Performance of the existing sewerage system of Dhaka city and Pagla sewage treatment plant

Abstract

The sewerage system of Dhaka city was initiated in 1923 with about 50 km sewer lines and an imhoff tank type treatment plant. Till to date about 531 km sewerage network, 20 sewage lift stations have been developed in different phases at different times to keep pace with the expansion of the city. The first conventional sewage treatment plant with 4 facultative lagoons built at Pagla in 1968 was renovated in 1977. The present sewage treatment plant at Pagla was rehabilited and expanded in 1992 to treat a capacity of 120,000 m /day sewage with influent BODs of 200 mg/l and effiuent BODs of 50 mg/1. About 150,000m3/day wastewater is presently being generated within the seweragecovered area of which only 28% find it's way to reach Pagla sewage treatment plant. Remaining 72% of sewage generated is discharged to the environment through leakages, overflows etc. of the network, creating tremendous pollution, hence affecting the human health. From the study it is found that under present situation about 50% of the city sewerage network is undersized, a large portion of the trunk mains are in low lying areas without service road. Some portion of the sewer mains is under the roads or even under structures. In addition, the sewer mains have construction defects like frequent improper sewer gradients, syphones, etc. Due to the defective construction of sewers manholes always remain flooded with several feet of wastewater. In this study it is also observed that there are several leakages in the sewerage network including completely blocked manholes. The outlet sewer of the Dhaka Medical College sewage lift station is broken and sewage is being pumped into the nearby surface drain. Most of the sewage lift stations were constructed or designed in such way that the back-flow/submersion of intake frequently takes place with subsequent septic condition in the sewage. This is one of the major reasons for high BODs value. Pumps of the sewage lift stations are very old and needs frequent repair. An average of 28 % (43,000 m3/day ) of generated wastewater is being treated by the Pagla Sewage Treatment Plant (PSTP) at present. From the study it is found that the performance of the primary sedimentation tanks are acceptable and that of facultative lagoons are not satisfactory. The influent BOD5 value of the sewage treatment plant is as high as 4,100 mg/l, which was designed for a influent BODs value of 200 mg/1. The effiuent BODs value is much higher, some 240 mg/l instead of designed value of 50 mg/1. The observed effective sewage depths of the lagoon are around 1.Omthough these were designed for an effective sewage depth of 2.0m. On the bottom of the lagoons about 0.30m height of suspended solids found to have settled. The laboratory of the treatment plant is not well equipped to conduct regular monitoring. The sludge lagoons are functioning well and are in good condition. There exists a liquid chlorination system at the outlet of the plant with a dosing of 3.0 ppm and performing twice a day from 6A.M. to 9 A.M. and 2 P.M to 4P.M. For the improvement of the sewerage system, rearrangement and redesign is recommended. Moreover, the entire Dhaka City should be covered under sewerage system. It is also recommended to specifically identitY the reasons of the high BODs value in the sewage including a proper inventory of effiuent quality, discharge points, etc. of different industries. Change of old pumps of the sewage lift stations with suitable new ones like variable speed pumps may be introduced to overcome the problems of the lift stations.