Characterization of combined sewage in Hatirjheel diversion sewer system and its implications on the dasherkandi STP

Abstract

Hatirjheel serves very important hydrological functions of draining and detaining storm water from a large area (about 24 sq. km) of Dhaka city. Its main diversion sewer system running along the periphery of Hatirjheel, carries domestic sewage during dry season, and mixture of storm water and domestic sewage during wet season, received through 9 major storm sewer outfalls, as there is virtually no operational sanitary sewer network in the city. The combined wastewater of main diversion sewer system of Hatirjheel would be transported to the under-construction Dasherkandi STP for treatment. The main focus of this research work was to assess characteristics of the wastewater flowing through the diversion sewers of Hatirjheel, and to assess impacts wastewater characteristics and ongoing/future projects of DWASA on the operation of Dasherkandi STP.

Wastewater samples from Hatirjheel main diversion sewer system were collected during April-May 2017 (summer), July-August 2017 (wet season) and December-January 2017-18 (dry season). Water samples were collected in on a weekday and on a holiday in each season; on each sampling day, samples were collected at 0600, 0800, 1000, 1200, 1400 and 1600 hrs.

It has been found that the characteristics of combined sewage of Hatirjheel main diversion sewer vary significantly with the time of the day. Concentration of most of the parameters, particularly Turbidity, TSS, Ammonia, Phosphate, BOD5 and COD increases as the day progresses, and reaches their peak values at around 1400 hours. Concentrations of Turbidity, TSS, BOD5 and COD appears to be slightly higher during the weekend, compared to weekdays; while concentration of ammonia and phosphate appear to be slightly lower during the weekend, compared to weekdays. BOD5/COD ratio of combined wastewater during dry season varied from 0.49 to 0.63, indicating that the wastewater does not contain significant non-biodegradable waste and it could be easily treatable by biological means.

The characteristics of wastewater coming from northern and southern sides of the main diversion sewer are comparable, although the organic load coming from the southern side appears to be slightly higher. The BOD5/COD ratio of wastewater coming from the northern side is slightly lower than that coming from the southern side, indicating relatively higher contribution of industrial sewage from the northern side of Hatirjheel.

The characteristics of combined wastewater change significantly during the wet season due to dilution with rainwater. The peak BOD5 concentration (420 mg/l) recorded in December 2017 (dry season) has been found to be over four times higher than that of the sample collected in August 2017 during heavy rainfall. The COD (672 mg/l) and Ammonia (21.2 mg/l) concentrations in December (dry season) were found to be over two times higher than those recorded in August 2017 (wet season). The operational parameters of the Dasherkandi

STP (e.g., detention times in aeration tanks and secondary clarifier; sludge management) need to be adjusted to accommodate these variations in wastewater characteristics. The flow coming through the “southern main diversion sewer of Hatirjheel” carries sewage mainly from Pagla and Rayerbazar catchments, would carry primarily storm water during wet season, after separating these two catchments. This would cause even more dilution of sewage during the wet season, requiring further adjustments at the Dasherkandi STP. Again separate and independent sanitary sewerage network for Dasherkandi could solve these difficulties.