Optimal design of water treatment processes

Predicted water shortages assign water treatment a leading role in improving water resources management. One of the main challenges associated with the processes remains early stage design of techno-economically optimised purification. This work addresses the current gap by undertaking a whole-system approach of flowsheet synthesis for the production of water at desired purity at minimum overall cost. The optimisation problem was formulated as a mixed-integer non-linear programming model. Two case studies were presented which incorporated the most common commercial technologies and the major pollution indicators, such as chemical oxygen demand, dissolved organic carbon, total suspended solids and total dissolved solids. The results were analysed and compared to existing guidelines in order to examine the applicability of the proposed approach

Algal blooms and membrane based desalination technology; Dissertation, UNESCO-IHE Institute for Water Education, Delft.

Seawater desalination is rapidly growing in terms of installed capacity (~80 million m3/day in 2013), plant size and global application. An emerging threat to this technology is the seasonal proliferation of microscopic algae in seawater known as algal blooms. Such blooms have caused operational problems in seawater reverse osmosis (SWRO) plants due to clogging and poor effluent quality of the pre-treatment system which eventually forced the shutdown of the plant to avoid irreversible fouling of downstream SWRO membranes. As more extra large SWRO plants (>500,000 m3/day) are expected to be constructed in the coming years, frequent chemical cleaning (>1/year) of SWRO installations will not be feasible, and more reliable pre-treatment system will be required. To maintain stable operation in SWRO plants during algal bloom periods, pre-treatment using ultrafiltration (UF) membranes has been proposed.This thesis addresses the effect of algal blooms on the operation of UF pre-treatment and SWRO. Experimental investigations demonstrated that marine algal blooms can impact the backwashability of UF and can accelerate biological fouling in RO. However, it is unlikely that algae themselves are the main causes of fouling but rather the transparent exopolymer particles (TEPs) that they produce. To better monitor TEPs, a new method capable of measuring TEP as small as 10 kDa was developed and showed that TEPs can be effectively removed by UF pre-treatment prior to SWRO. This work also demonstrated that although TEPs and other algal-derived material (AOM) are very sticky and can adhere to UF and RO membranes, adhesion can be much stronger on membranes already fouled with AOM. Moreover, a model was developed to predict the accumulation of algal cells in capillary UF membranes which further demonstrated that the role of algal cells in UF fouling is not as significant as that of AOM and TEPs. Overall, this study demonstrates that better analytical methods and tools are essential in elucidating the adverse impacts of algal blooms in seawater on the operation of membrane-based desalination plants (UF-RO). It also highlighted the importance of developing effective pre-treatment processes to remove AOM from the raw water and reduce the membrane fouling potential of the feed water for downstream SWRO membranes

Removal of Transparent Exopolymer Particles (TEP) in integrated membrane systems

Integrated membrane systems (IMS) had been successful in controlling particulate fouling in reverseosmosis (RO) treatment. However, it fell short in dealing with a more persistent problem of organic andbiological fouling. The discovery of a formerly unknown but abundant form of polysaccharide calledtransparent exopolymer particles (TEP), led to a better understanding of the role of polysaccharides inbiofouling of RO systems. The main objectives of this research were: (i) to develop a suitable method tomeasure TEP and its colloidal fraction in fresh and sea water; (ii) assess TEP removal by pre-treatment;and (iii) employ TEP as indicator of biofouling potential of RO feedwater in IMS.Modifications on the spectrophotometric method (Passow and Alldredge, 1995b) were proposed,evaluated and verified based on its applications to fresh and marine water samples. The majormodification was the introduction of smaller pore size filters (0.4ìm) and colloidal (0.05-0.4ìm) TEP. The TEP retained on the filters were stained withAlcian Blue (dye specific for acid polysaccharides), dissolved in acid and then absorbance at 787nm weremeasured. Based on the absorbance, TEP was semi-quantified by relating it with the absorbance of astandard. A commercially available polysaccharide (Xanthan gum) was used to standardize the weightof polysaccharides binding with Alcian Blue. TOC measurements were employed to estimate the dryweight of Xanthan, relating TOC removal by filtration to the amount of Xanthan (ìg) retained on thefilter. TEP concentrations were expressed in terms of milligrams Xanthan equivalent per liter (mg Xeq.L-1).Sample measurements showed significant TEP variations for different types of water. Total TEPconcentrations ranged from 1.13 mg Xeq.L-1 in canal water to 8.12 mg Xeq.L-1 in seawater, with highconcentrations recorded during warm seasons. Sequential filtration of water samples revealed thatcolloidal TEP (0.05-0.40ìm) was more abundant than particulate TEP (>0.40 ìm), highlighting theimportance of measuring the colloidal/dissolved TEP fraction.TEP measurements performed on 8 IMS plants recorded TEP removal of 80% by coagulationflocculation-sedimentation, 42-81% by ultrafiltration, 57-70% by in-line coagulation (FeCl3) + UF, 48-80%by microfiltration, 34-71% by coagulation (FeCl3) + rapid sand filtration and 53% by rapid sand filtration.The RO concentrate to feed ratio were analysed to predict TEP accumulation or depletion in the ROsystem and relate it to the extent of biofouling based on the RO cleaning frequency of 5 IMS plants. HighTEP accumulation coincided with high cleaning frequency for 3 plants (A, B, C) while it was not the casefor 2 plants (D and E). Bioufouling limitations for Plant D were attributed to nightly chlorine disinfectionin the MF and/or possible osmotic shock and osmotic backflush cause by the recycling of RO permeate,while the good performance of Plant E was probably due to the fact that it was running with a newlyinstalled RO system (3 months).LC-OCD results for Plant B showed that only 50% of biopolymers in the RO feedwater wererecovered in the concentrate as compared to >70% recovery of other NOM fractions, an indication thatbiopolymer accumulation likely occurred in the RO system. Biopolymer analysis revealed that more than95% of proteins were removed by ultrafiltration while just 60% removal of polysaccharides, suggestingthat the latter is the likely cause of biopolymer fouling in the RO system downstream. Comparison of theLC-OCD polysaccharide and colloidal TEP results in the RO system of 4 IMS plants showed closecorrelation for Plants B, D and E (±13%) but not for Plant A (± 96%). The latter could be a result of the sizelimitations of the two methods (proposed TEP method>0.05 ìm; LC-OCD< 0.45 ìm).The specific deposition rate (SDR) of TEP was determined on 5 IMS based on mass balancecomputations in the RO system. SDR of TEP ranged from -2.6ìg Xeq/m2h in April to +22.9ìg Xeq/m2h inMay. However, some of the SDR results did not coincided with the hypothesis that the higher the SDR,the higher the extent of biofouling and thus frequent RO cleaning. This was attributed to flowassumptions, which probably did not matched with the actual conditions during sampling.Improvements of data inputs for future studies are necessary to make SDR an efffective diagnosis tool offouling in RO systems

