Comparison of MFI-UF constant pressure, MFI-UF constant flux and Crossflow Sampler-Modified Fouling Index Ultrafiltration (CFS-MFI UF)

Understanding the foulant deposition mechanism during crossflow filtration is critical in developing indices to predict fouling propensity of feed water for reverse osmosis (RO). Factors affecting the performance on different fouling indices such as MFI-UF constant pressure, MFI-UF constant flux and newly proposed fouling index, CFS-MFI_UF were investigated. Crossflow Sampler-Modified Fouling Index Ultrafiltration (CFS-MFI_UF) utilises a typical crossflow unit to simulate the hydrodynamic conditions in the actual RO units followed by a dead-end unit to measure the fouling propensity of foulants. CFS-MFI_UF was found sensitive to crossflow velocity. The crossflow velocity in the crossflow sampler unit influences the particle concentration and the particle size distribution in its permeate. CFS-MFI_UF was also found sensitive to the permeate flux of both CFS and the dead-end cell. To closely simulate the hydrodynamic conditions of a crossflow RO unit, the flux used for CFS-MFI_UF measurement was critical. The best option is to operate both the CFS and dead-end permeate flux at flux which is normally operated at industry RO units (∼20 L/m² h), but this would prolong the test duration excessively. In this study, the dead-end flux was accelerated by reducing the dead-end membrane area while maintaining the CFS permeate flux at 20 L/m² h. By doing so, a flux correction factor was investigated and applied to correlate the CFS-MFI_UF measured at dead-end flux of 120 L/m² h to CFS-MFI_UF measured at dead-end flux of 20 L/m² h for RO fouling rate prediction. Using this flux correction factor, the test duration of CFS-MFI_UF can be shortened from 15 h to 2 h.     

Post-treatment of upflow anaerobic sludge blanket effluent by combining the membrane filtration process: fouling control by intermittent permeation and air sparging

In this study, the membrane filtration process was proposed as a post-treatment process to treat the upflow anaerobic sludge blanket (UASB) effluent. The flat-sheet membrane modules were submerged into the UASB reactor to retain the suspended solids in the UASB effluent under intermittent permeation and air sparging conditions. The results indicated that intermittent permeation enhanced the sustainability of the submerged membrane and an idle time of 4 min with a 10-min filtration was optimal for reducing membrane fouling under the experimental conditions. Air sparging could also alleviate membrane fouling, and the lowest transmembrane pressure (TMP) change rate (dTMP/d t ) was achieved at the moderate aeration rate of 2 L/min. It appears to be effective to alleviate membrane fouling and achieve high effluent production at a flux of 25 L/m² h and an idle time of 4 min with a 10-min filtration considering the energy consumption.     

Critical analysis of submerged membrane sequencing batch reactor operating conditions

To evaluate the Submerged Membrane Sequencing Batch Reactor process, several short-term studies were conducted to define critical flux, membrane aeration and intermittent filtration operation. Critical flux trials indicated that as mixed liquor suspended solids increased in concentration so would the propensity for membrane fouling. Consequently in order to characterise the impact of biomass concentration increase (that develops during permeate withdrawal) upon submerged microfiltration operation, two longer term studies were conducted, one with a falling hydraulic head and another with a continuous hydraulic head (as in membrane bio-reactors). Trans membrane pressure data was used to predict the maximum possible operating periods at 10 and 62 days for the falling hydraulic head and continuous hydraulic head respectively. Further analysis revealed that falling hydraulic head operation would require 21% more aeration to maintain a consistent crossflow velocity than continuous operation and would rely on pumping for full permeate withdrawal 80% earlier. This study concluded that further optimisation would be required to make this technology technically and economically viable.     

