To understand why SCWO is uniquely effective, it is important to examine how water behaves when pushed beyond its critical point.
Supercritical Water Oxidation (SCWO) is an advanced hydrothermal process that destroys organic contaminants by oxidizing them in water above its critical temperature (374°C) and critical pressure (221 bar). At these conditions, water becomes a single supercritical fluid where organic materials, oxidants and gases become fully miscible and react extremely rapidly.
SCWO is recognised globally as one of the most effective technologies for the complete destruction of:
The key to the SCWO process lies in the unique properties of water when it is pushed beyond its critical temperature and pressure. The phase diagram of water is illustrated below with the Solid (ice), liquid & gas (steam) phases shown. The top right-hand quadrant is the region where water becomes a supercritical fluid. 650°C and 275 bar is the operating point of the General Atomics SCWO system.
The extreme temperature and pressure conditions present when water is a Supercritical Fluid fundamentally change the physical properties of water, creating a reaction medium unlike anything observed in conventional liquid-phase chemistry.
This is what water looks like when it is 99 thousandths of a degree below the supercritical state.
When water is heated above 374 °C and simultaneously pressurised above 221 bar, it undergoes a profound transformation. At these “supercritical” conditions, the distinction between liquid and gas phases disappears and water becomes a single, homogeneous fluid with entirely different physical behaviour.
At supercritical conditions:
Source: NASA Technology Transfer Program - Advanced Supercritical Water Oxidation Reactor
The net effect of the conditions present in Supercritical Water is a reaction environment where oxidation chemistry proceeds orders of magnitude faster than in liquid water and without the emissions and maintenance difficulties associated with traditional thermal destruction.
With these physical changes in place, the chemistry inside a SCWO reactor becomes both rapid and highly predictable.
Inside a SCWO reactor, organic molecules are rapidly oxidised to stable end products. The carbon within the waste stream is converted to carbon dioxide, while hydrogen converts to water. Fluorinated compounds — including PFAS — are broken down into fluoride ions once the carbon-fluorine backbone is destroyed, and these fluoride ions are later neutralised into stable mineral salts. Sulphur and phosphorus in the feed material typically form sulphate and phosphate species, while metals are oxidised into inert oxides.
This chemistry occurs without the production of soot, dioxins, furans, or other incomplete combustion by-products. SCWO is not incineration — it is an oxidative chemical reaction that takes place in a supercritical aqueous medium and is capable of breaking down some of the most stable synthetic molecules currently known.
These characteristics distinguish SCWO from traditional thermal or physical treatment processes, and the differences become clear when directly comparing the most common available waste treatment methods.
| Criteria | Supercritical Water Oxidation | Incineration / Thermal Oxidation | Pyrolysis / Gasification | Adsorption (GAC/IX) |
|---|---|---|---|---|
| Primary Mechanism | Hydrothermal oxidation of organics in supercritical water | High-temperature flame combustion at 850-1,200°C | Thermal degradation in oxygen-limited environment followed by thermal oxidation | Physical adsorption |
| PFAS Destruction Capability | Excellent (>99.99%, complete mineralisation) | Variable, some PFAS survive unless >1,200°C with long residence time | Moderate, some PFAS may remain in char, the rest is present in syngas and destruction on thermal oxidizer | None, PFAS is only transferred to media |
| By-Products | Water, CO₂ and inert mineral salts | Flue gas (NOx, SOx, particulate), ash, possible PFAS in flue gas treatment streams | Char, tar, syngas requiring thermal oxidation | Spent media (PFAS concentrated) |
| Emissions to Air | Water Vapour, CO₂ & N₂ | Various acid gases (NOx, SOx), particulate requiring flue gas treatment | Similar to Thermal Oxidation | None (excluding manufacturing process) |
| Dioxin / Furan Formation | None - no flame chemistry | Moderate-high unless extremely well controlled | Possible - depending on feedstock | None |