Algal blooms and Membrane Based Desalination Technology

Seawater desalination is rapidly growing in terms of installed capacity (~80 million m3/day in 2013), plant size and global application. An emerging threat to this technology is the seasonal proliferation of microscopic algae in seawater known as algal blooms. Such blooms have caused operational problems in seawater reverse osmosis (SWRO) plants due to clogging and poor effluent quality of the pre-treatment system which eventually forced the shutdown of the plant to avoid irreversible fouling of downstream SWRO membranes. As more extra large SWRO plants (>500,000 m3/day) are expected to be constructed in the coming years, frequent chemical cleaning (>1/year) of SWRO installations will not be feasible, and more reliable pre-treatment system will be required. To maintain stable operation in SWRO plants during algal bloom periods, pre-treatment using ultrafiltration (UF) membranes has been proposed. This thesis addresses the effect of algal blooms on the operation of UF pre-treatment and SWRO. Experimental investigations demonstrated that marine algal blooms can impact the backwashability of UF and can accelerate biological fouling in RO. However, it is unlikely that algae themselves are the main causes of fouling but rather the transparent exopolymer particles (TEPs) that they produce. To better monitor TEPs, a new method capable of measuring TEP as small as 10 kDa was developed and showed that TEPs can be effectively removed by UF pre-treatment prior to SWRO. This work also demonstrated that although TEPs and other algal-derived material (AOM) are very sticky and can adhere to UF and RO membranes, adhesion can be much stronger on membranes already fouled with AOM. Moreover, a model was developed to predict the accumulation of algal cells in capillary UF membranes which further demonstrated that the role of algal cells in UF fouling is not as significant as that of AOM and TEPs. Overall, this study demonstrates that better analytical methods and tools are essential in elucidating the adverse impacts of algal blooms in seawater on the operation of membrane-based desalination plants (UF-RO). It also highlighted the importance of developing effective pre-treatment processes to remove AOM from the raw water and reduce the membrane fouling potential of the feed water for downstream SWRO membranes.Water ManagementCivil Engineering and Geoscience

Characterisation of algal organic matter produced by bloom-forming marine and freshwater algae

Algal blooms can seriously affect the operation of water treatment processes including low pressure (micro- and ultra-filtration) and high pressure (nanofiltration and reverse osmosis) membranes mainly due to accumulation of algal-derived organic matter (AOM). In this study, the different components of AOM extracted from three common species of bloom-forming algae (Alexandrium tamarense, Chaetoceros affinis and Microcystis sp.) were characterised employing various analytical techniques, such as liquid chromatography – organic carbon detection, fluorescence spectroscopy, fourier transform infrared spectroscopy, alcian blue staining and lectin staining coupled with laser scanning microscopy to indentify its composition and force measurement using atomic force microscopy to measure its stickiness. Batch culture monitoring of the three algal species illustrated varying characteristics in terms of growth pattern, cell concentration and AOM release. The AOM produced by the three algal species comprised mainly biopolymers (e.g., polysaccharides and proteins) but some refractory compounds (e.g., humic-like substances) and other low molecular weight acid and neutral compounds were also found. Biopolymers containing fucose and sulphated functional groups were found in all AOM samples while the presence of other functional groups varied between different species. A large majority (&gt;80%) of the acidic polysaccharide components (in terms of transparent exopolymer particles) were found in the colloidal size range

Biofouling in capillary and spiral wound membranes facilitated by marine algal bloom

Algal-derived organic matter (AOM), particularly transparent exopolymer particles, has been suspected to facilitate biofilm development in membrane systems (e.g., seawater reverse osmosis). This study demonstrates the possible role of AOM on biofouling in membrane systems affected by marine algal blooms. The tendency of AOM from bloom-forming marine algae to adhere to membranes and its ability to enhance biofilm growth were measured using atomic force microscopy, flow cytometry, liquid chromatography and accelerated membrane biofouling experiments. Adhesion force measurements indicate that AOM tends to adhere to clean membranes and even more strongly to AOM-fouled membranes. Batch growth tests illustrate that the capacity of seawater to support bacterial growth can significantly increase with AOM concentration. Biofouling experiments with spiral wound and capillary membranes illustrate that when nutrients availability are not limited in the feed water, a high concentration of AOM – whether in suspension or attached to the membrane – can substantially accelerates biofouling. A significantly lower biofouling rate was observed on membranes exposed to feed water spiked only with AOM or easily biodegradable nutrients. The abovementioned findings indicate that AOM facilitates the onset of membrane biofouling primarily as a conditioning platform and to some extent as a nutrient source for biofilm-forming bacteria.</p