Eliminating Wastewater Sludges, Biosolids and “Forever Chemicals” with Supercritical Water Oxidation

Most small to medium wastewater facilities in the United States are operating as end-of-pipe treatment plants with little or no recovery of resources. They are using about three percent of our electrical load, or about 2300 kW-h per million gallons of sewage treated. The majority are using an activated sludge process to degrade organics, nitrify ammonia to nitrite and nitrate. At some plants, denitrification is performed converting nitrite and nitrate to N2. In rare cases, phosphorus, a finite resource, is managed and recovered through enhanced biological phosphorus removal.

We, at 374Water, are commercializing a novel approach to Supercritical Water Oxidation (called “AirSCWO”). It has the potential to shift the wastewater paradigm from end of pipe treatment to resource recovery. It harnesses energy embedded in the waste stream undergoing treatment, while enabling recovery of clean water and minerals, paving the way for efficient resource recovery from all kinds of sludges and biosolids. The process is fast, clean and a net energy producer. Moreover, the benefits of AirSCWO empower organizations to achieve the United Nations Sustainable Development Goals (SDGs).

The value of AirSCWO to destroy organics and recover resources in wastewater sludge and biosolids has dramatically increased due to the ubiquitous presence of emerging contaminants such PFAS, 1,4-Dioxane, microplastics, pharmaceuticals, and other refractory chemical compounds. Landfills are refusing biosolids that do not meet the strict regulatory requirements for land application. A strong case can be made for AirSCWO as a superior treatment and resource recovery technology. AirSCWO can favorably replace existing anaerobic systems or can provide on-site solids handling with resources recovery at smaller plants that do not have anaerobic digestion.

Figure 1. Supercritical Water Oxidation of Wastewater Sludges and Biosolids

AirSCWO, the 3rd Generation of SCWO

For more than 25 years, technical challenges have slowed down commercial deployment of SCWO. Corrosion, plugging and fouling are the common issues and are linked to the complex nature of a high-pressure, high-temperature process. Critical design elements to overcome these challenges include reactor material, reactor shape and size. The form of oxygen used is another issue as pure oxygen handling and storage introduces a process hazard causing safety concerns and increasing capital expenditures to design in appropriate safety controls. Another obstacle to commercialization of SCWO has been an unrealistic goal of producing power at a competitive retail electricity rate ($0.3-0.5 per kWh). This has been out of reach and the cause of numerous failures.

AirSCWO is a novel approach to supercritical water oxidation. It is a continuous process designed for 24×7 operation. During recent PFAS elimination work with the US EPA, AirSCWO was labeled “the 3rd generation of SCWO.” It has pending U.S. patents that address prior hurdles by incorporating moderate preheating of the waste slurry, followed by mixing with supercritical water and air. Air serves as the oxidant, a much safer alternative to pure oxygen. The internal mixing rapidly brings the waste undergoing treatment to supercritical conditions thereby minimizing corrosion and the risks of waste charring and plugging. All organics in the sludge are rapidly oxidized to CO2, while the heat of oxidation is recovered to heat the influent waste. The other critical innovation instrumental in overcoming corrosion, plugging and energy efficiency challenges is a multi-stream tubular reactor configuration that enables efficient and sustainable treatment.

Each AirSCWO unit is containerized in shipping containers; a 20-foot for a one (1) metric ton, 40-foot for a six (6) metric ton, and three 40-foot for a 30 metric ton. This makes the AirSCWO compact and modular allowing it to integrate into the existing footprint of a wastewater plant. Units can be set up to operate in parallel, offering a number of configurations to satisfy the different throughput requirements of each unique wastewater plant. As part of our design and planning process, capital and operating expenses were evaluated for different sizes. The system is automated using sensors connected to a Supervisory Control and Data Acquisition (SCADA) system and process historian for real-time analysis of trends, key process indicators (KPIs) and performance measurements. Figure 2 shows the Nix1 pilot unit at Duke University; samples of typical influent and effluent; and a table with characteristics of influent and effluent for a typical biosolids run. A video of the operating Nix1 is available at here.

Figure 2. Duke University Pilot System (left), biosolids slurry and treated effluent and its characteristics.

AirSCWO has successfully processed primary, secondary, and digested sludge slurries, and a range of other feedstocks including landfill leachate, and sludge streams commingled with waste oil, food wastes, and plastics. The results so far show very rapid treatment with more than 99.9 percent conversion of organics to water, energy, CO2 and mineral salts. Total nitrogen and phosphorous removal were over 80 and 95 percent respectively. Pharmaceuticals, PFAS and other trace organic contaminants are destroyed with greater than 99.9 percent efficiency. All microorganisms and pathogens are eliminated.

The projected total for biosolids is $50 per wet ton at the Nix30 scale (approximately 30 wet tonnes/day) with a majority of the cost coming from amortizing the equipment. This cost is already competitive although the benefits of resource recovery (fertilizer, heat, and carbon credit) are not factored in those estimates, as their true value has not yet been evaluated. These cost projections make the SCWO process very competitive compared to anaerobic digestion and dewatering followed by offsite disposal by land application, landfilling or incineration.

Invent the Future – Water Resource Recovery with AirSCWO

AirSCWO has the ability to transform wastewater treatment plants (WWTPs) to Water Resource Recovery Facilities (WRRFs). The process recovers water, energy and phosphorus from wastewater sludge and biosolids while reducing the volume of solids by more than 97 percent. AirSCWO eliminates the methane generated by anaerobic digestion and GHG emissions from trucks hauling residuals offsite. It destroys emerging contaminants like PFAS and 1,4 dioxane, pharmaceuticals, microplastics and antibiotic resistant bacteria, eliminating public health concerns and financial liabilities.

We will ship our first commercial six metric ton units in the first half of 2022. Wastewater utilities in the western, northeastern and great lakes regions of the United States have shown the most interest in AirSCWO because of the pervasiveness of PFAS in their states. They like AirSCWO for its ability to treat sludges and biosolids onsite while eliminating PFAS completely.

Doug Hatler