Silt Density Index Values Research Articles

Silt density index as a fouling propensity parameter of various membrane materials using dissolved organic matter

Silt density index (SDI) is the most widely used parameter for predicting fouling potential that occurs during reverse osmosis (RO) processes to assess and reduce fouling; however, this index cannot accurately predict fouling, particularly, organic fouling. Therefore, this study analyzed the application of five different membranes (i.e., mixed cellulose nitrate and cellulose acetate (MCE), cellulose nitrate (CN), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polycarbonate (PC)) for measuring SDI and discerning the characteristics of organic foulants. The properties of the organic foulants on the membranes used to measure the SDI value were compared to those on the RO membrane. The quantitative fouling of the MCE, CN, PVDF, PC, and PTFE membranes in terms of the total organic carbon were 6.01 ± 0.03, 6.30 ± 0.14, 10.27 ± 0.10, 2.26 ± 0.12, and 3.08 ± 0.06 mg/L, respectively. In addition, liquid chromatography–organic carbon detection revealed that the MCE and CN were effective in predicting the fouling caused by building blocks and low-molecular-weight substances, respectively. The PVDF was effective in predicting fouling caused by biopolymers. The contaminants compositions on the PTFE and PC were similar; however, the PTFE was more effecting in detecting fouling caused by humic and low-molecular-weight substances. The composition of foulants on the PC was similar to that on the RO membrane, indicating that it can successfully predict the organic fouling of RO membrane. These results indicate that it is important to standardize the SDI value for PC membranes to make SDI a reliable index in the municipal wastewater reuse.

Correlations between silt density index, turbidity and oxidation-reduction potential parameters in seawater reverse osmosis desalination

The reverse osmosis method is one of the most widely used methods of seawater desalination at present. Hydrophilic and desalting membranes in reverse osmosis systems are highly susceptible to the input pollutants. Various contaminants, including suspended organic and inorganic matter, result in membrane fouling and membrane degradation. Fundamental parameters such as the turbidity, the amount of chlorine injection, and silt density index (SDI) are the most predominant parameters of fouling control in the membranes. In this study, the operation system included a water intake unit, a pretreatment system, and an RO system. The pretreatment system encompassed a clarifier, a gravity sand filter, pressurized sand filters, and a cartridge filter. The correlation between the amount of chlorine injection in terms of the oxidation-reduction potential (ORP) and the SDI value of the input water was investigated at a specified site next to the Persian Gulf. The results showed that, at certain intervals of inlet turbidity, injection of a certain amount of chlorine into the raw water has a distinct effect on the decrease of SDI.

Control of Silt Density Index of Osmosis Membranes Through Chlorine Injection and its Effect on Cartridge Filter Replacement Period

Background: Various suspended organic and inorganic contaminants including biological deposits, different types of algae and barnacles and causative factors of water turbidity result in irreparable damages to pretreatment systems and membranes in reverse osmosis systems. Objectives: In this study, the effect of injection of disinfectant materials such as calcium hypochlorite on the reduction of silt density index (SDI) value at the inlet of reverse osmosis system has been investigated. Methods: In this research, correlations between fundamental parameters in pretreatment system such as turbidity, chlorine injection, residual chlorine content and SDI values were thoroughly analyzed and evaluated at one of the desalination plants located in Persian Gulf. Results: The results indicated that there is a close link between the amount of chlorine injection and SDI inlet value. By adjusting the nephelometric turbidity unit values in specific ranges of 35 - 55, 55 - 80 and 80 - 150 and oxidation reduction potential in the ranges of 280 - 680, 315 - 680 and 420 - 680 mV, favorable SDI value of inlet water was obtained. Conclusions: SDI control via chlorine injection increased the lifetime of cartridge filters at reverse osmosis input units and reduced the operation costs significantly.

Effect of chemical pretreatment on the silt density index of brackish water from north and south Kuwait

In this study, the effect of chemical pretreatment with powdered activated carbon (PAC), ferric chloride, and aluminum chloride at the different concentrations of 10, 30, and 60 ppm on the silt density index (SDI) value of brackish water was examined. Generally, PAC pretreatment was found to be the best pretreatment method. Pretreatment with 10 ppm of PAC resulted in the lowest SDI value of 3.5, which is approximately 44.4% lower than untreated brackish water. After pretreatment with PAC, scanning electron microscopy (SEM) of the filter surface showed large graphite-like particles. An intermediate blocking mechanism produced the best fit for the filtration data from PAC pretreatment. The SEM images of the filter surface show a thick cake layer formation in the aluminum chloride and ferric chloride pretreatments. The cake filtration blocking mechanism was the best fit for brackish water treated with aluminum chloride and ferric chloride.

