XII Congreso Internacional de Aedyr Toledo, España, Octubre, 2018

Transcription

1 Reliability and efficiency assessment of a seawater desalination treatment scheme by means of pilot plant, in views of a subsequent full scale plant construction Authors: Bayona González, Carlos 1 (carlos.bayona.gonzalez@acciona.com), Sanroma Flores, Clara 1 (clara.sanroma.flores@acciona.com), Ferrer Mallén, Olga 1 (olferrer@acciona.com); Malfeito Sánchez, Jorge 1 (jorgejuan.malfeito.sanchez@acciona.com) 1 R&D Department, Acciona Agua S.A.U, Parc de Negocis Mas Blau II, Avda. de les Garrigues,, 88 El Prat de Llobregat, Barcelona, Spain Abstract: Despite seawater desalination by reverse osmosis (RO) is widely implemented due to its well-known advantages, its performance assessment by means of pilot plants provides highly valuable information for the succeeding full-scale plant construction and operation. In this study, a pilot plant consisting in dissolved air flotation (DAF), disk filters (DF), ultrafiltration (UF) and RO to desalinate seawater has been designed, constructed, operated and optimized during one year. This pilot plant mimics the 84. m3/d full-scale plant water treatment scheme constructed in the framework of the Umm Al Houl Independent Water & Power Project in Doha (Qatar). Influent seawater composition has fluctuated during the 1-year evaluation period, which has enabled testing the treatment scheme performance under different conditions. In particular, influent turbidity has ranged from NTU, total suspended solids (TSS) mg/l, conductivity ms/cm, total organic carbon (TOC) mg/l and SDI15 > 5 %/min. Additionally, algae threshold tests have been conducted, which have enabled pushing the technology to its limits and characterizing its hydraulic and quality performance. The assessed units have presented a stable operation, proving the system capacity to deal with extreme water quality episodes. Optimal operational conditions, associated chemical consumption and water yield have been defined per unitary process, so that full-scale plant CAPEX and OPEX values have been crosschecked. Moreover, treated effluent quality of each unit has been determined, demonstrating that the final permeate fulfilled the quality requirements during the whole operational period in a reliable way. Membrane autopsies and cleaning studies have provided some insights to further optimize the full-scale operation. This study shows the benefits in terms of costs and risks minimization when conducting pilot tests prior to a full-scale plant design and construction, as well as the suitability and robustness of the treatment scheme proposed. 1

2 1 INTRODUCTION In April 16, Acciona Agua designed, constructed commissioned and operated a seawater desalination pilot plant at Umm Al Houl Power Plant (Qatar). The pilot plant consisted of an ULTRADAF system followed by a UF and RO units with the objective of evaluating the performance of the designed system on the Umm Al Houl Power Plant RO desalination full scale plant. A neutralization system was also installed in order to neutralize the effluents generated during the chemical cleanings and to characterize backwashes from the pretreatment. The pilot plant was capable of treating 3 m 3 /h of seawater with a permeate generation of 3.5 m 3 /h (UF only treated half of the DAF effluent flow). The systems were sized in order to represent the full scale desalination plant. The pilot plant consisted of a first pre-treatment based on a DAF, equipped with a pollutants dosing system, to eventually simulate high incoming contaminants loads, ph adjustment, disinfection by sodium hypochlorite and coagulation-flocculation (being ferric chloride used as coagulant and polyelectrolyte as flocculant). Disk filtration ( µm) protected a pressurized PENTAIR UF unit (4 X-Flow modules) with a total filtration surface of 55 m per membrane. The RO system was protected by a cartridge filtration (5 µm) prior to the first pass with partial split, a pressure vessel with 7 membranes (TORAY TM8V). Both the first and second stage of the second pass were composed by two and one pressure vessel respectively, 5 membranes each (TORAY TM7D). A neutralization system was installed in order to neutralize the effluents generated during the chemical cleanings and to characterize backwashes from the pretreatment. 1.1 Background The Umm al Houl Water and Power Plant is located in Al Wakrah, in the Qatar Economic Zone 3, km south of the city of Doha along the Persian Gulf in the Emirate of Qatar. The plant construction began in 15, and was completed in 17. The project encompassed the construction of a natural gas-fired combined cycle power plant along with a desalination plant. The desalination plant has a capacity of 6, m 3 /day of net water capacity, 73, m 3 /day of which are produced through RO technology, while 347, m 3 /day will be produced using multiple-stage flash technology. An engineering, procurement and construction (EPC) contract was awarded to Samsung C&T to construct the complete facility. An EPC contract was also awarded to Hitachi Zosen for the construction of the potable water blending plant who bestowed the design and construction of the desalination plant to Acciona Agua, also responsible for its operation and maintenance (O&M). As a part of the scope of the Umm al Houl Qatar Project, Acciona Agua built a pilot plant in order to prove the reliability and efficiency of the designed pre-treatment and RO system, and to optimize operational parameters for the subsequent full scale plant operation. 1. Objetives The main objective of the pilot operation was to provide substantial information to support the operation of the full scale plant. On that sense, a compilation of a whole year operation containing seasonal changes that can affect different seawater parameters such as temperature, salinity, dissolved oxygen, entailing biodiversity alterations, and even turbidity to determine their impacts on the full-scale plant design. The operation was then focus on establishing the optimum, most cost-efficient operating conditions for the pilot system to aid the development of the full-scale plant design and operation criteria by assessing the fouling potential of the pre-treated water on RO process, the optimization of all chemicals involved (particularly the reagents involved in the coagulation/flocculation system) and the evaluation of this seasonal effects on pretreatment performance. Regarding the system reliability and robustness, the different process steps (DAF, UF and RO) should meet both the required water quality standards as well as contractual established system performance, enabling the verification of the bidder s technical proposals with reference to the pretreatment and RO process,

