Study on Enhanced Biological Phosphorus Removal ...

Int. J. Environment and Pollution, Vol.

Study on Enhanced Biological Phosphorus Removal using Membrane Bioreactor at different Sludge Retention Times Zhichao Zhang and Xia Huang* State Key Joint Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science and Engineering, Tsinghua University, Beijing, 100084, China E-mail: [email protected] E-mail: [email protected] *Corresponding author Abstract: An enhanced biological phosphorus removal process coupled with membrane bioreactor was proposed. Four runs were operated at different Sludge Retention Times (SRTs) of 20, 30, 40 and 50 d. Although average phosphorus removal tended to drop with SRT, the total phosphorus removal over 85% could be still achieved if the SRT was maintained under 40 d. Phosphorus-content in the sludge increased with SRT, but has a limit of around 5.6–5.8% mgP/gMLSS. Extracellular Polymeric Substances (EPSs) is a pool for phosphorus stored. Phosphorus-content in the EPS per EPS increased as the SRT increased. Keywords: EBPR; enhanced biological phosphorus removal; MBR; membrane bioreactor; SRT; sludge retention time; EPS; extracellular polymeric substances. Reference to this paper should be made as follows: Zhang, Z. and Huang, X. (2011) ‘Study on Enhanced Biological Phosphorus Removal using Membrane Bioreactor at different Sludge Retention Times’, Int. J. Environment and Pollution, Vol. Biographical notes: Zhichao Zhang is a PhD candidate in Department of Environmental Science and Engineering, Tsinghua University, China, from where he got his Bachelor’s Degree. He has more than five years of experience in study of wastewater treatment and membrane bioreactors. Xia Huang received her PhD in Environmental Chemistry from Tokyo Institute of Technology, Japan, in 1988. She is currently a Full Professor at the Department of Environmental Science and Engineering, Tsinghua University, China. Her current research interests include membrane bioreactors for wastewater treatment and reuse, and microbial fuel cells for simultaneous wastewater purification and electricity generation.

Copyright © 2011 Inderscience Enterprises Ltd.

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Z. Zhang and X. Huang

Introduction

Membrane Bioreactor (MBR), a new technology for wastewater treatment, has been well known for removal of carbonaceous and nitrogenous pollutants (Chiemchaisri and Yamamoto, 2005). With the stringent phosphorus regulations, some types of modified MBR coupled with Enhanced Biological Phosphorus Removal (EBPR) have been investigated in the past few years, and good performance for phosphorus removal has been achieved (Geng et al., 2007; Vocks et al., 2005; Lesjean et al., 2004). Theoretically, EBPR is achieved by groups of special microorganisms known as Polyphosphate-Accumulation Organisms (PAOs) in the activated sludge, which can take up phosphate far more than they need under aerobic condition (Fuhs and Chen, 1975). This means that drawing off the excess sludge full of PAOs is the only way to remove phosphorus from the wastewater treatment system. The amount of excess sludge is directly affected by Sludge Retention Time (SRT). It is well known that SRT is one of the important factors that can affect the biomass’s state (Knoblock et al., 1994) and concentration (Sperandio et al., 2005) in an activated sludge system. Hence, SRT plays a very important role in phosphorus removal. Unfortunately, in the Conventional Activated Sludge process (CAS), drawing off the excess sludge is mainly determined by the stability of solid–liquid separation, not the need for phosphorus removal. Generally speaking, the SRT of CAS is confined in a small range of 5–15 days to avoid sludge bulking and losing (Delai Sun et al., 2007). But, in MBR the SRT could be controlled in a wide range without regard to the stability of solid–liquid separation due to the membrane filtration. In addition, it was proved that MBR in a longer SRT could have high removal efficiencies for COD and nitrogen (Chae and Shin, 2007; Han et al., 2005). But, the SRT could not be increased without limit, since decreasing the discharge of the excess sludge may do harm to phosphorus removal. Thus, the suitable SRT needs to meet two targets at the same time: one is high and stable phosphorus removal efficiency; the other is minimising the excess sludge to cut down the cost for sludge disposal. On the other hand, Extracellular Polymeric Substances (EPSs) fill and form the space between the cells as a protective layer for the cells against the harsh external environment (Flemming and Wingender, 2001), and bind the PAOs in the flocs to avoid being washed out of the activated sludge system (Martin et al., 2006). Cloete and Oosthuizen used Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS) to determine the phosphorus-content in EPS and revealed that EPS in activated sludge contained on average between 23% and 30% phosphorus while cells contained on average between 57% and 59% (Cloete and Oosthuizen, 2001; Oosthuizen and Cloete, 2001). In the research of Jing et al. (1992) the results also showed that a portion of the polyphosphate stored during aerobic phase was located outside the cytoplasmic membrane. If EPS was another pool to store phosphorus, the difference of phosphorus-content in the EPS may affect phosphorus removal at different SRTs. In this study, an Enhanced Biological Phosphorus Removal Process Coupled with Membrane Bioreactor (EBPR-MBR) was proposed. Four runs were operated at different SRTs of 20, 30, 40 and 50 d to investigate the impact of SRT on phosphorus removal in the EBPR-MBR. Phosphorus removal efficiences, Food-to-Microorganism

