City of Beverly Hills Reverse Osmosis Water Treatment Plant

Transcription

1 Reverse Osmosis Water Treatment Plant Providing a safe and reliable supply of drinking water has always been a goal of the City of Beverly Hills. Today s growing municipal water needs require the treatment of millions of gallons of water each day. Through a partnership with the City of Beverly Hills and Earth Tech, a proven provider of Total Water Management, the Public Works Facility and Water Treatment Plant was built to enhance the community s water resources and provide its citizens sustainable, quality water services for generations to come. This successful partnership will continue to operate the facilities, benefiting everyone who lives, visits or works in Beverly Hills.

2 TABLE OF CONTENTS Page Definition of Terms... 1 Purpose of Water Treatment System... 1 Principles of Operation of the Overall Water System... 1 System Description... 2 Overview... 2 Antiscalant Chemical Dosing System... 4 Sulfuric Acid Chemical Dosing System... 4 One-Micron Cartridge Filters... 4 RO High Pressure Pumps... 5 Reverse Osmosis System... 5 Hydrogen Sulfide Stripper System... 5 Sulfuric Acid Chemical Dosing System... 6 Hydrogen Sulfide Scrubber System... 6 Caustic Soda Chemical Dosing System... 7 Sodium Hypochlorite Chemical Dosing System... 7 R.O. Configuration... 8 Reverse Osmosis Membranes... 8 Permeate Tube... 9 RO Element Housings (Pressure Vessels)... 9 High Service Pumps # Flushing/CIP System Hydrogen Sulfide Stripping Hydrogen Sulfide Scrubbing RO Cleaning Cycle RO Cleaning Cycle 1st Phase Shown Finished Water Clearwell Plant Overview Plant Flow Diagram Pretreatment Reverse Osmosis Air Strippers & Scrubber Finished Water i

3 Definition of Terms Brine - (1) Reject - normally, the waste stream from reverse osmosis system, which contains most of the dissolved solids in the feed in a concentrated form. (2) Regenerant - water saturated or strongly impregnated with salt. Concentrate - The reject (waste) from a reverse osmosis element. Membrane - A barrier, usually thin, that permits the passage only of molecules up to a certain size range and/or certain charge characteristics. Membrane Element - An individual membrane assembly, usually tubular, must be housed in some type of pressure vessel. Module - A single pressure vessel or housing containing membrane elements. Module Assembly - An array of membrane housings. Osmosis - The passage of water molecules through a semi-permeable membrane separating two solutions of different concentrations (the water passes into the more concentrated solution). Osmotic Pressure - Pressure required to prevent spontaneous flow of solvent (water) into the more concentrated solution when a semipermeable membrane separates two solutions of different concentration. Reverse Osmosis - Forced flow of solvent (water) from a solution through a semi-permeable membrane by means of pressure in excess of the osmotic pressure. Purpose of Water Treatment System Residents in the City of Beverly Hills presently receive their water service from the Metropolitan Water District (MWD) of Southern California. Through an agreement with MWD, the City has been developing supplemental groundwater well supplies. The City of Beverly Hills Reverse Osmosis Water Treatment Plant was designed to reduce total dissolved solids (TDS), total hardness, hydrogen sulfide, iron, and manganese concentrations in the groundwater supply wells that have been developed by the City. The final goal is to provide water similar to the MWD water supplied to the City of Beverly Hills. Principles of Operation of the Overall Water System The City operates and maintains the four water supply wells. Water from the wells are collected in a common water main and transmitted to the water plant. Well No. 6 is transmitted north separately. Both raw water transmission mains combine outside the treatment plant, providing a blended water supply. To initiate plant operations, the plant operator coordinates 1

