REMOVAL OF ALGAE, BACTERIA, AND ORGANIC CARBON BY THE WELL INTAKE SYSTEM AT THE SUR, OMAN SWRO FACILITY

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1 REMOVAL OF ALGAE, BACTERIA, AND ORGANIC CARBON BY THE WELL INTAKE SYSTEM AT THE SUR, OMAN SWRO FACILITY Authors: Rinaldi M. Rachman, Tony Merle, Sheng Li, Samir K. Al-Mashharawi, Thomas M. Missimer, Water Desalination and Reuse Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia Presenter: Rinaldi M. Rachman PhD Candidate, Water Desalination and Reuse Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia Abstract: Raw seawater extraction for desalination plants using wells has been considered as an alternative to open seawater intake use. This intake method is generally believed to deliver better quality feed water and also should reduce environmental impacts and the overall cost of desalination. An investigation was conducted at the Sur Desalination Plant in Oman to evaluate the water quality improvements produced by using wells to obtain desalination feed water. Analyses were made to assess water quality from the sea, the wells, and the aggregated inflow of the wells into the pretreatment process, focusing on algae, bacteria, and organic carbon removal. It was found that the well intake system produces positive impacts on plant operation by effective removal of algae, bacteria, and organic compounds that may potentially lead to membrane biofouling. Research on the effects of wells on raw seawater quality was conducted using flow cytometer measurements to assess changes in concentrations of algae and bacteria. Removal of algae by the wells was found to be quite effective with virtually no cells remaining in the feed water transported to the RO process. The cell counts in the raw water ranged from 1,900 to 113,040 (differs for each algae type), whereas algal concentration in the well water were <100 cells/ml. Total bacteria in the water had a concentration of just under 1,000,000 cells/ml. Concentrations in the produced well water ranged from 3,270 to 13,630 cells/ml which correlates to removal efficiencies ranging from 98.6 to 99.96%. Detailed analysis of the organic compounds in the raw seawater verse the well water also showed significant reduction in concentration. The DOC reduction was up to 67% with the biopolymer fraction reduction approaching complete removal and other organics fractions are in significantly reduced conditions. Also, the concentration of transparent exopolymer particules (TEP), a known precursor to biofouling, was reduced by 60 to 80%. The International Desalination Association 2013 / Tianjin, China

2 I. INTRODUCTION The intake is an integral part of the desalination process aiming to provide adequate and consistent flow of feed water [1]. Conventional marine seawater intake systems, such as an open-ocean type, have been the preferential method for supplying feed water for seawater reverse osmosis (SWRO) plants. Seawater collected through an open-ocean intake and pumped to a treatment plant contains high concentrations of marine debris, organic matters, and microorganisms. Seawater quality is commonly unstable depending on the seasonal variations (temperature or physical oceanographic change-induced) and periodic natural marine occurrences such as storms (high turbidity) and harmful algal blooms. Raw water collected using an open-ocean intake requires an extensive train of pretreatment processes before it can pass into primary RO process. The improvement of feed water quality eases the operation of the plant, requiring less pretreatment trains, increasing operation reliability, and reducing operation costs [2]. Advancement of the pretreatment methods, which typically require more energy and are chemically intensive, has been the trend over past years to enhance the RO feed water quality. Alternatively, research can be concentrated on the development of the water intake systems to achieve the same goal. The subsurface intake as an alternative method of supplying marine water is generally believed to deliver better feed water quality [1,2]. This intake method utilizes the natural geological properties of sediments or rock to perform physical and biochemical treatment, removing organic matter, suspended sediments, dissolved organic compounds, and microorganisms [3,4]. Most of the subsurface systems act similar to river bank filtration or bank filtration systems which have been around for over a century to treat freshwaters in Europe and the United States [5]. The intake construction and operation feasibility is highly dependent on the local geological and environmental characteristics in the coastal zone. Subsurface intake systems can be classified into two major categories: wells and galleries [4]. Gallery intakes typically involve construction of media filtration in the beach or the seabed. The treated water is extracted from the bottom of the filter bed. Conversely, wells essentially are vertical structures that tap into local aquifer recharged by seawater. A schematic of a well system is shown in Figure. 1 as an example, in which the well is located adjacent to the beach allowing the intake of seawater. The well systems can be subdivided, based on the water flow orientation, into conventional vertical wells, horizontal wells or drains, angle/slant wells, and Ranney wells or collectors [2]. Fig. 1 Schematic of well intake system