Analysis of the filtration curve and the effect of temperature on silt density index (SDI)

Filtration curves of samples obtained at three different seawater pretreatment plants were analyzed in order to identify the blocking mechanism during silt density index (SDI) measurements of seawater and filtered seawater. This was done in two ways. Firstly, by direct comparison of the measured filtration curves with the calculated curves based on different constant-pressure dead-end filtration models, the coefficient of determination R2 was analyzed. Secondly, by changing the sample water temperature the measured SDI was compared to the calculated SDI in each model. The R2 of both standard blocking models and intermediate blocking models show higher values than those of the other models in the case of seawater filtration curves, indicating that these fit the measured filtration curves. The R2 of the cake filtration model is clearly smaller than those of other models. The measured SDI value of the same filtered seawater sample increases with temperature rise, this tendency coincides with the results calculated by all the filtration models. More precise analysis through R2 comparison of each model shows that the standard blocking model and the complete blocking model, which indicate relatively strong dependence of water temperature on SDI, best coincide with the measured results. The temperature dependency on SDI of the cake filtration model shows poor correlation with test results. Consequently, blocking coefficient (ks) of the standard blocking model is the most appropriate fouling indicator. The indicator ks can be the consistent fouling index, widely adaptable from seawater to filtered seawater after pretreatment. It is also noted that the standard blocking model can be applied for the quantitative evaluation of the effect of water temperature on SDI.

Effects of different pretreatments on the performance of ceramic ultrafiltration membrane during the treatment of oil sands tailings pond recycle water: A pilot-scale study

Membrane filtration is an effective treatment method for oil sands tailings pond recycle water (RCW); however, membrane fouling and rapid decrease in permeate flux caused by colloids, organic matter, and bitumen residues present in the RCW hinder its successful application. This pilot-scale study investigated the impact of different pretreatment steps on the performance of a ceramic ultrafiltration (CUF) membrane used for the treatment of RCW. Two treatment trains were examined: treatment train 1 consisted of coagulant followed by a CUF system, while treatment train 2 included softening (Multiflo™ system) and coagulant addition, followed by a CUF system. The results indicated that minimum pretreatment (train 1) was required for almost complete solids removal. The addition of a softening step (train 2) provided an additional barrier to membrane fouling by reducing hardness-causing ions to negligible levels. More than 99% removal of turbidity and less than 20% removal of total organic carbon were achieved regardless of the treatment train used. Permeate fluxes normalized at 20 °C of 127–130 L/m2 h and 111–118 L/m2 h, with permeate recoveries of 90–93% and 90–94% were observed for the treatment trains 1 and 2, respectively. It was also found that materials deposited onto the membrane surface had an impact on trans-membrane pressure and influenced the required frequencies of chemically enhanced backwashes (CEBs) and clean-in-place (CIP) procedures. The CIP performed was successful in removing fouling and scaling materials such that the CUF performance was restored to baseline levels. The results also demonstrated that due to their low turbidity and silt density index values, permeates produced in this pilot study were suitable for further treatment by high pressure membrane processes.

해수담수화 전처리 및 역삼투막여과 공정의 유기물 제거특성

This study investigated removal characteristics of organic matters in pretreatment and reverse osmosis (RO) membrane processes for seawater desalination. Also, the influence of the changes in characteristics of organic matters on the membrane fouling was assessed. The pretreatment processes included dual media filtration (DMF), pressurized membrane filtration (MF), and submerged membrane filtration (SMF). Turbidity, UV absorption at 254 nm, dissolved organic carbon, size exclusion chromatography (SEC), fluorescence excitation emission matrix (FEEM), and transparent exopolymer particles (TEP) in raw and processed waters were analyzed. Ions and minerals were not removed by any pretreatment process tested, but were removed over 99% through the RO membrane process. Hydrophobic organics, which can play major role in organic membrane fouling, were relatively readily removed compared with hydrophilic ones. Membrane based pretreatment such as MF and SMF exhibited better removals of organics than conventional DMF. As the levels of organics in pretreated water decreased, the silt density index (SDI) decreased. MF treated water exhibited the lowest SDI value; this is possibly due to the lowest TEP () concentrations.

