Sensitivity of SDI for experimental errors

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

Removal of polar organic micropollutants by pilot-scale reverse osmosis drinking water treatment

The robustness of reverse osmosis (RO) against polar organic micropollutants (MPs) was investigated in pilot-scale drinking water treatment. Experiments were carried in hypoxic conditions to treat a raw anaerobic riverbank filtrate spiked with a mixture of thirty model compounds. The chemicals were selected from scientific literature data based on their relevance for the quality of freshwater systems, RO permeate and drinking water. MPs passage and the influence of permeate flux were evaluated with a typical low-pressure RO membrane and quantified by liquid chromatography coupled to high-resolution mass spectrometry. A strong inverse correlation between size and passage of neutral hydrophilic compounds was observed. This correlation was weaker for moderately hydrophobic MPs. Anionic MPs displayed nearly no passage due to electrostatic repulsion with the negatively charged membrane surface, whereas breakthrough of small cationic MPs could be observed. The passage figures observed for the investigated set of MPs ranged from less than 1%-25%. Statistical analysis was performed to evaluate the relationship between physicochemical properties and passage. The effects of permeate flux were more pronounced for small neutral MPs, which displayed a higher passage after a pressure drop

Short to medium term optimization of dead-end ultrafiltration

Dead-end ultrafiltratie is een veelbelovende waterzuiveringtechnologie. Echter, het membraan vervuilt tijdens de productie. De kosten van vervuilingpreventie en membraan­reiniging dragen flink bij aan de kostprijs van gefiltreerd water. Bastiaan Blankert onder­zocht hoe het ultrafiltratieproces efficiënter en goedkoper kan worden afgesteld. Het resulaat is een kostenreductie van 34 procent

Minimum Net Driving Temperature Concept for Membrane Distillation

In this study, we analyzed the heat requirement of membrane distillation (MD) to investigate the trade-off between the evaporation efficiency and driving force efficiency in a single effect MD system. We found that there exists a non-zero net driving temperature difference that maximizes efficiency. This is the minimum net driving temperature difference necessary for a rational operational strategy because below the minimum net driving temperature, both the productivity and efficiency can be increased by increasing the temperature difference. The minimum net driving temperature has a similar magnitude to the boiling point elevation (~0.5 °C for seawater), and depends on the properties of the membrane and the heat exchanger. The minimum net driving temperature difference concept can be used to understand the occurrence of optimal values of other parameters, such as flux, membrane thickness, and membrane length, if these parameters are varied in a way that consequently varies the net driving temperature difference

Dynamic optimization of a dead-end filtration trajectory: blocking filtration laws

An operating model for dead-end membrane filtration is proposed based on the well-known blocking laws. The resulting model contains three parameters representing, the operating strategy, the fouling mechanism and the fouling potential of the feed. The optimal control strategy is determined by minimizing the energy consumption for a fixed final time and produced volume.\ud \ud It was found that constant power filtration leads to minimal energy consumption. Constant flux and constant pressure filtration have equal energy costs. However, compared to strategies with a non-decreasing pressure and non-increasing flux, the relative savings are small. Only if the fouling mechanism resembles standard blocking and the fouling resistance is large compared to the membrane resistance, it may be attractive to implement the optimal trajectory

Development of a control system for in-line coagulation in an ultrafiltration process

A control system for the in-line coagulation applied in an ultrafiltration process was developed. The dosing strategy aims to apply a minimal coagulant dose while maintaining desirable process performance by regulating the initial filtration resistance. This is achieved by a feedback controller. It was found that the control system performs well; adaptation to changing conditions is achieved adequately and sufficiently fast. The initial resistance of the last filtration before the chemical cleaning phase can be controlled within an accuracy of approximately 3% (of the total resistance) or 9% (of the fouling resistance). Compared to the current dosing strategy, a significant reduction of coagulant consumption can be achieved

Dynamic optimization of a dead-end filtration trajectory: non-ideal cake filtration

A control strategy aimed at minimizing energy consumption is formulated for non-ideal dead-end cake filtration with an inside-out hollow fiber ultrafiltration membrane system. The non-ideal behavior was assumed to originate from cake compression, non-linear cake resistance and a variable pump efficiency.\ud \ud Constant gross power, constant flux and constant pressure filtration were considered as alternatives for the optimal operating strategy. It was found that the ratio between the initial and final total resistance determines whether a large difference between these strategies occurs. This is mainly determined by the specific cake resistance, the final state and the membrane resistance.\ud \ud When there is a large difference between the operating strategies, the pump characteristics determine which suboptimal strategies are attractive. For a pump with a low head and a large capacity, constant flux filtration is nearly optimal, whereas for a pump with a large head and a small capacity optimal operation is closer to constant pressure filtration. Under the investigated conditions there was no significant difference (< 0.5%) between the constant gross power and the optimal operating strategy