Initial improvement of a sewage system after embankment: a combined index of water and sediment quality

Abstract

To Lich River (TLR) system receives wastewaters from a population of nearly two million people and 99 manufacturers of five industrial zones in inner city Hanoi, Vietnam. To improve quality of TLR, the construction of an embankment has been carried out during the period 1998-2002. Water and sediment quality indices based on heavy metal concentrations were used to evaluate the current river environment compared to that of the pre-embankment. The mass balance model was employed to estimate total metal loads for specific river reaches, which correspond to various types of wastewater discharged along the river. The results indicated that domestic-discharged river reaches received much lower metal loads, roughly 8-28% compared to river reaches of both domestic and industrial inputs. Total load of all nine concerned metals (Cr, Mn, Fe, Ni Cu, Zn, As, Cd, and Pb) at the end of TLR is 161.7 kg/day. Water quality was improved after completion of the embankment, then it gradually deteriorated. Meanwhile, sediment quality became much worse after the embankment. Relative river quality index as an equal weight for both water and sediment quality indices indicated that quality of TLR was not much improved after the embankment. It even became worse due to urbanization in recent years. To improve quality of a sewage system, the embankment is not enough and is just an initial stage. The next stage which may become more important is a treatment plan to fully treat wastewater at sources following environmental standards before discharging to a sewage system.