Influence of detachment on substrate removal and microbial ecology in a heterotrophic/autotrophic biofilm

Competition between heterotrophic bacteria oxidizing organic substrate and autotrophic nitrifying bacteria in a biofilm was evaluated. The biofilm was grown in a tubular reactor under different shear and organic substrate loading conditions. The reactor was initially operated without organic substrate in the influent until stable ammonia oxidation rates of 2.1 g N/(m² d) were achieved. A rapid increase of fluid shear in the tubular reactor on day 156 resulted in biofilm sloughing, reducing the biofilm thickness from 330 to 190 μm. This sloughing event did not have a significant effect on ammonia oxidation rates. The addition of acetate to the influent of the reactor resulted in decreased ammonia oxidation rates (1.8 g N/(m² d)) for low influent acetate concentrations (17 mg COD/L) and the breakdown of nitrification at high influent acetate concentrations (55 mg COD/L). Rapidly increasing fluid shear triggered biofilm sloughing in some cases—but maintaining constant shear did not prevent sloughing events from occurring. With the addition of acetate to the influent of the reactor, the biofilm thickness increased up to 1350 μm and individual sloughing events removed up to 50% of the biofilm. Biofilm sloughing had no significant influence on organic substrate removal or ammonia oxidation. During 325 days of reactor operation, ammonia was oxidized only to nitrite; no nitrate production was observed. This lack of nitrite oxidation was confirmed by fluorescent in situ hybridization (FISH) analysis, which detected betaproteobacterial ammonia oxidizers but not nitrite oxidizers. Mathematical modeling correctly predicted breakdown of nitrification at high influent acetate concentrations. Model predictions deviated systematically from experimental results, however, for the case of low influent acetate concentrations.     

Removal of coliphages in secondary effluent by microfiltration-mechanisms of removal and impact of operating parameters

The efficacy of a microfiltration (MF) pilot plant in removing somatic coliphages (referred hereafter as coliphages) present in the secondary effluent was evaluated during this study. The impact of operating parameters such as feed coliphage concentrations, permeate flux and membrane fouling on the removal of coliphages by the MF plant was investigated. The study showed that membrane fouling was beneficial for removing coliphages by MF. It was also shown that the removal of coliphages by MF was initially governed by adsorption on membrane surface or in membrane pores. As the membrane fouled, however, the removal of coliphages was primarily governed by direct interception on the cake layer formed on the surface of the membrane. Increases in feed coliphage concentrations resulted in the passage of larger numbers of coliphages when the MF was clean but had little impact on the passage of coliphages when the membrane became fouled. Increasing permeate flux lowered log-removal values (LRVs) for the clean membrane but resulted in an initial increase in LRVs for the fouled membrane followed by a drop in LRVs with further increases in permeate flux.     

A comparison of cake properties in traditional and turbulence promoter assisted microfiltration of particulate suspensions

The use of turbulence promoter can effectively enhance the permeate flux in crossflow microfiltration (CFMF) of particulate suspensions. Flux enhancement which is generally attributed to the reduction in cake thickness, however, has still not been clearly understood. In this study, the effects of turbulence promoter on cake properties (thickness, porosity and particle size) were investigated during CFMF of calcium carbonate suspension. It indicates that turbulence promoter has important effects on cake properties that directly affect the cake resistance. The significant reduction in thickness and slight increase in porosity are positive to reduce the cake resistance. The remarkable decrease in particle size is the negative effect due to the increased specific resistance of a cake. As a whole, the overall cake resistance is still diminished by turbulence promoter and therefore permeate flux is improved. The theoretical calculation of cake resistance shows a good consistence with the experimental result. The cake properties in both cases (using a turbulence promoter or not) almost exhibit the similar trends under various operating conditions. Differently, the use of turbulence promoter can greatly alleviate the effects of transmembrane pressure or feed concentration on growth of cake layer and intensify the effects of inlet velocity on diminishing the particle deposition.     