4 with the City to select the desired combination of water supply wells to provide the required influent water quality and quantity. After treatment water discharges into a 16-inch dedicated water transmission main which is connected to Sunset Reservoir where MWD water enters the Beverly Hills system. The Beverly Hills Water Treatment Plant operates at a constant rate, with the MWD influent varying to match the system demand. System Description Overview The Water System consists of a single Reverse Osmosis (RO) Train and its associated pre-treatment and post-treatment equipment. Pretreatment equipment includes two horizontal 1-micron cartridge filters and one Antiscalant (Scale Inhibitor) injection system. In addition, a sulfuric acid injection system has been provided to ensure that the RO is operated at a high recovery ratio. The Reverse Osmosis Train is furnished with membrane housings, membrane elements, piping, energy recovery turbine, sampling, instrumentation and PLC controls. A CIP (Clean-In-Place) system is also provided to enable the membranes to be cleaned regularly in order to extend their life. Post Treatment consists of sending the permeate water through a packedtower stripper to provide removal of hydrogen sulfide. The blend by-pass water is also treated with acid and sent to a packed-tower stripper also to remove hydrogen sulfide. The foul air exhaust from both strippers is taken to a three-stage scrubber to remove the hydrogen sulfide using caustic soda and sodium hypochlorite so that the air can be released to the atmosphere. The water from each of the two strippers is blended in the clearwell and sodium hypochlorite added for disinfection. Three high service pumps are provided to pump the water to the City of Beverly Hills Water System. The function of each component in this system is as follows: Antiscalant Injection System: To delay the precipitation of the scaling salts such as Calcium Carbonate (CaCO 3 ), Barium Sulfate (BaSO 4 ) and Silica (SiO 2 ) within the RO membrane, thus allowing the RO system to operate at a higher recovery. Sulfuric Acid Injection: To reduce the feedwater ph to 7.0 thereby reducing the alkalinity and the tendency to scale by lowering the Langelier Index. This also assists in allowing the RO to operate at a higher recovery (design 78%). 1-micron Cartridge Filters: To remove particles in the feedwater larger than 1-micron diameter. 2

5 Reverse Osmosis System: To remove dissolved salts from the water. Permeate Air Stripper: To remove hydrogen sulfide from the permeate water. Blend By-pass Air Stripper: To remove hydrogen sulfide from the bypass raw water. Sulfuric Acid Injection: To reduce the by-pass raw water ph to 6.0 thereby increasing the fraction of the hydrogen sulfide in the gaseous form and improving the efficiency of the stripper Scrubber: To remove hydrogen sulfide from the foul air in order to allow discharge to atmosphere. Caustic Soda Injection to the first stage of the scrubber: To remove most of the hydrogen sulfide as sodium sulfide. Caustic Soda Injection to the second/third stages of the scrubber: To remove most of the remainder of the hydrogen sulfide as sodium sulfide. Sodium Hypochlorite Injection to the second/third stages of the scrubber: To remove the last traces of hydrogen sulfide by oxidation to sodium sulfate. Twin Tank Softener System: To provide softened permeate for scrubber circulating water make-up in order to avoid precipitation of hardness after several cycles of concentration. Sodium Hypochlorite Injection to the blended water to the clearwell: To sanitize the treated water and maintain the clearwell and distribution system free of biological activity. Caustic Soda Injection to the high service water pumps discharge: To increase the treated water ph to prevent corrosion in the distribution system. Ammonia Injection to the high service water pumps discharge: To combine with a proportion of the free chlorine to form chloramines for increased sanitizer stability in the distribution system. CIP System: To clean the RO membranes after fouling has occurred. Twin Tank Softener System: To provide softened treated water for cleaning tank make-up during periods when no permeate is available. Clearwell (40,000 gallons): To allow mixing and disinfection prior to pumping to distribution system Chemical Storage Area: To store and distribute chemicals to all process related injection points High Service Pumps (#1 and #2- Service to Sunset Reservoir and #3 Service to Zone 4): To pump treated water to the City of Beverly Hills Water Distribution System. Post production water analyzers: To monitor water parameters (quality related) including ph, turbidity, conductivity and residual chlorine. 3

