The key difference between the two is that black water has come into contact with fecal matter. Fecal matter is a haven for harmful bacteria and disease-causing pathogens. Additionally, this waste doesn't break down and decompose in water fast or effectively enough for use in domestic irrigation without the risk of contamination.

Gray water is exactly the kind of stuff a consumer puts down a drain. If there is a septic system the ultimate waste water is deposited in the back yard. It is best to understand what a household gray water contains in order to protect the family from accidental contamination.

The line between white and gray, however, comes down to a number of possible additions made in the acts of washing, bathing, cooking and cleaning. Unlike white water, gray water may contain soap particles, fat and oil from cooking, hair, and even flakes of human skin.

The basic structure of all soaps is essentially the same, consisting of a long hydrophobic (water-fearing) hydrocarbon "tail" and a hydrophilic (waterloving) anionic "head":

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 COO − or CH 3 (CH 2 ) n COO −

The length of the hydrocarbon chain ("n") varies with the type of fat or oil but is usually quite long. The anionic charge on the carboxylate head is usually balanced by either a positively charged potassium (K + ) or sodium (Na + ) cation. In making soap, triglycerides in fat or oils are heated in the presence of a strong alkali base such as sodium hydroxide, producing three molecules of soap for every molecule of glycerol. This process is called saponification and is illustrated in Figure 1.