Reverse osmosis is an effective, widely accepted and established technology for purification of water and has also become increasingly popular as one of the technologies used in desalination of seawater for human consumption .

Reverse Osmosis (RO) is an advanced technology to purify water for a wide range of applications, including production of bottled drinking water, production of ultra-clean water, electronics industries, semiconductors, food processing, biotechnology, pharmaceuticals, power generation, seawater desalting, and municipal drinking water (2) It is also used to recycle used water to drinking water on space shuttles and on International Space Stations.

However, RO is more expensive than conventional treatment techniques and generates concentrate that ultimately requires disposal. Disposal of the concentrate can present serious challenges. In areas where water resources are scarce, there is also a strong desire to get the most product out of the source water used. Therefore, maximization of recovery (the ratio of treated water produced to source water used) has become an important challenge to make RO economically feasible for communities. (5)

From classified reports, high recovery processes are broadly classified in to three claims which are for industrial RO systems and the other unique one for domestic RO purifiers

HIGH RECOVERY RO’s – Industrial scenario

1. CCD (Close circuit desalination )

In industrial RO systems, recoveries as high as more than 90% can be achieved. Semi batch or close circuit ROs provide high recoveries (4)
The CCD (Close circuit) RO process for high-permeate-recovery operation, uses single membrane pressure vessel representing multiple modules operating in parallel
Brine is recirculated without depressurization. When a desired recovery percentage is reached, brine is throttled out of the system. Brine rejection is executed without stopping the high pressure pump or the production of permeate. The process then returns to closed-loop operation, during which there is no brine reject stream. The resulting average membrane feed pressure in the CCD process is much lower than the feed pressure of typical RO or NF
Continuous brine recirculation maintains a maximum salt concentration in the membrane array, resulting in higher membrane feed pressure requirements, higher salt passage and permeate salinity, and higher risk of precipitation of sparingly soluble salts compared to CCD processes in which brine is completely purged regularly.

RO Close Circuit Desalination

2. UF pretreatment

UF system provides an adequate flow of high-quality permeate to the RO unit by removal of turbidity and particulate matter. UF enhances and ensures the performance of the RO system, resulting in optimal flux, high salt rejection, and high recovery. (6)

3. HERO

High Efficiency Reverse Osmosis (HERO) is a reverse osmosis technology specially designed to treat high silica, it can handle three to seven times higher silica levels than traditional RO systems (up to 2,000 ppm in concentrate stream)
It continuously operates above 90% water recovery – unprecedented for a typical reverse osmosis system – with little or no bio-fouling and organic fouling compared to a conventional RO. This also offers significant operational benefits. The process has an excellent track record of treatment of cooling tower blow down where conventional reverse osmosis (RO) is generally considered to be a less effective technology.
In some instances, HERO™ may be used to replace or supplement an evaporation process as a cost-effective method of achieving zero liquid discharge (ZLD).

Basic process consists of three steps: Hardness and suspended solids removal, Carbon dioxide removal, and RO treatment at elevated pH.

RO Treatment

4. A BREAKTHOUGH HIGH RECOVERY PROCESS– Domestic RO purifiers

The high recovery process that maximizes product yield from a single membrane
Background Reverse osmosis technology converts salty water to pure, good tasting water.
When there is no option for the customer and he has to be completely dependent on bore well- hard water, he realizes the importance of RO, since it removes TDS more than 90% and converts salty , bad tasting water to sweet palatable water
One may feel RO is a complete solution for drinking water purification , but may get disappointed when he knows the fact that while delivering pure water RO rejects or wastes lot of water .
Demand for water is high and global resources are limited, yet current RO systems waste both water and energy(1), a major drawback of this technology is that, it wastes lot of water while delivering pure water.
Often users of RO purifier collect this reject water and use it for flushing, in this era of water conservation , wasting water is highly questionable

Drinking water purifiers based on RO technology recovers good water to the extents of 20-25% remaining 80-85% water constituting contaminants and excessive salts is rejected /wasted.
Water being precious, this water wastage is always a matter of great concern.
The higher the recovery % means that you are sending less water to the drain as concentrate and saving more permeate water.

Let us understand what is percentage recovery of RO membrane.
Percent Recovery is the amount of water that is being 'recovered' as good permeate water compared to the wasted water. (3)
In case, the process design choose and operate this membrane at higher recovery more than 20%,, the membrane life is compromised consequently, due to depositions of hardness causing elements like calcium, magnesium and other salts on membrane surfaces, eventually pure water flow through membrane reduces.

RO Elements Cost Factor


Single RO membrane generally gives 20-25% recoveries
Recoveries as high as 80-90% are achievable using multiple RO membranes. However the challenge is to obtain high recovery from a single RO element. Multiple RO elements being a cost factor.

How can we reduce this water wastage in RO purifiers?

The present invention has enabled the recovery as high as 70-80% with single RO membrane thereby reduce water wastage as compared to conventional RO purifiers. (patent rights enforced )

High recovery is achieved by

a. Preventing excessive salt deposition on membrane.
b. Preventing fouling of membranes due to microorganisms in water.

The proprietary, ‘Water Saver Cartridge’ , WSC which is the key element of RO water purifier. It releases elements that prevent salt deposition on the membrane and thereby enhance RO membrane life.

The WCS also releases frequently small amount of disinfectant electrolytically which prevents biofouling of RO membranes and keeps the membrane sanitized.

Unlike conventional reverse osmosis purification system ,this system does not use any oxidizing chemicals for sanitization externally. WSC contains an in-built sanitizing system which avoids the use of oxidizing chemicals like chlorine, hydrogen peroxide, peracetic acid. This unique Water saver cartridge therefore not only prevents the membrane scaling but also protects the membrane surface from biofouling due to microorganisms in raw water . Thus High recovery RO purifier with 70-80% pure water recovery, with a feature of biofouling prevention is a revolution in the field of domestic reverse osmosis purifiers.

This novel concept is now commercialized , has evolved as Zero-B Eco-RO in domestic and institutional RO water purifiers

Domestic Reverse Osmosis Purifiers

References

  1. Author :Richard Stover Executive Vice President – Desalitech, Inc. – USA rstover@desalitech.com
  2. Hydranautics
  3. 2012-2014 Puretec Industrial Water
  4. The International Desalination Association World Congress on Desalination and Water Reuse 2013 / Tianjin, Richard Stover Executive Vice President – Desalitech, Inc. – USA rstover@desalitech.com
  5. Desalination and Water Purification Research and Development Program Report No. 119 Reverse Osmosis Recovery Maximization Dr. Graham Juby Carollo Engineers Fountain Valley, California
  6. Case History, Dow TM Ultrafiltration