3 confirming and fine tuning the chemical dosage and the related O&M costs. These parameters are shown in Table 1. Effluent Water quality Effluent Turbidity Effluent TSS > mg/l Turbidity <3NTU Total iron <1mg/L TOC <1mg/L Table 1. Contractual established system performance and water quality DAF UF RO Performance Saturation pressure 5-6 bar Recirculation flow rate - 1% Hydraulic loading rate 5 m/h Effluent Water quality 95% of time SDI<3 Turbidity <.1NTU Total particle counts</ml Total Iron <5µg/L Algae bacteria reduction >99,5% Performance Permeate Water quality TMP Boron <.5 Filtration flux 74.8 L/m/h BW flux 15 L /m /h BW interval 55 min BW duration TDS < 84 mg/l Chloride < 5 mg/l Bromide < 5 µg/l Sulphate < 5 mg/l Performance DP<bar CIP interval>6month salt rejection decay: 7%/year permeate flow decline: % from the initial value CEB interval 4 h Total Iron <4 CIP intervals µg/l 6 days 1 st Pass recovery 45% CEB total Duration 5 min CEB Flux 15 L /m /h TOC<.7mg/L nd pass recovery % MATERIALS AND METHODS.1 PILOT PLANT DESIGN AND INVOLVED EQUIPMENT A pressurized seawater flow entered the pilot plant regulated by a flow control valve at 3 m3/h towards the coagulation-flocculation and dissolved air flotation system, which was installed for the elimination of algae blooms and other events such as hydrocarbons and other organic contamination. For experimental purposes, a fouling agent was able to be injected at the pilot plant inlet for simulating any pollutant event occurrence. The chemicals dosage prior to the pre-treatment system was composed by sulphuric acid for ph control, sodium hypochlorite for disinfection purposes and ferric chloride to enhance the coagulation-flocculation process. A complete set of sensors was installed prior to the chemical dosing in order to characterize the seawater entering the plant. After the chemical dosing, ph and redox were also monitored. Two flocculation chambers with a total volume of 5 m 3, giving a nominal residence time of min working at 3m 3 /h, were equipped with a mixing turbine. The DAF consisted of two main regions, the contact zone and the separation zone. The contact zone, with a nominal residence time of 1.5 min was able to reach up to an hydraulic loading rate of around m/h equipped with exchangeable nozzles. The separation zone, worked at a nominal hydraulic loading rate of 4.9 m / h. The sludge removing system, based on a mechanical skimmer specially designed to avoid the floated to be broken. The air saturator, with a pressurized volume tank of. m 3 was able to increase pressure up to bar. A self-cleaning disk filter with a screen size of µm, according to the UF supplier requirements, equipped with a backwash system was installed downstream the DAF. Differential pressure switch and control system was installed to ensure its proper performance. The UF system consisted of 4 hollow fibre in-out pressurized 3

