Water quality modelling of the river Sava, Slovenia

Water quality of the river Sava nearby Ljubljana, Slovenia, was investigated. In that area, the river Sava receives two tributaries, the river Ljubljanica and the river Kamni ka Bistrica, the former having highly decreasing impact on water quality of the river Sava. Discharge of organic, degradable wastewater into the river Ljubljanica and further into the river Sava results in a decrease of dissolved oxygen concentration. A surface water quality model, QUAL2E, developed by United States Environmental Protection Agency, was applied to estimate the impact of discharged wastewater on quality of the river Sava. QUAL2E model was modified and sensitivity analysis was performed to determine the most important factors which affect dissolved oxygen concentration in river water. The model was calibrated and validated according to the field measurements. On the basis of the model predictions it was estimated that in time when summer river low flow period occurs wastewater should be treated to reach BOD value under 30 mg l−1 so that Slovenian water quality standards will not be violated (dissolved oxygen concentration higher than 5 mg l−1 in all sections of the river).