Assessment of surfactant concentration in selected water bodies in Dhaka city and its removal by alum coagulation

Abstract

Anionic surfactants like LAS (linear alkylbenzene sulfonic acid) are most commonly used in detergents and in household cleaners. The harmful effects of surfactant pollution in water include killing, reduction in growth rate, and reduction in the abundance of food available to fauna and depletion of DO. Surfactants find their way into water bodies through domestic sewage and industrial wastewater. Other than some occasional data on surfactant concentration at the Saidabad Water Treatment Plant (SWTP), there is virtually no data on surfactant for water bodies in Bangladesh. This study was conducted to assess the seasonal variation of surfactant concentration and other water quality parameters in selected four water bodies (Ramna lake, Dhanmondi lake, Hatirjheel and Shitalakshya river) from July, 2019 to December, 2019 in four cycles. The effectiveness of alum coagulation in surfactant removal from selected detergents under varying alum dosages and pH values was assessed. Among the four selected water bodies (Ramna lake, Dhanmondi lake, Hatirjheel and Sitalakhya river), the surfactant concentration was found to be maximum in Hatirjheel during the study period and the concentration peaked during the dry/winter season in the months of November and December, 2019.The lowest concentration of surfactant was detected in Dhanmondi Lake, followed by Sitalakhya River and Ramna Lake. It appears that detergent/soap is the main source of surfactant for Ramna lake, which is widely used for bathing and washing clothes but does not receive any significant wastewater). On the other hand, sewage/wastewater appears to be an important source of surfactant for Dhanmondi lake and Sitalakhya river, because of good correlation observed between surfactant and phosphate/ammonia/ COD. Hatirjheel receives mixtures of domestic sewage (and some industrial wastewater as well) and stormwater during the wet season. But, surprisingly, surfactant concentration in Hatirjheel did not show good correlation with any of the parameters (i.e., phosphate, ammonia or COD). During the alum coagulation process, the surfactant removal (%) increased with increasing alum dosage and highest removal was observed in the neutral pH range (6.5-7.5). However, very high doses (exceeding 100 mg/l) of alum is required for achieving significant removal of surfactant from water, and therefore surfactant removal by alum coagulation does not seem practical for water treatment plants. Therefore, efforts should be made to reduce surfactant concentration e.g., by ensuring treatment of domestic/ industrial wastewater, and reducing/ eliminating discharge of untreated sewage/wastewater in waterbodies.