6 Antiscalant Chemical Dosing System Antiscalant The electronic chemical metering pump draws chemical from the holding tank and delivers it at a controlled rate to the RO feed water. Antiscalant is a scale inhibitor chemical which, when injected at the specified levels, protects the downstream RO membranes from salt precipitation. Due to the process of salt concentration occurring on the concentrate side of the RO membranes, some salts reach saturation and would tend to come out of solution and deposit on the membrane surface. The antiscalant chemical delays the salt precipitation process until the concentrate has exited the system thus preventing scaling of the membrane surfaces. Scaling causes are due to the following: Salts such as CaCO3, CaSO4, BaSO4, SrSO4, etc (primarily CaCO3 at the Site). Metal hydroxides - common with high iron well water if air contact occurs before processing. Silica (the main limitation on the recovery of this water). A special anti-scalant must be used which is effective with silica as well as other salts. Sulfuric Acid Chemical Dosing System Sulfuric Acid is fed to the RO influent feed water to maintain a constant ph of 7.0. ph Control is critical in the processes following. This system will use Sulfuric Acid at a concentration of 93%. This product will meet N.S.F. approval standards and be fed at a rate of approximately 51 mg/l or 55.9 gallons per day. Sulfuric Acid Cartridge Filter Vessel One-Micron Cartridge Filters The cartridges remove suspended particles of 1 µm (micron) in size and greater. Water flows from the outside to the inside of the cartridges where it is collected inside the core. As water pushes its way out, particles in excess of 1 micron in size are trapped on the filter media. Eventually, more and more particulates plug the cartridge(s), the surface area available for filtration decreases, and the pressure drop ( P) between inlet & outlet of housing increases. It is typical to change cartridges when the P reaches 10 psi. Depending on the water quality, cartridges are typically changed every 2 weeks to 1 month. Differential pressure is indicated and monitored; a high differential pressure across the cartridge filter transmits an analogue signal to the PLC. An alarm is triggered on the RO panel. 4

7 RO High Pressure Pumps Two (One duty, One standby) vertical turbine pumps manufactured by Afton, from 316 SS tag # HPP & HPP Model # MPV, 6 X 8 X 10 complete with 150 hp high efficiency motors with variable speed drives. RO High Pressure Pumps The pumps are each rated for a maximum of a 1, ft. and 3621 rpm. Reverse Osmosis System The Reverse Osmosis System is designed to produce up to 1,028 gpm of product water from a feed of up to 1,318 gpm. The membranes are arranged as a 2-Stage system where the reject water exiting the first stage RO housings becomes feed water for the second stage thereby allowing increased system recovery. The RO vessels are arranged in three banks each containing 10 vessels arranged in parallel. The first two banks make-up the first stage and the third bank the second stage. RO System Hydrogen Sulfide Stripper System Permeate Stripper The Permeate Stripper Tower is designed to remove >91% of the 4 ppm of hydrogen sulfide gas present in the raw feed and permeate water at up to a maximum of 1,331 gpm. It is a countercurrent design 8' 0" diameter X 20' high on straight manufactured from FRP and lined with an NSF approved resin. The inlet distributor is a weir trough design to give good spreading characteristics with minimum head loss. The tower is packed with 12' 6" of 3.5" "Lanpac" although calculations require only a minimum of 6.3' to achieve design removal. The tower is designed to perform using 3,000-4,000 cfm of air, induced by the blower on the scrubber inlet. The calculated packing pressure drop is 0.01" W.C./ft. Total pressure loss through whole stripper system is calculated at 0.537" W.C. Strippers The Permeate Stripper tower air quality is monitored as per the South Coast Air Quality Management District (SCAQMD) permit. Blend By-Pass Stripper The Blend By-Pass Stripper Tower is designed to remove >91% of the 4 ppm of hydrogen sulfide gas present in the raw feed water at up a maximum of 1,125 gpm. It is a countercurrent design 7 0 diameter X 20 high on straight manufactured from FRP and lined with an NSF approved resin. The inlet distributor is a weir trough design to give good spreading characteristics with minimum head loss. The tower is packed with 12 6 of 3.5 Lanpac although calculations require only a minimum of 7.54 to achieve design removal. The tower is designed to perform using 3,000 3,800 cfm of air, induced by the blower on the scrubber inlet. The calculated packing pressure 5

