Micropollutants removal in tertiary moving bed biofilm reactors (MBBRs): Contribution of the biofilm and suspended biomass

Highlights

• The potential of tertiary MBBRs was investigated in terms of MPs removal.

• Biofilm's contribution was higher than the suspended biomass for MPs biodegradation.

• The dominant biodegradation mechanism of Estradiol was the competitive inhibition.

• Diclofenac, Naproxen, and Nonylphenol were biodegraded by the cometabolism mechanism.

• MPs sorption onto the suspended biomass was higher than the biofilm.

Abstract

The performance of tertiary moving bed biofilm reactors (MBBRs) was evaluated in terms of micropollutants (MPs) removal from secondary-treated municipal wastewater. After stepwise establishment of a mature biofilm, monitored by scanning electron and confocal microscopies, abiotic and biotic removals of MPs were deeply studied. Since no MPs reduction was observed by the both photodegradation and volatilization, abiotic removal of MPs was ascribed to the sorption onto the biomass. Target MPs i.e. Naproxen, Diclofenac, 17β-Estradiol and 4n-Nonylphenol, arranged in the ascending order of hydrophobicity, abiotically declined up to 2.8%, 4%, 9.5% and 15%, respectively. MPs sorption onto the suspended biomass was found around two times more than the biofilm, in line with MPs' higher sorption kinetic constants (k_sor) found for the suspended biomass. When comparing abiotic and biotic aspects, we found that biotic removal outperformed its counterpart for all compounds as Diclofenac, Naproxen, 17β-Estradiol and 4n-Nonylphenol were biodegraded by 72.8, 80.6, 84.7 and 84.4%, respectively. The effect of the changes in organic loading rates (OLRs) was investigated on the pseudo-first order degradation constants (k_biol), revealing the dominant biodegradation mechanism of co-metabolism for the removal of Diclofenac, Naproxen, and 4n-Nonylphenol, while 17β-Estradiol obeyed the biodegradation mechanism of competitive inhibition. Biotic removals and k_biol values of all MPs were also seen higher in the biofilm as compared to the suspended biomass. To draw a conclusion, a quite high removal of recalcitrant MPs is achievable in tertiary MBBRs, making them a promising technology that supports both pathways of co-metabolism and competitive inhibition, next to the abiotic attenuation of MPs.

Introduction

Nowadays, the high-risk occurrence of micropollutants (MPs), as priority hazardous substances in the aquatic environment, has created a global demand for developing innovative and cost-effective technologies to upgrade current wastewater treatment plants (WWTPs). Since most of the WWTPs are not designed to efficiently eliminate the majority of MPs (Barbosa et al., 2016), secondary-treated effluents have been world-widely recognized as the main source of these hazardous compounds in the water bodies (Margot et al., 2013). To overcome this anxiety, scientists have been trying various types of tertiary treatment technologies such as advanced oxidation processes (AOPs) (Homem and Santos, 2011; Lee et al., 2013), adsorption processes (Bonvin et al., 2016) and membrane filtrations (Taheran et al., 2016) throughout the last decade. As compared to such costly methods in the aspects of investment and operation (Zhang et al., 2015), lower attention has been so far paid to biological treatment of secondary-treated effluents probably due to the not-satisfactory growth of microbial strains at low available carbon sources and nutrients. In spite of this fact, recently, moving bed biofilm reactors (MBBRs) are under the sharp-eyed investigation to see their capability in tertiary treatment of wastewater (Tang et al., 2017a, Tang et al., 2017b). Indeed, the acceptable performance of these versatile reactors have been already proved for carbon oxidation, nitrification, denitrification, and deammonification (Boltz et al., 2017; McQuarrie and Boltz, 2011; Rusten et al., 2006). In addition, Torresi et al. (2016) have lately noticed high potential of tertiary nitrifying MBBRs in MPs removal. They concluded that the thickest nitrifying biofilm (500 μm), attached on Z-MBBR carriers, has the highest specific biotransformation rate constants for a broad range of organic MPs due to the high biodiversity found in thick biofilms. Despite this benefit, the time required for development of nitrifying biofilm is long because both types of “ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB)” are autotrophic, grow slowly and have limited abilities to produce extracellular polymeric substance (EPS) (Young et al., 2017) which is known as the main factor of biofilm formation (Boltz and La Motta, 2007). Furthermore, the thick nitrifying biofilm may confine substrate diffusion in the biofilm (Borghei and Hosseini, 2004) and can cause high levels of inorganic precipitates in the biofilm (i.e. scaling). Scaling can lead to the blockage of the biofilm surface by the precipitates, followed by enhancement of the carriers' weight for maintaining in suspension (Piculell, 2016). In view of these points, in this study, the formation of a heterotrophic biofilm was targeted, so the proper conditions for the development of autotrophic biofilm were not provided e.g. by adding the ammonia nitrogen to the influent. We, therefore, initially aimed at developing a heterotrophic biofilm in tertiary MBBRs followed by investigating its potential for MPs removal from secondary-treated effluent. At low substrate availability, however, generation of a thin biofilm was also expected which is logically encountered with lower problematic issues such as scaling. (Regarding the information received from manufacturer of the carriers used in this study (Z-MBBR carriers, AnoxKaldnes), 50 to 100-μm biofilms are relatively thin. Whereas, thickness of a thick biofilm can be from 400 to 500 μm). Meanwhile, conversely to autotrophic bacteria, heterotrophic bacteria can have a doubling time of a few hours, making the biofilm establishment faster (Alpkvist et al., 2007; Boltz and Daigger, 2010; Piculell, 2016).

