A triple fouling layers perspective on evaluation of membrane fouling under different scenarios of membrane bioreactor operation
© Pourabdollah et al.; licensee BioMed Central Ltd. 2014
Received: 16 February 2014
Accepted: 28 May 2014
Published: 6 June 2014
One of the main factors affecting membrane fouling in MBRs is operational conditions. In this study the influence of aeration rate, filtration mode, and SRT on hollow fiber membrane fouling was investigated using a triple fouling layers perspective. The sludge microbial population distribution was also determined by PCR method. Through various applied operational scenarios the optimal conditions were: aeration rate of 15 LPM; relaxation mode with 40s duration and 8 min. interval; and SRT of 30 days. The similarity between SMP variations in triple fouling layers with its corresponding hydraulic resistance confirmed the effect of SMP on membrane fouling. Among three fouling fractions, the upper (rinsed) layer found to have the most effect on membrane fouling which implies the critical role of aeration, but as for multilateral effects of aeration, the optimal aeration rate should be determined more precisely. Relaxation interval was more effective than its duration for fouling control. SRT variations in addition to influencing the amount of SMP, also affect on the structure of these material. At longer SRTs (20, 30 days) a greater percentage of SMP could penetrate into the membrane pores and for shorter SRTs they accumulate more on membrane surface. Results showed that there is a very good correlation between total hydraulic resistance (Log R) and protein to carbohydrate ratio at the rinsed layer (P1/C1). Considering significant effects of aeration and SRT conditions on this ratio (according to data), it is very determinative to apply the optimal aeration and SRT conditions.
Membrane bioreactors (MBRs) are widely used to treat municipal and industrial wastewaters. Solids’ separation by membrane provides unique advantages over conventional activated sludge (CAS) systems including a smaller footprint, less sludge production and better effluent quality[2–4].
Membrane fouling remains a major operational issue leading to higher operational costs compared to current treatment technologies. The main factors affecting membrane fouling include biomass characteristics (MLSS concentration, particle size distribution, concentrations of microbial products), operational conditions (aeration intensity, hydraulic retention time (HRT), solid retention time (SRT), operating flux, backwashing and chemical cleaning), and membrane physicochemical characteristics (pore size, surface characteristics, and chemical composition).[1, 5–9].
Exocellular materials excreted from cells are considered important membrane foulants[10–13]. Extracellular polymeric substances (EPS) and soluble microbial products (SMP) contain carbohydrates and proteins, and humic substances, uronic acids and nucleic acids are present in smaller quantities. According to study of Zhang et al., the initial stage of fouling includes passive adsorption of SMP and colloids on the membrane surface and initial pore blocking by feed particulates. After this stage, the membrane surface is expected to be mostly covered by SMP, promoting attachment of biomass particulate and colloidal material during next stage. The second stage consists of further pore blocking, biofilm growth due to accumulation of SMP and colloids, and cake formation by EPS bound within the biomaterials.
Previous studies have shown that humic and low molecular weight substances pass the membrane and therefore are not responsible for fouling, while polysaccharides (carbohydrates), proteins and organic colloids are retained almost completely. Comprehensive review by Le-Clech et al. indicates a direct relationship between soluble carbohydrates and fouling rate with a significant role played by the protein fraction. Effective operational parameters in decreasing importance include: aeration, sludge waste (which controls SRT), filtration mode, membrane cleaning, and imposed flux. Aeration has three major roles: providing oxygen, maintaining the activated sludge in suspension and mitigating fouling and SRT affects biological parameters like MLSS, SMP and eEPS concentrations.
According to the study of Wu et al., the rinsed fraction contains sludge flocs and biopolymers, originating from the cake layer on the membrane surface. The backwashed fraction is mainly composed of materials which block the membrane pores, while the desorbed fraction represents irreversible fouling. The resistance of the rinsed fraction contributed more than that of the desorbed fraction and had significant correlations with transmembrane pressure (TMP). Wu et al. also indicated that characteristics of the foulants on membrane surface were similar to those in the mixed liquor in bioreactor.
