Silt Density Index Research Articles

Comparative Assessment of Membrane Fouling Propensity Using Colloidal Silica as Foulant

Other than silt density index (SDI), modified fouling index (MFI) is claimed as one of most useful fouling predictive tools in the practice of reverse osmosis. In this study, the effect of colloidal silica particles on the MFI value with the presence of crossflow sampler (CFS) cell at the upstream of MFI setup was systematically investigated. Colloidal silica with the particle size ranging from 70-100 nm was used as the model foulant in this experiment. The feed silica suspension was 0, 50, and 200 mg/L. The effect of membrane pore size as the test membrane in the dead-end cell on the fouling indices was also studied. Polyvinylidene fluoride (PVDF) membranes with molecular weight cut-offs of 150 and 100 kDa were employed. As predicted, the experiment results demonstrated that the fouling indices increased significantly as the feed silica concentration increased. The CFS-MFI values were always lower than the MFI in all the fouling index tests due to the shear force effect generated under the crossflow filtration mode. The fouling index generated from PVDF100 was higher than that of PVDF150 due to the retention of smaller particles. For this reason, PVDF100 membrane was preferred to be used as the test membrane for fouling index tests using colloidal silica over the PVDF150.

Electrodialysis reversal desalination: monographs for the design parameters

Abstract Electrodialysis reversal (EDR) is known for its excellence to desalt Ca2+ and dominated brackish groundwater with a higher water recovery rate, an elevated silt density index (SDI 6-12), potential for biofouling, hard-to-treat, high hardness, and lower salinity feed waters with the ranges of 200–5000 mg/L TDS. The power index is the only currently available design and evaluating parameter for the EDR process; however, power index is only valid for the same total dissolved solid (TDS) concentrations of feed and product water. Since, ED/EDR is capable of treating a variable source of water quality while producing a consistent finished water quality, comprehensive design and evaluating parameters are required, identified and proposed in this study. The quantities of the design parameters were developed from 52 sets of literature experimental data that have 10 different types of feed water characteristics. Another one set of literature experimental data was used to validate the values of developed pa...

The sensitivity of SDI and MFI to a change in particle concentration and properties under saline conditions

The sensitivity of existing fouling indices of SDI (silt density index) and MFI (modified fouling index) to a change in particle concentration and properties was evaluated in this study. The MFI formula predicts an increased fouling tendency when the particle concentration increases and the particle size and density decrease. Consequently, concentration, size and density were selected as parameters to examine. For this purpose, silica particles and Aldrich humic acid (HA) were selected as target foulants because of the substantial difference in their size and density. According to this study, both SDI and MFI were sensitive to a change in particle properties of size and density, while MFI was more sensitive to a change in particle concentration than SDI. It is also found that the fouling tendency could be represented better by separate evaluation of T0 and T15 than SDI, at which the ratio of T0 and T15 is evaluated. T15 is found to be as sensitive as the MFI for both silica suspension and HA solution. The saline condition increased the fouling index. MFI and T15 were sensitive to a change in ionic strength. When inorganic and organic foulants were mixed, their relative proportion determined the fouling index values.

A statistical approach to analyze factors affecting silt density index

Abstract Silt Density Index (SDI) has been used as the most popular fouling index for reverse osmosis (RO) feed water to select a proper pretreatment option for RO processes. However, SDI lacks the fundamental consideration of RO membrane fouling, because SDI is supposed to be only sensitive to particles larger than 0.45 μm in diameter while fine particles (which can pass through a 0.45 μm filter) and dissolved organic matters can be potent foulants for RO processes. Our study started from the suspected performance of SDI based on its lack of the fundamental basis. Various sources of SDI data from nine literatures were collected and analyzed with turbidity and dissolved organic carbon (DOC). Interestingly, the result of our study shows that SDI can express the amount of particulate and organic fouling together. SDI can be described as a function of turbidity, DOC, and a categorical binary variable, M, for pretreatment type (i.e., M = 1 for membrane filtration and M = 0 for other methods). SDI increases if either of turbidity or DOC becomes higher and membrane filtration is not used as a pretreatment option according to the multiple linear regression method using various data sources. Therefore, our study concludes that SDI can measure the potential of fouling effectively.