3 In spite of the exceptional overall performance of the subsurface intakes to improve feed water quality, limited research has been done regarding the degree of removal of algae, bacteria, organic carbon, and specific compounds that may lead to biofouling. This paper will present the results of a well performance investigation at the Sur desalination plant in Oman, focusing on water quality improvements by looking into SDI, bacteria and algae counts, organic compounds, and transparent exopolymer particles (TEP). II. METHODS 2.1 Field Investigation The Sur Desalination Plant relies on the wells to obtain the raw seawater. The facility currently has the largest subsurface intake capacity in the world at about 160,000 m 3 /day. A series of 28 wells are constructed into a shallow Paleocene/Eocene age limestone that contains primary and secondary porosity (karstic). The location of the wells is divided into three areas i.e. west, center, and east relative to the plant location (Fig.2). Within each area, the location of the individual wells was carefully chosen by allowing adequate separation distance to ensure steady supply of water without exceeding maximum well drawdown criteria [6]. The plant is operating reliably with no major issues and had no membrane chemical cleaning since start-up two years ago. Water samples were collected from several locations to determine the water quality improvement by the wells. The samples were taken from the sea (1.5 m water depth offshore), the wells (numbers 1W, 9W, 12C, and 33E), and the aggregated wells inflow into the pretreatment process. 2.2 Water Quality Analysis Fig. 2 Diagram of wells location at Sur Desalination Plant Following the sampling campaign, a series of analyses were conducted to quantify the water conditions. General water quality was determined onsite by measuring the turbidity using a portable

4 turbidity meter (HACH 2100Q). The conductivity, salinity, TDS, and temperature were measured using a handheld conductivity meter (WTW Con 3210 SET1). In addition, Silt Density Index (SDI) analysis was also performed on each water sample. The organics and microbial quantifications were conducted at the Water Desalination and Reuse Center laboratory at the King Abdullah University of Science and Technology (KAUST). Laboratory measurement followed standard operating protocols to ashore quality control and measurement accuracy. The collected samples were appropriately treated, packed, and handled to ensure that preservation of the original condition was conserved. The total and dissolved organic carbon (TOC and DOC) were measured using a TOC Analyzer from Shimadzu. The fraction of organic carbon was determined using Liquid Chromatography Organic Carbon Detector (LC-OCD) from DOC Labor. The bacteria and algae quantification was performed using a BD Biosciences FACSVerse flow cytometer. The samples were previously fixed with glutaraldehyde once freshly collected at the site and were kept at 4 0 C at all times until the analysis. Direct measurement was performed to quantify the bacteria population, whereas comparative staining was performed for algae quantification. The measurement and identification of particle/cell population is based on the particle size. Identifier, 1µm beads, was added to each sample to standard size measurement. Triplet sampling was performed to assure stable and accurate measurement and sampling techniques (QAQC). Transparent exopolymer particles (TEP) analyses were also conducted. The combination of filtration and UV absorbance techniques was used. The filtration method was included to improve the result accuracy due to the low concentrations in the samples (near detection limit) when the UV absorbance method was used solely. Sample fixation was done by conditioning the water samples to contain 0.02%- w/v sodium azide. At least two liters of each sample was fed into vacuum filtration passing through a 0.4 um polycarbonate membrane. Subsequently, alcian blue dye was added to the membrane and kept for twenty seconds and was followed by DI water rinsing to flush the excess color. The membrane was then carefully collected and placed into a small beaker. A high concentration chloric acid solution then added to the beaker to oxidize the color. After at least 6 hours, the acid solution was collected to be analyzed with a UV absorbance at 752 nm. The concentration of TEP was then determined from a calibration curve which was previously made to relate specific UV absorbance with various concentrations of xanthan gum representing the TEP. III. RESULTS AND DISCUSSION 3.1 General Water Quality The general water quality results are presented in Table 1. The total dissolved solid (TDS) values are almost identical for all of the sampling locations. This observation indicates that major water recharge of the well aquifer is from the sea, which is the preferable condition of well intake system [6]. Some possible disturbances of well recharge can occur from high-salinity water originating in sabkhas or contaminated water from community sewage systems which can cause movement of water from the landward direction. This can cause hyper-saline water and/or high organics content water, respectively, to enter the wells

