A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge, that has to undergo further treatment before being suitable for disposal or land application.
Life expectancy is defined statistically as the average number of years remaining for an individual or a group of people at a given age.Longevity refers to the characteristics of the relatively long life span of some members of a population.athematically, life expectancy is the mean number of years of life remaining at a given age, assuming constant mortality rates. It is denoted by, which means the average number of subsequent years of life for someone now aged, according to a particular mortality experience.
The word  longevity  is sometimes used as a synonym for  life expectancy  in demography-however, the term longevity is sometimes meant to refer only to especially long lived members of a population, whereas life expectancy is always defined statistically as the average number of years remaining at a given age.he word  longevity  is sometimes used as a synonym for  life expectancy  in demography-however, the term longevity is sometimes meant to refer only to especially long lived members of a population, whereas life expectancy is always defined statistically as the average number of years remaining at a given age.