Destruction Refractory / Hazardous Organics | Excellent - including energetic materials, halogenated compounds | Good - Dependent on temperature & mixing | Limited - many compounds survive | None |
| Ability to Handle Liquids & Slurries | Excellent - liquids, slurries & concentrates | Good for liquids, poor for high solids slurries | Solids or slurries with pre treatment | Not applicable |
| Ability to Handle High-Salt Streams | Good - with quench/salt management | Moderate - Causes slagging & corrosion | Problematic - fusible salts | Not applicable |
| Residue / Solid Waste Generated | Excellent - Mineralised solution with recoverable nutrients | Moderate - Ash requiring disposal | Good - Biochar for beneficial reuse, can contain pollutants | Spent carbon/resin loaded with PFAS |
| Energy Requirement | Medium-High - depending on waste calorific value | High - For high temperature hazardous waste destruction | High - particularly for wet feeds | Low |
| Odour & VOC Control | Excellent - No organics in flue gases | Good - Requires flue gas treatment | Good - Requires flue gas treatment | Excellent |
| Complexity of Operation | Medium - High pressure but full control system | Medium-High - Emissions control systems can be complex | High - Multiple stages of processing & emissions control system | Low |
| Infrastructure Footprint | Compact relative to throughput | Large - Due to flue gas treatment train | Large - Due to dryers and flue gas treatment train | Compact |
| Regulatory Acceptance (PFAS) | Increasingly recognized as one of the few verifiable destruction technologies | Under scrutiny - Concerns regarding incomplete PFAS destruction | Under scrutiny - Concerns regarding incomplete PFAS destruction | No destruction - Concentration only |
| Overall Suitability for PFAS-Contaminated Wastes | ★★★★★ – Highly suitable | ★★★☆☆ – Only high-end incinerators | ★★☆☆☆ – Poor | ★★★★★ -For removal from water |
| Typical Use Cases | PFAS destruction, biosolids, leachates, concentrates, chemical wastes, energetics, chemical warfare agents | Solid hazardous wastes, municipal solid waste | Plastics, tyres, biomass | Potable water polishing |
Driven by requirements from the US Department of Defence (DoD) and NASA in the 1980s, General Atomics finalised development of a robust approach to SCWO waste destruction in 2000, with systems constructed and operated for the DoD until 2014. In 2012 an industrial version of the SCWO system was developed and has been in commercial scale operation since that time.
Utilising an approach which prioritises reliability, the PERSES system is simple with a low number of moving parts:
The PERSES system utilises the following features for effective destruction of slurries & wastes:
Corrosion control in the harsh supercritical environment is necessary. The PERSES system incorporates Titanium linings to prevent corrosion from high chloride, fluoride and acidic conditions.
The PERSES system has been tested to date on over 200 waste streams at full scale, which includes:
The full list of tested wastes can be found on the General Atomics tested waste list.
Below illustrates the layout of the PERSES system. The skid illustrated below is capable of destroying between 650 to 900 L/hr of organic waste in a liquid or slurry form.
In operation, the majority of gaseous emissions from the PERSES system is Water Vapour, Carbon Dioxide and Nitrogen.
The combined chemistry and engineering of SCWO results in several key technical, operational, and environmental advantages over traditional waste treatment methods.
The technical advantages of processing difficult waste stream are as follows:
From an operational perspective, advantages of this technology are:
Beyond the large volume of water vapour (which can be condensed and re-used), the effluent produced by SCWO is a mineralised solution containing oxidized forms of the original waste constituents. In the case of biosolids (or other organic streams), this can include:
These can be reused directly as additives in fertilizers or potentially recovered through crystallisation or nutrient extraction technologies, depending on the downstream requirements. The image below illustrates the minerals and metals that become available following SCWO processing of biosolids (Left - biosolids flocc'ed with Ferric Chloride, Right - flocc'ed with polymer).
The presence of Copper (green), Sulphur (yellow), Iron (red) is clearly visible as an example of the presence of minerals and metals.
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