Process flow diagram

[image] As shown on Process Flow Diagram, seawater desalination procedure is typical for seawater with high TSS and COD of above 50 ppm and includes 5 major steps: water pumping, 3-stage filtration, desalination by passing the feed water through reverse osmosis membranes, improving the permeate quality by passing it again through the reverse osmosis membranes, posttreatment and product delivery. The said filtration includes DAF and the lamella settler as a first stage, the dual media gravity filtration as a second one, and ultrafiltration as the third stage. The selected filtration process guarantees the required quality of water fed to the reverse osmosis unit.

67 generic P&ID symbols are described here.

Intake station

[image] [image] This P&ID shows implementation of seawater coarse filtration and pumping steps. Seawater goes through the rotating band screen separating coarse debris. Two screens are for double flowrate of seawater and of pool-out design. Each screen may be isolated with the stop logs to perform necessary inspection and maintenance of submerged parts. The rotating band screen is equipped with the cleaning system and the debris disposal one. The first cleans the screens by high-velocity jets produced in specially designed nozzles. They are fed with seawater taken from the discharge manifold of the main pumps, and additionally filtered through a 150 micron self-cleaning filters, and pressurized up to 4 Barg in the auxiliary pumps. The debris disposal system is a set of interconnecting shoots and conveyers (not shown on P&ID). Ancillaries include the pigging system, the chlorination system, and the compressed air one. Pigging is a batch process which is started every time the water level in the sump approaches the minimum NPSHr for the main pumps. The pressured water from the intake discharge is used to push the pig along the intake pipe towards the intake head. The latter is common for 2 intake pipes each designed for 75% nominal flow rate. The second subsystem - the chlorination system - is simple and rugged in design as it is used only on rare occasions. The shock-chlorination dosing rates are monitored at the station discharge manifold and after the last filtration stage. The compressed air is fed to the intake head to build the bubble screen that should scare away the jelly fish. The last subsystem is the sacrificial-anode protection of the screens against corrosion (not shown on P&ID). The pumping requested turn-down ratio and spare (standby and rotating) capacity are provided with a number of pumps connected in parallel, each pump being driven by the variable speed drive (VSD). The main pumps are of horizontal centrifugal design. The pumps are primed with the vacuum system driven by the water ring pump type. As polymer and flocculent are injected into the pump suction line the static mixing before pretreatment unit and the rapid mixing with agitator are not needed.

Pretreatment

[image] [image] [image] The selected standard fine filtration process and implementation match the target SDI of 3.5 for output at the high TSS (total suspended solids and COD above 50 ppm) of the initial seawater. First seawater goes through the slow mixing chambers before entering the direct air floatation (DAF) chamber. Besides the latter the DAF system includes the air saturator and the recirculating pumps. The main task of the DAF unit is to remove oil and grease, which might clog the membrane surface. Another target, especially in summer when the raw water quality is potentially more difficult, is the reduction of turbidity, organics and suspended matter to improve the feed water quality to the subsequent DMF system. From the DAF chamber the seawater stream enters the lamella clarifier - a rack of inclined plates, which cause flocks of suspended solids material to precipitate from water that flows across the plates. The flocks sludge - settle at the bottom and are collected and fed by a slow-rotating scraper to the progressive-cavity pump through the sludge outlet located at the clarifier center. Small part of settled sludge is used for seeding; it is extracted just above the scraper and re-circulated to the flocculation chambers. Clear water stream from the lamella is directed to the multi-media filters working at the atmospheric pressure. This type of filter requires periodical backwashing as the build-up of the filtrated material gradually plugs it. The selected backwashing system does not cause any dips in the plant production during its operation. The said system contains an open backwash tank located above the clear water tank, the backwash pumps constantly filling the backwash tank with filtered water, and the air scouring system. 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 filtration quality is periodically checked through water sampling to SDI- monitoring system.

From the multi-media filters the seawater is pumped to the self-cleaning filters of 150 micron of the ultrafiltration system. These filters protect the UF fibers from potentially harmful objects. After the filters the water is distributed equally between the ultrafiltration modules. They are operated in dead-end mode with a design flux of slightly more than 72 L/(m2•h) in filtration mode. The filtration takes place from inside to outside. The design filtrate cycle time is 25 minutes until the fibers are backwashed. The filtration quality is periodically checked through water sampling to SDI- monitoring system. The system auxiliaries include the backwash system and CEB one. Additionally the UF modules are connected to the CIP system of the plant.

SWRO unit

[image] Selected implementation of the seawater reverse osmosis desalination is built round PX300 - energy recovery device brand produced by ERI company. As shown on P&ID SWRO membrane array is fed with 2 streams of seawater. First stream is pressurized in the high pressure booster pump and the high pressure pump connected in train. The second stream is pumped by the low pressure booster pump to ERI where its pressure is further increased 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. 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 decrease the risk of SWRO membrane fouling from scaling formation, the antiscalant is constantly added to seawater streams. During the intake chlorination, the free chlorine control is engaged, injecting SMBS into seawater if needed. To shave off occasional flares in SDI values (triggered by hydraulic transients in the pretreatment system), after first-stage pressure boosting seawater goes through micron filters of cartridge type. 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 is of the ring-diffuser construction. The pump and motor bearings temperature and vibration are constantly monitored.

SWRO chemical dosing

[image] This P&ID shows the antiscalant and SMBS daily storage and dosing systems. Such an implementation is safe and simple in maintenance. Batch recharging of the dosing systems is quick and 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).

BWRO unit

[image] This P&ID shows Brackish Water Reverse Osmosis (BWRO) unit is needed to raise the quality of the rear-end permeate produced by SWRO unit. This unit consists of the feed pumps delivering the rear-end permeate to the first array of the membranes, which brine reject is fed to the second array by the booster pump. Reverse osmosis is conducted at the slightly increased pH-values to maintain the high Boron rejection. The permeate streams collected from the membrane arrays are mixed with the front-end permeate, the mixture being directed to the permeate surge tank (not shown here). The volume of the latter is enough to feed the SWRO membranes during direct osmosis following the plant emergency outage and the SWRO membranes de-pressurization.

Posttreatment

[image] This P&ID describes the re-mineralization of desalted water by adding soda ash to make it more stable and less corrosive. The process is controlled by a set of criteria such as alkalinity, hardness, pH and LSI. To make the process effective, the CO2 gas is injected into the influent. The mineral-enriched water stream is then discharged to the buffer tank, its volume being selected based on the residence-time criterion, the plant average outage duration (4 hours) and the predicted hourly variations in the product consumption. This product water flowrate, pH and LSI are constantly monitored.

Plant layout

[image] The plant layout provides minimum length of interconnecting piping, clearly defines the project areas, and meets work safety and O&M requirements. All chemical storage tanks have direct access for trucks. Neither SWRO membrane vessel ends nor BWRO ones overlook the maintenance areas which are mostly frequently visited.