What Is Wet Air Oxidation? A Professional Wastewater Term Explained
Wet Air Oxidation (WAO) is a novel method for treating organic wastewater. The process oxidizes organic pollutants into low-toxicity or non-toxic substances in the liquid phase under high-temperature and high-pressure conditions using oxygen as the oxidant. The WAO process was first studied and proposed by Zimmermann of the United States in 1944, who obtained multiple patents, so it is also called the Zimmermann process. In principle, the wet air oxidation reaction conducted under high temperature and high pressure can be divided into two stages: oxygen mass-transfer control and reaction-kinetics control, and temperature is the key influencing factor of the entire WAO process. The higher the temperature, the faster the chemical reaction rate. In addition, a higher temperature also increases the oxygen mass-transfer rate and reduces liquid viscosity. The main role of pressure is to ensure the liquid-phase reaction and keep the oxygen partial pressure within a certain range to maintain a high dissolved oxygen concentration in the liquid phase. In 1958, WAO was first used to treat paper black liquor, achieving a COD removal rate above 90%. To date, more than 200 WAO units in the world have been applied to industrial wastewater such as petrochemical alkaline waste liquor, olefin production wash liquor, acrylonitrile production wastewater, and pesticide production wastewater. However, WAO still has some limitations in practical application: for example, the WAO reaction must be carried out at high temperature and high pressure, requiring reactor materials with high-temperature, high-pressure, and corrosion-resistant capabilities, so the equipment investment is large; moreover, it is uneconomical for low-concentration, high-flow wastewater. To improve treatment efficiency and reduce treatment costs, in the 1970s a wet air oxidation technology based on WAO using efficient and stable catalysts was derived, namely catalytic wet air oxidation, abbreviated as CWAO. [1]
ROO· + R· → ROOR
(1) Chain initiation; in the wet air oxidation process, chain initiation refers to the process by which reactant molecules generate free radicals. In this process, oxygen generates H2O2 through a thermal reaction, as follows:
RH + O2 -> R middot + HOO middot (RH is an organic compound)
2RH + O2→ 2R· + H2O2
H2O2 + M -> 2OH middot (M is a catalyst)
(2) Chain propagation or transfer; the process in which free radicals and molecules interact alternately, rapidly increasing the number of free radicals.
RH + ·OH → R· + H2O
R· + O2→ ROO·
ROO· + RH → ROOH + R·
(3) Chain termination; if free radicals combine with each other to form stable molecules, the chain growth process is interrupted.
R· + R· → R-R
Current research results generally hold that the wet air oxidation reaction is a free-radical reaction, divided into three stages: chain initiation, chain propagation or transfer, and chain termination. [2]
ROO· + ROO· + H2O → ROOH + ROH + O2
A common industrial-scale wet air oxidation process flow is shown in Figure 1. The wastewater to be treated is pressurized by a high-pressure pump and heated in a heat exchanger to the reaction temperature, then enters the reactor; at the same time, air or pure oxygen is pressed into the reactor by an air compressor. Inside the reactor, the oxidizable pollutants in the wastewater are oxidized by oxygen. After the reaction products leave the reactor, they first enter the heat exchanger, where they are cooled while heating the raw water; then the reaction products enter a gas-liquid separator, where the gas phase (mainly N2, CO2, and a small amount of unreacted low-molecular organic matter) and the liquid phase are separated and discharged separately.
The salient features of the wet air oxidation process are its wide range of treatable organics, good effect, short reaction time, and small reactor volume, with almost no secondary pollution and the ability to recover useful substances and energy. The main constraints on the development of wet air oxidation are the high equipment requirements and large one-time investment. [3]
Since the 1970s, the wet air oxidation method has been widely used abroad. At present there are about 200 WAO units in the world, more than 50 of them in Japan, mainly used for sludge and industrial wastewater treatment. The basic information of several foreign WAO units is shown in Table 1. [4]
Table 1. Basic information of several foreign WAO units
Treatment capacity (m3/d)
Morris Northern Petrochemicals (USA)
Mitsubishi Petrochemical (Japan)
Kawasaki Chemical Co. (Japan)
