Effect of cold water temperature on membrane structure and properties

A Review of Temperature Effects on Membrane Filtration

Membrane technology plays a vital role in drinking water and wastewater treatments. Among a number of factors affecting membrane performance, temperature is one of the dominant factors determining membrane performance. In this review, the impact of temperature on membrane structure, fouling, chemical cleaning, and membrane performance is reviewed and discussed with a particular focus on cold temperature effects. The findings from the literature suggest that cold temperatures have detrimental impacts on membrane structure, fouling, and chemical cleaning, and thus could negatively affect membrane filtration operations and performance, while warm and hot temperatures might expand membrane pores, increase membrane flux, improve membrane chemical cleaning efficiency, and interfere with biological processes in membrane bioreactors. The research gaps, challenges, and directions of temperature effects are identified and discussed indepth. Future studies focusing on the impact of temperature on membrane processes used in water and wastewater treatment and the development of methods that could reduce the adverse effect of temperature on membrane operations are needed.

Interaction of operating HRT and temperature on fouling of tertiary membranes treating municipal wastewater

This paper presents the first study to quantify and demonstrate the interactions between SBR operating conditions (hydraulic retention time (HRT) and temperature) and soluble microbial product (SMP) generation, as well as the impact of SBR operating conditions and filtration temperature on fouling of membranes used in tertiary treatment. Reducing SBR operating HRT from 20 to 10 h resulted in an increase in SMP concentrations, however, the extent of the increase in high and low molecular weight (MW) organics was different for the effluents from SBRs operated at 8 and 20 °C. Results of SMP modelling demonstrated that a reduction in SBR operating HRT induced decreased utilization associated product (UAP) yields and the influence was greater at the SBR operating temperature of 8 °C. In contrast, biomass associated product (BAP) yields were relatively stable with SBR operating HRT but greater at lower SBR operating temperature. The effects of SBR operating HRT and temperature on fouling indices were also interactive. Reducing SBR operating HRT led to a lower increase in hydraulically reversible resistances and a greater increase in hydraulically irreversible resistances for the effluent from the SBR operated at 8 °C. Reducing the filtration temperature resulted in additional increase in membrane resistances, and the increase was greater at lower SBR operating HRT. The contribution of filtration temperature was observed to have the greatest impact on membrane resistances, followed in importance by SBR operating HRT and temperature. The comprehensive analysis undertaken in the present study provides insights into the interaction between secondary and tertiary operations on fouling development. The results can be employed to understand the limits of fouling control for tertiary treatment under challenging conditions.

Biocarriers facilitated gravity-driven membrane filtration of domestic wastewater in cold climate: Combined effect of temperature and periodic cleaning

In this study, two lava stone biocarrier facilitated gravity-driven membrane (GDM) reactors were operated at ~8 °C and ~22 °C in parallel for treating primary wastewater effluent. Although the biocarrier reactor at 8 °C displayed less efficient removals of biodegradable organics than that at 22 °C, both GDM systems (without cleaning) showed comparable fouling resistance distribution patterns, accompanying with similar cake filtration constants and pore constriction constants by modelling simulation. Compared to the GDM at 8 °C, more foulants were accumulated on the GDM at 22 °C, but they presented similar soluble organics/inorganics contents and specific cake resistances. This indicated the cake layers at 22 °C may contain greater-sized foulants due to proliferation of both prokaryotes and eukaryotes, leading to a relatively less-porous nature. In the presence of periodic cleaning (at 50 °C), the cleaning effectiveness followed a sequence as ultrasonication-enhanced physical cleaning > two-phase flow cleaning > chemical-enhanced physical cleaning > physical cleaning, regardless of GDM operation temperature. However, significantly higher cake resistances were observed in the GDM system at 22 °C than those at 8 °C, because shear force tended to remove loosely-attached foulant layers and may compress the residual dense cake layer. The presence of periodic cleaning led to dissimilar dominant prokaryotic and eukaryotic communities in the cake layers as those without cleaning and in the lava stone biocarriers. Nevertheless, operation temperature did not influence GDM permeate quality, which met EU discharge standards.

