Reverse Osmosis Process Explained: How Desalination Works

Desalination produces concentrated brine as a waste product, which cannot be discharged directly into the ocean without environmental consequences. Modern desalination plants typically recover about half of their intake flow, meaning the brine stream becomes approximately twice the concentration of normal seawater. Direct discharge would harm marine life, particularly organisms near the sea floor, since the dense, super-saline solution tends to sink. To mitigate environmental impacts, most plants use diffusers to spread out the brine so it dilutes faster, or blend it with other water streams like power plant cooling lines or wastewater effluent before release. When neither option is feasible, some plants inject brine into the ground, though this expensive approach adds significantly to operational costs.

This section covers brine discharge environmental management and physical treatment systems in desalation plants. Brine discharge creates environmental impacts through increased salinity and density, affecting marine organisms like Posidonia oceanica. Environmental monitoring uses Posidonia as a bioindicator with mortality rates indicating impact levels. Brine discharge mitigation strategies include diffuser systems with high-velocity jets, parabolic discharge pipes, and dilution with seawater before discharge. Achieving zero impact requires dilution but increases energy costs. Physical treatment systems include conventional multi-material filters (bicapa, tricapa), ultrafiltration, and pre-treatment requirements for membrane systems. Ultrafiltration has become increasingly popular as a pre-treatment for membrane systems due to its compact design and better water quality, though it requires careful system design. Coagulation and flocculation are used with caution as excessive chemical use can damage membranes.

Desalination plants releasing concentrated brine directly into marine ecosystems can cause mass mortality of marine species like turtles by disrupting the food chain; introducing halophytic plants (salt-tolerant plants) between the desalination plant and the ecosystem can absorb salt and prevent salinity spikes, protecting marine life.

Desalination produces hyper-concentrated brine (60 g/L salt versus normal 36-37 g/L) that returns directly to coastal waters. Researcher Fernando Espino documents dead zones extending 50 meters from discharge points, where no marine life survives. Beyond salinity, cleaning chemicals like sodium metabisulfite kill aerobic organisms, causing rapid fish mortality. Studies from the Red Sea, Florida, and Australia confirm long-term ecosystem damage. With 51 billion cubic meters of brine discharged globally in 2023, this invisible pollution threatens marine biodiversity and carbon-sequestering seagrass beds essential for climate regulation.

Desalination plants remove salt from ocean water to create drinking water, but they then dump the concentrated brine back into the ocean. This brine is denser than regular seawater and sinks to the ocean floor, creating a dead pocket of salty water that kills marine life along the food chain. This practice, while providing fresh water, causes significant environmental damage to ocean ecosystems.

Energy recovery devices (ERDs) are essential in seawater reverse osmosis (SWRO) plants because approximately 60% of energy is wasted across the brine discharge; these devices recover hydraulic energy from the high-pressure brine stream to supplement the main high-pressure pump, with three main types including turbine/centrifugal-based systems (Pelton turbines at ~87% efficiency, turbochargers at ~90% efficiency) and positive displacement pressure exchangers (PX) achieving over 90% efficiency but causing 3-5% salinity increase in feed water.

Energy Recovery Devices (ERD) capture hydraulic energy from high-pressure reject streams in reverse osmosis systems and transfer it back to the feed stream, significantly reducing energy consumption; while ERDs are essential for seawater desalination where high pressure (60+ bar) is required, they are typically not used in brackish water systems due to lower pressure requirements and higher implementation costs.

Energy recovery systems capture pressure energy from concentrate streams to reduce overall energy consumption. Two main types: (1) Isobaric energy recovery devices (ERDs) - transfer energy from concentrate to product stream, achieving ~30% energy savings; (2) Turbo chargers - use pressure differences to generate additional pressure, achieving ~31 kW savings per 360 m³/day plant. Selection depends on local energy costs and investment recovery time. In Peru, continuous operation may require 2-3 years to recover additional investment for isobaric systems.