Novel Monitors Enable Early Detection of RO System Fouling

AbstractNoninvasive process monitoring methods—such as ultrasonic time domain reflectometry (UTDR) and electrical impedance spectroscopy (EIS)— provide reliable information. Coupled with a side-stream reverse osmosis (RO) fouling monitor cell that simulates plant flows, these methods could preempt fouling problems through more sensitive process monitoring. This is more desirable than crude measurements of transmembrane pressures and flows that may fail to detect incipient fouling. When the RO process is operating, another way to preempt fouling problems is to measure the water's fouling propensity with a silt density index (SDI). However, SDI is empirical and potentially unreliable. A membrane integrity sensor (MIS) can evaluate the integrity of microfiltration or ultrafiltration membranes used in pretreatment, which in turn measures the RO feed's fouling propensity. This article reviews recent MIS and UTDR developments at the Singapore Membrane Technology Centre and EIS developments at the University of ...

Evaluating the efficiency of different microfiltration and ultrafiltration membranes used as pretreatment for Red Sea water reverse osmosis desalination

Conventional processes are widely used as pretreatment for reverse osmosis (RO) desalination technology since its development. However, these processes require a large footprint and have some limitation issues such as difficulty to maintain a consistent silt density index, coagulation control at low total suspended solids, and management of higher waste sludge. Recently, there has been a rapid growth in the use of low-pressure membranes as pretreatment for RO systems replacing the conventional processes. However, despite the numerous advantages of using this integrated membrane system mainly providing good and stable water quality to RO membranes, many issues have to be addressed. The primary limitation is membrane fouling which reduces the permeate flux; therefore, higher pumping intensity is required to maintain a consistent volume of product. This paper aims to optimize the permeation flux and cleaning frequency by providing high permeate quality. Different low-pressure polyethersulfone membranes with different pore sizes ranging from 0.1μm to 50 kDa were tested. Eight different filtration configurations have been applied including the variation of coagulant doses aiming to control membrane fouling. Results showed that all the configurations with/without coagulation, provided permeate with excellent water quality which improves the stability of RO performance. However, more stable fluxes with less-energy consumption were achieved by using the 0.1μm and 100 kDa membranes with 1 mg/L FeCl3 coagulation. The use of UF membranes, having tight pores, without coagulation also proved to be an excellent option for Red Sea water RO pretreatment.

Al Dur SWRO plant: a double challenge for pre- and post-treatments

The Al Dur Power & Water Company is a 2.1 billion USD gas-fired greenfield power generation and seawater reverse osmosis desalination plant that is located on the southeast coast of Bahrain. The company Degrémont has been selected to design and build the Reverse Osmosis (RO) desalination plant, for a drinking water production capacity of 218,000 m3/day. The plant design had to answer to particularly challenging requirements, both for the pre-treatment and for the post-treatment steps. The pre-treatment step is one of the key factors for the success of desalination using the reverse osmosis process. Its purpose is to remove the major foulants, which can be found in the seawater to allow for a reliable operation of the reverse osmosis units. It is even more essential within the Gulf area, where the seawater contains high concentrations of organics and is subject to algal blooms. Al Dur RO plant was thus designed with an efficient two-step pre-treatment process: a flotation step, including an enhanced coagulation stage, and a filtration step. In parallel, to assess the seawater variability and demonstrate the performances of the selected pre-treatment line, a pilot study was carried out on-site during one year. Al Dur drinking water objectives were particularly challenging and the design had to incorporate unusual guarantees such as a Silt Density Index below 3.0%/min, a turbidity below 0.2 NTU and a Langelier Saturation Index between 0.1 and 0.3. As a consequence, to comply with both the Bahraini drinking water standards and the tender specifications, the Al Dur mineralization unit design included a high velocity lime water saturator and a lime water filtration unit. The present paper describes the pre- and post-treatment performances based on the initial project design, the pilot study and the on-site results during the commissioning and the first few months of operation.

