Reverse Osmosis Membrane Desalination

Reverse osmosis in meeting global freshwater demands

Reverse osmosis (RO) is a pressure-driven membrane diffusion process and the dominant technology under membrane desalination processes. The reverse osmosis membranes retain up to 99% of the dissolved organic and inorganic solutes from the feed water into the concentrate, and the permeate produced is considered the product water. Reverse osmosis technologies have the highest utility rate globally, accounting for 69% of installations for use in middle to small-scale single-process plants responsible for freshwater production.

Reverse osmosis is one of the most energy-efficient desalination processes and greenest solutions for industrial wastewater, with no need for steam and heat exchangers to facilitate any phase changes in the purification process. Reduced energy consumption, the requirement for lesser floor space, and equipment designate reverse osmosis as a more economical process. 

Recent advancements have transited systems to a 'zero waste' system, eliminating water waste by returning concentrate water from the reverse osmosis system to other commercial purposes.

Differences between types of membrane desalination systems  

Seawater reverse osmosis (SWRO) and brackish water reverse osmosis (BWRO) membrane desalination are two systems with increased use by 6.8% yearly over the past decade and contribute up to 4.6 million cubic meters of water a day. 

The primary difference in treating seawater and brackish water is the amount of dissolved solutes present in the feed stream coming into the membrane systems. Seawater feed streams contain higher amounts of total dissolved solids, ranging from 15,000 milligrams per liter (mg/L) to over 40,000 mg/L, while brackish feed streams contain total dissolved solids ranging from 1,000 mg/L to 15,000 mg/L. 

The greater the salt content of a feed stream, the higher the pressure and energy consumption needed to treat water using membranes. The amount of dissolved solids influences the extent of supersaturation caused by changes in pressure, leading to scale precipitation. Different membranes used in reverse osmosis systems also have differing membrane burst pressure limits. 

Brackish water with lower amounts of dissolved solids allows for higher applied pressure in the system, achieving up to 70% to 96% recovery rates. Seawater reverse osmosis systems typically have a lower recovery rate of 35% to 45% due to lower applied pressure to avoid precipitation and membrane damage. 

Addressing fouling in membrane desalination systems

Fouling in membrane desalination systems can include inorganic scale formation, particulate fouling, biological fouling, and chemical membrane degradation. The causes of each fouling can vary but can be closely related, and mitigation methods applied should consider inhibitive actions in all aspects of fouling. 

a) Alkaline scale formations

Calcium carbonate is the most common scale formation that persists in membrane desalination systems and occurs when water concentrates through reverse osmosis. Upon supersaturation beyond the set solubility limits, precipitation leads to scale formation. These formations are typically observed firstly in the last stage and gradually manifest upstream. 

Apart from supersaturation, contributory factors can also affect the rate and extent of precipitation. High operating pH conditions (> 8.2), high operating temperatures, and the lack of permeate flushing encourage calcium carbonate scale formation, resulting in the decline of normalized permeate flow and salt rejection. 

Magnesium hydroxide is another alkaline scale species that can persist in membrane desalination systems, though less pervasive. Like calcium carbonate, its solubility decreases with an increase in operating temperatures. 

b) Non-alkaline scale species 

Calcium sulfate is a common non-alkaline scale species that occurs in membrane systems and is more unmanageable than alkaline scale species like calcium carbonate. Being very adherent and tenacious, removing this specific scale species typically requires a longer shutdown duration and affects the overall productivity of the treatment plant. 

Apart from calcium sulfate, less pervasive non-alkaline scale species that can persist due to supersaturation include barium and strontium sulfate. 

c) Other mentionable scale species 

Calcium fluoride scale species can occur when the feed stream originates from areas and environments with a higher concentration of fluorides. Fluoride in the water comes from natural processes like rock weathering and volcanic emissions or human activities, such as phosphate rock mining and drinking water fluoridation. 

Silica scale formations exist in an amorphous form and are classified further as polymeric, colloidal, and particulate. In the feed stream of low concentration, reactive silica (silicic acid) remains in its monomeric form. Polymerization occurs when the feed stream concentrates, and the silicate anion grows into a macromolecule. 

The presence of cations like calcium and magnesium upon the concentration of the feed stream also increases the rate of silica polymerization. Polymerized or colloidal silica is non-reactive and cannot be removed easily by traditional cleaning methods. 

The recommended scale inhibitor solution includes the one-for-all CrestoPro R494, which offers extensive inhibition action on carbonate, sulfate, fluoride, and silica scale species. In plants that require more mobility in applying treatment solutions, the composite solid formulation of CrestoPro R594 offers the same range of scale formation inhibitions, with improved hazard management. 

Accurate dosing guidelines

Accurate dosing of any scale inhibitor solution allows operators and plant managers to carry out cost-effective daily operations. The Online Guidance Tool helps interpret and translate plant operating conditions and water sample analysis, to recommend the appropriate scale inhibitor for treatment programs