Urea is an organic compound (CH₄N₂O) and a metabolite of mammals. It is excreted mainly as urine and so it reaches the sewage system and finally the sewage plant via the toilet.

Urea is mainly responsible for the nitrogen load in a sewage treatment plant. Already in the sewer, the majority of the urea breaks down into ammonium nitrogen. This process is called hydrolysis or ammonification. During hydrolysis all organic nitrogen compounds (e.g. also proteins) are converted into ammonium nitrogen. This process takes place without external help. At the beginning of the biological purification stage the organic nitrogen is usually already completely hydrolyzed and, thus, present as ammonium nitrogen.


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