Effect of hydraulic retention time on the performance of a hybrid moving bed biofilm reactor-membrane bioreactor system for micropollutants removal from municipal wastewater

This study evaluated micropollutants removal and membrane fouling behaviour of a hybrid moving bed biofilm reactor-membrane bioreactor (MBBR-MBR) system under four different hydraulic retention times (HRTs) (24, 18, 12 and 6 h). The results revealed that HRT of 18 h was the optimal condition regarding the removal of most selected micropollutants. As the primary removal mechanism in the hybrid system was biodegradation, the attached growth pattern was desirable for enriching slowing growing bacteria and developing a diversity of biocoenosis. Thus, the efficient removal of micropollutants was obtained. In terms of membrane fouling propensity analysis, a longer HRT (e.g. HRTs of 24 and 18 h) could significantly mitigate membrane fouling when


Introduction
Although the frequently detected micropollutants are at trace concentrations in our aquatic environment, they may cause long-term adverse effects, such as bioaccumulation and carcinogenicity (Klavarioti et al., 2009;Soares et al., 2008).Membrane bioreactors (MBRs), as a promising biological treatment technology, have been widely used to remove micropollutants (Tadkaew et al., 2011;Trinh et al., 2012;Wijekoon et al., 2013).However, two major concerns remain during the application of MBR systems, including membrane fouling, which could be alleviated by frequent physical and/or chemical backwashing or relaxation, and inconsistent removal of some polar and persistent hydrophilic micropollutants even at long hydraulic retention time (HRT).
Combination of moving bed biofilm reactor (MBBR) with MBR in series (MBBR-MBR) is one of recent advances in improving MBR performance, due to its potential for reducing membrane fouling and prolonging filtration duration as well as the possibility of prompting microbial degradation of certain organic compounds (Luo et al., 2015;Sombatsompop et al., 2006).It has been shown that the prolonged HRT decreased membrane fouling propensity and enhanced the formation of a diverse biocoenosis and biodegradation efficiency for certain micropollutants (De Gusseme et al., 2009;Schröder et al., 2012).Nevertheless, a previous research (Chen et al., 2008) concluded that the changes in HRT (HRTs of 8, 6, and 4 h) had little effect on removal efficiency of Bisphenol A in a lab-scale submerged MBR, probably due to its rapid biodegradation and low potential for bioaccumulation.
Up to now, there has been no comprehensive study regarding the impact of HRT on micropollutants removal.Our previous investigation indicated that MBBR as pretreatment to MBR exhibited better performance in removing micropollutants than single conventional MBR (CMBR), and more importantly MBBR could mitigate membrane fouling of MBR to a great extent (Luo et al., 2015).Therefore, in this study, the performance of MBBR-MBR hybrid system was investigated at four different HRTs (24 h, 18 h, 12 h, and 6 h) to determine the optimal HRT in terms of micropollutants removal and membrane fouling control.Soluble microbial products (SMP), extracellular polymeric substances (EPS), zeta potential, and relative hydrophobicity (RH) were used to examine the membrane fouling propensity.

Materials and methods
In this study, a set of 22 frequently detected micropollutants were selected to represent 11 pharmaceutical and personal care products (ibuprofen, acetaminophen, naproxen, ketoprofen, diclofenac, primidone, carbamazepine, salicylic acid, metronidazole, gemifibrozil and triclosan), 5 steroid hormones (estrone, 17-β-estradiol, β-estradiol 17-acetate, 17-α ethinylestradiol and estriol), 4 industrial chemicals (4-tert-butylphenol, bisphenol A, 4-tert-octylphenol and 4-n-nonylphenol) and 2 pesticides (fenoprop and pentachloro-phenol).The preparation of synthetic wastewater (containing micropollutants), characteristics of sponge carrier and analysis methods (e.g.concentrations of mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS)) could refer to our previous study (Luo et al., 2014 and2015).Influent and effluent samples were taken for analyses of dissolved organic carbon (DOC), chemical oxygen demand (COD), ammonium nitrogen (NH 4 -N), nitrate (NO 3 -N), nitrite (NO 2 -N), and orthophosphate (PO 4 -P).The hybrid MBBR-MBR system was composed of a 40 L MBBR unit and a 10 L submerged MBR unit.Four different HRTs of 24 h, 18 h, 12 h and 6 h were applied to the operation of MBBR unit, and the HRT of the MBR unit was kept at 6 h (i.e.constant flux of 8.33 L/m 2 •h) throughout the experimental period.The filling ratio of sponge cubes in the MBBR unit was 20% (v sponge /v reactor ).For MBR unit, a hydrophilic polyvinylidene fluoride (PVDF) hollow fibre microfiltration (MF) membrane module was used with a pore size of 0.2 μm and surface area of 0.2 m 2 .The operation conditions of the MBR unit were described in previous study by Luo et al. (2014).Chemical cleaning and fouling resistance analyses were performed when transmembrane pressure (TMP) reached 35 kPa (Deng et al., 2014).Measurements of zeta potential, RH, particle size, EPS and SMP were carried out on sludge samples which were directly collected from the MBR unit.The analytical methods and the process for determining micropollutant concentration using solid phase extraction (SPE) and gas chromatographymass spectrometry (GC-MS) quantification were reported in Luo et al. (2014).

