Effect of ciprofloxacin dosages on the performance of sponge membrane bioreactor treating hospital wastewater

Highlights

• Ciprofloxacin affected treatment performance and fouling propensity of Sponge-MBR.

• COD removal was 94–98% under the added CIP dosage less than 100 µg L⁻¹.

• High CIP dosages enhanced nitrification but significantly inhibited denitrification.

• Floc size and membrane fouling decreased at high CIP dosages of 100–200 µg L⁻¹.

Abstract

This study aimed to evaluate treatment performance and membrane fouling of a lab-scale Sponge-MBR under the added ciprofloxacin (CIP) dosages (20; 50; 100 and 200 µg L⁻¹) treating hospital wastewater. The results showed that Sponge-MBR exhibited effective removal of COD (94–98%) during the operation period despite increment of CIP concentrations from 20 to 200 µg L⁻¹. The applied CIP dosage of 200 µg L⁻¹ caused an inhibition of microorganisms in sponges, i.e. significant reduction of the attached biomass and a decrease in the size of suspended flocs. Moreover, this led to deteriorating the denitrification rate to 3–12% compared to 35% at the other lower CIP dosages. Importantly, Sponge-MBR reinforced the stability of CIP removal at various added CIP dosages (permeate of below 13 µg L⁻¹). Additionally, the fouling rate at CIP dosage of 200 µg L⁻¹ was 30.6 times lower compared to the control condition (no added CIP dosage).

Introduction

In recent years, the occurrence of antimicrobials has been emerged as a critical problem due to their risk of causing the undesirable ecosystem and human health (Kümmerer, 2009). Antimicrobials, namely antibiotics, are one of the most important drugs to prevent and treat infectious diseases (Tran et al., 2016). These substances were presented at various concentrations in the aquatic environment in Vietnam (i.e., sulfamethoxazole (2.5 ± 1.9 μg L⁻¹), norfloxacin (9.6 ± 9.8 μg L⁻¹), ciprofloxacin (5.3 ± 4.8 μg L⁻¹), ofloxacin (10.9 ± 8.1 μg L⁻¹), erythromycin (1.2 ± 1.2 μg L⁻¹), tetracycline (0.1 ± 0.0 μg L⁻¹), and trimethoprim (1.0 ± 0.9 μg L⁻¹) (Vo et al., 2016). Most wastewater treatment plants (WWTPs) were not intentionally designed for antibiotic removal (Luo et al., 2014). Especially, there were many antibiotic groups such as Sulfonamide, Fluoroquinolone and Macrolide found in real hospital wastewater. The most widely prescribed fluoroquinolone antibiotic is ciprofloxacin (CIP) (Santos et al., 2013). For instance, CIP was found in hospital wastewaters in Viet Nam, with the concentrations being 4–53 times higher than those in other Asian countries such as China and Australia (Vo et al., 2016). As well known, a recent study indicated that CIP was one of the major antibiotics registered for production in the 5-year (2008–2013) in Viet Nam (Thai et al., 2018). Their investigation reported that the high concentration of CIP (41 µg L⁻¹) was detected from the hospital wastewater. Another one, some studies showed that CIP was detected at the different concentration ranges, namely 7.9–87.3 µg L⁻¹ (Lien et al., 2016), 1.1–44 µg L⁻¹ in Viet Nam (Duong et al., 2008), 3.6–101 µg L⁻¹ in Sweden (Lindberg et al., 2007), 0.7–125 µg L⁻¹ in Germany (Hartmann et al., 1998), and 32–99 µg L⁻¹ in Brazil (Martins et al., 2008). Regarding the CIP removal, it has been previously reported with poor degradation of 32% after 48 h corresponded with the influent of 96.7 µg L⁻¹ (Li and Zhang, 2010). Another study indicated that despite a low concentration of 2.2 µg L⁻¹ in feed water the CIP removal was still negative when applied the conventional activated sludge (CAS) (Blair et al., 2015).

