Oxygen for wastewater treatment

High-purity oxygen activated sludge

A Water Research Foundation fact sheet (WRF 4973, ID 1121, updated 10 September 2022) explains that using high-purity oxygen in the aeration system of an activated sludge process provides a high oxygen transfer rate into the liquid and can accelerate the biological reactions, especially when the wastewater contains high concentrations of readily biodegradable organics. The higher biological oxidation rate can result in significantly smaller bioreactors while removing biochemical oxygen demand (BOD). The fact sheet calls the process attractive for certain industrial wastewaters with high soluble BOD or with volatile organics that require containment during treatment, where a small footprint is needed and nutrient removal is not required, and names large cities and industrial dischargers as good candidates.

Limits of the process

• HPO is a well-established process for BOD removal, but very few HPO plants remove nutrients (WRF, 2022).
• The pH is normally depressed to 6.0 to 6.5 because CO2 accumulates in the closed headspace, and the depressed pH reduces nitrifier growth rates (WRF, 2022).
• Pure oxygen generation equipment is more complex and requires specially skilled operators (WRF, 2022).
• A June 2021 wastewater training presentation by Larry W. Moore, hosted by Oak Ridge National Laboratory, lists a rate of oxygen addition 2 to 3 times greater than in conventional processes, high capital and operating costs, more complexity because of on-site oxygen generation, and additional safety concerns.

The same presentation lists the sources of high-purity oxygen as trucked-in liquid and on-site generation by pressure swing adsorption, vacuum pressure swing adsorption or a cryogenic system.

Oxygen in sewers and hydrogen sulphide

The background section of US patent 9,340,438 (published 17 May 2016) explains that under anaerobic conditions, bacteria naturally present in a sewer convert sulphate compounds into reduced sulphur compounds that cause bad odours. Hydrogen sulphide reacts with residual oxygen in the sewer headspace to form sulphuric acid, which corrodes steel and iron piping and other sewer components. Dissolving oxygen into the wastewater prevents the anaerobic condition, reducing odour and controlling corrosion.

ULTRAOX VSA

The ULTRAOX VSA datasheet lists wastewater treatment plants and hydrogen sulphide (H2S) reduction processes among the applications of this Ultra Controlo industrial oxygen plant. ULTRAOX VSA delivers 75% to 95% oxygen at 50 kPa, with an additional oxygen booster for higher pressures supplied on demand. Its datasheet describes a totally oil-free production system, a skid or container installation and the option to replace cryogenic systems with liquid oxygen tanks or work as the main source of supply. Its seven models deliver 18 to 240 m³/h at 93% oxygen.