4 modules PENTAIR X-Flow Seaguard 55) that enabled to work at a filtration flux range from 75 to 83 L/m h (LMH) with a maximum TMP of.1 bar at a maximum feed pressure of 6 bar. RO was equipped with low pressure centrifugal pump of 9 m 3 /h at 4 bar followed by a cartridge filter with a filtration size of 5 µm and a high pressure pump able to reach up to 7 bar. The RO first pass was composed by one 8 diameter pressure vessel containing 7 membranes (TORAY TM8V) with partial split in which the front permeate was blended with the second pass permeate. Depending on temperature, a fraction of rear permeate was sent to the second pass, fed with a centrifugal pump of 4 m 3 /h at a maximum inlet pressure of 11 bar. Two 4 diameter pressure vessels containing 5 membranes each (TORAY TM7D) followed by a second stage with one 4 diameter pressure vessels containing 5 membranes (TORAY TM7D) were installed in order to ensure boron concentration below.5 ppm. A complete cleaning-in-place (CIP) system over UF and RO systems was provided. Chemicals were located separately in bund tanks. The unit consisted of GRP bund tanks for each chemical (8 in total): Sodium hypochlorite dosing system, for either the UF chemical enhanced backwash (CEB) and for a continuous dosing previous to the DAF. Sulfuric acid dosing system for ph adjustment previous to DAF, for the UF CEB, for ph adjustment previous to the RO and for ph adjustment for the neutralization unit. Sodium hydroxide dosing system used for ph adjustment previous to the second pass RO and for ph adjustment in the neutralization unit. Ferric chloride dosing system for raw seawater coagulation. Sodium bisulphite dosing system for the neutralization of chlorine prior to RO first pass and for the neutralization of chlorine in the neutralization unit. Antiscalant dosing system for the RO system and for the RO second pass. Pollutant dosing system for DAF influent water in order to simulate an algae bloom. 4

5 Figure 1. Pilot plant blocks diagram. ANALYTICAL CAMPAIGNS The methods used to characterize the water quality are shown in Table. Table. Analytical methods used to characterize the water quality. Parameter Method Parameter Method Turbidity APHA 13B Conductivity APHA 5 B Total dissolved solids (TDS) Total Suspended Solids (TSS) Chlorophyll APHA 54C APHA 54D APHA H Boron Bromide Total Iron APHA 315 APHA 41 B According to standard ISO

6 3 RESULTS In this section a summary of the different process performance is presented. 3.1 Seawater characterization The following seawater quality parameters gave the basis for the design of the pilot plant (Table 3). These results are based on previous seawater analysis and were assessed during the whole operational period. Table 3. Seawater envelope characterization Parameter Unit Value Inlet temperature C TDS mg/l 459 TSS ppm 3 Turbidity NTU Algae Mcells/L ph mg/l 8.3 Total Hydrocarbons mg/l.5 TOC mg/l 1.4 Boron mg/l 5.3 Bromide mg/l 94 Oil and grease mg/l 5 Online sensors were continuously registering inlet water temperature, ph, redox potential, conductivity, turbidity, particle size and chlorophyll concentration while daily samples were collected and measured in situ for SDI 15, TSS, iron concentration. Samples were also collected once a week by an external laboratory in order to determine total ion concentration, TOC, HPC bacteria, E. Coli, total coliform, algae counts and hydrocarbons concentration. The most relevant online seawater quality parameters measured are presented below. Seawater turbidity distribution and TSS for the whole operational period is presented in Figure a) and b) respectively. % Seawater Turbidity distribution % Seawater TSS distribution % Time 6% 4% % % Time 6% 4% % % <.5 <1 < <4 < <3 <6 Turbidity (NTU) TSS (mg/l) Figure.a) (left-hand side) Seawater turbidity distribution..b) (right hand-side) Seawater TSS distribution. Seawater turbidity ranged from.31 NTU to 4.37 NTU while mean and median were 3.91 NTU and.1 NTU, respectively during the whole period of operation. Seawater TSS ranged from 1.3 to 6 mg/l while mean and median values were of 9.4 and 8. mg/l. Seawater conductivity and temperature distribution for April 16 - April 17 period are presented in Figure 3a) and b) respectively. % <5 <7 <9 <11 <13 <15 <3 6

