SECTION 5 CONCEPTUAL TREATMENT DESIGN

Transcription

1 SECTION 5 CONCEPTUAL TREATMENT DESIGN 5.1 INTRODUCTION The Town of Hull is considering the construction of a 2.5 to 5 MGD seawater desalting facility to meet its water demands within the Town s service area as well to other areas and provide a reliable, drought-proof water supply. The purpose of this section is to present conceptual designs of a desalination facility including conceptual plant layouts. The desalination facility will take its raw water supply from either angle-drilled beach wells or from an open ocean intake. The raw water will be pre-filtered (if necessary) to remove solids and the filtrate desalted using reverse osmosis membranes (RO). The Total Dissolved Solids (TDS) in the RO permeate (desalted product water) will consist mostly of sodium chloride. The permeate (finished water) will need post-treatment to meet water quality regulations and water quality goals prior to delivery to Hull's customers. Post-treatment will include addition of carbon dioxide and calcium carbonate (to add hardness and alkalinity) and free chlorine for disinfection. Adequate contact time to meet disinfection requirements will be provided at the treatment plant using an above-grade, pre-stressed concrete clearwell with internal baffling. Other technologies to desalt seawater such as thermal processes where not considered viable for defailed study because of high costs. 5.2 PROCESS DESIGN CONSIDERATIONS There are many components required for an RO water treatment facility (WTF). This section discusses those components as part of a conceptual treatment design. 1651A 5-1 Wright-Pierce

2 5.2.1 Definitions The desalination industry uses terms in specific ways to describe equipment and flow streams within the treatment process. Definitions of terms used in this report are discussed below. Permeate: Permeate is desalted water that has passed through the RO membrane prior to any additional treatment. Concentrate: Concentrate is the waste stream from an RO system, containing the salts removed from the permeate. Concentrate is also commonly referred to as brine. Brine will require disposal in Hull using an ocean outfall. Recovery: Recovery is the percentage of RO feedwater that is recovered as permeate. For a system with 5% recovery, half (5%) of the influent water is recovered as permeate, while half exits as concentrate. When it is possible to blend some of the raw water around the RO process, recovery can be expressed either as RO recovery or overall recovery. RO recovery is the percentage of RO feedwater that is recovered as permeate, while overall recovery is the percentage of raw water entering the treatment plant that is recovered as product water. Seawater RO plants typically do not blend raw water into the product water. Net Driving Pressure: Net driving pressure is the difference in feed and permeate pressures, adjusted for osmotic pressure. Net driving pressure determines the rate of permeate production through the membrane system; with permeate production increasing as net driving pressure increases. Osmotic Pressure: Osmotic pressure is an inherent chemical property of water with dissolved salts. Osmotic pressure is the amount of pressure required to prevent water from passing through a semi-permeable membrane in response to a concentration difference. As a rule of thumb, osmotic pressure increases 1 psi for each 1 mg/l of TDS. Thus, for a seawater RO system, osmotic pressure at the first membrane is about 33 psi (33, mg/l in the feed vs. about A 5-2 Wright-Pierce

3 mg/l in the permeate). At the last membrane, where the feedwater has concentrated to about 65, mg/l TDS, osmotic pressure is about 65 psi. Flux: Flux is the amount of permeate produced through a given membrane area. It is typically reported as gallons per square foot of membrane area per day Raw Water Sources There are two available options for a raw water source to feed the proposed RO WTF; (1) angle drilled beach wells and/or (2) a direct ocean water intake. As presented within Section 2 - Saline Groundwater Well Supply Investigation and Section 4 - Distribution System Analysis, there are preliminarily identified limitations in the capacity of water that could be withdrawn form the beach wells and hydraulically handled by the distribution system. Based on these limitations, the three capacities to be analyzed for the RO WTF are 2.5 MGD, 4. MGD and 5. MGD. Due to the volume of the waste streams that are created by the RO treatment process (described later in this section), actual raw water requirements are double that of the ultimate RO WTF capacity. That is, the 2.5 MGD WTF would need a raw water supply of 5. MGD, the 4. MGD WTF would need a raw water supply of 8. MGD, and so on Beach Wells Based on the preliminary test well investigation up to approximately 8. MGD will be able to be withdrawn with beach wells pending confirmation with subsequent hydrogeologic testing. Therefore, only the 2.5 and 4. MGD WTF capacities would be able to be served via the beach wells option. (Source water capacities of 4. MGD at 8. MGD, respectively). For estimating purposes, a configuration using 3-angles wells arranged radically from a single point location has been assumed for the purpose of the treatment evaluation. Water from beach wells would be pumped from submersible pumps installed in each angled well. The well casings would be extended above the ground surface for protection against storm surges and flooding. Electric equipment and motors for this type of installation are designed to 1651A 5-3 Wright-Pierce