Characterization and modelling of soluble microbial products in activated sludge systems treating municipal wastewater with special emphasis on temperature effect

Soluble microbial products (SMP) classified as utilization-associated products (UAP) and biomass-associated products (BAP) are the predominant foulants determining fouling in tertiary filtration. However, the exact mechanisms of BAP and UAP generation when treating real wastewaters under cold temperatures remain unrevealed. This paper presents the first study linking biological processes and SMP formation when treating real wastewaters through a combination of bioprocess modelling and advanced SMP characterization. Further, the impact of low operating temperatures on SMP production which has received relatively little attention was studied in detail. The use of liquid chromatography-organic carbon detection (LC-OCD) revealed a significant increase in protein and polysaccharide concentrations in the treated effluents as temperature decreased with a more sensitive impact on polysaccharides. The generation of SMP from biomass decay (BAP) and substrate utilization (UAP) was derived from the LC-OCD data on the basis of protein and polysaccharide mass balances. UAP and BAP yields were estimated as the ratios of the observed generation rates to the rates of substrate utilization and endogenous decay respectively, which both declined as temperature increased. A strong correlation was observed between temperature and BAP/UAP yields whereas the generation of BAP was more temperature sensitive than UAP. Such process modelling can be employed to assist with the optimization of the design and operation of membrane processes when treating wastewaters under challenging conditions like low temperature.

Membrane fouling performance of Fe-based coagulation-ultrafiltration process: Effect of sedimentation time

Pre-coagulation is commonly used with ultrafiltration (UF) to alleviate the membrane fouling. Compared to conventional coagulation-sedimentation-UF (CSUF) processes, the direct coagulation-UF (CUF) processes are widely believed to perform better due to the formation of a looser cake layer. It is however shown in this study that not only the density of a cake layer, but also its thickness as well, can affect the membrane fouling behavior, which therefore are influenced by both the sedimentation time and flocs characteristics. Herein, the membrane fouling performance of Fe-based coagulation-UF process was systematically investigated with different sedimentation times. A critical threshold of 30 min was observed at the lab-scale: if shorter than that, the membrane fouling depended mainly on the cake layer density, and thus CUF outperformed CSUF; but when the sedimentation time was over 30 min, the cake layer thickness turned to be the dominant factor, thereby resulting in CSUF performing better. Furthermore, it was shown that the critical sedimentation time was decided by flocs characteristics. A lower water temperature induced the formation of irregular flocs with a lower fractal dimension, and the corresponding cake layer exhibited an almost identical density with increasing sedimentation time. In this regard, CSUF processes were constantly superior to CUF as the cake layer thickness decreased. On the other hand, a critical sedimentation time reappeared because of the higher floc fractal dimension under acidic conditions. This work showed for the first time that the membrane fouling of CSUF was up to the sedimentation time, and it was possible to outperform CUF if the sedimentation time exceeded a critical threshold. Such a finding is crucial to the future development of coagulation integrated UF processes.

Influence of sedimentation with pre-coagulation on ultrafiltration membrane fouling performance

Coagulation plays an important role in effectively alleviating ultrafiltration (UF) membrane fouling due to the existence of flocs. In comparison to traditional filtration, direct filtration (no sedimentation tank) is generally considered to result in less membrane fouling due to the looser cake layer formed on the membrane surface. However, cake layer characteristics are easily influenced by operating conditions (e.g., sedimentation time, temperature), resulting in different fouling loads and the still very limited knowledge about the fouling behavior. Here, a detailed investigation of UF membrane performance was carried out to analyze the differences between traditional filtration and direct filtration. The results showed that a critical settling time indeed existed when flocs gradually settled, leading to severe membrane fouling induced by a dense and thick cake layer. Therefore, the traditional filtration performed worse when the settling time was shorter than the critical time, while it performed better when the settling time was longer. In comparison to room temperature water, the proportion of membrane fouling caused by the cake layer increased at low water temperature due to the contracted membrane pore size, and the cake layer thickness became the main fouling load. The longer the settling time, the fewer pollutants reached the membrane surface, the thinner the cake layer, and the lower the potential for membrane fouling. Thus, traditional filtration performed better than direct filtration, and the critical settling time was not obvious. Although solution pH played an important role in floc properties, the fouling load was influenced little and traditional filtration still performed better. This work shows that the cake layer properties/fouling loads vary with the settling time during the coagulation-UF process, and this finding is of great significance to the improvement of membrane technology in drinking water plants.

