The maximum allowable return air ratio in chilled ceiling (CC) and mixed displacement ventilation (DV) system for good air quality is regulated by acceptable levels of CO2 concentration not to exceed 700 ppm and airborne bacterial count to satisfy World Health Organization (WHO) requirement for bacterial count not to exceed 500 CFU/m(3). Since the CC/DV system relies on buoyancy effects for driving the contaminated air upwards, infectious particles will recirculate in the upper zone allowing effective utilization of upper-room ultraviolet germicidal irradiation (UVGI) to clean return air. The aim of this work is to develop a new airborne bacteria transport plume-multi-layer zonal model at low computational cost to predict bacteria concentration distribution in mixed CC/DV conditioned room without and with upper-room UVGI installed. The results of the simplified model were compared with layer-averaged concentration predictions of a detailed and experimentally-validated 3-D computational fluid dynamics (CFD) model. The comparison showed good agreement between bacteria transport model results and CFD predictions of room air bacteria concentration with maximum error of +/- 10.4 CFU/m(3) in exhaust air. The simplified model captured the vertical bacteria concentration distribution in room air as well as the locking effect of highest concentration happening at the stratification level. The developed bacteria transport model was used in a case study to determine the return air mixing ratio that minimizes energy consumption and maintains acceptable IAQ with and without UVGI. Results showed that the use of upper-room UVGI resulted in 35% in energy saving, whereas the use of in-duct UVGI achieved no more than 12% energy saving, both compared to 100% fresh air case. (c) 2014 Elsevier Ltd. All rights reserved.