Fate of Alkylphenolic Compounds during Activated Sludge Treatment

Environ. Sci. Technol. 2011, 45, 248–254

Fate of Alkylphenolic Compounds during Activated Sludge Treatment: Impact of Loading and Organic Composition EWAN J. MCADAM,† JOHN P. BAGNALL,† ANA SOARES,† YOONG K. K. KOH,‡ TZE Y. CHIU,§ MARK D. SCRIMSHAW,⊥ JOHN N. LESTER,† AND E L I S E C A R T M E L L * ,† Centre for Water Science, Cranfield University, Bedfordshire, MK43 0AL, U.K., Public Utilities Board, Technology and Water Quality Office, 40 Scotts Road No. 15-01, Environmental Building, 228231, Singapore, AECOM Design Build, Wentworth Business Park, Tankersley, Barnsley, S75 3DL, U.K., and Institute for the Environment, Brunel University, Uxbridge, Middlesex, UB8 3PH, U.K.

Received April 1, 2010. Revised manuscript received October 28, 2010. Accepted October 29, 2010.

The impact of loading and organic composition on the fate of alkylphenolic compounds in the activated sludge plant (ASP) has been studied. Three ASP designs comprising carbonaceous, carbonaceous/nitrification, and carbonaceous/nitrification/ denitrification treatment were examined to demonstrate the impact of increasing levels of process complexity and to incorporate a spectrum of loading conditions. Based on mass balance, overall biodegradation efficiencies for nonylphenol ethoxylates (NPEOs), short chain carboxylates (NP1-3EC) and nonylphenol (NP) were 37%, 59%, and 27% for the carbonaceous, carbonaceous/nitrification, and carbonaceous/nitrification/ denitrification ASP, respectively. The presence of a rich community of ammonia oxidizing bacteria does not necessarily facilitate effective alkylphenolic compound degradation. However, a clear correlation between alkylphenolic compound loading and long chain ethoxylate compound biodegradation was determined at the three ASPs, indicating that at higher initial alkylphenolic compound concentrations (or load), greater ethoxylate biotransformation can occur. In addition, the impact of settled sewage organic composition on alkylphenolic compound removal was evaluated. A correlation between the ratio of chemical oxygen demand (COD) to alkylphenolic compound concentrationandbiomassactivitywasdetermined,demonstrating the inhibiting effect of bulk organic matter on alkylphenol polyethoxylate transformation activity. At all three ASPs the biodegradation pathway proposed involves the preferential biodegradation of the amphiphilic ethoxylated compounds, after which the preferential attack of the lipophilic akylphenol moiety occurs. The extent of ethoxylate biodegradation is driven by the initial alkylphenolic compound concentration and the * Corresponding author tel: +0044 (1234) 758366; fax: +0044 (1234) 751671; e-mail: [email protected]. † Centre for Water Science, Cranfield University. ‡ Public Utilities Board, Technology and Water Quality Office. § AECOM Design Build. ⊥ Institute for the Environment, Brunel University. 248

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proportion of COD constituted by the alkylphenol polyethoxylates (APEOs) and their metabolites relative to the bulk organic concentration of the sewage composed of proteins, acids, fats, and polysaccharides. Secondary effluents from this study are characterized by low bulk organic concentrations and comparatively high micropollutant concentrations. Based on the biodegradation mechanism proposed in this study, application of high rate tertiary biological treatment processes to secondary effluents characterized by low bulk organic concentrations and comparatively high APEO concentrations is predicted to provide a sustainable solution to micropollutant removal.

