By IDE Technologies

PROJECT OBJECTIVES

• Compliance with product quality requirements at all times, under a wide range of operation conditions.
• Flexibility of operation as a result of the IDE modular approach
• Low energy consumption
• Chemical-free process
• Fully automated and highly reliable plant
• High-end materials of construction

The new Bonaire Seawater Reverse Osmosis (SWRO) desalination plant, based on IDE Technologies’ reverse osmosis technology, was recently successfully commissioned. It has added 5,600 m3/day to the existing desalination units in Bonaire, increasing the total water supply to 7,200 m3/day. Additional units can be added for future extension to 11,200 m3/day.

The plant is designed in a two (2) train concept – each train has a production capacity of 3,200 m3/day, and each train can operate independently of the other train.

LOW ENERGY CONSUMPTION

As low energy consumption was of great importance in plant design, measures were taken to ensure a low guaranteed value. These measures include process design at low recovery and flux, selection of low-energy membranes, and choosing equipment with high efficiency. The design also includes an Energy Recovery System (ERS) that maximizes the energy efficiency of the SWRO process.

MODULAR, PRE-ASSEMBLED DESIGN APPROACH

The preassembled and modular unit concept is a project delivery strategy developed by IDE Technologies, Ltd. The goal of this strategy is to provide desalination plants that:

• Are the best in class in terms of safety, quality, and performance
• Have the lowest total installed cost
• Can be installed in the shortest time
• Minimize site plant execution risks

In the preassembled and modular unit concept, a significant portion of the plant fabrication, assembly, and testing is completed off-site, under controlled plant conditions and precision tooling, using the same materials and designing to the same codes and standards as conventionally built facilities – but in much less time.

The preassembled and modular unit concept allows a reduction in installation schedules and costs, as modules are simpler to install than components delivered loose and assembled on site. In addition, since the total installed cost of the project is extremely well defined in the design process, and the installation scope is clear, the owner is at significantly less risk as a result of construction-related change orders and delays.

As module fabrication can occur simultaneously with site improvements and foundation work, projects can be completed sooner compared to traditional construction. In addition, the need for highly skilled construction personnel (i.e., welders, pipefitters, machinists, etc.) onsite is reduced, as are potential delays attributed to inclement weather during construction.

Shortening the onsite erection time offers considerable potential savings to the EPC contractor, and a shorter overall schedule, which results in an earlier start to the water production.

CHEMICAL-FREE PROCESS

The WEB Bonaire desalination plant provides the required product quality without the use of any chemicals. This is achieved by Direct Osmosis Cleaning (DOC) – a high-end, patented technology developed by IDE that utilizes osmotic pressure to clean the membranes in a short and repetitive process, removing biofouling and scaling from the membranes, and reducing the intervals between CIP.

MAIN PLANT COMPONENTS

Pre-treatment
Pre-treatment of seawater prior to the SWRO process, to remove suspended solids and minimize the growth of micro-organisms on the membranes, is of critical importance to the reliability of any SWRO process. Effective pretreatment is thus necessary to increase the efficiency and life span of the SWRO system. Selection of proper pretreatment minimizes fouling and membrane degradation, resulting in optimized product flow, salt rejection, product recovery, and operating costs.

The pre-treatment process includes fully automated Pressurized Multi-Media Filters (PMMF), followed by micronic filters.

Pressurized Multi-Media Filters (PMMF)
Each filter contains different filtration layers including anthracite, garnet, and sand. Each media (layer) has a different size and density. The arrangement of the layers is according to their size, with the larger and lighter layer – anthracite – on the top, and the heavier and smaller layer – garnet – on the bottom. This arrangement constrains the largest particles to be removed near the top of the filter, and the smaller at the end of the filtration process. This allows a longer filtration cycle and less filter clogging between backwashes.

