RO
Reverse osmosis (RO) is a pressure-driven membrane filtration process that reverses the natural osmotic flow by applying hydraulic pressure greater than the osmotic pressure of a solution, forcing water molecules through a semi-permeable membrane while retaining dissolved solutes, ions, and larger particles on the feed side.[1][2] In typical systems, feed water is pressurized against the membrane, producing permeate (purified water) and concentrate (reject stream containing impurities), with rejection rates often exceeding 95% for salts and contaminants like bacteria, viruses, and heavy metals.[3][4] The underlying phenomenon of osmosis was first observed in 1748 by Jean-Antoine Nollet, but practical RO emerged in the mid-20th century amid U.S. government efforts to address water scarcity through desalination research, with the first cellulose acetate membranes developed in the late 1950s and initial commercial plants operational by the 1960s.[5] Early advancements focused on asymmetric membranes to enable viable flux rates under pressure, evolving from laboratory experiments to large-scale applications by the 1970s, when RO supplanted thermal distillation as the dominant desalination method due to lower energy demands per unit of water produced.[6] RO's defining applications span seawater desalination—accounting for over 60% of global capacity, enabling fresh water production in arid regions like the Middle East—and brackish water treatment, industrial wastewater recycling, and household point-of-use systems that deliver high-purity drinking water.[3][6] Its efficacy in removing nearly all dissolved solids has made it indispensable for producing ultrapure water in pharmaceuticals, electronics manufacturing, and boiler feed, though limitations include high energy consumption (typically 3-5 kWh/m³ for seawater), substantial brine discharge requiring careful management to avoid marine ecosystem harm, and removal of beneficial minerals like calcium and magnesium, which some studies link to potential health concerns in exclusive long-term consumption despite dietary compensation.[7][8] These factors drive ongoing innovations in low-pressure membranes and energy recovery devices to enhance efficiency and sustainability.[3]Science and technology
Reverse osmosis
Reverse osmosis (RO) is a pressure-driven membrane filtration process that separates dissolved solutes, such as salts, minerals, and organic compounds, from water by applying hydraulic pressure exceeding the natural osmotic pressure to the feed solution side of a semi-permeable membrane.[9] This forces purified water molecules through microscopic pores in the membrane while rejecting larger ions and molecules, achieving rejection rates of 95% to 99% for dissolved solids in typical systems.[10] The underlying principle counters natural osmosis, where solvent flows from low to high solute concentration across a semi-permeable barrier; in RO, external pressure reverses this flow, concentrating contaminants in a reject stream known as brine or concentrate.[3] The concept of osmosis was first demonstrated in 1748 by Jean-Antoine Nollet using a pig's bladder as a membrane, observing water movement across it due to concentration gradients.[11] Practical RO development accelerated in the mid-20th century, with early experiments in the 1950s by researchers at the University of Florida and the U.S. Department of the Interior, leading to cellulose acetate membranes capable of seawater desalination by the 1960s.[12] Commercial viability emerged in the 1970s, driven by energy crises and water scarcity, with thin-film composite membranes improving flux and durability; today, polyamide-based membranes dominate due to higher salt rejection and pressure tolerance up to 1,200 psi for seawater applications.[13] In operation, feed water is pressurized by high-pressure pumps—typically 800-1,000 psi for brackish water and 1,000-1,200 psi for seawater—passing tangentially across spiral-wound or hollow-fiber membrane modules to minimize fouling from scaling, bio-growth, or particulates.[1] Pretreatment, such as microfiltration or antiscalant dosing, is essential to extend membrane life, which averages 3-5 years under optimal conditions. Energy consumption varies by salinity: seawater RO requires 2.5-4.0 kWh per cubic meter of permeate, primarily for pumps, while brackish water systems use 0.5-2.0 kWh/m³; recovery rates range from 40-50% for seawater to 75-85% for lower-salinity feeds, with the balance discharged as hypersaline brine.[14][10] RO's primary applications include large-scale seawater desalination, producing over 100 million cubic meters of fresh water daily worldwide as of 2020, particularly in arid regions like the Middle East, where plants such as those in Saudi Arabia account for half of global capacity.[15] It is also used for brackish groundwater purification, industrial process water (e.g., pharmaceuticals, electronics), and point-of-use drinking water systems, removing up to 99% of contaminants including heavy metals, nitrates, pesticides, and pathogens when paired with UV disinfection.[16] Efficiency metrics show RO outperforming thermal distillation in energy use for desalination, with modern systems achieving second-law efficiencies up to 60% through innovations like batch processing or pressure exchangers that recover 95% of hydraulic energy from brine.[17] Despite its efficacy, RO systems face limitations: high capital costs for membranes and pumps, operational dependence on electricity (vulnerable to outages), and production of mineral-stripped water that may require post-treatment remineralization to prevent health issues like reduced bone density from long-term demineralized consumption, as noted in World Health Organization guidelines.[18] Environmentally, brine disposal poses risks of hypersalinity harming marine ecosystems if not diffused properly, contributing to localized oxygen depletion and toxicity in discharge zones; global brine volumes exceed 142 billion cubic meters annually, necessitating zero-liquid discharge strategies in sensitive areas.[15] However, RO reduces reliance on energy-intensive bottled water or imported supplies, offering net environmental benefits in water-stressed contexts when brine is managed through evaporation or resource recovery.[19] Ongoing research focuses on fouling-resistant membranes and hybrid systems to lower energy to theoretical minima near 1 kWh/m³ for seawater.[14]Read-only (computing)
In computing, read-only refers to a permission or attribute that allows data to be accessed and retrieved but prohibits modification, deletion, or writing operations, serving to protect information integrity and prevent accidental or unauthorized changes.[20] This principle applies across hardware like memory chips, software file systems, and media formats, where enforcement occurs via physical design, firmware, or operating system controls. The concept is epitomized in read-only memory (ROM), a non-volatile semiconductor storage technology that retains programmed data without electrical power, even when the device is off.[21] Unlike volatile RAM, ROM's content is embedded during or shortly after manufacturing and resists alteration under normal conditions, making it ideal for immutable instructions such as boot firmware.[22] Semiconductor ROM emerged in 1965, when Sylvania Electric Products developed the first commercial 256-bit bipolar TTL ROM chip for Honeywell systems, programmed bit-by-bit at the factory.[23] ROM variants differ in programmability and erasure methods, balancing permanence with flexibility:- Mask ROM: Fabricated with fixed bit patterns via photomasks during production; offers high density and low cost for mass production but requires redesign for changes.[20]
- PROM (programmable ROM): Blank chips users program once using a device to blow internal fuses, disconnecting unwanted paths; suitable for custom firmware but irreversible.[20]
- EPROM (erasable PROM): Features a quartz window for ultraviolet light exposure to erase all bits uniformly (up to thousands of cycles), followed by electrical reprogramming; requires chip removal for erasure.[20]
- EEPROM (electrically erasable PROM): Allows byte-level electrical erasure and rewriting in-circuit without special tools, enabling limited updates while maintaining non-volatility; used in modern flash memory precursors.[20][22]