Study on Enhanced Biological Phosphorus Removal

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(F/M) ratio, Specific Oxygen Uptake Rate (SOUR), phosphorus-content in the sludge (TP-sludge), EPS amount and phosphorus-content in the EPS (TP-EPS) were investigated at different SRTs.

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Methods and materials

2.1 EBPR-MBR process The schematic diagram of EBPR-MBR process is shown in Figure 1. The total effective volume of EBPR-MBR was 21.7 L, including three reaction zones. Anaerobic (AN), anoxic (AX), and aerobic (O) zones were 4.7, 9.4, and 7.7 L, respectively. Mixed liquor R was recycled using peristaltic pumps (LONGER○BT00-100M) from AX to AN and from O to AX, at the ratios of 100% and 300–400% to the influent flow, respectively. The membrane filtration module (Korea Membrane Separation Co. LTD, Korea) was submerged in the aerobic zone, which was made of polyethylene hollow fibre, with an effective area of 0.2 m2 and a nominal pore size of 0.4 µm. The effluent was collected by R a diaphragm pump (PULSAFEEDER○KX1000-AAAA) in a constant flow rate but intermittent suction of 10 min ON and 2 min OFF to mitigate the membrane fouling. In aerobic zone, aeration was applied with a volumetric airflow rate of 15–20 m3/(m3 ⋅ h) and the Dissolved Oxygen (DO) level was in the range of 2.5–4 mg/L. The EBPR-MBR was seeded with activated sludge taken from the aeration tank of Qinghe municipal wastewater treatment plant of Beijing, China, where anaerobic–anoxic–aerobic process was adopted for enhanced nitrogen and phosphorus removal. The EBPR-MBR was fed with a synthetic domestic wastewater and operated for four runs at 25 ± 2°C. The Hydraulic Retention Time (HRT) was set at 9 h for all the runs, when only SRT was changed at 20, 30, 40 and 50 days, respectively. Figure 1

Schematic of the EBPR-MBR process

2.2 Analytical methods All samples were filtered through a 0.45 µm membrane filter. Chemical Oxygen Demand (COD), Total Nitrogen (TN), Ammonia Nitrogen (NH4+-N), Total Phosphorus (TP), Mixed Liquor Suspended Solid (MLSS) and Mixed Liquor Volatile Suspended Solid (MLVSS) were determined according to standard methods (APHA, 1992). Total Organic Carbon (TOC) was measured with a TOC analyser (TOC-5000, Shimadzu). SOUR representing sludge activity was measured according to the method as described by Surmacz-Gorska et al. (1996).

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2.3 EPS extraction The formaldehyde–NaOH extraction method of EPS was used in this study, which has been proved to be the most effective one without contamination by the intracellular materials of bacteria from activated sludge flocs (Comte et al., 2006; Liu and Fang, 2002). The extraction procedure described by Liu et al. was modified in this study (Liu and Fang, 2002). The raw sludge sample was centrifuged at 8000 g for 5 min, and then residues were recovered and resuspended in ultra-pure water. After that formaldehyde solution (36.4%, at 4°C for 1 h) was added, followed by the addition of NaOH solution (1 N; at 4°C for 1 h). Then, the EPS released to the supernatant from the sludge was separated from by centrifugation at 20,000 g for 20 min at 4°C. In this study, the time for EPS extraction using NaOH was modified to 1 h to minimise the risk of induced cell lysis.