Oil refinery wastewater treatment in biofilm reactor followed by sand filtration aiming water reuse

Oil refinery wastewater was sequentially treated in a moving-bed biofilm reactor (MBBR) and a slow-rate sand filter (SF) in order to obtain an effluent with adequate characteristics for downstream reverse osmosis (RO) operation. Experiments were conducted in bench scale units and the results showed that the MBBR was able to remove 90% chemical oxygen demand (COD), 75% NH4+, 95% phenols, operating with a hydraulic retention time (HRT) of 9 h. Additional removal of COD (15–40%) and ammonia (30–60%) was achieved in the slow-rate SF that was also effective for removing microorganisms. The silt density index (SDI) of the treated wastewater (4.5) was below the maximum limit recommended for RO operation. The quality of the effluent from the combined treatment system (MBBR+SF) was already adequate for cooling tower make-up. The RO produced an effluent with quality compatible with that required for use in boilers.

SDI: is it a reliable fouling index?

The ASTM considers the silt density index (SDI) test as a standard test for fouling potential of RO and NF feed waters. Up to date, the SDI is used at many full- and pilot-scale installations. The design and choice of the applied RO pretreatment is to a large extent based on the SDI test on the raw feed water. Comparing and monitoring UF/MF membrane performance is another SDI application. From a practical point of view, the SDI of RO feed water preferably should be lower than 3. The SDI has several disadvantages making it an unreliable test. The SDI has a non linear relationship with the colloidal concentration in the water and it is not corrected for the feed water temperature. Besides that, the SDI is not based on any filtration model. SDI is trying to simulate the RO fouling using dead-end MF membrane. The modified fouling index (MFI) is another fouling index. The MFI is based on the cake filtration model, can be corrected for pressure and temperature and is therefore used as a promising alternative for the SDI. Nevertheless, the procedure of measuring a MFI is more difficult and not directly suitable for carrying out “in the field”. The objective of this study therefore is to determine a theoretical relationship between SDI and MFI, and to validate this with experimental results. This relationship can be used to investigate the influence of membrane and testing parameters on SDI under cake filtration conditions, implying this model is valid for cake filtration mechanism and a particle rejection of 100%. In order to calculate the SDI, the times t 1 and t 2 for collecting the first and second sample are predicted using the measured MFI value and the MFI definition. In this research, the influence of several parameters (such as temperature, membrane resistance, etc.) on the SDI will be shown. The experimental results show a good agreement with the theoretical work, but only if the cake filtration start builds up directly at the beginning of the experiment. In general, this work clearly demonstrates that SDI currently is not reliable test for RO fouling. Either corrections for the SDI are necessary to give a more reliable index, or a new index has to be developed.

Full utilization of silt density index (SDI) measurements for seawater pre-treatment

Abstract In order to clarify the fouling mechanism during silt density index (SDI) measurements of seawater in the seawater reverse osmosis (SWRO) desalination process, 11 runs were conducted under constant-pressure (207 kPa) dead-end filtration mode according to the standard protocol for SDI measurement, in which two kinds of 0.45 μm membranes of different material and seawater samples from the Mediterranean including raw seawater and seawater pre-treated by coagulation followed by sand filtration (CSF) and coagulation followed by microfiltration (CMF) technologies were tested. Fouling mechanisms based on the constant-pressure filtration equation were fully analyzed. For all runs, only t/(V/A) ∼ t showed very good linearity (correlation coefficient R2 > 0.99) since the first moment of the filtration, indicating that standard blocking rather than cake filtration was the dominant fouling mechanism during the entire filtration process. The very low concentration of suspended solids rejected by MF of 0.45 μm in seawater was the main reason why a cake layer was not formed. High turbidity removal during filtration indicated that organic colloids retained on and/or adsorbed in membrane pores governed the filtration process (i.e., standard blocking) due to the important contribution of organic substances to seawater turbidity in this study. Therefore the standard blocking coefficient ks, i.e., the slope of t/(V/A) ∼ t, could be used as a good fouling index for seawater because it showed good linearity with feed seawater turbidity. The correlation of SDI with ks and feed seawater quality indicated that SDI could be reliably used for seawater with low fouling potential (SDI15 min seawater pre-treated by CSF > seawater pre-treated by CMF, indicating the better performance of CMF than CSF.

