Advanced Oxidation Processes
by Rami Elias Kremesti M.Sc., CEnv, CSci, CWEM
Kremesti Environmental Consulting Ltd.
Transmutare Substantiarum Basium In Aurum TM
Introduction and Definition
Advanced oxidation processes (abbreviation: AOPs), in a broad sense, refers to a set of chemical and sometimes physical treatment procedures designed to remove persistent organic (and sometimes inorganic) materials in water and waste water by oxidation through reactions with or a combination of hydroxyl radicals (·OH), Ozone or Hydrogen Peroxide/UV. When applied to waste water treatment plants, the processes are called quaternary treatment or the Fourth Treatment Step. Sometimes, AOP’s are combined with Active Carbon Filtration.
If Oxidant generation is Electrochemical, the process is named EAOP.
The Oxidation can be Catalytical as described below in the ZimCat or TiO2-UV process.
Sometimes, non thermal plasmas NTP’s are used.
This is one of the most exciting branches of chemistry to me because I can apply my knowledge to treat difficult to remove pollutants from waste water and there is a lot of chemistry involved.
The world’s largest rivers like the Thames in London and the Danube in the EU are polluted with unsafe levels of anti-biotics and persistent organic pollutants that need to be removed. It is predicted that the next epidemic will be caused by anti-biotic resistant “superbugs”. For more on this read the WHO article here.
Oxidation Potential
Oxidation is a property of chemical species that have a high oxidation potential, i.e., affinity for electrons. The below table summarises the Oxidation Potential of major oxidizers:
Oxidation Potential of Various Chemical Species
Achievable Treatment Goals
• AOPs can reduce the concentration of contaminants from several hundreds ppm to less than 5 ppb and therefore significantly bring COD and TOC down, which earned it the credit of “water treatment processes of the 21st century”.
• AOP’s breakdown organics into H2O and CO2 and sometimes other smaller molecules. The process is called mineralization.
• Some heavy metals can also be removed in the form of precipitated hydroxides M(OH)x
Specific Contaminants
• The AOP process is particularly useful for treating biologically toxic or non-biodegradable chemicals such as phenols, aromatics, pesticides, petroleum constituents, anti-biotics, various pharmaceuticals, EDC’s PFAS, and volatile organic compounds in wastewater.
• TriChloroEthylene (TCE) and TetraChloroEthylene (PCE) are also destroyed by AOP.
• PFAS and Pharmaceuticals
Applications
• Oil and Gas Waste Water
• Tertiary Sewage Effluent Polishing (Quaternary treatment)
• Swimming Pools
• Pharmaceutical Waste Water
• Removal of Pesticides and Biocides
• Pulp and Paper Waste
• Hospital Waste Water
• Ground Water Remediation
• PFAS and PFOS
• Hospital Waste Water Treatment
Conditions
• The reaction, using H2O2 for the formation of ·OH, is carried out in an acidic medium (2.5-4.5 pH) and a temperature between 10 °C – 50°C, in a safe and efficient way, using optimized ferrous catalysts and hydrogen peroxide formulations.
• OH. radicals are indiscriminate towards organics hence, it is useful to lower the TOC as much as possible before trying to remove the COD.
For example, PureBlue Water use anion exchange to remove humics and fulvics from treated waste water BEFORE they try to remove POP’s.
• Warning: A small percentage of Nitrates Absorb UV light and are converted to Nitrites under specific conditions.
Ozone (O3) + UV
• Ozone gas generation systems work by discharging a corona discharge into pure O2. Air used as feed needs to be concentrated as O2 otherwise toxic NOx are formed as a by-product.
• Ozone system suppliers: DeNora, Ozonia, OzoneTech by Mellifiq
Element Six (De Beers Group) boron-doped diamond (BDD) electrodes are used for ozone generation in water due to their high electrochemical stability, which allows them to be used in harsh wastewater treatment applications. They are effective in generating ozone through electrolysis, a process that uses electrical current to break down pollutants and oxidize organic compounds, and can be designed with features like perforations to improve performance. How they manage to generate O3 and not O2 is top secret…
Hydrogen Peroxide (H2O2) + UV
• Hydrogen Peroxide can be dosed as a chemical or generated in-situ by electrolytic systems that convert demin Water + O2 into H2O2. Such an electrochemical AOP system has been tried and tested by
https://www.hpnow.eu/ and shown to improve growth characteristics of plants in the agricultural industry.
Fenton’s Reagent
• A small concentration of ferrous ions catalyses the breakdown of H2O2 into Hydroxyl ions. This is classical chemistry.
• Fe2+ + H2O2 → Fe3++ HO· + OH− (initiation of Fenton’s reagent)
• Fe3+ + H2O2 → Fe2++ HOO· + H+ (regeneration of Fe2+ catalyst)
• H2O2 → HO· + HOO· + H2O (Self scavenging and decomposition of H2O2)
The Fenton’s reaction was first discovered by British chemical engineer H.J.H. Fenton in the year 1894.
TiO2 Photocatalytic Oxidation :
TiO2 + UV → e− + h+ (irradiation of the photocatalytic surface leads to an excited electron (e−)
and electron gap h+)
Ti(IV) + H2O ⇌ Ti(IV)-H2O (water adsorbs onto the catalyst surface)
Ti(IV)-H2O + h+ ⇌ Ti(IV)-·OH + H+
the highly reactive electron gap will react with water
Zimpro® Wet Air Oxidation (WAO)
Zimpro® wet air oxidation (WAO) is a highly proprietary wastewater treatment technology used primarily in the oil & gas industry. Wet Air Oxidation (WAO) and Wet Oxidation systems are designed for the treatment of highly toxic spent caustic, making the effluent amenable for discharge to a conventional biological treatment plant for polishing. The severe conditions increase reaction rates and enhance the solubility of oxygen, which in turn promotes the production of free radicals.