浸入式膜系统的操作条件对膜污染的影响研究

采用浸入式膜处理滦河水,考察了膜系统的操作条件对膜污染的影响。结果表明:适当降低膜通量对膜处理系统的稳定运行起关键作用,试验条件下,膜通量为53.3L/(m2.h)、过滤周期为30min较适宜。提高曝气强度和水反冲洗强度可有效提高反冲洗效果,改善膜污染。浸入式膜对原水水质有较强的适应能力,可适当减少反冲洗排放次数,试验条件下,采用四轮一排较为可行。相比三氯化铁,聚合铝更适合作为PVDF膜的混凝预处理药剂。     

Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts

We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from Etna (Italy) and columnar basalt from Seljadur (Iceland). Measurements were made in a servo-controlled steady-state-flow permeameter at effective pressures from 5–80 MPa, during both increasing and decreasing pressure cycles. Selected samples were thermally stressed at temperatures up to 900 °C to induce thermal crack damage. Acoustic emission output was recorded throughout each thermal stressing experiment.

At low pressure (0–10 MPa), the P-wave velocity of the columnar Seljadur basalt was 5.4 km/s, while for the Etnean lava flow basalt it was only 3.0–3.5 km/s. On increasing the pressure to 80 MPa, the velocity of Etnean basalt increased by 45%–60%, whereas that of Seljadur basalt increased by less than 2%. Furthermore, the velocity of Seljadur basalt thermally stressed to 900 °C fell by about 2.0 km/s, whereas the decrease for Etnean basalt was negligible. A similar pattern was observed in the permeability data. Permeability of Etnean basalt fell from about 7.5×10⁻¹⁶ m² to about 1.5×10⁻¹⁶ m² over the pressure range 5–80 MPa, while that for Seljadur basalt varied little from its initial low value of 9×10⁻²¹ m². Again, thermal stressing significantly increased the permeability of Seljadur basalt, whilst having a negligible effect on the Etnean basalt. These results clearly indicate that the Etnean basalt contains a much higher level of crack damage than the Seljadur basalt, and hence can explain the low velocities (3–4 km/s) generally inferred from seismic tomography for the Mt. Etna volcanic edifice.     

Zero net growth in a membrane bioreactor with complete sludge retention

A bench-scale membrane bioreactor was operated with complete sludge retention in order to evaluate biological processes and biomass characteristics over the long term. The investigation was carried out by feeding a bench-scale plant with real sewage under constant volumetric loading rate (VLR=1.2 gCOD L_react⁻¹ h⁻¹). Biological processes were monitored by measuring substrate removal efficiencies and biomass-related parameters. The latter included bacterial activity as determined through respirometric tests specifically aimed at investigating long term heterotrophic and nitrifying activity. After about 180 days under the imposed operating conditions, the system reached equilibrium conditions with constant VSS concentration of 16–18 g L⁻¹, organic loading rate (OLR) below 0.1 gCOD gVSS⁻¹ d⁻¹ and specific respiration rates of 2–3 mgO₂ gVSS⁻¹ h⁻¹. These conditions were maintained for more than 150 days, confirming that an equilibrium had been achieved between biomass growth, endogenous metabolism, and solubilization of inorganic materials.     

Influence of operating parameters on the arsenic removal by nanofiltration

Arsenic contamination of surface and groundwater is a worldwide problem in a large number of Countries (Bangladesh, Argentina, Italy, USA, New Zealand, etc.). In many contaminated areas a continuous investigation of the available arsenic removal technologies is essential to develop economical and effective methods for removing arsenic in order to meet the new Maximum Contaminant Level (MCL) standard (10 μg/l) recommended by the World Health Organization (WHO).In this work the removal of pentavalent arsenic from synthetic water was studied on laboratory scale by using two commercial nanofiltration (NF) spiral-wound membrane modules (N30F by Microdyn-Nadir and NF90 by Dow Chemical). The influence of main operating parameters such as feed concentration, pH, pressure and temperature on the As rejection and permeate flux of both membranes, was investigated. An increase of pH and a decrease of operating temperature and As feed concentration led to higher As removal for both membranes, whereas higher transmembrane pressure (TMP) values slightly reduced the removal achievable with the N30F membrane. In both cases, the permeate flux increased with temperature and pressure and reached its maximum value at a pH of around 8.Among the parameters affecting the As rejection, feed concentration plays a key role for the production of a permeate stream respecting the limits imposed by WHO.     