Sensitivity of SDI for experimental errors

Abstract Silt density index (SDI) testing is a widely-accepted method for estimating the rate at which colloidal and particle fouling will occur in water purification systems when using reverse osmosis (RO) or nanofiltration (NF) membranes. However, the SDI has several deficiencies. For example, the SDI has no linear relationship with the particle concentration, is not based on any fouling mechanism, and is not corrected for temperature, pressure and membrane resistance. The accuracy and reproducibility of the SDI is often questioned. In this study, mathematical models were developed to investigate the sensitivity of SDI for the following types of errors: errors due to inaccurate lab or field equipment, systematic errors, and errors resulting from artifacts and personal observations and experience. The mathematical results were verified experimentally. Both the mathematical models and experimental results show that the membrane resistance RM has the highest impact on the SDI results. The allowable ASTM variation in RM is responsible for a deviation in SDI between 2.29 and 3.98 at a level of SDI = 3. Besides that, a 1 s error in measuring the time to collect the second sample t2 results in ±0.07 at SDIO = 3. The artifacts and personal experience also influence the SDI results. The total error in measuring SDI was estimated to be equal to ±2.11 in the field and only ±0.4 in the lab in level of SDIO = 3. Furthermore, several recommendations are mentioned based on these theoretical results and our personal experience. This study demonstrates the sensitivity of the SDI for errors in RM and the accuracy of the equipments, and explains the difficulties in reproducing SDI results for the same water.

Flux dependency of particulate/colloidal fouling in seawater reverse osmosis systems

Abstract Fouling is the main operational problem in seawater reverse osmosis systems (SWRO). Particulate fouling is traditionally measured through the silt density index (SDI) and through the modified fouling index (MFI). In recent years, ultrafiltration membranes were used successfully at constant flux – MFI-UF – to measure particulate/colloidal fouling potential and tested in sea water applications. Furthermore, constant flux operation allows predicting the rate of fouling in RO systems. The objectives of this study are: (1) to measure the flux effect in MFI-UF with different membranes (100, 30 and 10 kDa) for raw seawater and pre-treated water before reverse osmosis in three different locations; (2) to study the particulate and colloidal fouling potential of seawater in reverse osmosis systems; (3) to project the increase in pressure due to cake resistance in reverse osmosis systems. In this research, flat ultrafiltration membranes (100, 50, 30 and 10 kDa) are used in a constant flux filtration mode to test and compare real seawaters from various locations (North and Mediterranean Sea) and from various full scale facilities including different pre-treatments (i.e., ultrafiltration and coagulation + dual media filtration). The operated fluxes range from 350 down to values close to real RO operation, 15l (m2 h)−1. After each filtration test, the MFI-UF is calculated to assess the particulate fouling potential. The obtained results showed that: (1) the articulate and colloidal fouling potential is directly proportional to the applied flux during filtration. This proportionality is related to the compression of the cake deposit occurring at high flux values; (2) the higher the flux, the higher the required pressure, the less porous the cake and therefore the higher the specific cake resistance; (3) particulate and colloidal fouling potential of seawater is site specific and is influenced by pre-treatment.

Filtration demonstration plant as reverse osmosis pretreatment in an industrial water treatment plant

Reverse osmosis (RO) has emerged as an alternative treatment in industrial water production. However, RO systems need suitable pretreatment to avoid membranes fouling. A demonstration study was conducted to assess improvements to RO pretreatment in the water treatment plant of a steel factory. Various pretreatment options, resulting from the combination of the existing treatment with different filtration stages, were tested. The silt density index (SDI) and the modified fouling index (MFI) were used to evaluate the performance of the different configurations. Throughout the experimentation period, it was not possible to assess the SDI of the filtration inlet water due to the high colloidal fouling, while the inlet MFI ranged from 9 to 458. Results showed that the dual-stage in-series filtration, including two pressurized filters, provided the best results, achieving SDI values lower than 5 and effluent MFI values lower than 1 under optimized operating conditions. Considering the fact that the MFI is a more accurate measurement as it takes into account the cake filtration mechanism, we conclude this configuration was able to provide enhanced RO pretreatment with a higher quality than the minimum required, despite the SDI not being lower than 3 with a reliability of 90%.

