Abstract

Alkylphenol ethoxylates, in particular nonylphenol ethoxylates, are widely used nonionic surfactants that are discharged in high quantities to sewage treatment plants and directly to the environment in areas where there is no sewage or industrial waste treatment. This article reviews the treatability of nonylphenol ethoxylates and nonylphenol in sewage treatment plants and their persistence in aquatic environments. Nonylphenol ethoxylates can be biologically degraded in sewage treatment plants and in natural environments. Some of the degradation products, including nonylphenol, are more persistent than the parent surfactants and they are found in receiving waters of sewage treatment plants. Nonylphenol in particular is found at high concentrations in some sewage sludges that may be spread on agricultural lands. While some sewage treatment plants discharge significant amounts of nonylphenol ethoxylate degradation products in their final effluents and digested sludges compared to what enters the plant, others degrade nonylphenol ethoxylates more or less completely. The differences in treatment efficiency of such compounds and their degradation products among different sewage treatment plants have been attributed to the load of the surfactants in influent streams, plant design and operating conditions, and other factors such as temperature of treatment. The highest nonylphenol ethoxylate elimination rates were achieved in plants characterized by low sludge-loading rates and nitrifying conditions. In natural waters, it appears that parent nonylphenol ethoxylates are not persistent, but some degradation products may have moderate persistence, especially under anaerobic conditions. Recent results from mesocosm experiments indicate moderate persistence of nonylphenol in sediments, with half-lives of 28 to 104 days. Microbial acclimation to the chemicals is an important determinant of persistence vis-à-vis biodegradation. Sunlight photodegradation of such products is also likely important. Further research on the persistence in natural environments of the lower ethoxylate and carboxylate degradation products, as well as nonylphenol, is necessary. Based on the limited data available, nonylphenol and the lower ethoxylates and carboxylates are persistent in groundwater. They are also persistent in landfills under anaerobic conditions, but they do not appear to be persistent in soil under aerobic conditions. Recommendations are made for further research in order to more fully characterize the treatability of nonylphenol ethoxylates and their degradation products in sewage treatment plants and their persistence in the natural environment.

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