Are anionic surfactants in untreated wastewater safe to aquatic ecosystem? A conservative risk assessment case study at That Luang Wetlands, Vientiane Capital, Lao PDR

Abstract

A few aquatic environmental risk assessment studies of anionic surfactants executed in developing countries have shown contradictory results on ecological safety. This study was initiated to understand the discrepancies among the previous studies. It was executed in Lao PDR, by analyzing methylene blue active substance (MBAS) from 7 sites representing a wetland receiving untreated municipal wastewater and associated sewage channels and it evaluated the ecological risk of the sites at local level.The measured MBAS levels in the dry season were converted to linear alkylbenzene sulfonate (LAS) and the risk characterization ratios (RCRs) were calculated. The RCRs of two sites at the sewage channels were 4.8 and 5.5, level of concern. The RCRs of two sites at the Houay Mak Hiao river of the That Luang Wetlands were 2.2 and 3.3, level of concern. The other two sites of the river were less than 0.06. The RCR at That Luang Lake showed 1.1.The levels were not as high as expected from the level estimated using data reported in the literature: over 10. On the other hand, the levels were not low enough to be considered as a level of less concern (lower than 1) as observed in some previous studies by other authors: around 0.1, mainly due to the low level of dissolved oxygen (DO) which could have hampered the normal biodegradation of anionic surfactants even at high temperature under slow water flow. It was recognized that the low DO might have been caused by the heavy coverage of the water stream by the aquatic plant, water hyacinth (Eichhornia crassipes) during the period of this experiment. Based on these findings, it is concluded that:(i) municipal wastewater containing anionic surfactants in developing countries might cause aquatic environmental disturbance at limited local distance of the public waterways in a wetland,And, it is suggested that:(ii) the disturbance could be technically ameliorated to a level of less concern by maintaining aerobic conditions at the concerned waterways.