4 be submerged in seawater. enclosure for protection against unintended access. The well heads would require fencing and an un-obstructive Ocean Water Intake Because the beach wells option cannot supply enough raw water for the 5. MGD WTF capacity, a direct ocean water intake having a capacity of 1. MGD would be required. Conceptually, this intake would consist of a directionally drilled boring constructed to intersect the ocean floor at an appropriate depth and fitted with a self-cleaning screen. Furthermore, the open intake must be designed to meet requirements for minimizing impact on sea life, and in particular to minimize fish impingement and entrainment of small marine organisms. Typically, this requirement is met by providing a fine screen on the intake and limiting inflow velocity below crossflow velocity. The intake should be sited far enough offshore to be out of the surf zone to minimize sediment loading, and deep enough to provide relatively consistent water quality and temperature and protection from water craft. The permitting impediments of a direct ocean intake are discussed in Section 6 of this report. Alternatively, additional wells could be constructed to provide the required 1 MGD. However, for costing purposes, our study will bracket the design using a direct ocean intake for the 5. MGD plant option. A raw water influent pump station would then be required to convey the raw water from the intake to the head of the RO WTF. An ocean intake would require a pumping station. The pumping station would likely consist of a small building housing three centrifugal pumps. The pumps would be located in a lower level of the pumping station to provide a flooded inlet for consistent pump priming. The pumping station would house an air clearing system which would periodically inject pressurized air into the intake pipeline to clean the intake screen. Variable frequency drive units would be provided in the upper floor to vary flows into the pre-treatment membrane system and a motor control center (MCC) described later in this report section. Controls and communication between the pumping station and treatment facility would be provided with telephone or radio telemetry equipment. 1651A 5-4 Wright-Pierce

5 5.2.3 Raw Water Quality The raw water quality of the supply source is an important factor in determining the type and level of treatment needed. In particular, differences in water quality between a beach well (which would receive some natural filtration) and a direct ocean source determine treatment requirements. For the purposes of this preliminary study, a full-suite of water quality tests were performed on samples collected during the hydrogeological investigation phase. Water quality samples were taken on February 23, 26 from both the open water and from a shallow point well (test well) within the beach area during an incoming tidal event. A summarized set of results of the analyses are provided in Table 5-1. A complete suite of sample results is included in Appendix J. TABLE 5-1 RAW WATER QUALITY RESULTS HULL, MASSACHUSETTS Constituent Open Water Test Well Calcium, mg/l 4 39 Magnesium, mg/l 1,3 1,4 Sodium, mg/l 9,3 8,6 Potassium, mg/l 6 61 Bicarbonate, mg/l Sulfate, mg/l 3,2 3,2 Chloride, mg/l 19, 21, Bromide, mg/l TDS, mg/l 31, 31, TSS, mg/l 3 84 ph Temperature, ºC The concentration of total dissolved solids (TDS) is the primary water quality parameter that drives the operating cost and membrane selection for a desalination facility. The TDS is a measurement of dissolved inorganic salts, dissolved gases and natural organic matter. In seawater, sodium and chloride ions are the primary constituents of the TDS concentration. 1651A 5-5 Wright-Pierce

6 In general, seawater can be expected to exhibit a TDS concentration ranging from 28, mg/l to 35, mg/l. For the samples collected in Hull, for both the open ocean water and test well samples were found to have a TDS concentration of approximately 31, mg/l which is within this range. The data in Table 5-1 also shows that the quality of water from the open ocean and from the test well is very similar, with the main exception being suspended solids. Suspended solids in the well water were assumed to be substantially higher than ambient groundwater conditions due to the test well not being fully developed. The suspected solids concentration in a properly developed beach well will be lower. Samples were also taken and analyzed for parameters that can be indicators of possible natural organic matter to help better define the potential reduction in this material via the natural filtration provided by a beach well option. The results of these analyses are summarized within Table 5-2. TABLE 5-2 ADDITIONAL RAW WATER QUALITY RESULTS HULL, MASSACHUSETTS Constituent Open Water Test Well Color, CU 5 5 TOC, mg/l UV 254, cm Non Detect SDI (1) HPC, CFU/mL 6 < 1 BOD, mg/l < 6 < 6 1. The open water sample result is presented as SDI 5 (due to faster plugging) and the test well data is presented as the standard SDI 15 result. The results from Table 5-2 indicate that there is an expected decrease in organic material within the water from a beach well compared to that of open ocean water. Except for color and BOD, all other samples results for the test well were lower than those of the open ocean water. Organic matter is a foulant which can affect RO membrane performance. This data was incorporated into the water quality model to size RO membranes for both the beach well and ocean intake supply options. 1651A 5-6 Wright-Pierce

7 The Silt Density Index (SDI) is a key parameter that determines pre-treatment requirements for a RO membrane process. In particular, the beach well's SDI was significantly better. The SDI test provides an empirical number obtained from a special test apparatus that is used to determine the fouling potential of water feeding a membrane filtration process (i.e., a reverse osmosis system). The specific test procedure is defined within ASTM standard D In summary, this index is calculated from the rate of plugging of a.45 µm membrane filter when water is passed through at a constant applied gauge pressure of 3 psi. Membrane manufactures typically require ocean water to have SDI 15 of no higher than 3 prior to entering their membranes. If the SDI is greater than 3, pretreatment with cartridge filters, ultrafiltration or media filtration may be required. The beach test well water quality sample had an SPI 15 value of 4. Our concept designs presented herein assume that a full scale well can be constructed properly and fully developed, to produce raw water with an SDI 15 result below 4. As will be explained in further detail later within this section, the beach well's capability to have a much lower SDI 15 will result in potential cost savings as pretreatment for removal of solids would not be required as compared to the open ocean water supply. Additional review of the water quality by the hydrogeologist suggests that water quality from properly developed beach well should provide lower solids concentrations as shown in Table 5-1. Complete water quality information is provided in Appendix J. It should be noted that treatment recommendations presented herein will require validation under a stressed pumping condition during subsequent development phases of the project. A more extensive sampling and analysis program is required to collect water quality data over an extended period of time and below the ocean floor where the final screened wellhead will be constructed as described in Section 2 of the report. Pilot testing would ultimately then be required to determine the suitability of the pretreatment processes selected. 1651A 5-7 Wright-Pierce