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Results and discussion

3.1 Phosphorus removal at different SRTs Figure 2 illustrates the typical changes of TP concentration in the EBPR-MBR system at different SRTs. The common profiles of phosphorus concentration were founded in all runs. In the anaerobic zone, phosphorus was released, and TP concentration in supernatant rose to 40.3–46.8 mg/L, about 8–9 times of the TP concentration in influent. After that, in the anoxic zone the TP concentration decreased, which was due to the Denitrifying Phosphorus Removal Organisms (DPAOs) (Kuba et al., 1997) and the dilution of back flow from aerobic zone. In the aerobic zone, TP concentration kept decreasing to about 0.12–0.87 mg/L, and approaching the final effluent concentration of the system. Figure 2

Typical profiles of phosphorus concentration in the EBPR-MBR process at different SRTs

TP removal efficiencies in the EBPR-MBR process varied with SRTs, which is shown in Figure 3. TP removal efficiencies over 80% were achieved in all the runs. But, when SRT was increased from 20 d to 50 d, TP removal decreased from 93.4% to 83.0%. In addition, TP removal at SRT of 50 d with the lowest value as low as


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15.1% was more unstable than that at other shorter SRTs. These results suggested that excessively prolonged SRT did harm to phosphorus removal. The limitation of SRT in the EBPR-MBR process for ensuring phosphorus removal over 85% was around 40 days. Figure 3

Average phosphorus removal in the EBPR-MBR at different SRTs

The overall treatment performance of the EBPR-MBR process is summarised in Table 1. The average concentrations of COD, NH4+-N, and TN in the EBPR-MBR effluent were less than 50, 5.0 and 20.0 mg/L, respectively, in all experimental runs. Table 1

Overall treatment performance of the EBPR-MBR process Effluent

Parameter

Influent

Run 1 SRT = 20 d

COD

273 ± 74

40 ± 14

49 ± 17

42 ± 16

43 ± 15

TN

50.2 ± 6.0

12.90 ± 5.33

17.39 ± 2.89

18.58 ± 4.07

12.27 ± 5.92

NH4+-N

45.5 ± 10.2

1.96 ± 7.7

nd

nd

3.10 ± 4.58

TP

4.9 ± 1.25

0.29 ± 1.37

0.34 ± 0.14

0.58 ± 0.21

0.87 ± 1.28

pH

7.8 ± 0.4

–

–

–

–

25 ± 2

–

–

–

–

Temperature (°C)

Run 2 SRT = 30 d

Run 3 SRT = 40 d

Run 4 SRT = 50 d

All values in mg/L unless specified; nd: not determined.

3.2 Sludge activity change The F/M ratio, which represents the ratio of the COD loading to the MLSS concentration, was calculated since a specific F/M ratio can be obtained by changing the SRT, which may cause the changing of microbial activity. And, to find out the microbial activity change at different SRTs, SOURs were determined as an indicator for metabolic activity of the activated sludge. Figure 4 shows F/M ratio and SOUR changes at different SRTs. With the SRT increased from 20 to 50 days, the F/M ratios decreased from 0.137 kgCOD/(kgMLSS ⋅ d) to 0.077 kgCOD/(kgMLSS ⋅ d). And, the SOUR also dropped, but the change was not significant. This was because all the SOURs were at a

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relatively low level in MBR system with prolonged SRTs as reported in the literature (Delai Sun et al., 2007). According to Figure 4, the reduced F/M ratio caused only a slight decrease in microbial activity as SRT increased. Figure 4

Specific Oxygen Uptake Rates (SOUR) and Food-to-Microorganism (F/M) ratios at different SRTs

3.3 Phosphorus-content in sludge The phosphorus-content in the sludge (TP-sludge) was regarded as a very important parameter for evaluating the performance of phosphorus removal. TP-sludge was affected by the changing of SRT. As shown in Figure 5, TP-sludge increased from 4.5% to 5.8% gP/gMLSS as SRT increased. But, regardless of SRT change, TP-sludge had a limitation of about 5.6–5.8% gP/gMLSS. Figure 5

Practical TP-sludge and calculated TP-sludge at different SRTs

According to the P mass balance, in the steady-state conditions, equation (1) is fulfilled. TPi × Qi  TPe × Qe  − + TP -sludge × Qw  = 0 1000  1000 

(1)


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where TPi: TPe: TP-sludge: Qi: Q e: Qw:

TP in influent (mg/L) TP in effluent (mg/L) P-content in the sludge (gP/gMLSS) influent flow (L/d) effluent flow (L/d) excess sludge (g/d).