Over time, filtered debris accumulates on the filter media creating a filter cake layer, clogging the filter, and increasing pressure loss. To remove this cake layer and reduce the pressure loss over the media, a backwash process is initiated. The BW process with SWRO brine is fully automatic and is determined by a timer set point in most cases, or according to a pressure difference set point as measured online by the Pressure Differential Indicating Transmitter (PDIT) installed on each filtration unit.

Micronic Filters
Downstream of the PMMF, the water is distributed to the SWRO units. Before the pretreated seawater enters the SWRO, it passes through micronic filters. One micronic cartridge filter is installed for each SWRO unit. The micronic filters serve as safety filters against fouling agents that could harm the RO membranes, HPP, and the ERS units (PX).

Seawater Reverse Osmosis (SWRO)
The seawater reverse osmosis (SWRO) stage is the core process of the desalination plant. The SWRO process is performed by forcing the seawater through a semi-permeable membrane at high pressure to produce permeate water.

The desalination section is comprised of the SWRO membrane units, High-Pressure Pumps (HPP), and Energy Recovery System (ERS). The system is divided into two (2) trains, one train contains two (2) SWRO units and the other includes three (3) units. Each train is hydraulically independent, and each unit operates with a high-pressure pump, energy recovery system, and ERS circulation pump. Fourteen (14) pressure vessels (each containing 8 elements) are installed on each unit, according to the capacity requirements.

SWRO permeate water quality is monitored by a conductivity meter on each SWRO unit. In the case of quality failure, the permeate will flow automatically into the brine gutter, until quality improves. The SWRO permeate from each unit flows into two suck-back (buffer) tanks.

The low-pressure brine from the ERS flows to the backwash tank, used to BW the PMMFs.

The SWRO units are fully automated and controlled by two set points: (1) Permeate flow rate and (2) SWRO recovery. The permeate flow meter controls the HPP VFD in order to reach the permeate flow rate setpoint. The flow rate of the low-pressure brine setpoint is calculated from the recovery and permeate flowrate set points. The low-pressure brine flow rate is then controlled by the low-pressure brine valve. The valve position is controlled by the brine flowmeter, in order to reach the brine flow rate set point.

Brackish Water Reverse Osmosis (BWRO)
The brackish water flows to the second pass from the suck-back tanks. The second pass is divided into five (5) units that include a 2nd pass pump and two stages: 1st stage with four (4) pressure vessels and 2nd stage with two (2) pressure vessels. The brine of the 1st stage is the feed of the 2nd stage and the brine of the 2nd stage blends with the seawater that feeds the same SWRO train.

The operation of the BWRO is automatically controlled by a permeate flow rate setpoint that controls the brine control valve, as well as by a feed flowmeter that controls the feed pump VSD. The feed flow rate setpoint is also corrected according to the level of the suck-back tank in a cascade PID loop.

The permeate of the 2nd stage of each unit is collected and connected to the tie-in point, in order to transport the BWRO permeate through the existing post-treatment towards the existing drinking water tanks of Hato. Before the tie-in point, the BWRO permeate pressure, flow rate, and conductivity are continuously monitored.

There is an online conductivity meter in each BWRO unit. If the permitted water conductivity is exceeded, the unit will automatically go off-spec until the quality is corrected.

SUMMARY

With onsite commissioning and training of the local team behind us, IDE is proud to have partnered with WEB Bonaire to provide the island of Bonaire with a reliable SWRO desalination plant to provide drinking water to the population in these challenging pandemic times.

About the Contributor

IDE Technologies specializes in the development, engineering, construction, and operation of some of the world’s largest and most advanced thermal and membrane desalination facilities and industrial water treatment plants. IDE partners with a wide range of customers – municipalities, oil & gas, mining, refineries, and power plants – on all aspects of water projects, and delivers approximately 3 million m3/day of high-quality water worldwide.

This case study was published in the annual “Finest-50 Global Case Studies” Special Digital Edition, March-April 2022. Click here to read the complete e-Magazine issue.

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