Treatment of Municipal Wastewater by Anaerobic Membrane Bioreactor Technology

103

Hale Özgün, MSc

Hale Özgün TU Delft Istanbul Technical University

Treatment of Municipal

Wastewater by Anaerobic

Membrane Bioreactor Technology

Introduction

Reclamation and reuse of wastewater for various purposes such as landscape and agricultural irrigation are increasingly recognized as essential strategies in the world, especially for the areas suffering from water scarcity. Wastewater treatment and reuse have two major advantages including the reduction of the environment contamination and hence the health risks and saving of the huge freshwater amounts.

Municipal wastewater is one of the main alternative sources for reclamation and reuse. Globally a major demand exists for more efficient municipal wastewater treatment with higher effluent quality and reduced energy consumption which will lead to a cost effective treatment system with effluent reuse for irrigation. Throughout different alternatives, anaerobic treatment systems have major potentials to meet the market demand with biogas production and almost no energy requirement. Anaerobic treatment is an energy generating process, in contrast to aerobic systems that generally demand a high energy input for aeration purposes. However, they have several bottlenecks such as the production of effluents with lower quality in comparison to aerobic treatment systems, retardation of sludge granulation, problematic sludge granule stability, limited feasibility in the treatment of complex wastewaters such as municipal wastewater at low temperatures (<15o_{C) and no biological nitrogen removal. Among anaerobic} treatment technologies, anaerobic membrane bioreactor (AnMBR) system is a promising technology as a means to retain all of the biomass in the reactor more effectively and achieve high-efficient solid-liquid separation producing superior effluent quality.

Various applications of AnMBRs based on different reactor types can be found in literature for the treatment of municipal wastewater. Membranes can be coupled to miscellaneous anaerobic reactor types such as completely stirred tank reactors (CSTR) (Lin et al., 2011; Martinez-Sosa et al., 2011), upflow anaerobic sludge blanket (UASB) (Kataoka et al., 1992; An et al., 2009) and expanded granular sludge bed (EGSB) reactors (Chu et al., 2005), etc. in different configurations.

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Treatment of Municipal Wastewater by Anaerobic Membrane Bioreactor Technology

In the proposed research, an innovative configuration of AnMBR, named AnMBR for Aqua Cleaning and Energy Recovery (A-Racer), will be investigated in order to solve the bottlenecks of AnMBRs and enable effluent reuse in irrigation and energy recovery (Figure 1). The system consists of UASB reactor integrated with a parallel operating digester, which can be operated at any required process temperature.

The performance of UASB reactors at low temperature climates (5-20o_{C) is highly limited by} the hydrolysis of entrapped solids, which accumulate in the sludge bed. This situation might then limit the growth of methanogens resulting in a poor soluble chemical oxygen demand (COD) removal and thus deterioration of sludge stability. In A-Racer system, the solids which are entrapped in the sludge bed, are conveyed to a digester operated at optimal conditions where they can be stabilized and the digested sludge is re-circulated to the UASB reactor to improve its methanogenic capacity.

In this study, the applicability of an innovative configuration of anaerobic membrane bioreactor will be investigated in order to produce pathogen free but nutrient rich effluent for re-use in irrigation and enable energy recovery. With the realization of this study flux enhancement in AnMBR technology by controlling the total solids load will be the major output of the program bringing anaerobic membrane bioreactor technology to a higher level of understanding.

Based on this general objective, one of the first specific aims is to identify the optimum upflow velocity that will result in an effluent with higher filterability values for the case of membrane coupled UASB systems. Within this concept, there will be two specific aims including the identification of the optimum upflow velocity that will result in an effluent with higher filterability values.

Methodology

The system consisted of UASB and digester reactors having 7 L and 4 L effective volume, respectively. Lab-scale reactors are equipped mainly with feed and vacuum pumps, pH and temperature sensors, pressure sensors, water baths, and gas meters. A three-phase separator was installed at the top of the reactor to separate the biogas from the mixed liquor as well as to retain suspended particles in the reactor. The reactors are controlled by computer running LabView software and reactor pH, TMP, membrane flux, biogas flow are measured on-line. Pictures of lab-scale set-ups are given in Figure 2.

The reactors will be coupled with Pentair ultrafiltration membranes (Figure 3). The membrane module is designed in tubular geometry. The module has filtering area of ~1100 cm2_.

COD and TSS were measured according to Standard Methods (APHA, 2005). Each analysis was performed in triplicate. The samples for soluble COD measurement were filtered through a 0.45

Figure 1 - The schematic representation of A-Racer configuration.

Figure 2 - Lab-scale anaerobic membrane bioreactor system.

