5. Sludge treatment, reuse and disposal. Sludge is produced from the treatment of wastewater in on-site (e.g. septic tank) and off-site (e.g. activated sludge) systems. This is inherently so because a primary aim of wastewater treatment is removing solids from the wastewater.
Definition of central. 1  1 : containing or constituting a center. 2  2 : of primary importance : essential, principal the central character of the novel. 3  3a : situated at, in, or near the center the central part of the stateb : easily accessible from outlying districts a central location for the new theater.
Example Sentences for central character. Once a bright reporter wrote a play, in which the central character was Madame Mandelbaum. Perhaps if the central character had been imaginaryand it was nearly thatthe melodrama would have been all the better for it. Socrates appears in the play as the central character, and the chief of the Sophists.