SWRO 해수담수화 공정에서 전처리된 수질조건이 SDI에 미치는 영향

Pretreatment process is the critical step of RO (Reverse Osmosis) membrane desalination plant in order to prevent RO membrane fouling. The pretreatment as a key component of RO process must be designed to produce a constant and high quality RO feedwater which has low silt density index (SDI). This experiment was conducted to assess parameters affecting SDI value, such as pH, seawater turbidity, temperature, and coagulant dose. The experimental results indicated that the source seawater turbidity did cause little effects on SDI values of filtered water. The 0.45 um hydrophilic membrane was more appropriate than the hydrophobic membrane for measuring SDI. The SDI value was increased with decreasing pH under the condition of below pH 7.0. In addition, the water temperature significantly affected the SDI values, showing higher SDI value with lower water temperature.

Limitations, improvements and alternatives of the silt density index

Reverse osmosis (RO) membrane systems are widely used in the desalination of water. However, flux decline due to fouling phenomena in RO remains a challenge. To minimize fouling, a reliable index is necessary to predict the fouling potential of the RO feed water. The ASTM introduced the silt density index (SDI) as a standard fouling index to measure the fouling potential due to colloidal and suspended particles. For decades, the SDI is worldwide accepted and applied. There are growing doubts about the predictive value of this parameter. In addition there are several deficiencies observed, affecting the accuracy and reproducibility e.g. no correction factor for temperature, nor for variations in membrane resistance, and no linear correlation with the concentration of colloidal/suspended particles. This paper gives an overview of our work on limitations, improvements and alternatives for the SDI. Firstly, the influence of the applied 0.45μm test membrane on the SDI will be investigated. Variations in SDI values can be attributed to differences in properties of these membranes. In order to quantify the influence of pressure, temperature and membrane resistance on the SDI a mathematical relation was developed between the SDI and the MFI0.45, assuming cake filtration. In addition, also other fouling mechanisms were incorporated in the model using the well-known blocking laws. Based on a cake filtration fouling mechanism and assuming 100% particle retention, the models were used to normalize the experimental SDI values for temperature, pressure and membrane resistance to the SDI+. By applying this normalization, the results of SDI tests carried out under different conditions and/or with different membranes can be compared easily as was proven experimentally in the lab and at a seawater desalination plant. Finally, an alternative filtration index will be introduced, the volume-based SDI_v. The SDI_v compares the initial flow rate to the flow rate after filtering a standard volume of feed water using MF membranes with an average pore size of 0.45 μm. Our experimental results show that SDI_v is independent of the membrane resistance. In that way, it eliminates most of the disadvantages of the SDI and has great potential to replace the SDI in the field.

Assessment of silt density index (SDI) as fouling propensity parameter in reverse osmosis (RO) desalination systems

Due to its simplicity, silt density index (SDI) is extensively used in reverse osmosis systems despite its limitations in predicting membrane fouling. Employing a reliable fouling index with good reproducibility and precision is necessary. The aim of this investigation is to assess the reliability of SDI in order to understand the reasons for the low level of precision and accuracy. Different commercial SDI membranes and feedwater quality were used in this study. Results showed the existence of membrane properties’ variation within manufacturers, which then causes a lack of accuracy in fouling risk estimation. The nature of particles during SDI filtration provides information that particle concentration and size play a significant role in SDI quantification with substantial representation given by particles with size close to membrane nominal pore size. Moreover, turbidity-assisted SDI measurements along with determination of ultrafiltration permeate and clean water fouling potential, establish the indication of nonfouling-related phenomena involved on SDI measurement such as natural organic matter adsorption and hydrodynamic conditions that alters during filtration. Additionally, it was found that the latter affects the sensitivity of SDI by being represented by some portions of SDI values.

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%.

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.

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.

Silt Density Index and Modified Fouling Index relation, and effect of pressure, temperature and membrane resistance

Particulate matter present in feed water of reverse osmosis and nanofiltration membrane elements tends to deposit on the membrane surface and spacers. This type of fouling results in permeate flux decline, loss of product quality and membrane damage. To characterize the fouling potential of RO feed water the Silt Density Index (SDI) and the Modified Fouling Index (MFI0.45) are commonly applied. SDI is applied worldwide for many years on a routine basis by operators since it is a simple and cheap test. Unfortunately, the SDI has several deficiencies e.g. it is not based on any filtration mechanism, has no linear relation with particulate matter and is not corrected for temperature, pressure and membrane resistance. This might explain the frequently reported erratic results obtained in practice, e.g. water treated with ultrafiltration showed in several cases high SDI values, which could not be attributed to failures of the UF membrane elements or systems. To overcome these deficiencies the MFI0.45 has been developed. This test is based on the occurrence of cake filtration during a substantial part of the test, has a linear relation with particulate matter content, and is corrected for pressure and temperature. However the manual procedure of measuring an MFI0.45 is somewhat more complicated and for this reason less suitable for application on a routine basis in practice. Fully automated equipment, measuring SDI and MFI0.45 at the same time is on the market. In this study a mathematical relation between SDI and MFI0.45 has been successfully developed, assuming that cake filtration is the dominant filtration mechanism during the tests. Based on the developed mathematical relation and experiments with an artificial colloidal suspension of aluminum oxide spheres (0.6 μm) as model water, it could be demonstrated that the SDI depends on pressure, temperature and membrane resistance. The effect of temperature and membrane resistance explains to a large extent the erratic results from the field. It is recommended to correcting SDI for temperature and membrane resistance and/or to making the guideline formulated by ASTM for the allowable range of membrane resistances much more stringent.