8 drop is W.C./ft. Total pressure loss through whole stripper system is calculated at W.C. The Blend By-Pass Stripper tower air quality is monitored as per the South Coast Air Quality Management District (SCAQMD) permit. Sulfuric Acid Chemical Dosing System Sulfuric Acid is fed to the blend by-pass stripper influent feed water to depress the ph to 6.0 for maximum efficiency of hydrogen sulfide removal. The lower the ph, the higher the percentage of sulfide removal. This stripping process should remove approximately 90% of the hydrogen sulfide. The remainder H 2 S will be oxidized to sulfate in the treated water clearwell. Sulfuric Acid This system will use Sulfuric Acid at a concentration of 93%. This product will meet N.S.F. approval standards and be fed at a rate of approximately 254 mg/l or gallons per day. Hydrogen Sulfide Scrubber System Packaged Three Stage Scrubber System The Scrubber system supplied by U.S. Filter is a three stage packaged odor control system designed to treat the foul air extracted from the permeate water and blend strippers. The system is fed by a blower with a normal rating of 6,000 cfm, which can be increased if necessary to 8,424 cfm by changing the belt drive pulleys (supplied). The guaranteed removal efficiency is >99.5%. Scrubber The three-stage design minimizes chemical consumption as it allows the bulk of the hydrogen sulfide to be converted to sodium sulfide in the 1 st stage using only caustic soda. The 2 nd stage uses fresher caustic soda plus some sodium hypochlorite and finally the 3 rd stage also uses fresh caustic soda and hypochlorite. The first stage uses fresh caustic soda plus caustic soda and sodium hypochlorite left over from the 2 nd stage & 3 rd stages. The 2 nd & 3 rd stage uses fresh caustic soda and sodium hypochlorite. The 2 nd /3 rd stage recirculating water is drawn from the 2 nd /3 rd stage sump and pumped via the 2 nd / 3 rd stage pump at 225 gpm. The pump flow is split 150 gpm to the 3 rd countercurrent stage and 75 gpm to the 2 nd co-current stage. Water make-up is soft water fed to the 2 nd /3 rd stage sump at 1-5-gpm. This continuously overflows, taking the excess chemical to the 1 st stage sump where it is consumed along with the fresh caustic soda. 6

9 The 1 st stage water is drawn from the 1 st stage sump and is recirculated via the first stage pump countercurrent to the incoming foul gas at 150 gpm. This water overflows to waste to maintain the level constant. The cycles of concentration are controlled by regulating the make-up rate, which is normally in the 1-5 gpm range. The caustic and hypochlorite additions are controlled by the analogue outputs of the respective ph and ORP meters. The Scrubber emissions are monitored as per the South Coast Air Quality Management District (SCAQMD) permit. Twin Alternating Softeners for Scrubber Make-Up The U.S. Filter model KF twin-alternating softeners are designed to remove the last traces of hardness from the stripped permeate before being fed to the scrubber as make-up. This will ensure that no hardness scale is formed within the scrubber system, which is at a high ph and undergoes several cycles of concentration in order to obtain minimum blow down. Softener In case of no permeate being available e.g. at start-up, the treated water connection can be used which supplies water from the high service pumps discharge. This will put a far higher load on the softener, which will reduce the capacity between regenerations to about 22 hours. Caustic Soda Chemical Dosing System Caustic soda will be primarily used in the air scrubber. Up to 5 gpm of makeup water is estimated to be required for the scrubber, and caustic soda will be added to the solution for maintaining a ph of approximately At this ph most of the H 2 S will be removed. Caustic Soda This system will use Caustic Soda at a concentration of 50%. This product will meet N.S.F. approval standards and be fed at a rate of approximately 33.6 gallons per day. Sodium Hypochlorite Chemical Dosing System Sodium Hypochlorite is fed to the 2 nd and 3 rd stage of the air scrubber system to assist in the breakdown of H 2 S. Sodium hypochlorite is used to convert H 2 S to elemental sulfur. Theoretical requirements: 2.1 parts of N 2 OCL to one part H 2 S. Sodium Hypochlorite This system uses Sodium Hypochlorite at a concentration of 12.5%. This product meets N.S.F. approval standards and is fed at a rate of approximately gallons per day. 7