7 % Seawater Conductivity distribution % Seawater Temperature distribution % Time 6% 4% % Time 6% 4% % % % <61 <6 <63 <64 <65 <66 <67 <68 <69 <7 Conductivity (ms/cm) % <15ºC <.5ºC <3ºC <37.5ºC Temperature (ºC) Figure 3.a) (left-hand side) Seawater conductivity distribution; 3b) (right-hand side) Seawater temperature distribution Seawater conductivity ranged from 61.5 ms/cm to 69.3 ms/cm with a mean value of 63.44mS/cm while seawater temperature ranged from 14.5 ºC to 38.8 ºC with a mean value of 7.56 ºC during the whole period of operation. 3. DAF Performance Results DAF optimization was focused on the coagulant concentration in order to meet the highest downstream quality targets. The following parameters were stablished at a constant value: Mixer 1 at 4s-1; Mixer at 5s-1; recirculation flow-rate: %; saturation pressure: 6 bar. Strong wind scenarios eventually occurred during the whole year of operation. It is believed that, as a consequence of the seawater currents, disruptions in the seabed were dragging suspended matter towards the intake proximity causing massive increases on seawater turbidity. Figure 4b) shows the turbidity removal achieved considering the wide range of turbidity values faced. Coagulant dosage was tested for the whole range of turbidity found during the operational period. Based on Figure 4a), coagulant concentration at each of the maximum removal turbidity values has been chosen in order to obtain the function which links optimum coagulant concentration dosage at any seawater turbidity value within the mentioned range. Coagulant dosage (mgfe/l) 4,5 4 3,5 3,5 1,5 1 Optimum coagulant vs DAF Influent Turbidity y =,56x + 1, Turbidity Removal (%) % 95% 9% 85% 75% 7% 65% 6% Achieved Turbidity removal Turbidity (NTU) DAF Influent Turbidity (NTU) Figure 4a) (left-hand side) Coagulant optimization for the DAF system; 4b) (right-hand side) Achieved DAF turbidity removal Along with other desalination plants in the region, the presence of harmful algal bloom (HAB) events occur in the water source to be treated. DAF systems have increasingly been used to tackle the algae problems suffered in the RO systems by being used as a pretreatment process. Acciona Agua installed an ULTRADAF 7

8 system followed by UF and RO systems in this pilot plant. For this reason, threshold tests with algae addition were included in the contract. During the threshold tests, raw water was artificially spiked with Tetraselmis sp. Besides the optimization of the DAF pretreatment technology towards the elimination of inorganic content according to the high turbidity events occurred, a simulation of a natural algae bloom event at Mcells/L was simulated. Algae were acquired from Reed Mariculture, a marine microalgae concentrates producer company in the US. Tetraselmis algae, a nonviable, nontoxic algae strain is a large green flagellate (cell size 9-1 µm) chosen as per its similarity to the naturally occurring algae bloom species found in the region and its easily detection for lab measurements. DAF influent and effluent water chlorophyll concentration values during threshold tests are presented in Figure 5. Chlorophyll (µg/l) DAF Influent/Effluent Chlorophyll % 6% 4% % % Chlorophyll Reomval % DAF Removal DAF inlet DAF outlet Figure 5. DAF Chlorophyll removal during algae supplementation test 3.3 UF Performance Results Table 4 describes the operational settings of the UF system during the testing period. Table 4. UF stablished operational conditions Parameter Unit Pilot plant Filtration flux LMH 7-85 Filtration time Min 45-5 Backwash (BW) flow m 3 /h 55 BW flux LMH 5 BW duration s 35 Chemically enhanced backwash (CEB) frequency Number of BW 5 Acid & Basic CEB flux LMH 5 Acid CEB rinsing time s 8 Acid & Basic CEB dosing s 35 Acid & Basic CEB soak time Min Basic CEB rinsing time s 6 Following the above operational strategy, UF transmembrane pressure (TMP) evolution and flux are presented throughout the whole year operation in Figure 6. 8

9 Permeability (lmh/bar) UF Permeability & Flux Flux (lmh) Permeability CIP UF Flux lmh Figure 6. UF Permeability evolution and UF flux Assessing UF behavior and while optimizing coagulant dosages and DAF performance, TMP increased during the first semester operating the UF system at extreme operational conditions out of Pentair specifications resulting on exhaustive chemical cleanings (cleanings in place: CIPs). Most suitable chemical configuration for the UF CIP was found to be citric acid % and sodium metabisulfite (SMBS) 1% at a ph of. Once UF membranes were recovered, 6 months operation at Pentair s recommendations were performed with no CIP required. UF system was operated at a flux range from 7 to 83 LMH as per full scale design meeting contractual parameters to feed the RO system: UF SDI 15 below 3.5 % of the time; UF filtrate turbidity was below.3 the 97% of the time and TSS were below 5 mg/l the 97% of the time. The simulation of the natural algae bloom event at Mcells/L was also monitored at the UF system. Results show that UF outlet chlorophyll concentration was removed by a 75% in average feeding RO with a low residual chlorophyll concentration that had no effect on RO membranes performance. Chlorophyll (µg/l) UF Influent/Effluent Chlorophyll (Algae Threshold Test) % 9% 7% 6% 5% 4% 3% % % % Removal (%) UF Removal UF Inlet UF outlet Figure 7. UF Chlorophyll removal during algae Bloom simulation 3.4 RO Performance Results RO performance was assessed by monitoring its normalized hydraulic operational parameters such as permeability, differential pressure decay, salt rejection and permeate water quality. Figure 8 shows both the first pass normalized pressure drop (DP) evolution throughout the operational period as well as the first pass 9