8 5.2.4 Product Water Quality Goals In regards to product water quality (i.e., finished water quality), the primary goal for the RO process is to meet all of the primary drinking water standards at a minimum. Additionally, as the RO process removes the majority of the water's constituents (including desired minerals such as magnesium, calcium, etc.), the natural good taste of the water is also essentially removed. Therefore, it will be important not only to bring the water back to a non-corrosive state (for regulatory purposes), but to also re-mineralize it to bring back the desired taste for it to be palatable to the consumers. Table 5-3 which follows summarizes the product water goals of the RO process for the Hull project so the water is non-corrosive and acceptable by the consumers: TABLE 5-3 PRODUCT WATER QUALITY GOALS HULL, MASSACHUSETTS Parameter Desired Range ph Alkalinity (mg/l) > 4 TDS (mg/l) 5< TDS < 5 Hardness (mg/l as CaCO 3 ) < 75 Calcium (mg/l as CaCO 3 ) > 4 Sodium (mg/l) < 2-75 Chloride (mg/l) < 25 Langelier Index >.2 Detailed descriptions of the chemical processes to re-stabilize the treated RO permeate to prior to distribution is discussed later in this report section Proposed Treatment Process The proposed RO desalination treatment process is illustrated in Figure 5-1, Process Flow Diagram. This simplified flow diagram shows the major treatment processes and flow streams. These flow streams include: 1651A 5-8 Wright-Pierce

9 Stream 1 Raw water is extracted from the beach wells or through an open ocean intake. If an open intake is installed, self-cleaning screens would be provided. Stream 2 - Influent water will be combined with clarified backwash water and sent to the pretreatment filters (required by the 5. MGD capacity WTF). If beach wells are used, water quality may be good enough that pretreatment filtration will not be required. Stream 3 - Sufficient filtrate will be produced to provide feed water to the RO system and backwash water for the filtration system (required by the 5. MGD capacity WTF). The RO desalting process requires extremely clean water (i.e., very low turbidity with a Silt Density Index (SDI) of less than 3.). For purposes of this conceptual design, it was assumed that membrane filtration will be used. Membrane filtration has been shown to provide superior performance for RO feedwater in other locations and is therefore included as part of this conceptual analysis. Both the microfiltration (MF) and ultrafiltration (UF) technologies would be appropriate. Stream 4 - Filtrate will be withdrawn from the filtrate storage tank to backwash the filters (required by the 5. MGD capacity WTF). The spent backwash water from the filters will be sent to clarifiers or, potentially, directly to disposal. Stream 5 - Clarified spent filter backwash water will be combined with the influent ocean water and sent to the filters (see Stream 2). At some times it may be necessary to discharge this water back directly to the ocean via the concentrate outfall when the clarifiers are down for maintenance or out of service. Stream 6 - A relatively small volume of wastewater will be produced by the clarifiers. The wastewater will contain the solids removed from the ocean water. It is expected that the Total Suspended Solids (TSS) concentration of the wastewater will be approximately 1, mg/l (1%) based on the 3 mg/l TSS raw ocean water results. The wastewater would be sent to the wastewater treatment plant via a sewer connection or other disposal method as required. Stream 7 - Filtered water (TDS ~ 31, mg/l) will be sent to the RO equipment from the filtrate storage reservoir. The RO feed water will be pressurized to between 1 psig and 3 psig by vertical turbine pumps equipped with variable frequency drives. It will pass through cartridge filters to prevent particulate matter from impacting the RO membranes, and then be pressurized to 9 1,1 psig by the fixed speed high-pressure 1651A 5-9 Wright-Pierce

10 pumps. The high-pressure pumps will be able to operate at their most efficient point because they will always be pumping a fixed amount of water at a constant head. Stream 8 Product water (permeate) (TDS < 25 mg/l) will be produced by the RO system. Depending upon final process configuration, a portion of this stream may be treated by a partial second RO pass to further reduce permeate TDS. The permeate will be chemically treated with carbon dioxide and limestone to reduce its corrosivity, and disinfected with sodium hypochlorite before being delivered to the distribution system. Stream 9 Concentrate (TDS ~ 68, mg/l) will be returned to the ocean through energy recovery devices. Almost one-half of the pumping energy used in the RO process is present in the RO concentrate stream. Most of this energy can be recovered for use in the plant. The amount of energy recovered depends upon what type of energy recovery device is used. At least 75% of the energy in the RO concentrate stream should be recoverable. This is further discussed in the energy recovery portion of this section. 1651A 5-1 Wright-Pierce

