Design challenges and strategy
The following points govern the modern SWRO desalination engineering.
- Operational reliability
- Operation maneuverability
- Construction modularity and flexibility
Biofouling tips the scale of the pretreatment type selection - ultrafiltration or multi-media filtration - in favor of the last (conventional pressure filters).
Biofouling concerns change the reliability strategy entirely as the installed spare equipment spurs biofouling. In turn, it destroys the equipment by accelerating corrosion rates.
The guiding approach is to move from static spare capacity to "rotating" one in pumps and filters, from "wet" mechanical spare to "dry" electric one, from spare items installed to items stocked.
Reliability is a "twin" of efficiency as the inverse of the product of reliability and efficiency defines the water price. In simpler terms it means that selection of reliable valves, filters and instruments is as important as selection of high efficiency pumps and motors.
The plant reliability and integrity during unplanned failures dominate in selecting the size and configuration of the trains and 16" SWRO membranes horizontally installed (instead of the 8" size in wide spread). The bigger membranes increase the plant reliability by more than 2 percent.
The start-up and shut-down procedures have been entirely changed to meet the request for diurnal variation in the water production.
Construction modularity makes the plant design eligible for "copy-paste" engineering and keeps the customization at minimum.
The first step to modularity is minimizing the concrete works volume. The intake station is known to have the biggest one. The proposed design has at least 50% less concrete volume due to application of pull-out MultiDisc screens. Next step is to move from the below-ground concrete tanks to the above-ground ones costing but a fraction of the first.
The concrete structures are a barrier to the construction flexibility. It addresses not only quick fixes of the inevitable design misses but – which is more important - the plant future extensions and refurbishments to leverage technological improvements. For example, this plant is engineered for the capacity extension as high as 20 percent.
Process flow diagram
[image] PFD shows a sequence of the processes needed to comply with the client's functional requirements. It includes coarse filtration and pumping of seawater to the filtration second stage – pressure multi-media filters. After it seawater is pressurized to the pressure above 60 Barg and fed to the first stage of the reverse osmosis. This pass produces two streams of product – of low salinity and higher one. As the low salinity product stream meets the client's requirements, it goes directly to posttreatment. The inferior quality product is pumped through the other 3 passes of reverse osmosis membranes and is sent to the final product tank. After posttreatment in limestone reactors the product is pumped to the final product tank.
67 generic P&ID symbols are described here.
[image] Three seawater pipelines each designed for 60% of nominal seawater flow, connect the intake heads with the pumping station.
The latter includes self-cleaning MultiDisc screens, vertical pumps, automatic active cathodic protection system, chlorination system, air compressor, and debris removal system.
All of the pumps are equipped with variable frequency converters. The latter and the motors are oversized by 25% to compensate for the lack of the standby pump. As seen only 7 pumps will be installed to cover the current feed loads.
[image] [image] Pretreatment system includes four trains of conventional multimedia pressure filters backwashed with the effluent brine. To maximize the filtration efficiency, sulphuric acid, polymer and flocculent are admixed to seawater before filtration.
The selected backwashing system does not cause any dips in the plant production during its operation. The said system contains an open tank with the three low-head pump, and the air scouring system. The tank is continuously filled with the brine rejected from the reverse osmosis process or the filtrated water. After backwashing the filter goes through maturing phase during which the filtrated water quality returns to the normal one. This water flow is diverted and fed again to the filters.
The backwash water after filters is collected in the tank and then transferred to the sludge treatment system before being pumped to the brine line. The filtration quality is periodically checked through water sampling to SDI- monitoring system. Free chlorine, turbidity and pH are monitored as well.
[image] Selected implementation of the seawater reverse osmosis desalination is built round the 14x14x19 pump and ERI PX-300 - energy recovery device brand produced by Energy Recovery company.
The pump in question is a 2-stage axially split type API-compatible. It comes with the oil lubrication system needed for sleeve bearings cooling. Its 50Hz version is manufactured by 2 companies – Sulzer and ClydeUnion. Ebara built the 60Hs version. Its efficiency is around 90%.
As the unavailability of the pump is by a factor of 10 lower(!) than the unavailability of the single membranes array, the latter is split into three subarrays connected in parallel to the pump. Even in the case of two subarrays out of operation, the pump may be operated trouble-free for a prolonged period (8 hours and more).
As shown on P&ID SWRO membrane array is fed with 2 streams of seawater
The first stream is pressurized in the high pressure booster pump and the high pressure pump connected in train. The second stream is pressurized in ERI through the energy recuperation from the brine reject. Due to the brine pressure being below the one at the SWRO membranes inlet, and inevitable energy losses in ERI, the second stream is additionally pressurized in the ERI booster pump before being fed to SWRO membranes.
What makes unique this stream configuration is that they enter the membrane array from opposite sides. It means that roughly 50% of the membrane vessels is fed with the feed after the energy recovery accompanied by the inevitable brine admixture.
This mode of SWRO array feeding and operation is pioneered and used exclusively by IDE Technologies since 2003 (Ashkelon plant). It has been replicated in all the company plants afterwards. Surprisingly, this important discovery went unnoticed by IDE and the rest of the world. Its true value is yet to be re-assessed as it paves the way to new fast start/stop procedures needed to follow the diurnal water consumption variation and leverage the low electricity tariffs. These procedures are already documented for this project (patent pending).