10 Feed Water Permeate Reject Permeate Reject City of Beverly Hills R.O. Configuration The RO is designed as two stages in series: The reject from the 1 st stage becomes feed to the 2 nd stage. RO Configuration In the 1 st stage, there are twenty- (20) RO housings in parallel. In the 2 nd stage, there are ten (10) RO housings in parallel. Permeate from both stages are collected in a common manifold. In every housing, there are five (4 + 1) membranes (or elements) connected in series. Reverse Osmosis Membranes The Reverse Osmosis process can be regarded a filtration process on the molecular scale. The membrane acts as a barrier to all dissolved salts and inorganic molecules, as well as organic molecules with molecular weight greater than 100. Dissolved salts of <100 MW are rejected according to their ionization characteristics. Water molecules (H 2 O) and gases such as CO 2 or H 2 S pass freely through the membrane. This creates a purified product stream known as permeate. Rejection of ionized dissolved salts is typically greater than 98%; gases must be removed by degassing with a packed tower. 8

11 Permeate Tube The permeate tube is a plastic pipe at the center of the membrane perforated with rows of holes to which the membrane leaves are glued. Water that spirals around the permeate flow channels inside the membrane enters the tube through the holes. To connect permeate tubes of RO membranes in series, interconnectors are provided for that purpose. To connect the 1 st & last RO membranes inside RO housing, a permeate end adapter is used. Feed Spacer Brine Seal (O'ring) Inside U-Cup U-Cup Without Brine Seal Permeate Tube Sketch of RO Membranes Feed Spacer RO Element Housings (Pressure Vessels) RO housings are designed to accommodate RO membranes on the inside of the housings. At each end of a RO housing, there are two connections: "Feed/Concentrate" and Permeate. High Service Pumps 1 & 2 Feed water enters the housing under pressure through the "Feed/Concentrate" connection on one side of the housing (in this case, 1½" 316 SS Victaulic Connection). The "Feed/Concentrate" connection on the other side of the housing is used to collect reject or concentrate water, which becomes feed to the 2 nd stage housings. Reject of the last stage is sent to the sewer system. The water that permeates through the membranes into the center tube, called the permeate water (or product), is collected and exits from each PVC permeate port in the middle of the housing (in this case, 1" FNPT PVC connection). High Service Pumps # 3 One vertical turbine pumps manufactured by Johnston Pump, from 316 SS tag # HSP 603 Model # 5KS449ST6066P complete with 225 hp high efficiency motors with variable speed drives. The pumps are each rated for a maximum of a ft. and 1750 rpm. 9

12 Flushing/CIP System Flushing/CIP System A combined Flushing/CIP ( Clean-In-Place ) system is provided to flush & clean the RO system. The system consists of a 316 SS CIP tank for the preparation, mixing and recirculation of chemicals, a HDPE flushing tank (by customer) for storing clean permeate or softened service water a HDPE scavenger tank (by customer) for storing and neutralizing chemical cleaning waste prior to discharge and a "cleaning skid" with pump and 5 micron cartridge filter. The design flowrate for the CIP skid is 400 gpm. The flushing system is used as part of the normal shutdown operation in order to feed permeate water into the feed/concentrate side of the membrane and eliminate any tendency to scale during shutdown. Membrane Interconnectors Permeate Tube Feed Last Membrane Intermediate Membranes First Membrane Permeate Reject Brine seal prevents brine from one side of the membranes to mix with feed on the other side of the membranes End Adapters Flow Path Inside RO Housing Permeate water travels through the permeate tubes connecting all membranes and leaves the system from the permeate port of the RO housing where it is collected and discharged through the permeate manifold. The water that does not permeate the membranes, the Reject Water, is collected in the reject Manifold and discharged to drain. During service, the feed water enters the duty cartridge filter and is fed to the H.P. pump suction where it is pressurized and fed to the RO system. The pumping rate is controlled by the VFD which modulates the pump in order to keep the permeate flow rate constant. The concentrate from the RO is also modulated by the concentrate control valve which hold the concentrate flow constant to achieve a recovery of 78%. 10