Improving Fouling Indices Measurements and Modeling Their Relevance

Reliable methods to predict feedwater fouling potential are important in preventing and diagnosing fouling at the design and operation stages of ultrafiltration (UF) and reverse osmosis (RO) plants, as well as monitoring pretreatment performance during operation. The desalination industry uses the silt density index (SDI) extensively to characterize feedwater fouling tendency. However, SDI measurements commonly lack the required precision, especially for high-quality pretreated waters, such as UF permeates. A novel performance model predicted how much transmembrane and feed-brine fouling will occur in the RO stage of a desalination plant based on SDI and modified fouling index (MFI) results. The SDI and MFI observations can now be translated into enhanced parameters that describe how much transmembrane and feed-brine resistance would be created by a certain fraction of water passing through the membrane. This article describes developments in the equipment and methodology used to measure fouling tendency of RO feedwater. Recommendations regarding the testing equipment and procedures are also provided.

The influence of membrane properties on the Silt Density Index

The Silt Density Index (SDI) is commonly applied as a measure for the fouling potential of particles in RO and NF feed waters. The accuracy and reproducibility of the SDI test is increasingly questioned. In this work, the influence of membrane properties on the SDI value is investigated. Eight commercial ‘0.45 μm’ membrane types made of different materials (PVDF, PTFE, acrylic copolymer, nitro cellulose, cellulose acetate, nylon 6,6, and polycarbonate) were used to measure the SDI. Three samples were randomly chosen from each membrane type (same lot), and several membrane properties were studied (pore size distribution, pore shape, surface and bulk porosity, thickness, surface charge, contact angle and surface roughness). SDI values for an artificial feed, composed of a solution of α – alumina particles of 0.6 μm diameter, were determined. The characterization of these membranes shows variation between the membranes used in this study (M1–M8), and within a batch of one membrane type. Substantial differences were found in the SDI values for the different types of membrane filters used. Pore size, porosity and thickness are the most important membrane properties and determine the membrane resistance. Using a membrane with high a membrane resistance results in a low SDI value. The variations in measured SDI values between batches and within a batch are large and explain, at least partly, the problems encountered in practice with unacceptable variations in SDI values. These observed differences make the test unreliable. The variations are attributed to differences in properties of the membranes used. In order to make the SDI a reliable fouling index, there is a very strong need for membrane filters with uniform and constant properties.

SDI normalization and alternatives

Estimating the fouling potential is a prerequisite for controlling membrane fouling in reverse osmosis systems. Two different tests are routinely used to that end: the Silt Density Index (SDI) and the Modified Fouling Index (MFI0.45). SDI test, however, has disadvantages which make it unreliable. One disadvantage is that no linear relationship exists between SDI and the colloidal concentration in the water. Besides that, the SDI is not based on a filtration model. Finally, the SDI is not corrected for the testing condition parameters such as temperature, pressure and membrane properties. The SDI test is simple and does not require advanced professional skills, but the above mentioned disadvantages make it unreliable. This work offers practical tools to resolve these disadvantages, such as a slide tool to calculate the SDI from the measured collection times. It also proposes a normalized SDI (SDI +), and offers a line chart and slide wheel charts for this conversion, thus correcting for different test conditions and membrane resistance. Finally, this paper introduces a new fouling index. This volume-based SDI, SDI_v, compares the initial flow rate with the flow rate after filtering the standard volume. It has a linear relationship to the particle concentration if complete blocking is the dominant fouling mechanism during the test, and is independent of testing conditions and membrane resistance. The mathematical model and experimental results show that SDI_v eliminates most of the disadvantages of the traditional SDI. The SDI_v is the second fouling index developed at the University of Twente, 30 years after the MFI0.45.