With TP data in the influent and excess sludge obtained from the experimental results, if the goal for P removal is to let the effluent TP concentration down to 0.5 mg/L, we can calculate the theoretical TP-sludge by equation (1). Figure 5 shows the theoretical TP-sludge data at different SRTs compared with the practical TP-sludge data. At SRTs of 20, 30 and 40 d, the practical TP-sludge data were similar with the theoretical TP-sludge data, suggesting that the EBPR system performed well. But, at SRT of 50 days, the practical average TP-sludge data cannot reach the theoretical TP-sludge data, suggesting that the EBPR system became unstable. This implied that the activated sludge has reached their maximum capacity to store phosphorus when SRT was over 40 d, and will not be able to uptake more phosphorus from the mixed liquor to ensure the effluent TP concentration down to 0.5 mg/L. Therefore, the decreased TP removal at excessively prolonged SRT may be related to limitation of TP-sludge.

3.4 Phosphorus-content in EPS As the important part of the sludge flocs, EPS also acted as a P pool like PAOs. Figure 6 shows that with the increase in SRT, phosphorus-content in the EPS (TP-EPS) increased from 20.81 mgP/gMLSS to 31.35 mgP/gMLSS. This implied the prolonged SRT promoted phosphorus stored in EPS. The amount of EPS was analysed by determining TOC content in the extracted EPS sample. The impact of SRT on EPS amount was investigated by some researchers in different conditions, but no common result was achieved. Some studies demonstrated that EPS increased as SRT increased (Masse et al., 2006; Chang and Lee, 1998), while others showed the opposite result (Ng and Hermanowicz, 2005; Lee et al., 2003). As illustrated in Figure 7, EPS amount did not changed obviously as SRT increased in the EBPR-MBR system. But, TP-EPS per EPS increased from 115.34 mgP/gTOC to 173.85 mgP/gTOC when SRT increased from 20 to 50 days. The increase in TP-EPS per EPS might be related with the composition change of EPS. According to the literatures, PAOs are more predominant at longer SRTs (Lee et al., 2007), and it has been realised that PAOs have EPS produced gene clusters, and the produced EPS may play an important role in EBPR (Martin et al., 2006). We speculated that at longer SRT the EPS was more produced by PAOs, which contained more phosphorus than the EPS produced by other groups of bacteria. But, the EPS composition change will slow down at longer SRT due to the small change of F/M ratio. This explained the small change of TP-EPS per EPS when SRT increases from 40 to 50 days. Further studies will be continued to understand the role of EPS in luxury phosphorus uptake process.

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Figure 6

Phosphorus-content in EPS at different SRTs

Figure 7

Amount of EPS and TP-EPS per EPS at different SRTs

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Conclusions

An EBPR-MBR was proposed. Four runs were operated at different SRTs of 20, 30, 40 and 50 d to investigate the impact of SRT on phosphorus removal in the EBPR-MBR process. Although with SRT increased, average phosphorus removal efficiencies tended to drop, the TP removal over 85% could be still achieved if the EBPR-MBR was operated with an SRT limitation of about 40 d. TP-sludge was found to increase from 4.5% to 5.8% mgP/gMLSS with SRT, but has a limitation of around 5.6–5.8% mgP/gMLSS. The decreased TP removal at excessively prolonged SRT might be related to limitation of TP-sludge. Furthermore, as another pool for phosphorus to be stored, TP-EPS increased from 20.81 mgP/gMLSS to 31.35 mgP/gMLSS as SRT increased. The significant difference was not found in the amount of EPS at different SRTs. But, the TP-EPS per EPS was found to increase as SRT prolonged. This may be attributed to the composition change of EPS as SRT changes. It was presumed that at longer SRT the EPS was more produced by PAOs, which contained more phosphorus than the EPS produced by other groups of bacteria. Further study will focus on the role of EPS on phosphorus removal along the phosphorus luxury uptake process.


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Acknowledgement This work was financially supported by the National Science Fund for Distinguished Young Scholar of China (No. 50725827) and special fund of State Key Joint Laboratory of ESPC.

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