Traditional desalination is energy-intensive, but Israel's Sorec plant uses isobaric energy recovery devices that function similarly to regenerative braking systems in Formula 1 cars. Instead of discarding high-pressure brine (which retains nearly 70 bar of pressure), the system transfers this energy directly to incoming seawater. This technology recovers up to 98% of mechanical energy from waste streams, dramatically reducing energy costs. As a result, Israel achieved a world-record price of approximately 50 cents per cubic meter for desalinated water.

A eletrodiálise reversa (EDR) é uma tecnologia de tratamento de água avançada que utiliza princípios eletroquímicos para separar íons e reduzir sais e condutividade em águas e efluentes, oferecendo eficiência superior em comparação com outras tecnologias. Diferentemente da osmose reversa que filtra a água, a EDR dissocia iões através de membranas catiônicas e aniônicas, permitindo operação com baixa pressão (2-3 bares) e consumo energético reduzido de aproximadamente 0,09 kWh por kg de sal removido. A tecnologia consegue recuperar até 95% do volume tratado em águas e 85% em efluentes, com membranas de alta durabilidade que podem durar até 20 anos. A EDR é indicada para remoção de cloreto, nitrato, flúor, cromo, dureza e sulfato, sendo aplicável em dessalinização de água do mar, reuso industrial em torres de resfriamento e caldeiras, recuperação de concentrados de osmose reversa, e tratamento de efluentes industriais. No entanto, a tecnologia tem limitações com ferro e manganês, que podem danificar as membranas, exigindo pré-tratamento adequado.

The Electrodiálisis Reversible (EDR) process is a desalination technique that uses electrical energy to remove dissolved salts from water by forcing ions to migrate through ion-exchange membranes under applied voltage; the process demonstrates that higher salinity concentrations require more energy for ion migration, and reversing the polarity of the power source changes the direction of ion movement, allowing for controlled water purification.

Electrodialysis (ED) is a membrane separation technology using alternating cation and anion transport membranes between electrodes. Under DC current, cations move toward the cathode through cation membranes while anions move toward the anode through anion membranes, creating dilute and concentrate channels. ED suffered from scaling and fouling because accumulated ions couldn't be removed. Electrodialysis Reversal (EDR) solved this by reversing electrode polarity every 10-20 minutes, swapping channels to self-clean membranes. Advanced innovations include electrode blockers preventing electrode fouling, homogeneous membrane casting for smoother surfaces, modular stack designs with leak-free compression, and monoselective coatings for organic resistance. EDR economics are driven by outlet TDS level, required salt flux, and brine concentration limits (typically 120,000-150,000 mg/L). EDR-RO hybrid systems provide synergistic benefits: EDR softens water upstream by preferentially removing scaling ions before RO, eliminating chemical softening needs. Industrial applications include produced water treatment for enhanced oil recovery, achieving 2,000-45,000 mg/L inlet to 2,000 mg/L optimal re-injection range. EDR handles dissolved and free organics effectively without extensive pre-treatment.

Electrodialysis (ED) uses electrical current to separate ions from water. When voltage is applied, cations (positive ions) move toward the cathode and anions (negative ions) move toward the anode, effectively removing salts. However, Electrodialysis has limited applications because it requires very high electrical current and has high operational costs. Before 2002, thermal desalination was the only widely used method. After 2002, membrane desalination became more prevalent. Currently, thermal desalination accounts for about 46% of global capacity, while membrane methods have gained significant market share. Regional preferences vary: Qatar uses 94% MSF (due to high natural gas), UAE uses 89% MSF, Kuwait uses 95% MSF (highest globally), and Japan uses 86% RO (due to lack of oil/gas resources). Countries with abundant fossil fuels prefer thermal methods.

Multi Stage Flash (MSF) distillation is a thermal desalination process that purifies seawater by passing it through multiple stages (typically 30) where seawater is heated and flashed into steam in chambers with controlled low pressure and temperature; the steam then condenses on cold seawater entering from the opposite side, transferring heat and producing fresh water while concentrating salts in the remaining brine, which is continuously removed as waste.