Why does high-purity oxygen treatment lower the pH?

WRF (2022) says dissolved carbon dioxide builds up because of CO2 accumulation in the closed headspace, normally depressing the pH to between 6.0 and 6.5, which reduces nitrifier growth rates.

How much plant energy does aeration use?

EPA (2010), citing WEF (2009), says aeration ranges from 25 to as much as 60 percent of total plant energy use, depending on the site.

Pure oxygen vs air aeration in wastewater treatment

Q: Pure oxygen vs air aeration in wastewater treatment

A 2022 Water Research Foundation fact sheet says high-purity oxygen (HPO) in activated sludge aeration gives a high oxygen transfer rate and can mean significantly smaller bioreactors; a 2021 training presentation hosted by Oak Ridge National Laboratory adds that HPO is more complex, because of on-site oxygen generation, with high capital and operating costs.

A 2010 US EPA report on energy conservation in wastewater treatment describes the two routes: wastewater is aerated either by bubbling air or high-purity oxygen through it, or by mixing it. The two most common aeration systems are diffused aeration and mechanical surface aeration, and the report notes that aeration can account for the largest energy demand of any operation at the facility.

How the two routes differ

	Air aeration	High-purity oxygen (HPO)
Gas supply	In diffused aeration, air from blowers, which EPA (2010) describes as low pressure, high volume air compressors	Trucked-in liquid oxygen, or on-site generation by PSA, VPSA or a cryogenic system (Moore, 2021)
Main equipment	Air intake (typically fitted with filters), blowers, air piping, diffusers and controls in diffused aeration (EPA, 2010)	Pure oxygen generation equipment, which WRF (2022) describes as more complex and needing specially skilled operators

What the sources say about high-purity oxygen

Moore (2021) gives a rate of oxygen addition 2 to 3 times greater than in conventional processes, and lists high capital and operating costs and additional safety concerns.
WRF (2022) says HPO can accelerate the biological reactions, especially when the wastewater contains high concentrations of readily biodegradable organics, and that the higher oxidation rate can result in significantly smaller bioreactors while removing BOD.
WRF finds HPO attractive for certain industrial wastewaters with high soluble BOD or volatile organics that need containment, for sites that need a small footprint and do not need nutrient removal, and names large cities and industrial dischargers as good candidates.
HPO is well established for BOD removal, but very few HPO plants remove nutrients. In the closed headspace, carbon dioxide accumulates and the pH is normally depressed to between 6.0 and 6.5, which reduces nitrifier growth rates (WRF, 2022).

What the sources say about air aeration

EPA (2010), citing WEF (2009), puts aeration at 25 to as much as 60 percent of total plant energy use, while noting that the demand is site-specific and varies widely from plant to plant.
EPA notes that energy savings can be gained by designing and operating aeration systems to match, as closely as possible, the actual oxygen demand of the process.
EPA (2010) also notes that fine-pore diffusers have the highest efficiency of any diffused air or mechanical aeration system, and that the closer the dissolved oxygen is to saturation, the lower the oxygen transfer efficiency.

For more on each route, see oxygen for wastewater treatment and blowers for wastewater aeration.

Ultra Controlo equipment for each route

The ULTRAOX VSA industrial oxygen plant lists wastewater treatment plants and hydrogen sulphide (H2S) reduction processes among its applications. It delivers 75% to 95% oxygen at 50 kPa, higher pressures can be reached with an additional oxygen booster supplied on demand, it is supplied as a skid or container system, and its table runs from 18 to 240 m³/h at 93%. For air, Ultra Controlo distributes and services Gardner Denver and Elmo Rietschle, whose lines in the Industrial 2026 catalogue include blowers.