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12 Pretreatment Reverse osmosis treatment systems require water with very low particulate contamination. If the water source has excessive particulates (SDIs greater than about 3), than the source would require pretreatment filtration. In addition, pretreatment using anti-sealant chemicals are required to allow high recovery and reduce operating costs. For the Hull treatment system, it is expected that beach wells should be capable of providing up to about 8 MGD of water to supply the treatment plant. Considering that the plant is expected to operate at a recovery of approximately 5%, beach wells will be able to support a treated water capacity of about 4 MGD. If the plant capacity is larger than 4 MGD, an open intake will be required. If the raw water is supplied by beach wells, it is anticipated that the raw water SDI will be 3 or less. In this scenario, pretreatment filtration will not be required. However, if plant capacity is set at greater than 4 MGD, pretreatment filtration will be required, assuming that an open seawater intake is used. For the purposes of this evaluation, it is assumed that the 2.5 MGD and 4. MGD cases will not be equipped with pretreatment filtration. The 5. MGD plant capacity will require pre-treatment. However, the designs of the smaller plants should include space for inclusion of pretreatment filtration (through building expansion) in case it is found that water quality from the beach wells is found to be inadequate for direct RO treatment. The pretreatment requirements for either scenario are delineated on Figure 5-1. Pretreatment filtration, if required, is assumed to use membrane filtration technology. Pressure Membrane Pre-Treatment - This membrane option consists of a pressurized cartridge system containing all required membrane components including service pumps, instrumentation and other control equipment. A pressure system forces raw or pretreated water through the pores of hollow membrane fibers located within a sealed cartridge from the either an inside-out or outside-in configuration depending on the manufacturer. 1651A 5-12 Wright-Pierce

13 The nominal pore size in these hollow tubes is normally around.2 µm (microns). Membranes are typically manufactured of polysulfone. Hollow fiber, ultrafiltration membranes have approximately a 1, molecular weight cutoff (nominal) and a 35 mil (.35 inch) internal diameter. Pressure membranes can be operated over a wide ph range (1.5-13) and can tolerate strong oxidizing agents. The expected useful life of their membranes in a properly operated system is estimated to be over 5 years. An ultrafiltration skid has two basic operating modes; (1) production and (2) cleaning. During normal operation, feed pumps move raw or pretreated water through the in-line pre-filters and into the inlet valve of a pressure membrane assembly. By increasing flow across the membrane surface, the flux (the volume of water passing through a set membrane area per unit of time) increases. Flux is an important consideration which determines the number of pressure membranes and total surface area required for the system. The higher the flux, the fewer membranes will be needed to treat the same flow rate. To maintain a steady flow, variable speed drives on the feed pumps ramp up during production mode to compensate for the increasing transmembrane pressure caused by a build up of filtered material inside the hollow fiber tubes. The typical transmembrane pressure operating range for a pressure membrane system is between 5 and 35 psi. To remove solids filtered by the membrane, a periodic backflush is initiated. Backflush helps to maintain a stable permeate flow rate by physically removing the fouling layer from the membrane surface. The ability to complete a backflush is a unique benefit of hollow fiber membranes that allows the system to run for a maximum period of time between off-line chemical cleanings. In some plants, the direction of the feed flow is reversed in the module after execution of a backflush. The direction of flow is switched from upflow to down flow or vice versa. This enables the membrane to be utilized more uniformly by distributing the fouling layer evenly along the membrane. On a fixed interval, membranes require a chemical cleaning. The frequency can vary from every two to three weeks to twice a year depending on water quality. The cleaning is necessary to 1651A 5-13 Wright-Pierce

14 remove any accumulated fouling on the membrane surface which cannot be removed via physical means (i.e., backflush). Typical cleaning solutions used are caustic, sodium hypochlorite (or a combination), and citric acid. Immersed Suction or Vacuum Pretreatment Membranes - The immersed suction membrane process contains a series of cassettes containing hollow fiber membranes immersed in a tank of water. The membranes operate under a low pressure vacuum that is induced within the hollow membrane fibers. The cassettes consist of thousands of horizontally oriented hollow fibers mounted between two vertical headers. Shrouds enclose the fibers, leaving only the bottom and top open to create a vertical flow upwards through the fiber bundles. The pretreated water enters from the outside of the membrane fibers and flows through small pores to the inner hollow core of the membrane fibers. Membrane treated water flows from the membrane core and exits the treatment skid through piping located on top of the unit. During the filtration cycle, permeate is withdrawn through the membranes by applying vacuum to the permeate piping. The water removed by permeation is replaced with feed water to maintain a constant level in the tank. Similar to a pressure system, a backwash is performed at prescheduled intervals The membranes are simultaneously aerated and backpulsed to dislodge solids. During backpulsing, filtered water from the backpulse tank is passed from the inside to the outside of the membrane to disrupt any particles that may be physically lodged in the membrane interstices. Either a tank drain or feed flush is used to purge the solids from the tank. In the latter case, feed continues to be supplied at the bottom of the cassette and the solids are removed through backwash troughs, operating in a manner similar to sand filters. Periodically, the process tank is emptied to perform a maintenance clean, whereby the tank is drained and a chemical solution (typically sodium hypochlorite or citric acid) is either recirculated or backpulsed through the membranes. Chemical residuals are flushed from the membranes and process tank before the system resumes production. 1651A 5-14 Wright-Pierce