Novel filtration mode for fouling limitation in membrane bioreactors

A novel filtration mode is presented to reduce fouling propensity in membrane bioreactors (MBR). During this mode, an elevated high instantaneous flux (60Lm(-2)h(-1)) is initially applied for a short time (120s), followed by a longer filtration (290s) at lower flux (10.3Lm(-2)h(-1)) and a backwash in each filtration cycle. The mixed mode is expected to limit irreversible fouling as the reversible fouling created during the initial stage appears to protect the membrane. Hydraulic performance and the components of foulants were analyzed and compared with conventional continuous and backwash modes. It was found that the mixed mode featured lower trans-membrane pressure (TMP) after 24h of filtration when compared to other modes. The mixed mode was effective in preventing soluble microbial products (SMP) attaching directly onto the membrane surface, keeping the cake layer weakly compressed, and reducing the mixed liquor suspended solids (MLSS) accumulation on the membrane. This strategy reduced the resistances of both the cake layer and the gel layer. A factorial experimental design was carried out for eight runs with different conditions to identify the major operational parameters affecting the hydraulic performances. The results showed that the value of the flux in the initial high-flux period had the most effect on the performance of the mixed mode: high initial flux (60Lm(-2)h(-1)) led to improved performance.     

Pore fabric shape anisotropy in porous sandstones and its relation to elastic wave velocity and permeability anisotropy under hydrostatic pressure

To understand the relationship between pore space anisotropy and petrophysical properties, we developed a novel apparatus capable of simultaneously measuring permeability, porosity and ultrasonic velocities at hydrostatic pressures up to 100 MPa. First, we use magnetic susceptibilities and acoustic wave velocities to identify the principal anisotropy axes under ambient laboratory conditions. This directional anisotropy data is then used to guide experiments on two sandstones (Bentheim and Crab Orchard) under hydrostatic pressure from 5 to 90 MPa. We find the structural anisotropy formed by the void space is well described by velocity anisotropy in both cases. Under hydrostatic pressure, the acoustic anisotropy of Crab Orchard sandstone (COS) decreases from 3% and 7% at 5 MPa (P-wave and S-wave) to 1.5% and 1%, respectively, at effective pressures over 40 MPa; for Bentheim sandstone the decrease is considerably less. Permeability of COS is 125×10⁻¹⁸ m², decreasing rapidly as effective pressure increases, with permeability parallel to bedding approximately twice that normal to bedding. In contrast, permeability of Bentheim sandstone is 0.86×10⁻¹² m², and varies little with effective pressure or coring direction. We relate many of our measurements made under hydrostatic pressure to the contrasting pore fabric between the two rock types, and infer that a critical pressure is required for the initiation of crack closure.     

Mesoporous materials for water treatment processes

This paper presents results for the application of M41S materials as alternative adsorbents and as potential catalysts for catalytic ozonation processes for potable water treatment purposes. It was found that mesoporous silicates and aluminosilicates all possess BET surface areas in excess of 700 m² g⁻¹ and possess well defined pores of uniform dimensions, with the exceptions of the aluminium-substituted highly microporous silicate. Adsorption capacities for the model compounds, cyanuric acid and p-chlorophenol, are over 150 mg/g of material.     

浸入式膜系统回收膜反洗水的试验研究及工程应用

为提高超滤膜系统的产水率,采用混凝/粉末活性炭/浸入式膜组合工艺,对中试超滤膜反洗水进行了回收处理。结果表明,膜反洗水中的有机物浓度较高,以DOC表征的有机物主要分布在MW30 ku和MW1 ku区间内;回收系统的平均出水浊度为0.07 NTU;当FeCl3和PAC的投量均为15 mg/L时,出水CODMn平均为2.81 mg/L,平均去除率达到了50.7%;回收系统出水的pH和微生物指标均满足《生活饮用水卫生标准》(GB 5749—2006)的要求。将该反洗水回收工艺应用于杨柳青水厂的膜处理示范工程,可使系统的产水率从79.85%提高到98.03%,可减少废水排放量达33×104m3/a。     