Reverse osmosis pretreatment alternatives: Demonstration plant in the seawater desalination plant in Carboneras, Spain

This paper presents the results of a demonstration study for the RO pretreatment that was performed on an AcuaMed open intake desalination plant in Carboneras (Almería, Spain). The main goal was to test different configurations of conventional pretreatments in order to establish the limit of each process as well as their reliability. The pretreatment processes tested were Actiflo® (a process that consists of coagulation, polymer and microsand injection, maturation and settling of microsand ballasted flocs), multimedia filtration at atmospheric pressure, pressurized dual-media filtration and pressurized three-layer filtration. During the tests, the silt density index (SDI) of the filtered water was regularly measured and then evaluated in probability graphs. The raw seawater SDI ranged from 4.5 to 25. The results from this study showed that the Actiflo® system followed by two pressurized multimedia filters (dual-media and three-layer filters) in series, provided effluent SDI values lower than 3 with a reliability of 95% under the optimized operating conditions. These results were obtained when the Actiflo® system included coagulation, flocculation (with flocculant and microsand addition) and maturation stages or just working as a coagulation stage.

Dissolved air flotation of surface water for spiral-wound module nanofiltration pre-treatment

Bench-scale dissolved air flotation (DAF) of Tagus River surface water (Valadas, Portugal) was investigated as pre-treatment for spiral-wound module nanofiltration (SWNF). Before the DAF, a coagulation/flocculation of the surface water was performed using aluminium sulphate, ferric chloride and chitosan coagulants and several commercial coagulant aids. The coagulation/flocculation experiments were carried with 2 L of surface water in a jar-test equipment at room temperature. The DAF performance to remove colloidal matter and suspended solids was evaluated through the measurement of the silt density index (SDI) and the modified fouling index (MFI) of the treated water. The air saturation pressure was varied between 300 and 600 kPa and the recycle ratio between 5 and 50%. The DAF without coagulation/flocculation was not effective in the reduction of both the SDI and MFI, even at a high DAF recycle ratio of 50%. The use of coagulants improved the DAF efficacy in reducing the fouling indexes and, for the optimal operating conditions, the MFI of the treated water was reduced by more than one order of magnitude. However, it was not possible to obtain treated water with SDI and MFI values below the recommended ones for SWNF. Further filtration of the DAF treated water with a 5 micron capsule filter was necessary to reduce the SDI below 5%/min and the MFI below 10 s/l 2. The integration of DAF with another process that removes the residual suspended solids and colloidal matter is, therefore, necessary to provide a good pretreatment for the SWNF of Tagus River surface water.

The sensitivity of SDI analysis: from RO feed water to raw water

The water characterisation is generally challenging regarding the pre-treatment operation up-stream reverse osmosis. The classical parameters are not applicable due to analytical limitations, and/or the sensibility of those parameters is too low compared to the water fluctuation behaviour measured on full-scale water treatment plants. The evaluation of the water fouling potential could be a more sensible parameter, supplementing the analytical tools actually used for design purpose. The silt density index (SDI) is a parameter characterising the fouling potential of water. It is one of the most important parameter for the design and operation of RO membrane process. SDI analytical protocol is standardized in the ASTM D 4189-95 (Reapproved 2002). This method evaluates the quantity of matter in water, based on fouling variation of a 0.45-μm membrane during a filtration time. It is typically applied on low fouling potential water such as pre-treated water feeding reverse osmosis or nanofiltration (SDI 15 min < 5). SDI could become a good parameter to appreciate the fouling potential of water with low turbidity and suspended solids. However, there is a high risk for generating some imprecision on the result. As a consequence, this parameter has to be defined more precisely. The different methods to evaluate the fouling tendency of water are presented. Essentially based on ASTM D 4189-95, SDI standard method can be reviewed regarding the reproducibility and precision taking into account the possible deviations observed on site. The ASTM text is discussed with the presentation of its limits especially for high SDI values. Different factors, which can interfere on the SDI results are detailed (quality of the 0.45 mm membrane, percentage of plugging…). Some comparative tests results are presented with data collected from pilot studies and literature. Knowing the field of application and the limits of SDI-ASTM, the method is extended to higher fouling tendency water, based on 75% plugging limit. The SDI 75% is then applied to the characterization of surface seawater, brackish water and results compared with other more usual parameters such as turbidity, suspended solids, algae, particles count, etc. This characterization is particularly helpful for pre-treatment operation and to detect some failure on its efficiency.