Introduction Alkylphenol polyethoxylates (APEOs) are commercially important nonionic surfactants used in both industrial and domestic detergent and emulsifier formulations (1). These commercial APEOs are a mixture of nonylphenol ethoxylates (NPEOs) and octylphenol ethoxylates (OPEOs) with ethoxylate chain lengths ranging from 6 to 20. An increase in the toxicity of APEOs with decreasing ethoxylate chain length has been observed (2). To illustrate, LC50 values for long chain ethoxylates and alkylphenol are 1.5 mg L-1 and 0.1 mg L-1, respectively (3). The short chain ethoxylates, carboxylates, and alkylphenolic compounds possess a strong capability to mimic natural hormones by interacting with estrogen receptors (4). It is this capacity for the biotransformation of long chain ethoxylate compounds to form metabolites under environmental conditions that has created scientific and regulatory concerns. In addition, the risk of metabolite bioaccumulation in receptor tissue has been quantified. Concentration factors between ten and several thousand have been reported dependent upon receptor anatomy (2, 3). This further increases the potential risk of endocrine function disruption by alkylphenolic compounds in both wildlife and humans (5, 6). Alkylphenolic compounds are ubiquitous in wastewater discharges and in the aquatic environment with reported concentrations ranging NP6EO, a plateau was reached corresponding to ca. 70% removal for the longer chain oligomers, NP7-12EO. Similar plateaus were reached at NP5EO and NP10EO in the ASPnit. and ASPnit./denit. corresponding to ca. 92% and ca. 62% removal of long chain oligomers ranging NP5-12EO and NP10-12EO, respectively. The significance of the percent removal data reported is not to emphasize differences in compound removal, rather it is to demonstrate the attainment of an analogous data trend for each site. Negative removal was evident for short chain ethoxylates (NP1-3EO) and carboxylates (NP1-3EC) at all three ASPs. This was a consequence of their formation and is

indicative of long chain oligomer compound biotransformation which is well established (18). Removal of NP1-3EC, for example, ranged between -775% and -2150% compared to -100% and -1100% at the ASPcarb.. The parent compound nonylphenol was also formed at the ASPcarb. and ASPnit./denit. with -25.5% and -54.1% removal recorded, respectively. In contrast, positive nonylphenol removal of 42.6% ( 30.4% was observed at the ASPnit. indicating cleavage of the benzene ring consistent with observations described previously (3). Statistical analysis of the difference between settled sewage and secondary effluent nonylphenol composition confirmed that removal of nonylphenol at the ASPnit. was significant (p < 0.1, t(26) ) 1.74). The long chain ethoxylate compounds in the settled sewage were of similar composition at all three sites ranging from 78% to 90% for NP4-12EO and 8% to 16% for NP (see Figure S1, Supporting Information). Initial long chain ethoxylate settled sewage concentrations did vary, however, with NP4-12EO values of 9.2, 17.5, and 9.7 µg L-1 recorded for the ASPcarb., ASPnit., and ASPnit./denit., respectively. Despite the high initial NP4-12EO settled sewage concentration, the ASPnit. achieved the greatest long chain ethoxylate removal (Figure 1). The total mass of all nonylphenolic compounds (ΣNPE, sum of NP, NP1-12EO, and NP1-3EC) biodegraded at the ASPnit. was 101 300 µg d-1 or 59% of the settled sewage composition in comparison to 37% and 26.8% at the ASPcarb. and ASPnit./ denit., respectively (Figure 1). Retention of nonylphenolic compounds through adsorption onto the return activated sludge solids was also quantified (see Table S3, Supporting Information). Short chain NP1-3EO compounds were significantly adsorbed recording log Kp values between 3.2 and 3.9 (eq 1). Conversely, sorption of nonylphenol was highly variable ranging from log Kp values of 1.5 to 3.9 corresponding to the ASPnit./denit. and ASPcarb., respectively, however, this range of values is similar to that reported by Koh et al. (9) of 1.4 to 3.2. Evaluation of Biomass Activity and the Effect of NPEO Loading on Biodegradation. The pattern of biomass activity (calculated based on NP4-12EO removed/total MLSS concentration) across the ASPs was closely matched to the order of ΣNPE biodegradation efficiency observed. To demonstrate, NP4-12EO biomass activities were 7327 ( 3809, 11572 ( 1724,and 2629 ( 577 µgΣNP4-12EO kg biomass-1 d-1, compared to the ΣNPE biodegradation determined across the ASPs of 37%, 59%, and 27% (Figure 1) for the ASPcarb., ASPnit., and ASPnit./denit., respectively. Statistical analysis of the relationship between initial settled sewage ΣNPE concentration and biomass activity for the three sites (df ) 43) indicated a weak correlation (r2 ) 0.46, p ) 0.02). ΣNPE loading (sum of NP, NP1-12EO, NP1-3EC) provided a normalized parameter that accounted for flow variation. Averaged ΣNPE loading values for each ASP were 50.6 ( 16.1, 96.5 ( 15.9, and 10.4 ( 2.0 mgΣNPE m-3 d-1 for the ASPcarb., ASPnit., and ASPnit./ denit., respectively. The lower loading at the ASPnit./denit. was due to the extensive tank volume (53380 m3) and imposed HRT (ave. 21.7 h). Using ΣNPE data, a significant correlation between biomass activity and ΣNPE loading was determined (r2 ) 0.81, p ) 85% for NP3EO). From this result, it may be hypothesized that in the absence of competing substrate such as the amphiphilic ethoxylated compounds, micro-organisms will preferentially attack the lipophilic nonylphenol moiety. Tanghe et al. (31) made a similar assertion following observations in high concentration nonylphenol experiments with a semicontinuous ASP. Koh