Pre-acidification in anaerobic sludge bed process treating brewery wastewater

The effect of pre-acidification on anaerobic granule bed processes treating brewery wastewater was the focus of a comparison study employing two configurations, (a) a single stage upflow anaerobic sludge bed (UASB) and (b) an upflow acidification reactor in series with a methanogenic UASB. The pre-acidification reactor achieved 20±4% SCOD removal and 0.08±0.003 L of methane produced per gram of SCOD removal at a hydraulic retention time (HRT) of 0.75–4 h. Butyric acid was not detected and short chain fatty acids (SCFAs) were mainly acetic and propionic acids. The acidification ratio was about 0.42±0.02 g SCFAs as COD/g of influent COD.

Both systems’ critical loading rate to achieve 80% COD removal was established at 34–39 kg COD/m³ of total sludge bed volume per day. SCOD removal efficiency of 90±3% was achieved by both systems at an organic loading rate of 25±1 kg COD/m³ of total sludge bed volume per day, indicating that the installation of an acidification reactor had no effect in terms of the maximum granular activity, biomass granulation and the settleability of granules. At an organic loading rate of 67 kg COD/m³ of total sludge bed volume per day at an HRT of 1 h, the series system outperformed the single UASB by a removal of 62 compared to 57%.     

Recovery and concentration of polyphenols from olive mill wastewaters by integrated membrane system

The purpose of this work was to analyse the potentialities of an integrated membrane system for the recovery, purification and concentration of polyphenols from olive mill wastewater (OMW). The proposed system included some well-known membrane operations such as microfiltration (MF) and nanofiltration (NF), as well as others not yet investigated for this specific application, such as osmotic distillation (OD) and vacuum membrane distillation (VMD).The OMW was directly submitted to a MF operation without preliminary centrifugation. This step allowed to achieve a 91% and 26% reduction of suspended solids and total organic carbon (TOC), respectively. Moreover, 78% of the initial content of polyphenols was recovered in the permeate stream.The MF permeate was then submitted to a NF treatment. Almost all polyphenols were recovered in the produced permeate solution, while TOC was reduced from 15 g/L to 5.6 g/L.A concentrated solution enriched in polyphenols was obtained by treating the NF permeate by OD. In particular, a solution containing about 0.5 g/L of free low molecular weight polyphenols, with hydroxytyrosol representing 56% of the total, was produced by using a calcium chloride dihydrate solution as brine. The obtained solution is of interest for preparing formulations to be used in food, cosmetic and pharmaceutical industry. Besides the OD process, VMD was applied as another way for concentrating the NF permeate and the performance of both processes was compared in terms of evaporation fluxes.     

Quantitative study of the effect of electromagnetic field on scale deposition on nanofiltration membranes via UTDR

Ultrasonic time-domain reflectometry (UTDR) as an in situ, non-invasive real-time technique has been successfully utilized to quantify membrane fouling and cleaning. This study describes an existing ultrasonic technique for quantitative study of the effect of magnetic fields on CaCO₃ scale deposition on the membrane surface during crossflow nanofiltration (NF). The fouling experiments were conducted with CaCO₃ solution containing Ca²⁺ of 1.8 and 3 mmol/L. The experimental electromagnetic field (EMF) was 0.02 T. A commercial NF membrane and a highly sensitive ultrasonic sensor were used in all the experiments. Results show a good relationship between the ultrasonic measurements and the development of CaCO₃ scale on the NF membrane surface. An increase in the amplitude of differential signal with operation time results from the deposition of the CaCO₃ scale layer. In addition, the movement of the differential signal in the time domain observed is associated with an increase in the thickness of the fouling layer. The UTDR technique is capable of measuring the rate of fouling layer formation under different treatment conditions, i.e. with non-magnetic field (NMF) and EMF. Scale layer of deposited CaCO₃ obtained in the experiment with NMF is denser and thicker than that with EMF. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses imply that the magnetic treatment suppresses the formation of calcite crystals and prefers vaterite and aragonite. Furthermore, the ultrasonic technique is sensitive to the different fouling rate between experiments carried out with 1.8 and 3 mmol/L CaCO₃ solutions. The thickness of the fouling layer measured by weight measurement is consistent with that obtained by UTDR in real time. The thinner and less dense scale layer is the main reason for the slower flux decline. Overall, independent measurements such as the flux-decline data, SEM analysis and weight measurement corroborate the ultrasonic measurements.     

