softeners which require weekly recycling with brine water.

My consumption is 500 lt/day. Can you suggest an appropriate one?



14. R.O Reject management of textile dyeing house waste water

Dear All

R.O Reject management is still a great task, especially R.O Reject from dye house which have high concentration of salt. The main objective is how to convert high salty water to solid form. I think if any filter (hope nano filter) will be able to recover salt then final reject water will have very less quantity of salt. In this way, possible to use that water for plantation, toiletry, washing work.

15. What is best way to reuse and minimize the R.O reject water

Dear All,

I have a R.O. plant in Roorkee, Uttrakhand having a capacity of treating 50.00 Cum of water per day & approximate qty. of reject water is nearly 40.00 Cum. per day & the TDS of raw water is 320 & that of R.O.Reject water is 500. I want to know what could be the best way to dispose/reuse/minimize the R.O reject water without affecting the quality & quantity of water level of under ground water table.

It is also to inform that it is not possible to connect the waste/reject water with proper public drainage system because presently there is no proper drainage system is available and can only be connected to an open road side nala which is connected to water pond of the village.


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