Influence of accumulated sediment/ sludge in DND conveyance canal on the water quality of Saidabad water treatment plant

Abstract

Saidabad Water Treatment Plant (SWTP) is one of the vital sources of potable water for Dhaka city. Raw water for this treatment plant is drawn from the highly polluted Sitalakhya river and before reaching the SWTP the water is conveyed through the 4.6 km long DND (Dhaka-Narayangang-Demra) open conveyance canal. In this study the sediment and water quality of the DND conveyance canal has been assessed for determining its role in regulating raw water quality of the SWTP, particularly focusing on the estimation of possible contaminant flux from canal sediment to water column. Sediment samples from both longitudinal and traverse sections of the canal have been collected and analyzed for detailed characterization. Contaminant (selected nutrients, COD and selected heavy metals) flux from sediment to overlying water column has also been assessed through laboratory experiments to assess the impact of sediment quality and physical disturbance of sediment on water quality of the canal. It was found that the water quality of the DND canal deteriorates during dry season and pre-monsoon, and improves with the onset of monsoon. Dry season water quality of the DND canal is characterized by low concentration of DO (3.24-3.64 mg/L) and high concentration of BOD5 (1.2-5.4 mg/L), COD (29-36 mg/L), and ammonia (1.71-2.88 mg/L). Very low concentration of dissolved oxygen was observed in all seasons with the lowest value being 2.05 mg/L during pre-monsoon (May). Ammonia concentration exceeding the national permissible limit for drinking water was found in the water samples of the DND canal in all seasons. Gradual increase in BOD5 value was observed from upstream towards downstream of the canal during dry season. This indicates possible release of organic matter from the accumulated sediment/sludge of DND canal. Cadmium concentration exceeding the national permissible limit for drinking water was detected at all sampling points during the dry season. Highest cadmium concentration of 0.094 mg/l was detected during monsoon which suggests possible ingress of cadmium from the source water (i.e., water of Sitalakhya River). The DND canal has accumulated significant sludge on its bed since operation of the SWTP commenced in 2002. . Depth of sludge in the canal varied from approximately 381 mm to 762 mm, with estimated accumulation rate of 24 to 48 mm per year. Higher accumulation was found along the mid-section of the canal. High concentration of organic matter varying from 3.51% to 17.62% was found in the sediment of DND canal. Sediment from all sampling points were found to be polluted with copper (up to 84 mg/kg) and samples from downstream point of the canal were “highly polluted” with lead (up to 91 mg/kg) and cadmium (up to 18.5 mg/kg), and “moderately polluted” with chromium (up to 24.5 mg/kg), according to USEPA guideline for natural soil. Very high concentration of potential sediment oxygen demand (pSOD, as high as 13,374 g O2/m3 sediment/day) found in the study indicates the presence of high concentration of biodegradable organic matter in the sediment. Contaminant flux estimated from laboratory experiments suggests significant flux of organic matter (COD, as high as 2858 mg/m2.day) and nutrient (particularly ammonia, as high as 444 mg/m2.day) from sediment to the overlying water column. Decrease in ammonia concentration in water coupled with appreciable rise in pH and DO value during the experiment indicates significant algal uptake of ammonia. This study suggests that fluxes of nutrients from bed sediment are responsible for promoting algal bloom in the DND canal and thus contributes to the poor raw water quality at the SWTP. The experimental results and physical observation also suggests significant impact of temperature and weather on algal bloom and contaminant flux. Flux of heavy metals (lead, cadmium, chromium and copper) was not found to be significant from sediment to overlying water. The results of this study suggest that the resuspension of bottom sludge/sediment through any disturbance like dredging operation could turn the DND canal anoxic and promote release of nutrient (e.g., ammonia), organic matter (COD), suspended solids, and heavy metals (especially copper).. Hence for cleaning up of the DND canal, a sludge removal process that minimizes the resuspension of bottom sediment should be adopted.