Aqueous Urea Solution (AUS), also known as urea water solution or Diesel Exhaust Fluid (DEF), is a chemical compound that is used in the maritime industry as part of the exhaust gas cleaning process, particularly in selective catalytic reduction (SCR) systems.

SCR systems are used to reduce nitrogen oxide (NOx) emissions from ships by converting them into harmless nitrogen and water vapor. AUS is a key component of the SCR process, as it reacts with the NOx in the exhaust gas to form nitrogen and water.

In the SCR system, AUS is stored in a separate tank on board the vessel and is injected into the exhaust gas stream using a metering system. The AUS is then mixed with the exhaust gas in the SCR catalyst, where the chemical reaction takes place.

One of the key advantages of AUS is that it is a relatively safe and environmentally friendly alternative to other chemicals used in exhaust gas cleaning. AUS is non-toxic, non-flammable, and non-hazardous, and it does not produce harmful byproducts or emissions.

In addition, AUS has a lower operating cost compared to other exhaust gas cleaning technologies, making it an attractive option for ship operators looking to reduce emissions and comply with regulations. Overall, AUS is a critical component of the SCR system in the maritime industry, helping to reduce NOx emissions and promote sustainability.