PFAS Elimination Using Super Critical Water Oxidation
Some contaminants such as PFOA and PFAS are current candidates for regulation in the United States, and supercritical water oxidation may be a solution to address the so-called “forever chemicals.”
What are Contaminants of Emerging Concern?
The term “contaminant of emerging concern” (CEC) has been used to identify chemicals and other substances that are currently unregulated and are usually not included in routine monitoring programs under the U.S. Safe Drinking Water Act or Clean Water Act. Some such as PFOA and PFAS are current candidates for regulation in the United States.
CECs have been detected in natural water bodies and have a detrimental effect on fish and other aquatic species. Some CECs bioaccumulate up the food web, putting even non-aquatic species at risk when they eat contaminated fish. Eventually, CECs end up in humans. Many have shown to be toxic and are causing widespread public health concerns. CECs include several classes of chemicals including but not limited to the following:
- Persistent Organic Pollutants such as:
- Polychlorinated Biphenyls, Dioxins, and Organochlorine & Organophosphorus Pesticides
- 2,4,6-Trinitrotoluene (TNT)
- Dinitrotoluene (DNT)
- Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)
- N-Nitroso-dimethylamine (NDMA)
- Perfluorooctane Sulfonic Acid (PFOS), Perfluorooctanoic Acid (PFOA) and other Per- and Polyfluoroalkyl Substances (PFAS), also known as “forever chemicals”
- Polybrominated Biphenyls (PBBs)
- Polybrominated Diphenyl Ethers (PBDEs);
- Nanomaterials such as carbon nanotubes or nano-scale particulate titanium dioxide, of which little is known about either their environmental fate or effects.
- Pharmaceuticals and personal care products (PPCPs), including a wide suite of human prescribed drugs, over-the-counter medications, bactericides, sunscreens, and perfumes;
- Veterinary medicines such as antimicrobials, antibiotics, antifungals, growth promoters and hormones; and
- Endocrine-disrupting chemicals (EDCs), including synthetic estrogens and androgens, naturally occurring estrogens as well as many others capable of modulating normal hormonal functions and steroidal synthesis in aquatic organisms and in some cases humans.
Why are we Concerned?
The use and occurrence of CECs are varied. People frequently use and dispose of chemical-based products. The chemicals are disposed as trash, which is buried in landfills or incompletely combusted in waste incinerations. Humans excrete PPCPs and EDCs in urine which is flushed into wastewater systems. Industries process and use chemicals and release them via air emissions, water discharges, and land disposal. Eventually, the CECs end up in the environment and have been detected in drinking water supplies and the food chain.
Non-biodegradable CECs such as PFAS are released directly into the environment after passing through conventional wastewater treatment processes, which are not designed to address these pollutants. Wastewater solids from secondary treatment processes in some cases land-applied as biosolids, so the CECs leach into groundwater or stormwater run-off and eventually wind up in nearby bodies of water.
Plant uptake of CECs is also problematic, as farm animals ingest the plants causing the chemicals to enter their system and ultimately the human food chain. Moreover, pharmaceuticals used in animal feeding operations may be released to the environment in animal wastes via direct discharge of aquaculture products, the excretion of substances in animal urine and feces of livestock animals, or the wash off of topical treatments from livestock animals.
Most of the current wastewater and solids processing technologies like anaerobic digestion, thermal hydrolysis process (THP), and hydrothermal liquefaction process may be negatively contributing to the problem. They tend to break down the CECs into more toxic and recalcitrant byproducts.
For example, long-chain PFAS molecules have been found to be broken down into short-chain PFAS molecules that are more mobile in water. Many water and wastewater treatment plants, as well as groundwater remediation systems, create CEC-contaminated residuals that require further treatment such as spent filters, membranes, resins, and granular carbon.
What can we do about CECs?
First and foremost, the use of CECs with known environmental and human health effects should be banned, just as DDT, parathion, and PCBs have been banned from manufacture and use. This will eliminate new sources of CECs leaving the legacy contamination for remediation and natural attenuation.
Second, and most importantly, the waste, water, and wastewater industries, including the engineering consultancies, must adopt innovation and a new way of addressing CECs and waste, in general. There are emerging technologies that are commercially viable and ready for deployment.
Supercritical Water Oxidation
One such technology is supercritical water oxidation (SCWO). SCWO has the ability to transform the simplest and most complex wastes like CECs and forever chemicals into valuable recoverable resources. SCWO is a physical-thermal process that uses water above its critical point (374°C and 221 bar) as a highly energized solvent fluid.
In these conditions, in the presence of oxygen, organic molecules are rapidly oxidized and converted to clean water, energy, inert gasses, and minerals. SCWO systems have been around for a few decades but have not had widespread commercial success because of shortcomings in the design of earlier generations of the technology.
374Water, of Durham, NC, offers a new approach to SCWO that has been coined as “the 3rd generation of SCWO.” AirSCWO, its trademarked name, uses air as the oxidant, which makes the technology safer and more cost effective than pure oxygen or chemical oxidant alternatives. The way in which AirSCWO preheats and feeds waste into its reactor rapidly brings the feedstock to supercritical conditions thereby minimizing risks of corrosion, charring, and plugging; the three main shortcomings of earlier versions of SCWO.
It is also a continuous 24/7 flow system that effectively treats high strength waste streams like sludges and biosolids, including those containing CECs, forever chemicals, and aqueous industrial wastes. The technology has the potential to significantly alter how society manages the waste it generates by shifting from conventional linear waste treatment and disposal to circular waste elimination, resource recovery, and reuse.
374Water’s AirSCWO Systems are prefabricated, compact, and modular and can be readily integrated into new and existing sites. Orange County Sanitation District of California (OC San), a wastewater authority located in Fountain Valley, California, purchased an AirSCWO system. OC San provides wastewater collection, treatment, and recycling for approximately 2.6 million people in central and northwest Orange County, California.
A 6-ton-per-day system will be deployed at one of OC San’s wastewater treatment plants to evaluate the potential for the unit to solve several longer-term challenges facing OC San. These challenges include solids processing costs, air emissions requirements for methane and power generation equipment, and emerging contaminants such as PFAS compounds and microplastics, food waste utilization, and efficient use of OC San’s treatment plant properties.
How do we Solve the Problem?
Addressing CEC contamination of the environment and human food chain is a serious challenge facing the world. Overcoming these challenges will require more than water and wastewater utilities, who suffer from human and financial resource constraints and are extremely risk averse. Utilities, especially public entities, will need the help of the private sector for additional resources and risk sharing. Public-private partnerships can help transfer risk from taxpayers to investors, support the implementation of innovative treatment technology, and provide expanded access to capital.
About the author:
Doug Hatler is chief revenue officer of 374Water. Doug is an environmental scientist and engineer who has spent over 35 years applying technology to solve the most pressing global environmental problems that impact human health and the beneficial use of water.