Dear Iresha,

In industrial settings, RO is often used to remove total dissolved salts (TDS) from industrial wastewater or treated industrial wastewater and yield permeate with relatively low TDS concentrations.

While RO is an effective method for handling wastewater, what to do with the concentrate or reject water (typically as much as 20-50% of the RO feed water volume) can be an issue. Specifically, the high salinity in RO reject / RO concentrate can make it prohibitive for discharge to a local sewer facility. Hauling the wastewater to a treatment facility can also be prohibitively expensive.

ENCON Mechanical Vapor Compression (MVC) Evaporators and ENCON Thermal Evaporators have proven to be effective technologies for dewatering RO reject / RO concentrate waste streams. In brief, evaporation is a time-tested methodology for reducing the water portion of water-based waste. The evaporator converts the water portion of water-based waste to water vapor, while leaving the higher boiling contaminants behind. This greatly minimizes the amount of waste that needs to be hauled off-site.

Evaporation technology has always been more “hands off” than other wastewater treatment methodologies resulting in a dramatically lower labor cost. Evaporation technology can handle a much wider range of waste streams compared to membranes and traditional physical / chemical treatment methodologies. Finally, evaporation does a much better job of concentrating waste streams compared to other methods, thereby yielding a lower disposal volume and cost.

With my best regards

Prof. Bachir ACHOUR

Thanks Prof.Bachir for your valuable comment.

The reject water used to  mopping floor 

I use it to mop tiled floors at home but it does leave marks and stains when the salts are high.

The answer depends on the cost you are waling to pay for concentrate disposal and the volume of concentrate that has to be disposed of per day.  For small volumes (less than 1,000 m3/day) the lowest cost disposal is the discharge to the closest sewer system.  For larger volumes it usually is onshore or offshore discharge in a water body of lower salinity - preferably the ocean.

The best solution depends on the volume and the cost you can pay for brine disposal. The recovery of salts and valuable materials from brine is one of the novel methods reducing environmental concerns and water desalination cost. The following paper can help you.

http://www.sciencedirect.com/science/article/pii/S0959652615002486

I think that given the high conductivity of the RO reject , electrodialysis could also be considered. we are working on a unique capacitive electrodialysis technology that in the lab setting has been demonstrated to have a current efficiency of over 80% while using very small voltages. Using low voltages minimizes the required energy in electrodialysis systems. Such methods not only reduce the waste volume but will also provide for additional feed for your RO system. 

For large scales (i.e. industries):

With the increasing cost of water and waste discharge, more companies are looking to recover and reuse RO reject water. While it is possible to reuse the reject water from an RO system by feeding it directly into a 2nd RO unit, the likelihood of scaling or fouling the 2nd RO without pretreatment is rather high.

A Tubular Membrane Filter (TMF) system coupled with lime softening will reduce scaling and enhance the operation of the 2nd RO. Using this treatment technique, recovery rates can be >95% while assuring substantial reduction of scaling to the RO units. This is an efficient and cost effective method for either retrofitting existing RO systems or incorporating the process into newly designed systems.

For small scale setting (Household/residential):

The rejects from RO are generally not diverted to 2nd treatment unit and hence the salinity (i.e. TDS) of final concentrate effluent is just a bit higher than normal.

RO reject/RO concentrate can be used for any non-drinking and non-cooking purposes. The usage may range from utensil cleaning, flushing, floor mopping, gardening (not so good in the long run or can be good for salt resistance species) etc.

As this water is really not dirty (in terms of quality), it's just that its hardness/mineral content is different. It is okay to feed this to a nearby rainwater harvesting system.

Important points mentioned by Shiva Kumar in case of household RO units.

The large industrial RO systems can also divert their RO concentrates to the near-by farmlands for irrigation purpose. Most of the crops grow faster in mineral-rich water. However, regular check up of salinity must be checked for discharging it into irrigation. Unethical discharge can cause drop in the fertility of the farmlands.

This idea can be applicable in the drought-hit and low-rainfall regions.

RO effluent can be used as an irrigation source for some agricultural crops.

https://www.researchgate.net/publication/328828204_Effect_of_Different_Diluted_Reverse_Osmosis_Rejection_Water_as_Irrigation_Source_for_Different_Crops?_sg=v3Cpe8YKaoPfOeBhTbnW1T2OJmRPmyeiTJ1_-xb_0giFySP1MYKvtm086EYmtovxenvx2QzJf4ZvMQ.v-G1xv0eXUhDWgfbr9kSp9UBB7kLjegAUOsbt5AAkgvJfbzYXdLhtIeHL7nqg8_VeH2JrSS7P0spMGgVfIDkVA&_sgd%5Bnc%5D=2&_sgd%5Bncwor%5D=1

RO waste water can be used for Recovery of desired minerals easily.

why cant waste water treated as desalination.

The best method for disposal of reject water depends on its volume. for low flowrates, the best method is the discharge to the municipal wastewater collection network. but for high flowrates, Evaporation lagoons are recommended, If it is far from coastal areas.

Need to study he suitability for cultivating major corps and for horticultural crops.

I order to reduce the pollutant load due to RO reject wastewater, a pretreatment is necessity. If we reduce the influent load as well as reduce the effluent load. The remaining effluent will use to make steam and it partially recovered from condensation process. The most of textile sectors need to steam for ironing process. So, the operation of mass steam generation plant near to common wastewater treatment plant is flexible for reduce cost efficiency also.

Long detention in a deep well would help stratifying dilute layer at the top and conc. layer at the bottom. This difference in conc. of two layers may be used for generating energy. Highly concentration Reject can be supplied to industries using valuable salts for their processes e.g., tanneries. In remote areas, where land is available easily, shallow lined ponds may help rapid evaporation for harvesting salts.

RO water rejected water is called brine. This is new area for management of waste water. In RO system more than 60 per cent laid as waste water as brine. Hence, its management is very essential. The brine has TDS more than 850 ppm with gray water quality indices. Hence more research needed to study for suitability of these water irrigation to suitable vegetable or horticulture crop and effect on health, otherwise it is laid in the soil as such, which could polute the ground water.

Freeze Desalination is the best solution to recover RO waters of any salinity. We have successfully recovered RO waters TDS of 70,000 to 250,000 ppm to potable water, using repellents and catalysts. Power consumption will be about 5 kwh/cum. No pretreatment is required and scope to recover mineral salts.

Our results have to be consolidated with Pilot plant studies. Our work is held up due to pandemic.

Sir,

Pls explain Freeze desalination process and flow diagram.

Sir,Is any update on freeze desalination

The use of RO water for domestic and industrial use is tremendously increase. The 65 to 70 per cent rejected water release in filed/ nala as sewage water, which is having very high TDS i.e more than 800 to 1500. This release water must be have better management practices to protect ground water quality. If possibility to be check, can this brin used for rearing imc or some crops will have tremendous scope in future and its effect on health is the most advance study area in the urban agriculture.