Effects of Ineffective Wastewater Drainage System on Quantity of Wastewater Reaching Rivers and Treatment Plants in Addis Ababa
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Date
2021-12
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Addis Ababa University
Abstract
There have been rapid land use change both in the center and in the outskirts of Addis Ababa.
However, the existing conventional wastewater drainage system has been in use in the
central part for around 40 years while sewer system has not yet developed in the expanding
residential areas. In view of that, this research was conducted to identify how ineffective
existing sewer system and onsite wastewater management facilities are contributing for the
discharge of untreated wastewater into rivers in addition to assessing inflow and infiltration
quantity in the existing sewer. For these purposes, Senga Tera and Tulu Dimtu study areas
were selected to represent urban densification and urban expansion respectively. The
existing land use plans of both areas, and as-built data and sewer master plan for the existing
sewer system were collected and reviewed. Flow rate were manually measured at selected
manholes on field for model calibration in addition to analyzing six months of ArcGIS water
use data’s. Moreover, the number of stories and floor areas of high-rise buildings which are
now under construction were surveyed in Senga Tera. Similarly, field research was conducted
to identify the existing onsite wastewater management facilities inTulu Dimtu. Then, steady
state analysis and modeling for the existing sewer system and for the proposed nonconventional
sewer system in Tulu Dimtu were conducted using Sewer GEMS V8i.
Accordingly, it was revealed that 87% and 53% of the existing manholes in in Senga Tera
could be surcharged and overflowing due to rapid land use change during wet weather
season respectively. It was also found that around 15.59 l/s of wastewater was cross
connected into the nearby storm water drainage system. As a result, overflows and leakage
through the walls of the existing cesspools in Tulu Dimtu and Sanitary Sewer Overflows
(SSOs) from manholes and sewer cross connections in Senga Tera area have been found to
be factors for the discharge of untreated wastewater first into the nearby storm water drainage
system and then outfall into nearby rivers. Moreover, the quantity of average inflow and
infiltration which was estimated in Senga Tera was around 71%. This in turn could contribute
to induce SSOs on the existing sewer system in addition to become a factor for the hydraulic
overloading of the existing Kality Wastewater Treatment Plant. Therefore, it is recommended
to enhance the existing sewer system through master plan review and to use non-
conventional sewer system instead of cesspools in the expansion areas.
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Keywords
Addis Ababa River, sanitary sewer overflows, non-conventional sewer, Inflow and Infiltration, cesspool