The performance of MBBR at different HRTs
At HRTs of 24, 18, 12 and 6 h, the MBBR unit was able to achieve effective removals of DOC (> 94%), COD (> 94%) and NH 4 -N (> 82%), but unstable total nitrogen (T-N) (45.2-72.3%)and PO 4 -P elimination (26.4-49.9%).It was noteworthy that the MBBR showed the highest performance efficiency for removing DOC, COD, NH 4 -N, PO 4 -P and T-N at HRT of 18 h, which were 96.1 ± 0.4%, 97.4 ± 0.8%, 91.1 ± 1.6%, 49.9 ± 7.2%, and 72.3 ± 6.9%, respectively.This could be explained by the food to microorganisms (F/M) ratio in the MBBR unit.The F/M ratios were 0.72, 1.83, and 3.06 g COD/g MLVSS•d at HRTs of 24, 12 and 6 h, respectively, while at HRT of 18 h, the feed supplied for microorganisms was adequate for their growth with F/M ratio of 0.91 g COD/ g MLVSS•d, which was very close to the value (1.05 g COD/ g VSS•d) as mentioned by Pozo et al.

Removal and fate of the selected micropollutants in MBBR unit
Significant variations on micropollutants removal among each individual compound were observed over the study period (Fig. 1).At HRT of 18 h, the removals of most micropollutants remained constantly high over the experimental period except for carbamazepine, fenoprop, and metronidazole, because these three compounds are more recalcitrant due to the possession of electron withdrawing groups such as amide (CONH 2 ) carboxylic acid (COOH) and nitro groups (NO 2 ).Particularly, the removal of diclofenac increased by at least 30% along with better removals of ketoprofen, gemifibrozil, acetaminiphen, bisphenol A, and pentachlorophenol as compared with those at other HRTs.

Fig. 1.
The equations used to evaluate the fate of micropollutants in the MBBR unit could refer to our previous study by Luo et al. (2015).It is noted that biodegradation was the main pathway for the removal of most micropollutants in the MBBR.At the prolonged HRT of 18 h, major compounds of pharmaceuticals (especially salicylic acid, acetaminophen, naproxen, gemifibrozil, promidone, and ibuprofen), steroid hormones (except 17α-ethinylestradiol), industrial chemicals (biophenol A and 4-n-nonylphenol) and pesticides (pentachlorophenol) were mainly removed by biodegradation (> 80%) (Fig. 2).Sorption accounted for much less proportion of most micropollutants removal when compared to biodegradation since sorption is a faster process.Moreover, sorption of micropollutants onto biosolids may prolong longer residence time, prompting further removal via biodegradation (Tadkaew et al., 2011).In addition, biomass could fully occupy the acclimatized sponge over time, which reduced the sorption site and sorption efficiency (Luo et al., 2014).As a result, although most micropollutants were effectively removed at HRT of 18 h, the decreased sorption of 17α-ethinylestradiol, triclosan and 4-tert-octylphenol was observed.When HRT was shortened to 6 h, the biodegradation proportion declined, particularly for removals of acetaminophen, naproxen, gemifibrozil, pentachlorophenol, and biophenol A (< 75%).Although the proportion of sorption varied at different HRTs, it is notable that the percentage of sorption increased after HRT was changed to 6 h, especially for removals of some highly hydrophobic compounds (4-tert-ocytlphenol, triclosan) and some persistent compounds (fenprop, carbamazepine).

