Modelling of micropollutant removal in biological wastewater treatments: A review

Abstract

Modelling the fate of micropollutants through wastewater treatment plants is of present concern. Indeed, such a tool is useful to increase the removal of micropollutants and reduce their release to the environment. In this paper, 18 literature models describing micropollutant removal in activated sludge processes were reviewed. Investigated micropollutants were mainly volatile organic compounds, metals, surfactants, pesticides and pharmaceutical compounds. This work provides a detailed insight about the main mechanisms leading to the micropollutant removal (volatilisation, sorption, biodegradation, cometabolism), the associated mathematical equations and the parameter values found in the literature. A critical analysis was carried out to evaluate the conditions and the domain of validity for which each model was set-up. We also propose (i) an inventory of the experimental methodologies applied to determine the values of model parameters, (ii) a critical study of the main differences between models and (iii) suggestions for a standardisation of calibration methodologies. Finally, this review highlights the lack of explanation concerning the domain of validity of the models and proposes future developments to improve modelling of micropollutant removal in wastewater treatment plants.

Introduction

The effluents of municipal wastewater treatment plants (WWTP) are an important way of discharge of micropollutants into the environment. Micropollutants are suspected to have a potential ecotoxic impact on aquatic ecosystems (Halling-Sorensen et al., 1998, Ternes et al., 2004). The adoption of the European Water Framework Directive (EC, 2000) and of the daughter Directive 2008/105/EC (EC, 2008) drove states and industries to reduce micropollutant releases, including at present 41 priority substances for the evaluation of the chemical status of water masses. A new daughter directive (EC, 2012) proposed an update of the list of priority substances, including some emerging compounds such as pharmaceuticals. As a consequence, numerous research projects and monitoring programmes aimed at evaluating sources and occurrence of priority and emerging substances at national and European scale (e.g., projects Poseidon, Neptune, Knappe, Score-PP, and PILLS).

Numerous literature data have pointed out that many micropollutants are removed from the wastewater during treatment, even though WWTPs were not originally designed for this objective (Carballa et al., 2004, Salgado et al., 2012). For instance, results of the AMPERES project (“Analysis of priority and emerging pollutants in wastewater and surface waters”; Martin Ruel et al., 2012) indicated that about half of the studied organic and inorganic contaminants were removed at more than 70% by biological treatment. A large majority of contaminants was quantified in treated wastewater at significant concentrations (> 0.1 μg/L) (Choubert et al., 2011). Their presence in effluents is due to their relatively high concentration in raw wastewater and/or to their insufficient elimination in WWTPs. Also, the review of Verlicchi et al. (2012) provided a snapshot of the removal efficiency of pharmaceutical compounds achieved in biological treatments (conventional activated sludge and membrane bioreactor). They found that removal efficiency depended on properties of compounds and operational conditions. If regulation in the commercialization and the use of these substances is a possible reduction strategy, many studies suggested that performances of treatment processes could be optimized, particularly by offering more favourable operating conditions for degradation and sorption, the two main removal mechanisms with volatilisation (Lee et al., 1998).

In fact, removal efficiency depends strongly on the physicochemical properties of a substance. Elimination of hydrophobic compounds first occurs while suspended solids are removed, such as during the primary stage. Micropollutants can also be removed during secondary treatment, such as activated sludge, by sorption on biological sludge and biodegradation. Moreover, removal efficiencies vary depending on WWTP operating conditions, such as sludge retention time (SRT), hydraulic retention time (HRT) and temperature; even though the influence of these parameters is not always clearly understood. For instance, higher removal efficiency was observed for most micropollutants studied when nitrification occurs due to high SRT (Choubert et al., 2011). Previous studies showed that higher SRT increased the removal efficiency of pharmaceutical compounds (Strenn et al., 2004, Clara et al., 2005, Carucci et al., 2006). Indeed, a longer SRT may promote the diversity of bacterial communities, as well as the presence of slower growing species, thus increasing the biodegradation potential of the biomass (Kreuzinger et al., 2004, Suarez et al., 2010). Moreover, a high SRT combined with a reduced food/microorganism ratio seemed to favour the biodegradation of antibiotics (Gobel et al., 2007). However, some authors did not observe any improvement of removal efficiency of pharmaceutical compounds for SRT between 10 and 80 days (Joss et al., 2005, Vieno et al., 2007). The role of HRT was also pointed out by Joss et al. (2005) and Maurer et al. (2007): longer HRT involved longer contact time between wastewater and sludge, which seemed to favour biodegradation reaction. Gros et al. (2010) and García-Galán et al. (2011) reported better removal efficiency for micropollutants with a half life lower than HRT for pharmaceutical compounds.

Information about the influence of operating conditions on micropollutant removal is a key point to better understand and improve removal mechanisms. But as shown above, experimental results presented in the literature do not provide unambiguous conclusions. Modelling could help to resolve these questions because it enables to simulate many operating conditions. Moreover, this approach allows moderate investing and operating efforts that could complement a source reduction strategy supported by the authorities.

Understanding the fate of micropollutants through WWTP includes the knowledge of the influence not only of the WWTP operating conditions but also of the physicochemical properties of the micropollutants on the three main removal mechanisms: volatilisation, sorption, and biodegradation. For macropollutants (carbon, nitrogen, phosphorus), Activated Sludge Models (ASM; Henze et al., 1987) have been set-up and are now well mastered. They concern activated sludge processes that account for the larger part of European WWTPs. For micropollutants, specific models are under development and several modelling contributions have been published to date. This paper presents an overview of the different concepts and models found in the literature to simulate micropollutant removal mechanisms. A focus on mechanistic models is proposed. Other concepts like fugacity have been developed in the 70s (Mackay, 1979) to describe micropollutant behaviour in the compartments of the environment (including sediments, surface water, and air), but they are not included in process engineering. In this paper, first we present the general features of the models and detail the micropollutants modelled (such as volatile organic compounds (VOCs), metals, surfactants, pesticides and pharmaceuticals). Second, we discuss a state of the art of micropollutant modelling in activated sludge treating municipal sewage: the different equations and parameters used in the identified models are presented for each removal mechanism. Finally, this study focuses on modelling practice: experimental protocols set up to determine the values of the model parameters, the calibration and the validation methodologies are studied. We propose possibilities of improvement for future models.