15 Because water is induced to flow through the hollow fiber tubes by vacuum, the transmembrane pressures are much lower than that of the pressure system. An immersed suction membrane system typically operates with transmembrane pressure ranging between -1 and -11 psi. Power requirements are usually lower than a pressure system. While media filtration has historically been the filtration method of choice, membrane filtration has recently been shown to provide superior performance of the RO system, reducing RO membrane cleaning frequency and reducing overall cost of treatment. For the purposes of this evaluation it is assumed that pressure membrane filtration will be used. However, immersed membranes should also be considered when final design of the treatment plant commences. The filtrate from the pretreatment filters will be stored within an on site 1, gallon (nominal) filtrate storage tank (approximately 4' diameter and 12.25' height) which is sized for a minimum ten minute detention time. The filtrate storage tank will serve to supply the booster pumps and pretreatment filter backwash pumps. Pre-stressed concrete is preliminarily selected as the preferred construction material because of its lower long term costs due to reduced maintenance and repainting that traditional steel tanks require. Additionally, the tank is preliminarily proposed to be above grade (as opposed to below grade construction) due to the anticipated high groundwater elevations during construction. Elevated tanks reduce requirements for dewatering activities during construction, but may create an aesthetic concern. The design of this facility should be re-assessed by the design development phase of the project. Pretreatment chemical addition will consist only of a scale inhibitor added at a concentration of about 5 mg/l. Scale inhibitor delays the precipitation of sparingly soluble salts to allow the RO system to operate at a higher recovery than it would otherwise be able to operate. Based upon software projections of the treatment process, acid addition is not required to obtain a recovery of 5%, which is considering excellent for a seawater application in colder climates. Since the acids used for ph adjustment in RO treatment plants are acutely hazardous materials, elimination of acid addition is considered to be an important safety element for the Hull treatment plant. 1651A 5-15 Wright-Pierce

16 Cartridge Filters Cartridge filters capable of removing particles 5 µm and larger are recommended upstream of the RO system as a protective measure should, soil fines, turbidity or other solids be inadvertently invaded into the raw water flow stream. The cartridge filters will capture this material, thereby fouling the cartridge filters themselves (not the RO membranes) and causing the system to shut down because of high pressure loss. Cartridge filters are not intended to remove materials on a consistent basis. Replacement costs would be excessive if they are replaced more than three or four times annually. Sufficient filter redundancy would be provided to allow a system to be removed for cleaning while the treatment facility remains operational Reverse Osmosis Feed Pumps A RO system will typically operate at pressures ranging from about 9 to 1,1 psi, depending upon water temperature and membrane age. Since the required feed pressure will change over time, it is necessary to provide a means of increasing the pressure while maintaining consistent flow rate. This is best done using variable frequency drives (VFDs). However, since the power requirement for each feed pump would be very large (say 1,1 Hp or more), VFDs for these pumps would be very expensive requiring 416 V service voltage to the facility. We therefore suggest that feed pumping be split into two stages. The first feed pump stage would be the booster pump, and would supply about 1 to 3 psi boost depending upon final feed pressure requirements. This pump would be equipped with a VFD. Because it is a relatively small pump, the VFD cost would not be excessive. For the purposes of this evaluation, this pump is assumed to be a double-suction centrifugal pump. The second feed pump stage would be the RO feed pump ("high pressure pump"), and would supply an additional pressure 8 psi to feed the RO membranes. This second feed pump would be a constant-speed pump designed to operate at its most efficient operating point. No VFD's for the RO feed pumps reducing capital cost and increasing overall energy efficiency operations. For the purposes of this evaluation, this pump is assumed to be a vertical turbine pump. An inline can-type vertical turbine pump would be the most economic pump configuration. This type 1651A 5-16 Wright-Pierce

17 of pump does not require a pumping well, but draws such directly for the pipeline in which it is installed. A booster pump and a feed pump will be required for each RO train. In summary, the booster pump would pump through the cartridge filters. The feed pump would draw suction from the discharging point of the cartridge filter and pump flows through the RO membranes. A break tank is not desired between the two pumping systems to minimize tankage where undesirable biological growth could develop and ultimately foul the RO membranes Reverse Osmosis Systems RO systems are typically supplied as two or more trains of equal capacity. This allows plant output to be adjusted to meet demands, since train production rates are typically fixed at design time. When distribution system demands do not match train capacities, trains can be turned on and off as required. However, since RO systems typically provide best performance when run continuously, it is best to allow them to run as long as possible and use distribution system storage to provide for peaking. Two to four RO trains of 1.25 to 1.33 MGD capacity each are recommended for the Hull facility, depending upon the required finished water plant capacity at the plant (refer to Table 5-5 later in this section). While larger seawater RO trains have been constructed elsewhere in the U.S. and around the World, this size limits the size of the feed pumps to a point where electrical equipment requirements become excessive. Also, at least two trains should be supplied to provide redundancy. The RO system consists of RO membrane elements contained with a pressure vessels (PV), a rack to support the pressure vessels, and instrumentation to monitor performance. Typical seawater membrane elements are 8 inches in diameter, 4 inches long, and contain about 35 square feet of membrane area in a spiral-wound configuration. A significant amount of development has been underway to produce larger diameter elements up to 18 inches in diameter, which would have more membrane surface area and require fewer pressure vessels. 1651A 5-17 Wright-Pierce