et al. (9) also identified nonylphenol degradation at a full scale ASPnit./denit. which was heavily loaded with alkylphenolic compounds (242 mg m-3 d-1), operated under low bulk COD primary effluent concentrations (252 mg L-1), and achieved 95% ΣNPEO degradation (sum of NP0-12EO). The data presented here suggest that at low bulk COD/ΣNPEO ratios, the kinetic rate at which ethoxylated compounds are biotransformed increases, and in turn, increases the likelihood of metabolite and nonylphenol removal. Further work is required to ascertain the impact of multicomponent micropollutant matrices on the biodegradation kinetics of individual micropollutant compounds. Adsorption of Nonylphenol and Final Effluent Quality. The nonylphenol concentrations recorded in the effluent of the three ASPs correlated closely to median effluent values of 1120 to 2235 ng L-1 for a full-scale conventional ASP in Italy. However, NP effluent concentrations far exceeded the 44 to 55 ng L-1 recorded in a recent UK survey of two fullscale ASPs (9). The calculated partitioning coefficients were scattered around the octanol-water coefficient (Kow) for nonylphenol of 4.48 (32) and demonstrate the affinity of these compounds for adsorption to hydrophobic surfaces (11). Interestingly, while ASP effluents were characterized by low suspended solids concentrations (7 to 23 mg L-1) in this study, the dissolved phase nonylphenol concentrations, partitioned at 1.2 µm, ranged from to 390 ng L-1. The effluent solids fraction will typically yield lower particle sizes which can exhibit higher specific surface area thus increasing the potential adsorptive capacity. Consequently the significance of low solids concentration on effluent micropollutant quality cannot be discounted and it can be inferred from the results of this study that higher micropollutant retention may be facilitated through advanced particulate retention (cf. secondary clarification). The impact of alkylphenolic loading and organic composition on ΣNPE compound removal in secondary treatment has been assessed. From this study, it is postulated that high rate tertiary treatment processes comprising biodegradation and filtration can deliver effective, sustainable micropollutant removal. To illustrate, secondary effluents are characterized by low bulk organic concentrations and comparatively high micropollutant concentrations thus based on the results presented, tertiary biological treatment could yield high specific activities. Furthermore, filtration advances particulate retention, thereby enhancing retention of the adsorbed phase. This hypothesis seemingly accords with recent full-scale observations from tertiary sand filters which have indicated biological degradation of recalcitrant compounds using limited contact times (33). The potential to circumvent advanced tertiary treatment technologies (e.g., ozone) for removing endocrine disrupting micropollutants is highly desirable as these processes present both financial and environmental constraints particularly with reference to energy and carbon footprint (34).

Acknowledgments Y.K.K.K. is grateful to the Public Utilities Board of Singapore for the award of a PhD scholarship. We thank the following companies: Anglian Water Ltd., Severn Trent Water Ltd., Thames Water Utilities Ltd., United Utilities Plc., and Yorkshire Water Services Ltd. for providing their support and funding. Finally, Dan McMillan at Waters Ltd. is acknowledged for analytical support.

Supporting Information Available Three tables (Table S1, S2, and S3) describing the process characteristics of the works, the sanitary determinand concentration data. Settled sewage and final effluent nonylphenolic compound concentrations are provided in Figures S1 and S3 with the correlation of ethoxylate shortening/

carboxylic acid transformation outlined in Figure S2. The nonylphenolic compound biomass sorption coefficients for secondary activated sludge are presented in Table S3. This information is available free of charge via the Internet at http://pubs.acs.org/.

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