Membrane bioreactors (MBR) has been widely known as a potential technology to advance the water sustainability. The MBR system is a combination of a suspended growth in a bioreactor with a filtration on a porous membrane unit, which allows an operation at high biomass retention, microbial diversity and brings a promise for the degradation of micro-pollutants thus improving treated water quality (Cheng et al., 2018). Compared to CAS process, MBR exhibited an enhanced elimination of several pharmaceutical residues e.g., Sulfonamides, Macrolides, Tetracyclines, Indomethacin, Diclofenac, Propyphenazone, Pravastatin and Gemfibrozil (Radjenović et al., 2009). The long sludge age maintained in MBR helps to improve the removal of slowly degradable antibiotics. Furthermore, the longer sludge retention time (SRT) favored for the growth of nitrifying bacteria which helps to enhance degradation of the antibiotics (e.g. Ofloxacin, Sulfamethoxazole, Trimethoprim, Erythromycin, Roxithromycin) (Tran et al., 2016). In literature, the CIP removal was investigated at different feed concentration as well as various technologies used from the previous works. In detail, conventional MBR, anoxic–oxic MBR and aerobic granular sludge MBR showed efficiency about 51 ± 13% (Kovalova et al., 2012); 58.6 ± 6.2% (Hamjinda et al., 2017) and 15.8 ± 5.1% (Zhao et al., 2014), corresponding with the feed concentration of 2.33–3.75 µg L⁻¹, 17.92–46.04 µg L⁻¹ and 50.29–54.19 µg L⁻¹, respectively. Kim et al. (2014) studied on the real wastewater with the CIP concentration of 1.2–1.38 µg L⁻¹ based on the MBR process; their findings indicated that the CIP removal referred to the strong sorption to sludge (i.e., 98% removal). Another one, Dorival-García et al. (2013) reported that an improvement of the CIP biodegradation efficiency could obtain about 52.8% in the MBR system when operating the appropriate conditions i.e., SRT of 30 days, the temperature of 38 °C and MLSS of 15,000 mg L⁻¹. Another hand, for the key drawbacks of MBR application this system could not improve the total nitrogen removal due to a lack of anoxic zone whilst the membrane fouling became a crucial challenging for the wide practical application. Clearly, membrane fouling in MBR is a major obstacle that needs to be resolved (Chen et al., 2016, Teng et al., 2018). A study of Shi et al. (2017) reviewed the important role of extracellular polymeric substances (EPS) in controlling fouling in the MBR process. Their findings showed that concentration and characteristic of EPS are two vital factors that determine the degree and severity of fouling condition. Meanwhile, membrane fouling with the presence of pharmaceuticals/antibiotics has been paid to attention to a topic of discussion recently for understanding and the fouling control. The past studies revealed that the presence of pharmaceutical products caused an effect on the membrane fouling by inducing the microbial effects on activated sludge or changing the character of soluble microbial products (SMPs) and extracellular polymeric substances (EPS) (Li et al., 2015). However, for particular CIP a little attention has been directed to its role in influencing membrane fouling of the MBR process. A study of (Meng et al., 2012) indicated a role of certain CIP on the fouling propensity in the MBR. Their findings indicated that the fouling rate exposed the CIP of 1000 µg L⁻¹ was much lower compared to that of MBR control, i.e. the absence of CIP, thereby making a positive role in MBR fouling control.

To enhance the nitrogen removal as well as the fouling control, sponge carrier, thus, has been introduced as an ideal attached growth media coupling with the MBR process (Ngo et al., 2006). As well known, sponge-MBR was demonstrated with less fouling rather than the conventional MBR i.e. 10–40 times reduction (Nguyen et al., 2016). Another one, sponge-MBR, a strong candidate, gave a better alternative to conventional MBR with an enhancement of total nitrogen removal (Thanh et al., 2013). For antibiotic removal, Nguyen et al. (2017) recently indicated that the CIP removal in Sponge-MBR (i.e., hollow fiber membrane module) reached 70% treating real hospital wastewater i.e. low CIP concentration of 23.84 µg L⁻¹. As mentioned beforehand, however, the concentration of CIP in wastewater was dependent on area, i.e., urban and rural or even country (125 µg L⁻¹ in Germany). For example, Lien et al. (2016) reported the highest CIP concentration was 40.4 µg L⁻¹ and 87.3 µg L⁻¹ detected at the urban hospitals and the rural hospitals in Vietnam, respectively. Although Sponge-MBR has brought the positive removal and the fouling reduction when it was investigated with real hospital wastewater, i.e. low CIP concentration, in some cases CIP was detected with the high concentration as aforementioned. Moreover, it is noted that a systematic investigation of Sponge-MBR under different CIP gradients has not been sufficiently reported in the literature yet; therefore this need to be clarified. In this study, as a low CIP concentration of 12.85 ± 10.9 µg L⁻¹ in hospital wastewater was studied, we added different CIP dosages (20; 50; 100; 200 µg L⁻¹) to evaluate the change of biomass (in sponge carrier via SEM visualization, suspended sludge via particle size distribution (PSD)), the performance (COD, nitrogen, CIP removal) and the fouling propensity (correlation between PSD and TMP data). To be the best our knowledge, this is a first research to clearly make that point.