18 Consideration should be given to use of these larger elements during final design. At this time, further installed experience is needed before these larger membranes could be recommended. Water flux through the membrane depends upon the net driving pressure, temperature, and water salinity. Higher flux rated result in lower capital cost due to the need to purchase less membrane area for a given production rate. Permeate quality improves slightly as flux rates increases. However, higher flux requires higher feed pressure and has increased potential for fouling. Selection of the design flux can have significant impact on plant economics and should be confirmed with piloting. As water temperature increases, less net driving pressure is required to produce a given flux. This fact can lead to process savings if distribution system demands are significantly higher during warmer months than colder months. During the warmer months when high production is required, the RO system can operate at a flux of say 9 gallons per square foot per day (gfd). Then during colder months when water temperature and water demand have dropped, the flux can be reduced to say 7 to 8 gfd, offsetting the increased pressure requirement from the cold water. If winter flux were reduced to 7 gfd (assuming a cold water temperature of 3. C or 37.4 F) while summer flux is 9 gfd (assuming a warm water temperature of 12. C or 53.6 F), a 2.5 MGD plant (rated for summer) would have a capacity of only 1.95 MGD during winter. Careful analysis of seasonal water temperature variations would be required before implementing this type of scheme, since water temperatures typically lag seasonal changes and it might be that significant summer demands could appear before water temperatures rise enough to allow increased flux. For the purpose of this evaluation, design flux is set at 9 gfd, using summer temperature conditions. The RO trains should be individually instrumented to allow effective analysis of the condition of the RO membranes. At a minimum, instrumentation should include: Permeate and Concentrate Flow Meters 1651A 5-18 Wright-Pierce

19 Feed, Permeate and Concentrate Pressure Transmitters Feed, Permeate, and Concentrate Conductivity Transmitters Pressure Monitoring for Energy Recovery Devices Sample Taps For Feed, Combined Concentrate, Combined Permeate, and Each Vessel Permeate When seawater RO treatment trains are shut down, it is important to flush the seawater out of the membranes. This helps to prevent biogrowth, while minimizing corrosion of the piping and pumping systems. In addition, if concentrated seawater remains in the feed channels of the membrane elements while low-tds permeate is in the permeate channels, the permeate will attempt to flow back into the feed channels to dilute the seawater (this is osmosis which the reverse osmosis system is designed to reverse). This can cause high pressures in the feed channel, and can damage the membranes particularly if insufficient permeate is available. In order to provide the flush water and to supply permeate in case of an emergency shutdown where flushing cannot occur, a permeate storage tank will be provided with sufficient capacity to fill the RO membranes and the piping. Recent developments in RO membrane technology have improved the membranes' ability to reject salt. As a result, single-pass RO systems are able to produce permeate with TDS concentrations of 2 mg/l or lower. However, this TDS consists almost entirely of sodium chloride, with sodium concentration about 7 mg/l and chloride concentration about 13 mg/l. If these concentrations are acceptable, no second pass RO is required. However, if lower sodium or chloride concentrations are desired, a partial second pass RO system could be supplied to further treat the first stage RO permeate to lower TDS. No second pass RO system is included in this preliminary evaluation Energy Recovery The concentrate stream exits the RO at a pressure about 3 psi lower than the feed water pressure (i.e., 87 to 1,7 psig). Since the concentrate flow is about half of the feed water flow, there is 1651A 5-19 Wright-Pierce

20 substantial hydraulic energy available in this stream to be recovered to reduce overall plant energy costs. Energy recovery devices have been developed to put this energy to use. While there are a number of types of energy recovery devices available on the market, the most efficient currently in use are the hydraulic turbocharger and work (or pressure) exchanger. The hydraulic turbocharger consists of close-coupled turbine and pump impellers mounted on a single shaft and contained in a single housing. High-pressure concentrate enters the turbine, where energy is extracted. The rotation of the turbine is transferred directly to the pump impeller, which increases the pressure of the feed water stream fed into it. These devices are reported to have overall efficiencies as high as 8 percent, although somewhat lower efficiencies are typically seen. They can have flow capacities as high as 2, gpm. They have limited control capabilities to allow adjustment of concentrate pressure. Work (or pressure) exchangers are rotational devices that directly transfer the pressure energy of the concentrate stream to the feed stream. They operate at exceptionally high efficiencies, often in excess of 9 percent. However, they typically have lower hydraulic capacities, although capacities have increased over the last few years. In order to obtain the capacities needed for Hull, several of the devices would need to be connected in parallel for each RO train. The devices are typically installed in parallel with the RO feed pump, and are able to pump a feed water flow equal to the concentrate flow almost up to the RO feed pressure. A small pump equipped with a VFD provides the additional pressure required for this flow stream. This pump provides a method of controlling the energy recovery device and concentrate flow. At this level of design neither method of energy recovery has been selected. However, it is assumed that energy recovery will be employed, and that at least 75 percent of the energy in the concentrate will be recovered. 1651A 5-2 Wright-Pierce