Reversibility of membrane performance and structure changes caused by extreme cold water temperature and elevated conditioning water temperature

The effects of extreme cold water temperature, elevated cleaning water temperature, cleaning time on membrane performance, and pore size distribution were studied using flat-sheet polyvinylidene fluoride (PVDF) membranes. Extreme cold water temperature led to a decrease in membrane permeability, and membrane pore size shrinkage. The use of periodical warm water (35 °C) membrane conditioning could almost completely recover the membrane permeability and pore size structure loss caused by extreme cold water temperature, while the use of room temperature (23±1 °C) conditioning recovered majority of the membrane permeability and structure loss caused by extreme cold water temperature. Changes in membrane performance correlate well with changes in membrane structure. Membranes periodically conditioned in warmer water temperature showed a higher permeability than that periodically conditioned at room temperature from a long-term operation of three months. The results suggest the change in PVDF membrane performance and structure caused by extreme cold water temperature is almost completely reversible after periodical warm water conditioning, and thus the use of warm water conditioning and/or chemical cleaning will benefit the recovery of membrane performance and structure change caused by extreme cold water temperature in cold regions.

Selective effect and elimination of antibiotics in membrane bioreactor of urban wastewater treatment plant

Analyzing the temporal dynamics of antibiotics, antibiotic resistance genes (ARGs) and the functional community could contribute to the regulation and optimization of wastewater treatment plant (WWTP) operation to achieve better antibiotics and ARGs removal performances during different seasons. However, there is little research in this area. Therefore, in this study, samples were collected from the influent, activated sludge (AS), and permeate of the membrane bioreactor (MBR) in a WWTP located in Beijing, China, biweekly over 13 months, and then analyzed systematically. The removal efficiency for all detected antibiotics through biodegradation and adsorption was 59.25 ± 2.79%, with the highest rate of 64.79 ± 4.68% observed in summer, indicating that the higher temperature in summer may promote biodegradation in MBR. In contrast, the elimination of antibiotics through microfiltration was negligible and unfavorable, with negative overall removal rates. However, a positive rejection rate of 9.48 ± 8.92% was only observed in winter, indicating that a colder temperature might lead to better, but still limited, antibiotics rejection. Sulfonamides (SAs) were more likely to impose a selective pressure on their corresponding ARGs. However, due to the degradability of tetracyclines (TCs) and potential selection of ARGs in wastewater before entering WWTP, there were still TC resistances with non-detectable TCs. Significantly negative relationships were observed between the relative abundance of nitrifying bacteria (Nitrosomonas and Nitrospira) and the concentrations of certain antibiotic classes, indicating that nitrifying bacteria could be involved in the co-metabolic biodegradation of certain antibiotics through enzyme catalyzation during nitrification.

Bioflocculants' production in a biomass-degrading bacterium using untreated corn stover as carbon source and use of bioflocculants for microalgae harvest

BACKGROUND

Bioflocculation has been developed as a cost-effective and environment-friendly method to harvest multiple microalgae. However, the high production cost of bioflocculants makes it difficult to scale up. In the current study, low-cost bioflocculants were produced from untreated corn stover by a biomass-degrading bacterium Pseudomonas sp. GO2.

RESULTS

Pseudomonas sp. GO2 showed excellent production ability of bioflocculants through directly hydrolyzing various biomasses. The untreated corn stover was selected as carbon source for bioflocculants' production due to its highest flocculating efficiency compared to that when using other biomasses as carbon source. The effects of fermentation parameters on bioflocculants' production were optimized via response surface methodology. According to the optimal model, an ideal flocculating efficiency of 99.8% was obtained with the fermentation time of 130.46 h, initial pH of 7.46, and biomass content of 0.64%. The relative importance of carboxymethyl cellulase and xylanase accounted for 51.8% in the process of bioflocculants' production by boosted regression tree analysis, further indicating that the bioflocculants were mainly from the hydrolysates of biomass. Biochemical analysis showed that it contained 59.0% polysaccharides with uronic acid (34.2%), 32.1% protein, and 6.1% nucleic acid in the bioflocculants, which had an average molecular weight as 1.33 × 10⁶ Da. In addition, the bioflocculants showed the highest flocculating efficiency at a concentration of 12.5 mg L^-1 and were stable over broad ranges of pH and temperature. The highest flocculating efficiencies obtained for Chlorella zofingiensis and Neochloris oleoabundans were 77.9 and 88.9%, respectively.

CONCLUSIONS

The results indicated that Pseudomonas sp. GO2 can directly utilize various untreated lignocellulolytic biomasses to produce low-cost bioflocculants, which showed the high efficiency to harvest two green microalgae in a low GO2 fermentation broth/algal culture ratio.