10 rst pass normalized salt rejection 6, 5,5 5, 4,5 4, 3,5 3,,5, 1,5 1,,5, CIP,% 99,8% 9 99,6% 8 99,4% 7 99,% 6 99,% 5 98,8% 4 98,6% 98,4% 3 98,% 98,% 1st pass Temperature (ºC) 1rst pass salt rejection 1rst pass Normalized permeate Flow 1st pass Temperature (ºC) 1rst pass Normalizez perm Flow, m3/h normalized salt rejection. These two figures summarize the first pass behavior as per membranes integrity and water quality. Feed Temp 1st Pass Salt Rejection Feed Temp Figure 8. RO 1st Pass normalized Permeate flow and salt rejection for the whole operational period As per contractual operation two CIP were conducted as shown in the above figure totally recovering membrane performance to the initial values Boron RO inlet (mg/l) TDS (mg/l) TDS permeate pilot vs projections TDS projections TDS pilot Boron pilot plant vs projections 1,8 1,6 1,4 1, 1,8,6,4, Boron blended permeate (mg/l) Projections were calculated and compared with external laboratory results for seawater ions and TDS at permeate samples in order to evaluate salt rejection of the RO system. All analyzed final permeate ions fulfilled contractual parameters in all analyzed samples. Figure 9 shows final permeate TDS and boron concentration comparison between analytical (empty symbols) and projected (solid symbols) results. Boron RO inlet pilot plant Boron blended permeate projections Boron blended permeate pilot plant Figure 9. TDS and boron projections comparison with permeate water In order to evaluate the performance of the RO membranes, standard tests (hydraulic tests) and autopsies were carried out in order to evaluate DP, permeate flow and salt rejection of each membrane and compare them to supplier specifications. This procedure was carried out after 6 and 1 months of operation.

11 4 CONCLUSIONS 4.1 DAF Conclusions The optimal coagulant dosage range in terms of mgfe/l was 1.75 mgfe/l at sea water turbidity of 1 NTU up to 4.3 mgfe/l at sea water turbidity 5 NTU. This can be used as baseline for implementation of coagulant dosing in the main plant during high turbidity scenarios. 4. UF Conclusions: UF was able to work at 7 85 LMH successfully, with filtration cycles of 45-5 min, leading to high water yields. Acid and alkaline CEBs were performed every 5 BWs. The most suitable UF CIP chemical configuration was found to be citric acid % and Sodium Meta Bisulfite 1% at a ph of. It was able to recover the membranes. UF filtrate met all the contractual parameters to feed the RO system. 4.3 RO Conclusions: Regarding first pass performance parameters, inlet conductivity from 59 to 68 ms/cm corresponded to feed pressures from 61 to 74 bar (at temperatures from 16 to 39 ºC). Two CIP were conducted during the whole period of operation dating on 9th September 16; 6 months after starting RO performance, and 7th March 17; 6 months after first CIP was conducted. CIP performance was based on Toray guidelines and it is described below: 1h recirculation with Na4EDTA 1% at ph 1 at a flow of 4.5m3/h followed by 1 hour soaking 1h recirculation with Na4EDTA 1% at ph 1 at a flow of 9m3/h followed by 1 hour soaking 1h recirculation with citric acid 1.% at a flow of 4.5m3/ followed by 1 hour soaking 1h recirculation with citric acid 1.% at a flow of 9 m3/ followed by 1 hour soaking Regarding second pass performance parameters, feed pressure ranged from 6.5 to 8 bar at a stable differential pressure between 1.1 and 1. bar. Boron removals from 73% to 98% were achieved (at temperatures ranging from 17 to 39 ºC). Final permeate fulfilled contractual parameters for the whole period of operation: Boron <.5 mg/l Bromide <.1 mg/l TDS < 84 mg/l Obtained results for the twelve months operation autopsy indicated that no chemical or mechanical degradation was observed in any analyzed membranes although first pass first position was slightly fouled by 45 % organic 55% inorganic fouling. More specifically, the analysis of the foulants indicated that it was not possible to be quantified, therefore, it is not considered an issue. In addition, the characterization of each type of fouling indicated presence of bacteria, Fe and Si, which can be perfectly removed by a basic/acid CIP. 11