21 Post Treatment RO permeate has low hardness, alkalinity, and ph, and is corrosive as a result. Post treatment is required to reduce the permeate's corrosively and improve its flavor before delivery to Hull's water customers. Typical post treatment includes addition of alkalinity and hardness to reduce corrosiveness and to re-stabilize the treated water. In addition, the permeate must be disinfected to meet the requirements of the surface water treatment rules. Alkalinity can be added by bubbling carbon dioxide (CO 2 ) into the permeate. The CO 2 dissolves into the water and converts to carbonic acid. The carbonic acid will then react with and dissolve limestone (calcium carbonate), which produces calcium bicarbonate according to the reaction: H 2 CO 2 + CaCO 3 Ca(HCO 3 ) 2 Calcium bicarbonate provides the hardness and alkalinity needed to passivate the permeate and make it non-corrosive to the distribution system. The Hull desalination plant will most likely be classified as a surface water treatment plant, even if beach wells are used to supply the water. The plant must therefore meet the requirements of the surface water treatment rules for pathogen removal/inactivation. At a minimum, these rules require the following: 3-Log Removal/Inactivation of Giardia 4-Log Removal/Inactivation of Viruses 2-Log Removal of Cryptosporidium While the RO system (membranes) will provide some removal of pathogens, it is not designed to meet the requirements of the surface water treatment rules, and will not provide adequate removal of regulated pathogens. Therefore, chlorination (or alternative disinfection) of the plant product water is expected to be required to meet the removal/inactivation requirements. At this stage, it is estimated that approximately 2-log removals will be granted for either the beach well 1651A 5-21 Wright-Pierce

22 or open ocean water process and that 1-log will remain for inactivation with a disinfectant. Since the 1 log inactivation of Giardia would control the sizing of a final disinfection system, it was used to preliminarily size the required volume of a baffled storage tank (i.e., finished water clearwell) described in a later section. No degassifier is included in this process. Degassifiers are typically installed in brackish water RO treatment to remove excess CO 2 and increase ph of the finished water. In Hull's case where seawater is being treated, it will be necessary to add CO 2 to provide alkalinity to buffer the corrosivity of the finished water Plant Design Parameters The anticipated water qualities of the various production and waste streams that are part of the RO process have been calculated using a computerized water quality model. Table 5.4 presents projected qualities of the raw water, permeate, concentrate, and plant product waters as based on modeling results. TABLE 5-4 QUALITIES OF PLANT WATER STREAMS HULL, MASSACHUSETTS Constituent Raw Water Permeate Concentrate Product Calcium, mg/l Magnesium, mg/l 1,3 2 2,6 2 Sodium, mg/l 9, ,53 71 Potassium, mg/l Bicarbonate, mg/l Sulfate, mg/l 3,2 3 6,4 3 Chloride, mg/l 19, ,9 117 Bromide, mg/l 57 <1 116 <1 TDS, mg/l 33,9 2 67,7 24 TSS, mg/l 3 - ph Temperature, ºC A 5-22 Wright-Pierce

23 Based on the process design considerations discussed, conceptual plant design parameters have been developed and are presented in Table 5-5. These design parameters were developed for each of the three plant capacities under consideration. All of this data should be refined and reassessed as more complete water sampling data is collected in subsequent project phases. TABLE 5-5 PLANT DESIGN PARAMETERS HULL, MASSACHUSETTS Parameter 2.5 MGD 4. MGD 5. MGD Number of trains Train Capacity, MGD Number of passes Recovery, % Raw water flow, MGD Concentrate flow, MGD Flux, summer, gfd Feed pressure, summer 1,2 1,2 1,2 Feed pump Hp, each (1) 1,1 1,2 1,1 Installed treatment Hp 2,5 4,2 5,2 Number of PV (2) per train Number of elements/train Total PV (2) Total membrane elements 1,536 2,448 3,72 1. Sum of boost and feed pump Hp for individual train and includes consideration of energy recovery device. 2. PV = pressure vessel Waste Streams As described previously, various waste streams generated by the RO WTF will require treatment and convergence to a point of disposal. A brief description of primary waste streams follows Concentrate The primary waste produced by the RO facility will be the RO concentrate or brine. Concentrate will be produced at a flow equal to the product water flow for each of the WTF capacities with 1651A 5-23 Wright-Pierce

24 TDS approximately twice that of the influent seawater. The concentrate will ultimately need to be returned to the ocean through an outfall. Options for an ocean outfall will be driven primarily by permitting considerations discussed in Section 6 of the report. In summary, the preliminary seismic profiling data of subsurface conditions around the Hull peninsula suggest that sufficient soil depth exists in several locations to accommodate a directionally drilled out fall. As the concentrate will be produced at a constant rate when the treatment plant is in operation, it shall be directed through the energy recovery devices (previously described) and into a transmission system for ultimate disposal via an outfall. Waste flows exit the RO membrane system with sufficient residual energy to pressurize a pipeline for disposal, without the need for pumping. Most excess energy (not utilized for disposal) is recovered with the energy recovery devices. A final disposal plan for brine is expected to create significant debate in the regulatory community because of the precedence that will be set with this project in the Commonwealth of Massachusetts. Management of wastewater flows may be required on site with equalization tanks to balance flows to the environment. Costs for brine storage will be factored into the cost economics CIP Despite the level of pretreatment provided, the RO membranes will foul over time and require cleaning to regain any lost capacity. This is typically performed via a process referred to as cleaning-in-place (CIP). For a CIP, the RO pressure vessels are isolated from the process and cleaned with various chemical solutions. Typical chemical solutions for CIP process consist of caustic and citric acid. Because the CIP processes produce wastes that typically result in high or low ph values, they are collected and neutralized on site prior to ultimate disposal. They can then either be blended in with the concentrate stream or sent to the sanitary sewer for disposal. For this project, we have assumed that a discharge directly to the local sanitary sewer makes the best economic sense. 1651A 5-24 Wright-Pierce

