Abstract: The trommel assembly (1) with a spiral assembly having a decaying spiral pitch (3) comprises rotating drum structure with the decaying spiral assembly (2) and screen panels (6) made of polymeric material or rubber compounds. The pitch (3) is at a maximum at the inlet and at a minimum at the outlet providing increased screening efficiency. Maximum pitch produces low media height with increased inlet velocity and minimum pitch increases media height. The spiral (2) is of single start, double start or multi start configuration. The pitch (3) decreases along the trommel such that screening efficiency is improved without affecting the retention time.