Metzger et al. mentioned that three different fractions have different compositions and consequences on the fouling resistance. The upper cake layer consists predominantly of loosely bound biomass flocs and attached SMP. The layer is characterized by a low specific biopolymer resistance and a high permeability. That is assumed to have a porous structure allowing water to permeate easily. An intermediate layer, is composed equally by SMP and biomass flocs or EPS clusters and features a higher specific biopolymer resistance than the upper layer. Soluble carbohydrates are accumulate in this layer. The layer has a denser matrix, is expected to fill up the pores and act like a gel-like layer between the lower membrane fouling layer and the upper cake layer. The desorbed layer is composed predominantly of SMP. This layer is intimately attached to the membrane and forms a total coverage of the surface and its pores. Compared to the two other layers, it contains a higher concentration of soluble proteins strongly bound to the membrane. It features a very dense structure and has a very low permeability, resulting in the highest specific biopolymer resistance.
Considering the results of these studies, the importance of measuring triple fouling layers is better understood. By this measurement, firstly the most effective layer on membrane fouling and thus its relevant control method (operational) could be determined. Also determination of effective components of SMP (protein, carbohydrate) at each layer is an important parameter for improving the interaction between these components and membrane surface, such as improving physical and chemical structure of the membrane and so on.
Differences seen between the results of these studies, could be due to different applied operational conditions (SRT, aeration rate …). So Further research is needed to highlight the impact of other operating conditions like aeration rate and SRT. In this study, the effects of different operating conditions (aeration rate, SRT, and filtration modes) on the membrane fouling were investigated by fractionating the triple fouling layers. The correlation analysis was used to find the more important parameter (protein, carbohydrate) affecting transmembrane pressure (TMP) variation, in each layer, and identify the effect of each layer on the total fouling resistance. The optimal operating condition is also determined.
Materials and methods
Experimental set up and tests
The bioreactor temperature was maintained at 29 ± 0.5°C during the experiments using an electric heater. TMP was continuously monitored by pressure transmitter.
The bioreactor was originally seeded with sludge collected from a local municipal wastewater treatment plant. The microbial population distribution of sludge was also determined by PCR (Polymerase chain reaction) method. In this regard, first a DNA extraction was done by QIAamp DNA Mini Kit (Qiagen, Hilden, Germany). Then the DNA was quantified using an ultra violet spectrophotometer. After executing PCR stages, the products was electrophoresed by agarose gel electrophoresis, stained with ethidium bromide, and documented using a gel documentation system. ABI 3730X capillary sequencer was used for DNA sequencing and finally the sequences were analysed by GeneRunner program. The distribution of bacteria’s identified were: Enterobacter amnigenus (83.5%), Bacillus thuringiensis (12.6%), Aeromonas hydrophila (3.9%).
The bioreactor was fed with synthetic wastewater. Typical conditions consisted of influent COD of about 450 mg/L, TN of about 24.6 mg/L, and TP of about 5.1 mg/L (C:N:P ≈ 100:5:1).
Before running the experiments, the intrinsic resistance of the membrane (Rm) was determined by clean water test. After an adaptation stage, each experiment was executed in a 24 h period. For each series of scenarios, the optimal operational parameters of the previous series were applied. The first series of scenarios (aeration) were executed under typical operating conditions (SRT = 30 days, backwash duration and interval = 40 s and 8 min.).
Where J is the flux and Rtotal is the resistance after 24 h. TMP and μ are the trans-membrane pressure and the dynamic viscosity of permeate (water), respectively.
After each filtration period of 24 h, the fouled membrane was cleaned following a three-step protocol: (1) rinsed with 200 mL distilled water (2) backwashed with 1000 mL distilled water (3) desorbed in 1000 mL NaOH solution (pH 12) for 24 h. By applying this specific protocol, the fouling layer could be separated into three fractions, i.e. rinsed, backwashed and desorbed. After each step, a suction test was applied again to measure the resistance of each fraction and the three cleaning solutions were analyzed in terms of carbohydrate and protein concentration (SMP calculated as the sum of carbohydrate and protein concentrations).
Protein concentration was measured by the Lowry method, modified by Peterson using bovine serum albumin as standard. Samples were measured at 720 nm[22, 23]. Polysaccharide concentration was measured by the phenol–sulfuric acid methods with glucose used as standard. Samples were analyzed at 490 nm.
