Probably, the most relevant difference between grey and black water is the different degradation velocity of the pollutants. Black water, in fact, contains organic substances that have had one of the most degradation processes present in nature: the process of human gastro-intestine. Therefore it is easy to understand that residues of that process can't be rapidly decomposed in water, that is an inadequate environment for the microbial populations present in the residues. For example, in 5 days of biological degradation process, only 40% of organic matter has a complete mineralization, while for grey water in the same period a removal of 90% is obtained. This rapid loss of organic matter in grey water can be explained with the abundance of sugars, proteins and lipids, easily available for bacteria.

Grey water compared with black water contains only 1/10 of total nitrogen and less than a half of organic load.

After natural depuration treatment with constructed wetland, the treated grey water easily reaches quality standards that make it employable for toilet flushing or for other less noble uses (washing of squares, irrigation, etc). Moreover, black water separated from grey water are treated with more efficiency and less costs, and can be reused for irrigation of garden or cultivated areas. In case of a further separation of faeces from urines – by special urinators with low consume – urine can be directly used as fertilizer.

(Source: SWAMP Project - www.swamp-eu.org)