Zimpro® Electro-oxidation (ZEO) E-AOP
Zimpro® electro-oxidization technology (ZEO) uses electricity and long-lasting synthetic electrodes to oxidize/destroy highly toxic spent caustic, and other wastewaters. Only requiring electricity and cooling water, the process is economically attractive for smaller facilities. In General for E-AOP:
Boron Doped Diamond electrodes can generate Hydroxyl radicals at the anode that can oxidise recalcitrant organics
Suppliers:
Clear Fox https://clearfox.com/clearfox-eo-diox/
Arvia https://arviatechnology.com/
Siemens
Joshi Water in India for Fenton’s Reactors
Zimpro® Catalytic Gasification (ZimCat)
The Zimpro® Catalytic Gasification (ZimCat) system can treat high-strength and toxic wastewaters without producing solids, all with low energy requirements, and while producing fuel that can be used for combustion processes, such as engines and boilers, elsewhere in your plant – all in a small footprint when compared to conventional wastewater treatment processes. It uses primarily uses ruthenium (Ru) supported on an alumina (Al2O3) catalyst. It is a thermochemical process for converting carbonaceous materials into syngas (synthesis gas), primarily composed of hydrogen and carbon monoxide. It is not an AOP process per se but included here as part of a suite of Zimpro processes.
Zimpro® Wet Air Regeneration (WAR)
The Zimpro® Wet Air Regeneration (WAR) system is based on the world-leading Zimpro® Wet Air Oxidation process. Pairing a WAR system with a PACT® biological treatment system simultaneously reactivates powdered activated carbon, significantly reducing the amount of fresh carbon needed in the PACT system; and destroys non-biodegraded organics and biological sludge, eliminating the need for costly sludge dewatering and disposal. Again not an AOP process per se.
Plasma Vortex
A plasma vortex, harnesses the high energy fourth state of matter, plasma.
Within a plasma vortex reactor, a high voltage gradient is applied between two electrodes, through an external power supply. This creates an electric field that strips electrons from the inflowing gas molecules, creating charged ions and releasing a plasma discharge. The ions, which are charged particles, are highly chemically reactive, and are capable of breaking down PFAS molecules into relatively harmless by-products like fluoride, sulfate, carbon dioxide and water.
https://iopscience.iop.org/article/10.1088/1361-6463/ad2b22
Disadvantages of AOP
• Most prominently, the cost of AOPs is fairly high, since a continuous input of expensive chemical reagents is required to maintain the operation of most AOP systems. As a result of their very nature, AOPs require hydroxyl radicals and other reagents proportional to the quantity of contaminants to
be removed.
• Some techniques require pre-treatment of wastewater to ensure reliable performance, which could be potentially costly and technically demanding. For instance, presence of bicarbonate ions (HCO3−) can appreciably reduce the concentration of ·OH due to scavenging processes that yield H2O and a
much less reactive species, ·CO3−
As a result, bicarbonate must be removed from the system or AOPs are compromised. Also TOC needs to be as low as possible to avoid the extra cost of removal of TOC.
Note: Ozone can oxidise Bromide ions to form Bromate which is toxic
Special Materials/Piping:
• Need SS piping. OH. Radicals will attach plastic piping.
• Need to use chemical resistant gaskets (like Viton).
Sewage Treatment Plants with AOP Quaternary Treatment Processes
Data from a 2023 European Environment Agency dataset indicates Germany is a leader in waste water treatment plants with a quaternary treatment step (54 plants), followed by Italy (25 plants), Switzerland (25), Holland (12), UK (4) and Denmark (1).
Severn Trent appears to be the leader in the UK with WWTW’s that have AOP quaternary steps:
Frankton STW (Warwickshire) – Ozone
Itchen Bank STW (Warwickshire) – Ozone
Ludlow STW (Shropshire) – Ozone
Severn Trent uses non thermal plasma technology NTP as an Advanced Oxidation Process AOP to clean waste water treatment effluent at Chipping Campden STW’s.
Suppliers/System Designers
• http://www.escouk.com/
• https://www.watertechnologies.com/products/disinfection-oxidation/aopsystems
• https://genesiswatertech.com/blog-post/how-gwt-aop-advancedoxidation-systems-helped-companies-to-achieve-better-water-treatmentresults/
• https://www.poolmagazine.com/features/products/aop-advancedoxidation-process-what-it-is-how-it-works/
• https://pureblue.nl/en/
• https://www.lenntech.com/processes/pfas-removal-byozonation/advanced-oxidation.htm
Onvector is science-based cleantech company on a mission to destroy PFAS
Online Technical Lectures
• https://www.youtube.com/watch?v=ni6NtEjrFLA
• https://www.youtube.com/watch?v=ZcuXU9dbnYc
• https://www.youtube.com/watch?v=uTLh0tdfiN4
References
https://pubs.acs.org/doi/10.1021/acsestwater.4c00863
PFAS Destruction in IX Still Bottoms with Plasma Vortex Technology
https://drexel.edu/news/archive/2020/february/cold-plasma-pfas-water-treatment
About the Author:
Rami Elias Kremesti is a passionate chemical and water treatment engineer. He is always looking to troubleshoot or improve water treatment systems, disseminate knowledge and best practices, arouse passion in people about environmental protection and learn new things.
Rami Elias Kremesti Portrait