Three different conditions for aeration rate (A1 = 0.5, A2 = 1.2, and A3 = 4 m3/m2.h or A1 = 2.5, A2 = 6, and A3 = 15 lit/min (LPM)) were selected based on typical values mentioned in a literature review on aeration of MBRs. After 2 days of adaptation for each scenario, the 24 h test was done.
Conditions applied in the filtration experiments
After completing these experiments and selecting the optimal filtration mode, the SRT scenarios were initiated. Three different solid retention times including S1 = 10 days, S2 = 20 days, and S3 = 30 days were applied, based on typical SRT values in previous studies[4, 26, 27]. The optimal aeration rate and filtration mode determined in the prior experiments were used.
Results and discussions
In addition, backwash scenarios with shorter intervals (240 s) showed better performance in fouling control than longer intervals (480 s). But in relaxation scenarios, longer intervals (480 s) exhibited a more efficient effect on TMP control than the shorter interval (240 s).
But totally, the similarity observed between SMP and R variations, confirms the relationship between fouling and SMP content in MBR system (according to previous studies).
Significant correlations (Pearson coef.) between some parameters
0.885 (very good)
0.899 (very good)
0.859 (very good)
0.898 (very good)
0.859 (very good)
0.898 (very good)
As seen, total hydraulic resistance (Log R) showed the strongest correlation (r = 0.899) with protein to carbohydrate ratio at the rinsed layer (P1/C1). Comparing Pn/Cn ratios of different operational scenarios indicated that filtration scenarios had no significant effect on this ratio variations, but different aeration and SRT scenarios could alter this ratio significantly. In this regard, selecting optimal aeration and SRT conditions will be very important.
There was also a good correlation between Log R and SMP1 at the rinsed layer and especially its protein content. These findings (together with above paragraph) imply that the rinsed layer plays a major role in the membrane fouling and SMP content and its components (especially protein) at this layer should be considered as the main parameters for fouling control.
Furthermore it observed that among three fouling fractions, the rinsed and backwashed layers (Log R1 and R2) had very good correlations to the total hydraulic resistance (Log R), but desorbed layer (R3) did not show such relationship. So fouling control actions should be concentrated enough on these two layers.
In this study, the effects of different conditions of aeration rate, filtration mode, and SRT were assessed on fouling mitigation in MBR system. Important conclusions could be drawn:
Optimal operational conditions found among executed scenarios were: scenario A3 as aeration rate (15 LPM), scenario Rc as filtration mode (Relaxation with 40 s duration and 8 min. interval), and scenario S3 as SRT (30 days).
Comparing SMP variations with hydraulic resistance variations in different operational scenarios (especially in Figure 13), totally showed a similarity between these two variations, which confirms the relationship of membrane fouling with SMP in MBR system (according to previous studies).
The rinsed layer found to be the most effective fraction of membrane fouling considering SMP and hydraulic resistance graphs (Figures 4,5,7,8,10, and11). Also the good correlation between SMP content (especially protein) in this fraction confirms its major role in membrane fouling. In this regard, the aeration should be considered and applied as a very important practice with the ability of controlling this fouling layer.
Aeration exhibits multiple effects with different aspects on membrane fouling, so at each aeration rate some specific effects were dominant and hence its corresponding fouling behavior was not uniform for all of the aeration rates. This situation is clearly observed in transition of aeration rate from A1 to A2 scenario (comparing SMP and R variations). So the optimal aeration rate should be determined more precisely.
Relaxation in comparison with air backwashing showed a more positive effect on fouling control, and also its interval was more important than its duration for fouling control.
SRT variations in addition to influencing on the amount of produced exocellular materials, also affect on the structure of these material, so that at longer SRTs (20, 30 days) a greater percentage of SMP could penetrate into the membrane pores and for shorter SRTs they accumulate more on membrane surface. These effects thought to be due to the amount of protein production at each SRT (section “SRT scenarios”).
Results showed that there is a very good correlation between total hydraulic resistance (Log R) and protein to carbohydrate ratio at the rinsed layer (P1/C1). Considering significant effects of aeration and SRT conditions on this ratio, it will be very determinative to apply the optimal aeration and SRT conditions.
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