105

Hale Özgün, MSc

micrometer fiberglass filter (Whatman, Spartan 30). The turbidity was measured by HACH 2100N Turbidimeter. At Delft University of Technology Roorda (2004) has developed Specific Ultrafiltration Resistance for evaluation of filtration characteristics during dead-end ultrafiltration of wastewater treatment plant effluent. A specific ultrafiltration resistance (SUR) setup was applied to test the filterability of the effluent (Figure 4).

Results

After a 40 day acclimation period operating the UASB at 0.6 m/h, the measuring period was started at day 0 with upflow velocity of 1.2 m/h. After the decrease of the upflow velocity from 1.2 m/h to 0.6 m/h at day 34, a gradual decrease was observed

in total COD concentration to the range of 130-160 mg/L. However, soluble COD concentrations were found to be similar at both upflow velocities. At day 88 the upflow velocity was restored to 1.2 m/h in order to confirm whether the effect of upflow velocity on total COD concentration is reproducible. During this period, an increase in total effluent COD concentration was observed again, similar to the first period of 1.2 m/h. In order to verify this assumption, TSS and turbidity were measured in both supernatant and effluent of the UASB reactor. There was no significant change in effluent TSS concentrations as a result of changes in upflow velocity. However, the average turbidities were found to be 55±10 NTU and 35±10 NTU at the upflow velocities of 1.2 m/h and 0.6 m/h, respectively. Variations observed in

Figure 3 - Membrane module.

Figure 4 - Experimental set-up for the SUR measurement (A: demineralized water vessel, B: wastewater vessel, C: magnetic stirrer, D: three-way valve, E; H; I; K: valves, F: flow meter, G: membrane module, J: computer with measuring program, L: permeate vessel, P: pressure sensor).

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effluent turbidity at different upflow velocities seem contradictory with TSS results which do not show significant differences between each upflow velocity. Likely, this is attributable to the washout of colloidal particles from the system instead of suspended particles, since turbidity is an indicator of both suspended and colloidal particles. SUR of samples obtained at different upflow velocities was measured (Figure 5). Filterability deterioration at higher upflow velocity was related to the presence of colloidal material during the operation at 1.2 m/h.

Conclusion

Upflow velocity can be considered as an important parameter which affects the characteristics of the effluent especially in terms of physical characteristics. Upflow velocity of 1.2 m/h was not as appropriate as 0.6 m/h since high shear force in operation of 1.2 m/h allows the washout of colloidal materials. According to these results, optimum upflow velocity was selected in order to proceed with the membrane addition.

Acknowledgements

This research is conducted as part of the A-Racer project, with Pentair, Saxion, TU Delft, Water Board Regge & Dinkel as partners. The project (IWA10007) is partly funded by the Dutch Government via AgentschapNL under the InnoWator program.

References

1. An, Y.Y., Yang, F.L., Bucciali, B., and Wong, F.S. (2009). Municipal wastewater treatment using a UASB coupled with cross-flow membrane filtration. Journal of Environmental Engineering 135(2), 86-91.

2. APHA, 2005. Standard Methods for Examination of Water and Wastewater, 21st ed., American Public Health Association, Washington, USA. 3. Chu, L.B., Yang, F.L., and Zhang, X.W. (2005).

Anaerobic treatment of domestic wastewater in a membrane-coupled expanded granular sludge bed (EGSB) reactor under moderate to low temperature. Process Biochemistry 40, 1063-1070. 4. Kataoka, N., Tokiwa, Y., Tanaka, Y., Fujiki, K.,

Taroda, H., and Takeda, K. (1992). Examination of bacterial characteristics of anaerobic membrane bioreactors in three pilot-scale plants for treating low-strength wastewater by application of the colony-forming-curve analysis method. Applied and Environmental Microbiology 58(9), 2751-2757. 5. Lin, H., Chen, J., Wang, F., Ding, L., and Hong,

H. (2011). Feasibility evaluation of submerged anaerobic membrane bioreactor for municipal secondary wastewater treatment. Desalination 280, 120-126.

6. Martinez-Sosa, D., Helmreich, B., Netter, T., Paris, S., Bischof F., and Horn, H. (2011). Anaerobic submerged membrane bioreactor (AnSMBR) for municipal wastewater treatment under mesophilic and psychrophilic temperature conditions. Bioresource Technology 102 10377-10385.

7. Roorda. J. H. (2004) Filtration characteristics in dead-end ultrafiltration of wwtp-effluent. PhD thesis, Department of Sanitary Engineering, Technical University of Delft, Delft, The Netherlands, 2004.