Modelling Effects of Combined Sewer Overflow (CSO) Discharges on Receiving Environment

Mr. Bheem Bahadur Dhant

Abstract

Most of rivers in cities and towns in urban areas are under serious threat of degradation resulting from constant discharge of polluted effluents stemming from industrial, agricultural, mining, domestic and sewage activities. The Ljubljanica River which flows through the Ljubjiana city, Slovenia is one such river that is polluted mainly from industrial effluents upstream of the city and from numerous combined sewer overflows (CSOs) within the city. During heavy rainfall, CSO spillage to the river from the manhole and weirs causes a severe increase in ammonia concentration in the river and makes direct impact to the aquatic life. The objectives of this research are to study the water quality impact in the river as well as to minimize flooding within the city area and to study the effects of spatial variability of rainfall on receiving environment. A mathematical model MOUSE TRAP including the water quality module has been setup for both the river and sewer system in order to assess the impact from the combined sewer overflows on the water quality in the river in terms of ammonia concentration, backwater effect from river into sewer network and flooding in urban area. The present study describes how the receiving environment behaves with spatial variability of rainfall events upstream cathments to the city. It has been proposed though this report some remedial measures to maintain the ammonia concentration in the river within the acceptable limit and to improve the flooding effect within the city. Further ArcGIS tools with associate mapping techniques have been utilized to analyze and replicate the model results developed from MOUSE TRAP model.

The high rainfall occurring at upstream catchment to the city increases the discharge in the river which eventually decreases the ammonia concentration in the river due to dilution effect and vice versa as the rainfall is not the source of ammonia. Also this high rainfall at upstream catchment leads a severe backwater flow from river into the sewer network and serious flooding problem within the city whereas low rainfall intensity does not have a serious impact in terms of backwater flow into the sewer network and flooding within the city.

CSO overflow spillage problems through manholes and weirs can be resolved by either raising the weir crest level, sewer pipe amplification, introducing a new sewer pipe and building a detention storage or combination of these remedial measures built together within sewerage system. An option of adding a new pipe together with raising a weir crest levels in some weirs is the optimal solution in terms of the cost, environmental and other aesthetic aspects.

Keywords: combined sewer overflows, MOUSE TRAP, pollution impact, backwater effect, spatial variability, ammonia concentration