Energy impact assessment for the reduction of carbon dioxide and formaldehyde exposure risk in air-conditioned offices

Treatment of fresh air in ventilation systems for air-conditioning consumes a considerable amount of energy and affects the indoor air quality (IAQ). In this study, energy impact on ventilation systems was examined against certain IAQ objectives for indoor formaldehyde exposure risk in air-conditioned offices of Hong Kong. Thermal energy consumptions for ventilation systems and indoor formaldehyde exposure concentrations based on some regional surveys of typical offices in Hong Kong were reviewed. The thermal energy consumptions of ventilation systems operating for CO₂ exposure concentrations between 800 ppmv and 1200 ppmv for typical office buildings and the corresponding formaldehyde exposure risks were evaluated. The results showed that, for a reference indoor environment at a CO₂ exposure concentration of 1000 ppmv, the average thermal energy saving of ventilation system for a unit increment of the acceptable formaldehyde exposure limit of 1 h (loss of life expectancy of 0.0417 day) was 280 MJ m⁻² yr⁻¹; and for a unit decrement of the exposure limit of 1 h, an additional average thermal energy consumption of 480 MJ m⁻² yr⁻¹ was expected. This study would be a useful source of reference in evaluation of the energy performance of ventilation strategies in air-conditioned offices at a quantified exposure risk of formaldehyde.     

Removal of humic substances (HS) from water by electro-microfiltration (EMF)

Humic substances (HS) represent the common agents contributing to flux decline during membrane filtration of natural water. In order to minimize the fouling during microfiltration (MF) of HS, modifying the operation of MF presents a promising alternative. A laboratory-scale electro-microfiltration (EMF) module was used to separate Aldrich HS from water by applying a voltage across the membrane. The presence of an electric field significantly reduced the flux decline. A flux comparable to that of ion-free water was attained when the voltage was near the critical electric field strength (Ecritical), i.e., the electrical field gradient that balances the advective and electrophoretic velocities of solute. At an applied voltage of 100 V (approximately 110 V/cm), it was able to reduce UV absorbance at 254 nm (UV254), total organic carbon (TOC) and trihalomethane formation potential (THMFP) by over 50% in the permeate. Results from 1H nuclear magnetic resonance (1H NMR) analysis suggest that the aromatic and functionalized aliphatic fractions decreased significantly in the permeate. The charged HS have large molecule weight compared with those passing through membrane. Results clearly indicate that a combination of electric force with MF can increase HS rejection and decrease flux decline. Electrophoretic attraction was the major mechanism for the improvement of flux and rejection over time.     

Thermophilic treatment of acidified and partially acidified wastewater using an anaerobic submerged MBR: Factors affecting long-term operational flux

The long-term operation of two thermophilic anaerobic submerged membrane bioreactors (AnSMBRs) was studied using acidified and partially acidified synthetic wastewaters. In both reactors, cake formation was identified as the key factor governing critical flux. Even though cake formation was observed to be mostly reversible, particle deposition proceeds fast once the critical flux is exceeded. Very little irreversible fouling was observed during long-term operation, irrespective of the substrate. Critical flux values at the end of the reactors operation were 7 and 3L/m(2)h for the AnSMBRs fed with acidified and partially acidified wastewaters, respectively, at a gas superficial velocity of 70m/h. Small particle size was identified as the responsible parameter for the low observed critical flux values. The degree of wastewater acidification significantly affected the physical properties of the sludge, determining the attainable flux. Based on the fluxes observed in this research, the membrane costs would be in the range of 0.5euro/m(3) of treated wastewater. Gas sparging was ineffective in increasing the critical flux values. However, preliminary tests showed that cross-flow operation may be a feasible alternative to reduce particle deposition.