Approaches on phenoxy acid herbicides behavior in wastewater treatment plants and treatment relevance upon river biota

The purpose of this study is to evaluate the risk contribution of wastewater treatment plants (WWTPs) to the contamination of receiving surface water with phenoxycarboxylic herbicides, focusing on the efficiency of WWTPs in removing these micropollutants. These compounds are widely used in agriculture and have moderate persistence, making them more likely to be present in the environment. However, there is a lack of significant studies regarding their presence in Romanian waters. The concentration of phenoxycarboxylic acids in inflows and outflows ranged from below 3 ng/L to 247 ng/L. The efficiency of WWTPs showed a dynamic variability, beginning with a high removal rate of 4-chloro-2-methylphenoxyacetic acid (MCPA, 77.5%), followed by an accumulation of 2,4-dichlorophenoxyacetic acid (2,4-D) in the effluents, which increased by 247.5%. To move one step closer to solving the mechanistic puzzle, key factors affecting herbicide behavior in WWTPs were investigated, specifically hydraulic retention time and hydrolysis rate during the biological treatment phase. The comparison between estimated and measured results indicates that 2,4-D is removed slowly from WWTPs. Additionally, other compounds may be transformed from lower to higher hydrolyzable derivatives due to recirculation in biological treatment. The ecotoxicological index indicates that both acute and chronic exposure to 2,4-D and MCPA may exert a very high risk to certain phytoplankton and macrophyte species. However, the grey water footprint of WWTP effluents contaminated with herbicides suggests a low overall impact, attributed to the high assimilation capacity of the Jiu and Danube rivers.