13 RO Service Cycle Hydrogen Sulfide Stripping The acidified water from the blend by-pass and the permeate from the RO system are both fed to individual induced draft packed towers to remove the bulk of the H 2 S gas present in both streams. The inlet level of H 2 S can be up to 4 ppm and in it's gaseous molecular form (at low ph), cannot be removed by the RO membrane. Hydrogen Sulfide Scrubbing The foul air from the strippers cannot be discharged into the environment because of the odor problem. The air is therefore passed through a triple stage scrubber system to effect >99.5% removal. This is a relatively more expensive process than stripping since all the H 2 S has to be removed chemically. The chemical doses are continuously modulated under ph control for the two caustic soda injections and under ORP control for the hypochlorite injection. RO Cleaning Cycle As described before, due to the size of the RO, the CIP skid is designed to clean this RO in three cycles: 1. The 1 st cycle consists of cleaning the 1 st seven housings in the 1 st stage (first bank) 2. The 2 nd cycle consists of cleaning the 2 nd six housings in the 1 st stage (2 nd bank) 3. The 3 rd cycle consist of cleaning the last six housings in the 2 nd stage (3 rd & last bank) 11

14 Reject Return Line CIP Feed ft head Permeate Return Line City of Beverly Hills A typical flowrate for cleaning of each RO-housing is 35 to 40 gpm at maximum temperature of 35 ºC. The cleaning feed pressure should not exceed 60 psi and must be above 50 psi. RO Cleaning Cycle 1st Phase Shown 1st Stage 316SS Manifold 1st Stage 316SS Manifold 1st Stage PVC Manifold Mov PSL 123 PI V-525 1st Stage 1st Stage HPP V-510 CIP Tank T-901 Cartridge Filter F-901 CIP Pump CFP-901 RO Cleaning Cycle Suitable cleaning chemicals are re-circulated through each bank of RO membranes, and then rinsed and finally the system re-started. The spent chemical solutions must be neutralized and then sent to the sanitary sewer. Finished Water Clearwell The Reverse Osmosis permeate is gravity fed from the air stripper, blended with by-pass raw water, and treated with sodium hypochlorite where it is then discharged into a 46,686 gallon clearwell for immediate discharge to Sunset Reservoir or Zone 4 of the distribution system. The clearwell is a rectangular concrete structure located within the treatment plant facility. The inside height is feet. Two screened 18 overflow connections are 9.5 feet from the floor of the clearwell extending outside the clearwell with elbows directed into a storm drain box. An appropriate air gap is provided at the overflow discharge location. Two vertical turbine high service pumps are installed on the top of the clearwell, one of which serves as standby. The two pumps have the capacity of 1805 gpm and are powered by electric 150-hp motors with variable frequency drives. Each pump is able to support the 12

15 maximum flow requirements through a 16 ductile iron pipe, at the design maximum distribution system pressure of 70 psi. A third high service pump is installed on the top of the clearwell. This third high service pump has the capacity of 2240 gpm and is powered by an electric 250-hp motor with a variable frequency drive. This pump is designed to delivery-finished water to Zone 4 of the distribution system. Plant Overview The Plant Overview screen, as depicted below, gives a general overview of the plant and the different areas of interest. Plant Overview 13

16 Plant Flow Diagram If the operator needs to view the plant overall operational functionality the operator will need to access the Flow Diagram screen as depicted below. Plant Flow Screen In this screen the Treatment Plant Summary is available for the operator to view. The treatment plant summary gives the operator the current measured values of some of the key parameter for the water treatment system, such as conductivity, product flow rate and tank levels. 14

17 Pretreatment The Pretreatment Screen allows the most control of any of the screens controlled by the DCS. This screen monitors many of the process parameters such as the overall pump the flow rate, the cartridge filters differential pressure, the NTU, conductivity, temperature and ph after the filters and the ph of the blend. Pretreatment Screen 15

18 Reverse Osmosis The Reverse Osmosis system processes can be monitored from the RO System screen. However, no control is available from this monitor. To access the controls for the RO System go to the control panel 103. Reverse Osmosis Screen 16

19 Air Strippers & Scrubber The Air stripper and Scrubber system processes can be monitored from the system screen. However, no control is available from this monitor. To access the controls for the system go to the scrubber control panel. Air Strippers Screen 17

20 Finished Water The screen, similar to that shown below, is a graphical representation of the Finished Water Tank water level. However, no control is available from this monitor. Finished Water Screen 18