Experimental Investigation and Modeling of Denitrification of Water in a Column Bioreactor Using Immobilized Microorganisms on Modified Zeolite oleh Ehsan Zamani Abyaneh, Mohammad Hosein Heidary, Mahkame Rafaatinia, Arash Darzian Rostami, Fatemeh Yazdian, Behnam Rasekh, Navid Mostoufi