STOAT has been extensively used for the dynamic simulation of a trickling filter based Sewage Treatment Plant Some alternative schemes have been suggested for the modernization of the existing sewage treatment works Different schemes have been compared with respect to the removal of Total Suspended Solids (TSS). b-COD, ammonia, nitrates etc in them The humus tanks (settling tanks associated with trickling filters) were modeled using the Takacs settler model [I Takacs, G G Patry and D. Nolasco A dynamic model of the clarification-thickening process Water Research 25. 1263-1271, 1991]. The results emphasize that COD based models are preferred over the BOD based ones. A compact bio-film with variable film thickness was proposed for the new schemes Suggested The newly proposed Scheme 3 was found most effective for reducing the mean b-COD level to as low as 2 3 mg/l. while the mean TSS level was brought down to 0 5 mg/l as compared to the mean levels of TSS (42 0 mg/l) and b-COD (48.0 mg/l) in the existing plant (C) 2009 Elsevier B V. All rights reserved.