Fate and behaviour of NPEOs and their metabolites during wastewater treatment process

壬基酚聚氧乙烯醚（NPEOs）是一种常见的表面活性剂，广泛应用于生产和生活中。NPEOs能够被微生物降解生成壬基酚（NP）等代谢产物。NP作为一种持久性有毒污染物，其内分泌干扰等特性受到关注。本文以污水处理工艺为研究对象，对NPEOs及其代谢产物的迁移转化规律进行研究，为构建安全转化的控制技术系统探索有效途径。 对北京的四座采用不同工艺的城市污水处理厂进行了调查研究，结果表明，四座污水处理厂进水中NPEOs及其代谢产物的总浓度为0.116-0.348 μmol/L，其浓度高低与污水来源有关，生活污水的浓度高于生活污水与工业废水的混合污水的浓度；污水处理工艺对于NPEOs及其代谢产物的总去除率为75.7%-90.8%，去除率因污水处理工艺而异；温度升高能够促进NPEOs的生物降解，降雨会因稀释作用降低NPEOs及其代谢产物在污水厂进水中的浓度，但不会影响此类物质的去除率。 NPEOs及其代谢产物在污水处理工艺中的迁出途径包括：生物降解、污泥吸附和随出水排放。其中，生物降解所占比例较大（52.4%-78.2%），其余则通过污泥吸附或随出水排出。对污水处理工艺中各单元的监测表明，沉砂池对NPEOs及其代谢产物无去除作用，初沉池有助于NPEOs及其代谢产物向污泥转移；NP和短链NPnEOs（n=1,2）主要吸附于污泥上，而壬基酚聚氧乙烯酸（NPECs）主要存在于出水中。 为了阐明NPEOs及其代谢产物在城市污水处理厂中的迁移转化规律，建立了实验室模拟系统。模拟试验结果表明，在厌氧和缺氧条件下，NPEOs主要发生EO链缩短的反应，即，长链NPEOs转化为短链NPEOs，最终转化为NP，少量短链NPEOs发生EO链的羧酸化反应而生成短链NPEC；在好氧条件下，NPEOs主要发生EO链的羧酸化反应和EO链的缩短反应，生成短链NPECs，最终壬基链发生羧酸化反应，生成双侧羧酸化的烷基酚聚氧乙烯酸（CAPECs）。在生物反应过程中，每种NPEOs的代谢产物都具有降解和由长链物质代谢生成的双重作用，导致这些物质在不同污水厂的相同处理阶段的增量可能不一致。在厌氧、缺氧和好氧条件下，NPEOs及其代谢产物的降解速度由快到慢依次为：NPnEOs（n=3-15）、NPECs、NP2EO、NP1EO和NP；而每种物质在厌氧、缺氧和好氧条件下的降解速率均逐渐增大。硫酸根和硝酸根的存在能够促进NPEOs及其代谢产物的生物降解。 臭氧氧化对城市污水处理厂二级生物处理出水中残留的NPEOs及其代谢产物有良好的去除作用。当臭氧进气浓度为17mg/L时，投加时间为10min时，可以完全去除水中的NPECs和NPnEOs（n=3-15），对NPEOs及其代谢产物的总去除率为94.3%。将污水生物处理与臭氧氧化技术相结合，对污水厂进水中NPEOs及其代谢产物的去除率可达98.6-99.5%。Nonylphenol ethoxylates (NPEOs) are an important group of non-ionic surfactants that are widely used in many commercial and household functions. NPEOs can be transformed into their metabolites such as nonylphenol (NP) after biodegradation. NP is a kind of persistent toxic substances (PTS), which constitute a class of man-made chemicals with a pronounced persistence against biological degradation and mutagenic and estrogenic activities, so their significant impacts on human health and the environment have raised the society’s serious concern. In this study, the wastewater treatment process was taken as the research object, and the fate and occurrence of NPEOs and their metabolites during biological wastewater treatment process were investigated, in order to explore effective approach for the safe control techniques. Four Beijing wastewater treatment plants (WWTPs) with different treatment processes were selected to investigate behaviours of nonylphenol polyethoxylates and their metabolites in different treatment process. The results showed that the total concentrations of nonylphenolic compounds in the influents of the four WWTPs ranged from 0.116 to 0.348μmol/L, and the concentration was related to the wastewater source: higher in the domestic wastewater than in the mix of domestic and industrial wastewater. The removal efficiencies ranged from 75.7% to 90.8% and were different according to different treatment processes. Higher temperature may accelerate microbial transform of long-chain NPEOs. The rain weather decreased nonylphenolic compounds input to WWTPs as well as COD by dilution, but had no remarkable impact on the removal efficiences compared to those in the dry weather. NPEOs and their metabolites entering WWTPs were output mainly through three pathways: biodegradation, adsorption onto sludge and release via effluents. 21.8-47.6% of the input amount was emitted into environment via effluents and sludge, leaving a considerable portion of nonylphenolic compounds available for microbial transformation. The grit chamber had nothing to do with elimination of nonylphenolic compounds, while the primary setting tank played an important role on the adsorption pathway. NP and NPnEOs (n=1,2) tended to adsorb onto sludge flocs because of their high hydrophobic properties, on the other hand, NPECs occupied most of nonylphenolic compounds in effluents due to their relative hydrophilic properties, both of which could raise environmental risk. The lab-scale simulation system was set up to clarify the transformation of NPEOs and their metabolites. The results showed that the biodegradation routes of nonylphenolic compounds under the anaerobic and anoxic conditions were: long-chain NPEOs were mainly biodegraded with EO-chain shortened, diverted to short-chain NPEOs, finally leading to the accumulation of NP. With time, NP was further degraded. Only a small amount of short-chain NPEOs were oxidized to their corresponding NPECs, which could disappeared subsequently. Biodegradation routes of nonylphenolic compounds under the aerobic condition were: The first process is a rapid ω-oxidation of the EO-chain of both short-chain and long-chain NPEOs. The next step occurs more slowly, in which the long-chain NPECs are degraded to short-chain NPECs. The third process is the oxidation of the nonyl-chain of NPnECs (n≤2) produced. Under the anaerobic and anoxic conditions, NP and NPnEOs (n=1,2) were not only degraded, but also formed; This is also the case for NPECs under the aerobic condition. The two processes led to the possible discrepancy of the increments of these compounds under the same condition of the different plants. Under the various conditions, NPnEOs (n≥3) was degraded at the highest rate, followed by NPECs, and NPnEOs (n=1,2) were degraded at a lower rate, then NP the lowest. Each group was degraded more and more faster when they were transferred from anaerobic condition, via anoxic, to aerobic condition. The existence of sulphate and nitrate accelerated the biodegradation of nonylphenolic compounds. Ozone oxidation can highly treated the remaining nonylphenolic compounds in the effluents. When the concentration of ozone was 17mg/L and the reaction time was 10min，NPnEOs (n≥3) and NPECs were almost removed, and the removal of the total nonylphenolic compounds was up to 94.3%. If the ozone oxidation was combined with the biological wastewater treatment process, 98.6-99.5% of the total nonylphenolic compounds in the influent could be removed