The high pressure booster pump serves 2 purposes; it accommodates the pressure variation in the SWRO process and, secondly, it boosts the pressure before the high pressure pump to avoid cavitation incipience. To protect the booster discharge piping from the pressure surge during the power outage, the check valves are installed at the inlet of the membrane array. In comparison to a single check valve on the high pressure pump discharge, smaller valves are 4 times faster in catching the pressure surge.
Application of the 16" membranes substantially decreases the array height – the key parameter in sizing the suck-back tank. Only the water volume above this height may be used safely for membrane back-flushing during the plant outage.
The permeate streams extracted from the membranes front end and the rear one differ in quality generally defined by remained TDS and Br content. By varying the ratio between the permeate streams the front-end quality may be tailored to that of the final product delivered to the client.
the high pressure pump forced oil lubrication system serves both the pump and the motor. For higher reliability of this system, one oil pump is coupled to the shaft of the high pressure pump. At power supply interruption this pump continues pumping oil to the bearings till the complete stoppage of the pump set. To cool the oil, the lubrication system is plugged into a cooling system common for all water-cooled motors.
To decrease the risk of SWRO membrane fouling and scaling formation, the antiscalant is constantly added to seawater streams. During the intake chlorination, the free chlorine control is engaged, injecting SBS into seawater if needed.
Another unique feature of this SWRO unit is horizontal double-split micron filters. Comparing to the conventional vertical single-flow design they have the following advantages.
- The smallest footprint and compact design
- The same axial direction for inlet and outlet
- The easy access to the cartridge filters headers for replacement
- No ladders and pedestals
- In cartridge headers replacement, piping is not dismantled at all
- The shortest time for the cartridge headers replacement
SWRO membrane array
[image] This P&ID shows SWRO membrane arrays arrangement and manifold fittings. The membrane stand may accommodate extra 5 membrane vessels. All the membrane are already registered with the membrane tracking software.
[image] This P&ID shows the antiscalant and SBS daily storage and dosing systems. Batch recharging of the dosing systems is fully automatic. The typical storage system includes an open tank with a spill berm, the group of transfer pumps with 100% reserve capacity, and the strainer installed at the pumps common suction line. The storage system is common for all SWRO units. The dosing system has 50% redundant capacity and a means to check the metering pump calibration (measuring bucket and/or mass-meter).
[image] This P&ID shows brackish water reverse osmosis (BWRO) unit needed to raise the quality of the rear end permeate produced by SWRO unit (from above 300ppm TDs to below 75 ppm), and the permeate suck-back (surge) tank. The BWRO unit includes three passes.
The first pass membrane array and the pump sizes are defined by the motor VSD optimal size of around 1000kW; each kW added above this value is priced substantially higher. Eight first-pass arrays and six arrays of combined second and third passes are needed to process 80% of the total product flow from the SWRO pass. The brine after the last pass is fed to the SWRO unit.
BWRO membrane array
[image] This P&ID shows BWRO membrane arrays arrangements for the first pass and for the combination of the second and third passes. The ratio between the passes is 7 : 2.6 : 1. Unlike the first two passes, the last one uses the dense seawater membranes.
[image] This P&ID shows details of remineralization of front-end permeate in the pressurized limestone reactors to make it more stable and less corrosive. The process includes the CO2 dosing before the reactor and admixture of NaOH after it to correct the pH value.
After remineralization the product water stream is mixed with the permeate from the BWRO unit before entering the final product tank.
The limestone reactor shall be periodically backwashed. As the water used for the reactor backwashing has high content of solids, it is treated by the sludge treatment system before returning back to reactors.
[image] CIP system is an auxiliary system used to clean the SWRO and BWRO membranes before they being replaced for new ones. Before being fed to SWRO membranes the CIP solution is passed through the micron cartridge filter. The CIP interconnecting piping is designed for maximum allowable velocity to cut down the piping volumes to be flushed after each cleaning.
The CIP system is sized for parallel cleaning of 3 SWRO arrays. The client may choose another strategy – so called weak CIP cleaning. It has substantially shorter duration but executed every 15 - 30 days. This mode may be run automatically without the high pressure pump stoppage. Implemented reverse flushing of the CIP piping and tank make the neutralization tank redundant.
This process is a challenge in mega-projects as the sludge transport over the distances above 20 - 40 meters is unreliable. So the sludge treatment location and the piping flushing mode shall be carefully selected.
Shown in P&ID typical process of waste water treatment includes flocculation and settling in the lamella thickener equipped with rotating scraper. After the lamella settler the clean stream is pumped to the brine outfall line, while the liquid sludge (over 5 percent by weight) is further concentrated (above 24%) in the centrifuge for final dewatering and disposal.
[image] Shown in the figures the plant preliminary layout meets the following requirements.
- Bulky equipment installation requirements
- Technological requirements (such as sludge treatment)
- Work safety and operation and maintenance requirements
- Minimum length of interconnecting piping
- Direct access to all the chemical storage tanks
- Installation requirements for electronic devices
- Noise abatement requirements
- Future extension and refurbishment requirements