The fate of MPs during the activated sludge processes is controlled by the abiotic and biotic reactions. Photodegradation, air stripping and mostly sorption onto the biomass constitute the abiotic removal of MPs (Jelic et al., 2011), whilst metabolism and co-metabolism are recognized as the biodegradation mechanisms involved in the biotic MPs removal (Margot, 2015). To date, the importance of the biotic MPs removal has been attracted much higher attentions than the role of its counterpart (Andersen et al., 2005), probably due to this fact that MPs biodegradation is a sustainable process and potentially can form end products consisting of inorganic compounds, i.e. mineralization (Maeng et al., 2011). Additionally, MPs biodegradation is often the dominant removal process for the majority of compounds, as compared with abiotic removal drivers (Stevens-Garmon et al., 2011). According to the review paper published by Verlicchi et al. (2012), sorption onto the secondary activated sludge is reported up to maximum 5% for most of the analgesic and anti-inflammatory pharmaceuticals, beta-blockers, and steroid hormones which is too much lower than the role of biodegradation in MPs removal (even up to 100%). On the contrary, the removal percentage of some antibiotics like Ciprofloxacin and Norfloxacin is reported in the range of 70–90% due to the sorption, while below than 10% of these compounds were abated by the biodegradation mechanisms (Golet et al., 2003). Some studies have pointed out the significance of MPs sorption onto the biomass, as this factor is found to have an impact on the MPs bioavailability (Maeng et al., 2011) and causes the occasional negative mass balance of MPs, where MPs desorption from the suspended or attached biomass occurs during the treatment process (Blair et al., 2015). When the waste sludge is going to be used as a fertilizer on an agricultural land, this factor should be also taken into account, knowing that sludge digestion is likely not able to remove the most of persistent MPs (Ternes et al., 2004a, Ternes et al., 2004b).

In MBBRs, today's knowledge on the mechanisms of MPs removal is still insufficient in terms of the abiotic and biotic aspects (Casas and Bester, 2015; Grandclément et al., 2017; Torresi et al., 2017). Apart from that, individual contributions of the biofilm and suspended biomass have been rarely studied in MPs removal. To distinguish such contributions, studying the MBBRs would be a good approach as they contain both types of the attached and suspended biomass. Considerable potential of the biofilm for MPs removal is already shown by Falås et al. (2013) who studied the removal of organic MPs in a hybrid biofilm-activated sludge process. They concluded that the attached biomass can contribute significantly to the removal of recalcitrant compounds, such as Diclofenac (Falås et al., 2013).

The main objective of this study was to evaluate the removal of four MPs including two analgesic and anti-inflammatory pharmaceutical compounds (Diclofenac and Naproxen), a steroid hormone (17β-Estradiol) and an endocrine disrupting compound (4n-Nonylphenol) by means of tertiary lab-scale MBBRs, and thereby assess the distinct role of the biofilm and suspended biomass in abiotic and biotic elimination of MPs.

To describe an outline for this research, we firstly tried to develop an efficient biofilm in the reactors that ever worked on the continuous mode. At the same time, the steady-state situation of the reactors fed by the MPs-bearing secondary-treated municipal wastewater was achieved. Subsequently, distributional removal of MPs was comprehensively studied.