5 Parameter Table 1. General water quality results. Seawater Well 1W Well 9W Well 12C Well 33E TDS [=] ppm Turbidity [=] NTU SDI The SDI is based on five minutes of filtration Improvement of water quality is observed in the turbidity and SDI values. The turbidity of well discharge was reduced compared to the seawater. A similar trend is also observed in the SDI data which is one of the most practical tools used in determination of fouling potential of the membrane system. Considering the value of SDI below 3 as the most common requirement of pretreatment outflow before it goes to membrane system, the well intake system delivers exceptionally high quality feed water. Solely based on the SDI, the quality of the discharge water is found comparable with the pretreated feed water at other seawater reverse osmosis (SWRO) systems. It illustrates the degree of pretreatment taking place in the well system. Thus, there is a possibility that subsurface intakes can replace most of the pretreatment trains required when operating a plant using an open-ocean intake system. 3.2 Bacteria and Algae Removal The flow cytometer (FCM) results for algal quantification are shown in Table 2 with graphical representation given by Fig. 3. Table 2 FCM analysis results for algae(values are in cells/ml). Location Prochlorococcus sp. Synechococcus sp. Pico/nano plankton Seawater 4, ,040 1,900 Well 1W < 100 < 100 < 100 Well 9W < 100 < 100 < 100 Well 12C < 100 < 100 < 100 Aggregated Well < 100 < 100 <

6 Fig. 3 FCM analysis results for algae. A is seawater, B is well 9W, C is well 12C, and D is aggregated wells (in A, (1) is Procolorococcus marinus, (2) is Synechococcus sp., and (3) is pico/nano plankton). The seawater was found to contain Synechococcus sp. as the major algae cluster with smaller portion of Proclorococcus marinus and pico/nano plankton. Significant removal was found for Synechococcus sp. cluster, representing 1-3µm algae, which reduced in concentration from 113,000 cells/ml in seawater to <100 cells/ml (indication of near detection limit value) in the well discharge streams. Similar trends of complete removal also were found for the other algae clusters. This observation can be extended further to an assumption that the well intake system is able to protect the desalination plants in the problematic case of algal bloom and red tides. The quantification of bacterial contents by the FCM method is shown in the Table 3 with graphical representation depicted in Fig. 4. The total bacteria concentration in the seawater was 995,000 cells/ml and reduced to a range from 3,000 to 13,000 cells/ml in the well discharges, confirming significant removal of bacterial content by the well system. Table 3 FCM analysis results for bacteria. Values are in cells/ml. Location Total Bacteria LNA Bacteria HNA Bacteria Seawater 995, , ,850 Beach Well 1W 3,270 2, Beach Well 9W 8,540 6,110 2,230 Beach Well 12C 13,630 9,520 3,900 Aggregated Well 11,000 7,540 3,

