Select the correct statement from the following

Activated sludge is a process dealing with the treatment of sewage and industrial wastewaters. Atmospheric air or pure oxygen is introduced to a mixture of primary treated or screened sewage (or industrial waste water) combined with organisms to develop a biological flock, which reduces the organic content of the sewage. Sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria because small amounts are used as inoculum in secondary treatment or biological treatment stage of sewage treatment.