Pretreatment for desalination of seawater from an open intake by dual-media filtration: Pilot testing and comparison of two different media

The results of pilot testing of dual media filtration on seawater from an open intake in the Thermaikos Gulf, northern Greece, are presented in this study. Two different filter materials, FILTRALITE MC, a processed expanded clay material with particle range 1.5–2.5 mm, and anthracite coal with a similar particle range, were tested in parallel operation under various operating conditions (coagulant type and dose, filtration rate). Attention was focused on filtrate quality parameters relevant to feeding RO membrane desalination systems, that is, turbidity, silt density index (SDI), total organic carbon (TOC). In order to evaluate the effect of temperature variations on pretreatment performance, similar experiments were performed during a winter and a summer campaign of the same year. Despite the poor quality of feedwater, through an open intake, very low turbidities and SDI values lower than 5 could be obtained by means of coagulation and dual media filtration. Both filtration materials demonstrated almost similar performance in removing particulates from the feed water and produced permeates of acceptable quality for feeding RO systems (SDI 15 values lower than 5) during the winter and summer campaigns. It was observed that both filters were much more effective during the summer period due to the higher temperatures prevailing, which allowed the formation of larger aggregates and their subsequent effective capture within the filter media. Laboratory and on site pilot tests showed that optimal performance was achieved when polyaluminum chloride (PACl) was added to the feed water as a coagulant medium at a dose 1.8 mg Al/L. In the winter period similar SDI 15values were obtained by both filters when operating at a flow velocity of 10m/h, whereas at 5 m/h the sand/filtralite column produced water with lower SDI 15values and better quality compared to the one from the sand/anthracite column. On the other hand, during the summer period, and for the same coagulant dose, similar SDI 15values (usually close to 3) were obtained by both filters at all the tested flow velocities (5, 10, 15 m/h). Furthermore, due to the effect of temperature on aggregation, in the winter period the duration of filtration cycles was found to be determined by particle breakthrough. In contrast, in the summer no breakthrough was observed and the filtration cycles were determined by the available free space for hydrostatic head buildup above the filter media. Finally, regarding the hydrostatic head development, it was observed that the sand/filtralite column had a slower head buildup than the sand/anthracite column for the same duration of filter operation.

Evaluation of organic matter fouling potential by membrane fouling index

The membrane fouling index, widely used in the reverse osmosis (RO) membrane industry, is developed mainly to assess the fouling potential of particulate matter. However, the fouling potential by organic matter should also be accurately evaluated as the productivity loss by organic fouling is often more severe than particle fouling, despite its low concentration in feed water. This study was performed to investigate the influence of organic characteristics and feed water solution chemistry on the membrane fouling index, such as the silt density index (SDI). The results showed that the feed solution chemistry (i.e. pH, ionic strength and hardness) affected the SDI values of organic rich feed water to some degree. In addition, Aldrich humic acid (AHA) exhibited higher fouling potential by SDI than Suwannee river humic acid (SRHA), emphasising the importance of organic properties. Lastly, it is shown that although SDI values measured were the same, the degree of NOM fouling was significantly different for the membranes with different surface properties.

An assessment of the Silt Density Index based on RO membrane colloidal fouling experiments with iron oxide particles

RO membrane colloidal fouling experiments were performed in the laboratory under well controlled and realistic conditions. Iron oxide was selected as a typical inorganic colloidal foulant, due to its importance, as evidenced from well known manufacturer recommendations on iron concentrations in feed waters and from frequently encountered problems in membrane installations. A range of iron concentrations was identified where a linear relationship existed between flux reduction rate and concentration. The performance of the Silt Density Index (SDI) was tested on the basis of the RO fouling data obtained. The range of iron concentrations where measurable and meaningful SDI values could be obtained was remarkably close to membrane manufacturer recommendations. A notable sensitivity of the SDI was also observed with particles for which retention is negligible. However, on the basis of the RO fouling data obtained, it appears that the SDI is not conservative enough. Furthermore, since the SDI cannot predict fouling rates, it cannot discriminate between different types of membranes.