7 Fig. 4. FCM analysis results for bacteria. A is seawater, B is well 9W, C is well 12C, and D is the aggregated wells. A simple classification is given to the bacterial fraction defining low nucleic acid (LNA) and high nucleic acid (HNA) bacteria fractions. The classification represents the relative size; the HNA bacteria are bigger in size compared to LNA bacteria. The LNA bacteria were found at higher concentrations than the HNA bacteria in the seawater. The preferential removal of the HNA fraction is also indicating a size exclusion removal mechanism by the wells. The removal of bacteria and algae by the well intake system supports the assumption that a significant degree of pretreatment is performed by the wells. A potential elimination of some pretreatment trains when comparing the operation of a SWRO plant using a subsurface intake compared to an open-ocean intake once again shows that the subsurface intake system use leads to a reduction in SWRO operating cost. 3.3 Organic Carbon Compounds Removal The organics content characterization is important to identify the potential of membrane (bio)fouling. Many of the organic compounds occurring on seawater are precursors to persistent biofouling of the RO membranes which can significantly affect the system performance and integrity [7]. The bulk organic parameter, DOC, was found to be reduced up to 67% in the well discharges compared to the seawater. The results of LC-OCD analysis of the organic fraction is provided in Fig

8 Location Aggregated Low Molecular Weight Acids Low Molecular Weight Neutrals Bldg. Blocks Humic Substances Biopolymer BW 12C BW 9W BW 1W Seawater Organic Concentrations [=] ppb Fig. 5 Organics content characterization in wells discharge and seawater The fractions of organic matter detected by the instrument are biopolymer, humic substances, building blocks, low molecular weight neutrals, and low molecular weight acids. Aforementioned fractions are in ordered based on their relative molecular size with a descending trend. Complete removal is experienced by biopolymer substances, which is the biggest size fraction of organics undergoing reduction by some size exclusion mechanism. The remaining organic fractions are also significantly reduced in concentration. Thus, the well system successfully attenuates the key organic compounds, reducing the potential for membrane biofouling and illustrating that wells perform a pretreatment function for SWRO systems. The TEP content was also quantified to assess changes in a key compound that may induce membrane conditioning which leads to biofilm formation [8]. The filtration-uv method results are given in Fig. 5. The TEP concentration was found to be reduced in similar manner with aforementioned analyses between the seawater and the well discharges (60-80% removal) to produce water quality improvement in relation to biofouling potential

9 Fig. 6 TEP concentration in wells discharges and seawater IV. CONCLUSIONS A site visit to the Sur Desalination Plant was conducted to obtain water samples for analysis that would assess the performance of the well intake system, focusing on general water quality improvement, and algae, bacteria, and organic compounds removal. Significant reduction in the SDI was found from in the seawater to 0.8 to 1.4 in the wells sampled. The algae, bacteria, and organic compound concentrations were significantly reduced by the well system, resulting in a comparable water quality to that occurring using a complex pretreatment system applied to open-ocean intake supplied feed water. The size exclusion mechanism was clearly demonstrated by the preferential removal of larger particles (algae, HNA bacteria, and biopolymer fractions). Water quality enhancement, based on observed reductions in concentration of various organic compounds, illustrates that specific biochemical mechanisms are also occurring in the well system. The well intake system produces positive impacts on the plant operation by delivering low potential fouling water and performs virtually all of the pretreatment processes without the use of chemicals. This observation can be extend to a possibility of eliminating some or all pretreatment processes if a subsurface intake system is used instead of an open-ocean intake. The subsurface intake system at Sur, Oman, conventional vertical wells system, provides a high quality feed water to the SWRO plant which reduces biofouling potential and the frequency of membrane cleaning. This is a clear example of the viability of using a subsurface intake system for a plant of significant capacity. ACKNOWLEDGEMENTS This research was funded by the Water Desalination and Reuse Center and from baseline research funding provided by the King Abdullah University of Science and Technology. The authors thank the Water Desalination and Reuse Center laboratory for use the analytical equipment and Sur Desalination Plant in Oman for access of operation. For exceptional field assistance, the authors would express high appreciation to Marcelino Linas, Saravan Subramanian, S.P. Logeswaran, Mohd. Said Al Farsi, and Saud Al Fannah

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