Fouling in a novel airlift oxidation ditch membrane bioreactor (AOXMBR) at different high organic loading rate
Highlights
► A novel airlift oxidation ditch membrane bioreactor was introduced for wastewater treatment. ► Biofilm formation has a decreasing effect on membrane pore blocking and fouling. ► Reversible polysaccharide deposit due to cell lysis malfunctioning membrane performance. ► The viability of activated sludge affected on membrane fouling in MBR systems.
Introduction
The membrane bioreactor (MBR) has been widely used to treat both urban and industrial wastewater [1], [2], [3], [4]. The advantage of an MBR, compared to a conventional activated sludge process, is that it has a small footprint and produces a highly quality effluent [5], [6], [7]. A new scheme which combines the MBR with an airlift reactor and oxidation ditch is now used for this process and has been named AOXMBR. The advantages of AOXMBR are sufficient propulsion, efficient aeration, long hydraulic retention time and complete mixing. On the other hand, the membrane separation step permits high sludge retention time with the possibility of increasing OLRs at low operation costs compared to other processes [5], [6], [7].
The major limitation of AOXMBR, which it shares with MBR, is membrane fouling which induces membrane flux decline or trans-membrane pressure (TMP) increase [8], [9], [10]. There are different possible factors influencing fouling such as; operating conditions (hydraulic conditions, aeration) [11], [12], [13], sludge characteristics including floc size [9], [14], [15], [16], [17], [18], [19], Mixed Liquor Suspended Solids (MLSS) concentration [14], [15], [16], [17], [18], [20], [21], [22], [23], viscosity [15], [17], [21], soluble and bound exopolymeric substances [9], [15], [23] and membrane materials [24], [25]. These parameters are related to MBR operating conditions such as sludge retention time (SRT) and OLR which affect the mixed liquor characteristics and induce changes in the physiological state of microorganisms [9], [15], [22], [26], [27], [28]. For more than a decade, many studies have tried to investigate the respective role of each fraction of the biofluid in terms of filtration resistance [9], [12], [15], [22], [24], [28], [29], [30].
Membrane fouling consists of two different types: reversible and irreversible. In the membrane filtration literature, reversible fouling is considered to be the loosely bounded fraction of the membrane foulant which can be removed by physical means, such as relaxation and back-washing. Accumulation of macromolecules and solids on the membrane surface are usually considered as the reversible fouling. In contrast, irreversible fouling is caused by strong adherence to the membrane such as adsorption, gel layer formation or biofilm formation, which can only be removed by chemical means [32], [33], [34], [35], [36], [37]. A decrease of soluble microbial products (SMP) or extracellular polymeric substances (EPS) which has accumulated both in the mixed liquor and on the membrane surface, [8], [12], [20], [30], [31] generally causes a decrease in both reversible and irreversible fouling propensity.
Researchers have discovered that in aerobic reactors, the production of SMP can be reduced by adjusting the operating parameters such as; SRT [38], [39], hydraulic retention time (HRT) [40], OLR [41], [42], [43] and/or influent concentration [44]. Relatively short SRT and HRT, high OLRs and high influent COD in the aerobic reactors usually cause accumulation of SMP [45]. On the other hand, many studies have examined the effect of OLR on the COD removal efficiency under constant aeration rates or the dissolved oxygen (DO) concentration. Results obtained with various wastewaters showed that an increase in the OLRs could cause a slight decrease of COD removal efficiency [46]. It has been reported that with increase in OLR, the consequent rise in the biomass content, colloidal hydrophobicity and mean particle diameter caused the membrane to foul more rapidly [10].
High OLR and high influent COD usually caused significant accumulation of SMP. However, previous research has been usually limited to low and medium OLR and the biological viability has not been taken into consideration. There is a lack of information about how the quantity and quality of the activated sludge can change during transient OLR conditions. Both the efficiency and control of a biological wastewater treatment process depends on the quantity and quality of biomass that is directly affected by the nature and concentration of the influent wastewater when other operational conditions are defined [47], [48], [49]. The use of on-line sensors that provide information on key process parameters appears crucial for the effective treatment of wastewater.
Cell suspension dielectric permittivity could also be an indication of viable biomass concentration. Dielectric permittivity measurements have been applied to observe changes in the physiological state of several microorganisms [50], [51], [52], [53], [54]. The measurement of dielectric properties of microbial cell suspensions is based on the ability of the biological cells to accumulate charges when they are exposed to an electrical field [53]. Dead cells, whose plasma membrane is permeabilized, are not detected because they do not polarise and therefore not detected [51], [52], [53], [54], [55].
Despite numerous studies, knowledge on the fouling mechanisms and fouling compounds is still a key issue. Furthermore, as recently reviewed by Drews [9], results are still controversial due to the large variety of experimental methods (lab or full scale experiments) and due to the definition of the relevant fouling fraction (biopolymers, SMP and transparent exopolymer particles) [15]. Also, studies have been limited to measuring quantitative and qualitative changes of the activated sludge and have not taken into account the effect of system viability on fouling in MBR.
The purpose of this study was to investigate the influence of the variation in OLR and the viability of activated sludge on membrane fouling in the novel AOXMBR. Other objectives of our research focused mainly on the development of the secondary membrane formation as a dynamic layer during operation at high OLRs in this system. In addition, this study aims to find whether the OLRs and the viability affect the membrane fouling.
Section snippets
Experimental setup and operating conditions
The activated sludge from the MBR wastewater treatment plant of Nime (France) was used as the inoculums for the AOXMBR. The AOXMBR setup with a total volume of 65 L (0.8 m × 0.4 m × 0.5 m length × width × height) is shown in Fig. 1. It is composed of three distinct sections: a 65-litre oxidation ditch, a submerged flat sheet membrane module (Microdyn-Nadir GmbH, Germany) and an air injection system placed under the membrane module between two baffles acting as the riser (airlift) of AOXMBR [59]. Some of
Suspended solid
Fig. 2 shows the variations of MLSS, MLVSS and TS concentrations during the experimental period, three phases have been specified on this figure. Phase 1; could be considered the start-up period during which OLR was gradually increased from 0 to 5 kg COD m−3 d−1.
It could be seen that the MLSS and TS were increased during the 1st phase except for the 20th day where the activated sludge was accidently withdrawn from the system. In phase 1, the experiment was followed until the steady-state condition
Conclusions
More information on the state and nature of fouling in MBR could be useful to improve COD removal efficiency and effective operational of MBR. The role of the membrane barrier appeared significant when the COD in supernatant was high, confirming the great interest of MBR in comparison with conventional activated sludge that is unable to retain such colloid matters by a simple secondary settling step.
The analyses of membrane fouling highlighted the different fouling mechanisms (cake layer, pore
Acknowledgement
Authors would like to give special thanks to Prof. Christian Drakides, Dr. Babak Bonakdarpour, Prof. Samuel Elmaleh and Ms. J. Wilson for their useful comments. This work was partially supported by the Centre for International Research and Collaboration (CIRC) and the French Embassy in Tehran.
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