Evaluation of Wastewater treatment plant in Sana'a, Yemen

Abstract

The activated sludge- extended aeration wastewater treatment system in Sana'a is considered a flexible
and powerful technology expected to achieve high performance and eventually produce effluent with
a pollutant concentrations safe for the environment. However, this treatment system is operated and maintained at high cost compared to other technology. This is due to the underestimatic of design parameters applied in 1985 and not revised when updating the design in 1994. The applied design
parameters have changed due to a drought period Yemen experienced which led to low water consumption causing higher concentrations of pollutants in the incoming wastewater. The existing BOD5 is higher than 1000mg/L which is two times higher than the design value of 500mg/L. although it has not yet reached the design hydraulic limit. This overloading has resulted in lower efficiency and therefore
high pollutant concentration in the effluent compared with the design values. Additionally, the drying beds are malfunctioning due to inadequate operation and maintenance. In spite of the addition of polymer to the wet sludge in order to coagulate and ease the separation of water from sludge, the drying period is 20 days compared to the design drying period of 10 days. Therefore, the drying beds at the moment are not able to absorb all the excess sludge at the aeration tank. This causes accumulation of sludge in the aeration tank and re-suspension of sludge from the final settling tank with effluent water.
These problems require precise and scientifically sound steps for operation in order to achieve effluent at concentrations safe for the environment. The aim of this paper is to assess Sana'a's wastewater treatment plant performance in order to overcome the malfunctioning of the plant and to correct the operational problems'therefore achieving better performance. The results are summarized in the following operational measures for the correction of malfunctions in the treatment plant such as: A movable trap with crane could be installed at the inlet chamber; Conducting an awareness campaign among people to explain the problems created at the WWTP from disposing big solids, oils and storm water into the
wastewater network; An extension of the inlet pipe to separate the return sludge from the influent; Desludging frequently at 40% DS and the sludge cake is tendered to a private company to be composted and sold as fertilizers; Replacing the top layer of sand in the drying beds to speed up the drying process; Impose to sulations on the workshops to install oil traps or pretreatment before discharging into the
network; Install Oil separation at the inlet of WWTP; Decrease the MLSS concentration to reach the design value of 4000mg/L.