A correlation method for the estimation of retention times at full-scale sewage treatment plants

The theory is given of a correlation function method for measuring retention times in operational sewage treatment process units. Diurnal and random variations in influent B.O.D. and S.S. are used as a tracer, and results are presented of an analysis by this correlation method of three B.O.D. data sets collected for a primary sedimentation unit. The theory demonstrates that, for a unit in which the retention time can vary. e.g. with flow rate. the method yields an average retention time, and thus provides information on the average hydraulic performance of the unit. The average retention times derived for the primary sedimentation unit are shown to be mutually consistent but of a magnitude that suggests that approx. 50% of the sedimentation tank volume is not exploited by the flow and represents a “dead volume”.