Effect of testing conditions and filtration mechanisms on SDI

The Silt Density Index is applied world-wide for many decades to determine the fouling potential of feed water of reverse osmosis systems and more recently to judge the performance of micro- and ultrafiltration systems. However there are growing doubts about the reproducibility and accuracy of this test. Currently, the Silt Density Index (SDI) is applied without any correction for temperature, applied pressure and membrane resistance. Besides that, the SDI is not based on any fouling mechanism which affects its reproducibility and accuracy. To identify opportunities for improvements, existing mathematical fouling models were further extended to study the effect of temperature, applied pressure and membrane resistance on the SDI value under four different fouling mechanisms. Significant variations in SDI values are observed mathematically as a result of differences in temperature and membrane resistance for the same water quality. The fouling mechanisms are described by the relationship between the filtrated volume w and the total resistance R. The sensitivity of the SDI for variations in the testing parameters theoretically increases when the relation between w and R is stronger. The SDI increases with an increase in feed temperature and applied pressure. Temperature has a substantial effect on SDI. As a consequence it is not recommended to compare SDI values measured at different temperatures. The SDI value decreases when membranes with a high resistance are used. To achieve a more reliable SDI, the use of a standardized membrane with constant properties, in particular having a narrow resistance range, is recommended.

Pilot study on the ceramic membrane pre-treatment for seawater desalination with reverse osmosis in Tianjin Bohai Bay

One of the critical factors for a successful seawater reverse osmosis (SWRO) process is the pre-treatment of the feed so that a stable, high-quality feed is supplied to the reverse osmosis (RO) membranes. Membrane filtration has been accepted as the preferred pre-treatment method in RO processes in recent years. In this paper, a pilot study of a pre-treatment process using a ceramic membrane for the desalination of seawater from Tianjin Bohai Bay, in the Bohai Sea of China, is presented. Different coagulation methods were compared, and the flocculation-natural sedimentation operation was determined to be the optimal method for ceramic membrane filtration. The ceramic membrane was found to work adequately in a long-term run, even under low-temperature conditions (3–6 °C). Based on the permeate qualities, such as turbidity, SDI 15 (Silt Density Index ) value, and concentration of Fe, it was concluded that this ceramic membrane technology can be successfully used in SWRO processes.

Impact of Seawater-Quality and Water Treatment Procedures on the Active Bacterial Assemblages at Two Desalination Sites

Inorganic and organic compounds, particles and microorganisms in intake waters are mainly responsible for fouling of reverse osmosis membranes, which reduces the efficiency of the desalination process. The characterization of seawater quality to better predict its fouling potential remains a challenge for the desalination field and little is known about the seasonal variability of water quality parameters in the coastal waters used to supply desalination plants. In this study, standard water quality methods were combined with flow cytometry and molecular methods (16S rRNA sequencing and fingerprinting) to assess in parallel, the physicochemical properties, the microbial abundance and the active microbial community composition of the intake waters and their associated pretreated waters at two desalination sites from July 2007 to July 2008. The overall assessment of quality parameters revealed that microfiltration followed by slow sand filtration were the most efficient in removing microorganisms than the conventional dual media filtration routinely used in full-scale desalination plants, and that all treatments were inefficient for organic matter reduction. Temporal variation of the environmental parameters such as temperature, turbidity and silt density index only moderately affected the bacterial community structure in raw waters, but that interestingly, water treatment compartments changed the composition and diversity of the metabolically active bacterial populations and thus create distinct ecological post-treatment niches.

An Integrity Sensor for assessing the performance of low pressure membrane modules in the water industry

There is a need for a low cost, sensitive, on-line sensor to assess the integrity of membrane modules used in the water industry for water treatment, membrane bioreactors and for pre-treatment prior to using nanofiltration or reverse osmosis. A sensor addressing this need is described whose operating principle is based on measuring the differential pressure across a membrane that intercepts a portion of the permeate from the pre-treatment module(s) relative to the differential pressure across a valve. The latter can be adjusted to maximize the sensitivity of the instrument. A dimensionless metric based on the differential pressures is defined and corroborated with the Silt Density Index. The sensitivity of this Integrity Sensor is demonstrated by its response to cutting one fiber in a UF pre-treatment module that contains 1500 hollow fibers.