25 Preliminary volumes of waste generated per CIP are estimated to be approximately 8, gallons per train per cleaning event. For each of the three WTF capacities, the following volumes are estimated: 2.5 MGD: 16, Gallons/Event (64, Gallons/Year) 4. MGD: 24, Gallons/Event (96, Gallons/Year) 5. MGD: 32, Gallons/Event (128, Gallons/Year) For each of the three WTF capacities, a CIP cleaning system with a 1, gallon holding tank has been included. Based on the estimated cleaning frequencies and estimated volume of approximately 8, gallons per cleaning event, the CIP processes will be able to be staggered over time for better flow equalization Filter Backwash Wastewater Because the beach wells will not have the capacity to provide the volume required for the 5. MGD plant capacity, a direct ocean water intake will be required for this scenario. As discussed previously, the raw ocean water will require pretreatment to remove excess solids prior to desalting by the RO membranes. This pretreatment is preliminarily determined to be pressure membrane filtration. Over time, the membranes will also need to be backwashed to flush out the accumulated solids. This backwash water will be sent to onsite clarifiers for additional solids concentration. The clarified water from this process will be directed to the head of the plant for additional treatment and the concentrated solids (~ 1%) from the clarifiers would be sent to the Hull wastewater treatment plant via a sewer connection. Alternatively, mechanical sludge drying equipment could be provided at extra cost to produce dry solids for land filling. However, this option is not practical in Hull because of limited land availability. For a 5. MGD treatment facility, approximately 9, gallons of backflush wastewater is estimated from the pretreatment system to receive and equalize this flow, a 12, gallon wet well would be required to provide approximately 2 minutes of detention time for solids settling 1651A 5-25 Wright-Pierce

26 and clarification solids would be periodically decanted on the tank bottom and pumped to the sanitary sewer system Sanitary Wastewater Sanitary wastes from the building restrooms, etc. will be delivered to the sanitary sewer system serving the area Product Water Storage and Pumping On site storage for product water is also included as part of the WTF. Upon exiting the RO membranes, the product water will receive the post treatment for ph and alkalinity adjustment as well as chlorination for final disinfection. As discussed previously, a clearwell for additional disinfection credit is anticipated to be required. Therefore, the design includes an above ground storage tank to act as a baffled clearwell and provide the desired product water storage volumes. Based upon the required volumes calculated for inactivation credit, the sizes of the storage tanks for each of the three flow capacities are as follow: 2.5 MGD: 175, Gallons (Approximately 45' Diameter and 14.75' Height) 4. MGD: 28, Gallons (Approximately 5' Diameter and 19' Height) 5. MGD: 35, Gallons (Approximately 55' Diameter and 2' Height) Due to expected high groundwater tables and potentially undesirable soils, the product water storage tanks are proposed to be above ground pre-stressed concrete type tanks. In general, concrete has been preliminarily selected as the preferred construction material because of its lower long term costs, lower maintenance costs. Steel tanks require periodic repainting and cleaning. Flows to fill above-ground tanks will not require pumping because of residual pressure in the permeate downstream of the energy recovery system. Two or three finished water pumps are envisioned to pump disinfectant permeate to the distribution system. The pumps would be located in the water treatment facility. 1651A 5-26 Wright-Pierce

27 5.2.8 Chemical Usage As described previously, various chemicals can be expected to be used at varying points within the process. Chemicals used in the treatment process include: Scale Inhibitor to Allow Increased Recovery in the RO Process Carbon Dioxide (CO 2 ) to Increase Product Water Alkalinity Limestone to Increase Product Water Hardness And Ph Sodium Hypochlorite (Naocl) for Disinfection Cleaning Chemicals for RO Membrane Cleaning (CIP Process) Cleaning chemicals typically consist of surfactants, sodium hydroxide to increase ph, and acids (such as citric) to reduce ph. Additional proprietary cleaning formulations may be used, consisting of combinations of these and other specialized chemicals. Preliminary projected chemical use for the RO WTF in Hull is described in Table 5-6. TABLE 5-6 PROJECTED CHEMICAL USE HULL, MASSACHUSETTS Chemical % Conc. by Wt. Dose (mg/l) Consumption at Max Production 2.5 MGD 4. MGD 5. MGD Sodium Hypochlorite gpd 56 gpd 69 gpd (NaOCl) Antiscalant gpd 33 gpd 42 gpd Limestone 1 (dry) lb/d 1,334 lb/d 1,67 lb/d Carbon Dioxide (CO 2 ) lb/d 5 lb/d 625 lb/d 1651A 5-27 Wright-Pierce