Removal of organic carbon, nutrient and the selected micropollutants of MBBR-MBR hybrid system
The MBBR-MBR system was able to achieve effective removals of DOC (94.7-96.7%),COD (95.0-98.1%)and NH 4 -N (83.1-92.1%),but low PO 4 -P (32.5-51.7 %) and TN reductions (46.8-68.7%).The initial MLSS and MLVSS in the MBR unit were similar to the concentrations of MLSS and MLVSS in MBBR tank, which gradually increased to 0.91 and 0.89 g/L (HRT of 24 h), 1.02 and 0.98 g/L (HRT of 18 h), 1.09 and 1.04 g/L (HRT of 12 h), and 1.05 and 0.96 g/L (HRT of 6 h) at the end of the experiment, respectively.
During the MBBR-MBR treatment, even the micropollutants removal varied significantly (11.0%-99.5%),most compounds were eliminated to large extents (> 70%) as the attached growth pattern provided a desirable environment for slow growing bacteria and facilitated a diverse biocoenosis.On one hand, the removal of micropollutants by MBR was limited due to low MLSS concentration and the utilization of 0.2 μm pore size MF membrane (Luo et al., 2015).On the other hand, both metronidazole and carbamazepine are nitrogen bearing compounds, and the enhanced removal could be the result of the infinte sludge retention time (SRT) applied in the MBR unit.Additionally, compounds like gemfibrozil, pentachlorophenol and diclofenac were most likely to be removed by biodegradation rather than by MF filtration.As to the fate of micropollutants removal in the MBBR-MBR, noting that the hydrophobic compounds were mostly detected at low concentration in biosolids at all HRTs, while some persistent hydrophilic compounds (e.g.carbamazepine) consistently presented very high solid phase concentration.

Membrane fouling analysis in MBR unit
Fig. 3 depicts the time course of TMP increase in the MBR unit.The TMP in the MBR unit reached up to 35 kPa, 36.5 kPa, 37.5 kPa and 36.5 kPa in 89 days, 74 days, 42 days and 20 days of operation at HRTs of 24, 18, 12, and 6 h, corresponding to fouling rates of 0.39 kPa/d, 0.49 kPa/d, 0.89 kPa/d, and 1.83 kPa/d, respectively.Therefore, a longer HRT (e.g.HRT of 24 h) could significantly mitigate membrane fouling when compared with a relatively short HRT (e.g.HRT of 6 h).Especially, the TMP value maintained less than 15 kPa for 60 days (HRT of 18 h) and 68 days (HRT of 24 h) of operation.Total fouling resistances (R T ) were 12.59 × 10 12 , 12.08 × 10 12 , 11.55 × 10 12 and 11.28 × 10 12 m -1 at the end of experiments under HRTs of 24, 18, 12 and 6 h, respectively.
The cake resistance (R C ) consisted of about 70% of R T at all HRTs, indicating that the main contributor to membrane fouling was formation of cake layer.Pore blocking resistance (R P ) was higher at shorter HRT of 6 h (1.92 × 10 12 m -1 , 12.0% of R T ) than that at longer HRT of 24 h (1.51 × 10 12 m -1 , 17.0% of R T ) and 18 h (1.90 × 10 12 m -1 , 15.7% of R T ).The zeta potential increased while RH decreased when the HRT was changed from 24 h (-15.6 ± 3.5 mV and 79.2 ± 4.2%, respectively) to 18 h (-15.7 ± 3.9 mV and 77.0 ± 4.0%, respectively) and further 6 h (-19.5 ± 1.4 mV and 72.0 ± 3.6%, respectively).Hydrophobic sludge flocs (high RH value) enhanced flocculation and limited their interaction with the hydrophilic membrane.The decline in negative charge on sludge floc surface due to the increased zeta potential could prompt the aggregation of flocs by charge neutralization mechanism, resulting in formation of larger flocs at longer HRTs of 24 and 18 h (15-40 μm) than those at HRT of 6 h (10-25 μm) (Ji et al., 2010).Thus, less fouling was observed at longer HRTs.
As shown in Fig. 4, the levels of EPS were similar at the beginning of all HRTs, and then gradually increased to 15.24 mg/L, 16.43 mg/L, 19.88 mg/L and 22.93 mg/L at the end of operation at HRTs of 24 h, 18 h, 12 h, and 6 h, respectively.The SMP concentration varied for different HRTs but showed minor variation under the same HRT.SMP concentrations were lower at HRTs of 24 and 18 h (4.89-6.32 and 4.93-9.96mg/L, respectively), whereas signficantly higher SMP concentations were observed at HRT of 6 h in the range of 16.60-29.68mg/L.Thus, the accumulation of SMP and EPS at shorter HRT aggravated membrane fouling (Guo et al., 2012).

Conclusion
In this study, at 4 different HRTs was investigated in terms of micropollutants removal and fouling control.The MBBR appeared to be an effective process for removing most of the selected micropollutants at all HRTs.At HRT of 18 h, the MBBR-MBR system was not only able to achieve comparable organic and nutrient removal, but also improve the removal of micropollutants compared to other biological treatment processes (i.e.activated sludge and MBR) and prolong the operative time due to efficient fouling reduction.