By Ajay K. Nair, Principal Process Engineer, MWH UK, and Bernard Clancy, Project Manager, MWH Australia Pty
The majority of water supplied in Australia is extracted from surface water rivers, lakes and reservoirs; however, the requirement to find alternative water sources to alleviate pressures on existing supplies has led seawater desalination to become a possible solution to today's problems.
The NSW Central Coast is currently experiencing its most severe drought on record. Gosford and Wyong Councils' Water Authority is proceeding with planning for a seawater desalination scheme as one of its drought contingency measures.
MWH were engaged to review and develop the most appropriate conceptual design for a new seawater reverse osmosis plant to produce a minimum of 7000Ml/year of drinking water to supplement supply and improve security. The purpose of the design was to enable a rapid implementation of a design, operate and construct contract in the event of continuing drought conditions. The component options reviewed and technically evaluated for the SWRO treatment plant included:
• The intake system
• RO configuration
• Post treatment
• Energy recovery systems
• Residuals disposal
Overview of the conceptual design
Seawater intake
The selection and design of the seawater intake system is a key consideration in the development of a desalination plant. The complexity and interaction of the selected intake method can have a profound effect on the design of both the pre-treatment and reverse osmosis plant.
The option adopted comprised Beach wells drawing water from the ocean from under Lakes Beach. A horizontal beach well arrangement extending under the beach into the tidal zone can access the seawater needs of the desalination plant without impact on nearby groundwater-dependent ecological communities. The seawater would be filtered through the beach sands providing high quality source water.
Design water quality
Information relating to the influent water quality was limited. Water quality data representing the specific attributes of this system with an intake several meters below the sand bed at lakes beach are not available. However, 4 years of historic seawater quality data from nearby Cabbage Tree Harbor were available, and formed the basis for the feed water quality assumptions used in this concept design.
Experience from previous membrane desalination plants such as Tampa, (Florida, USA) and Port Lisas, (Trinidad and Tobago) highlights the importance of including appropriate risk mitigation measures particularly in the pre-treatment processes.
Why reverse osmosis selected for water treatment
Reverse Osmosis operates by passing feedwater through a membrane that concentrates the dissolved solids (e.g., salts) into one stream (saline reject) and the purified water into another (permeate). Feedwater is pumped at high pressure through permeable membranes, separating salts from the water. To achieve reverse osmosis, the feedwater must be at high enough pressures to overcome the osmotic pressure gradient that naturally exists between waters of different salinity. The membranes require the feedwater to be of consistently high quality, necessitating pretreatment facilities. The quality of the water produced depends on the pressure, the concentration of salts in the feedwater, and the salt permeability of the membranes. Passing the RO permeate through a second set of membranes in an arrangement called 2 pass RO can improve the product water quality.
The water treatment plant can be broken down into four basic components:
• The Pre-treatment Process
• The Reverse Osmosis Plant
• The Post Treatment Process
• Residual Treatment, Handling and Disposal
Pre-treatment process
The purpose of the pre-treatment process is to prolong the life span of the membranes by providing RO feedwater of sufficiently high quality to minimize fouling potential and, in turn, membrane cleaning frequency.
The pre-treatment process train required to meet the RO feedwater quality given the identified range of seawater characteristics consists of:
• Chlorination
• Inlet Reception
• Grit & Sand Removal
• pH Correction, Coagulation and flocculation
• Media Filtration
• RO Feed Chemical Conditioning (including dechlorination, pH correction, antiscalant and secondary filtration)
Reverse osmosis treatment
The specific process design of the RO membrane system will be dependent upon the philosophy adopted by the DBO contractor in developing the unit price for treated water. It is likely that the membrane manufacturers will target the optimization of the membrane flux rates in any tendered design.
It is anticipated that a two pass RO system will be required to achieve the product water quality targets. This system would use half of the first pass RO as feedwater to the second pass to create a very high quality second pass permeate. This second pass permeate is then recombined with the first pass permeate to create a product water as required.
Table 1. broadly presents how the membrane flux rate impacts the capital and operating considerations of the plant.
Energy recovery
The saline reject stream created in the membrane process is still at a high pressure, and represents a significant resource . A range of energy recovery devices exist that take the residual energy and convert it to useful power to assist in the desalination process.
Energy recovery will be effective on the first pass RO only. The second pass operates on much lower pressures and has less energy available for recovery.
Like all energy conversion processes, a degree of wastage occurs in each link. Efficiency is the key to energy recovery, as are both capital and operating costs. While energy recovery devices (ERDs) have been available for some time, recent improvements in efficiency (or recovery across the device), plus new products, have seen marked improvements in this field. Three technologies were considered as compared in Table 2 below:
Without energy recovery a single-train first-pass RO would require 2500kW of power. All of the devices can significantly reduce the energy requirements of the first pass RO process. The work exchangers provide for high energy recovery efficiency over a broad operating range and can be set up to retain a defined residual pressure in the saline reject stream. The residual pressure in the saline reject stream allows its transfer to the Norah Head Outfall Dosing tank without re-pumping. The work exchanger (Pressure Exchanger and the DWEER systems) is the preferred energy recovery device in this application. It should be noted that the Water Corporation Perth desalination plant will utilize the ERI PX system for its energy recovery.
Inclusion of a PX system significantly reduces the installed high pressure RO feed pump capacity (by greater than 50%). However, the pressure exchange requires the saline reject flow to be in operation. This leaves a shortfall of pump capacity on plant start up. The shortfall could be addressed by utilizing the stand-by RO feed pump capacity until the pressure exchange has reached working capacity, providing RO booster pumps to facilitate start up or by setting the initial plant recovery to zero until the operating pressure and flow has been established.
Post treatment process
The RO permeate will have low levels of salts and minerals and be very "soft" and could lead to corrosion of some metals and pipe cement linings. This RO permeate will need to be conditioned to make it suitable for conveyance in the drinking water reticulation system.
Residuals handling
Treatment will generate residuals that will require suitable disposal. Table 3 provides a summary of the residual management options for each of the residual streams.
Saline reject
Saline return water will be released through the existing Norah Head Ocean Outfall. The return water will be mixed with treated sewage effluent at the dosing tank at Toukley STP.
Treatment costs
The plant operating and capital costs were developed based upon the selected plant design, with estimations being made for the level of manning necessary to operate the complex plant. Manning schedules were developed based on a number of considerations, including:
• Level of automation on ancillary equipment such as chemical dosing for switch over to standby systems on detection of failure.
• A significant level of instrumentation will be used in the process monitoring and control of the pre-treatment, RO membrane processes. Instrumentation requires continued calibration and maintenance for successful operation.
• A requirement for extensive manual intervention for start-up of the RO system following shutdown due to unplanned power shortages or abnormal pre-treatment performance.
• Shutdown of the membrane systems requires the RO membranes to be flushed with high quality feed water until clear of any concentrate. This operation requires manual intervention rather than an automated action.
With these considerations, it is anticipated that the plant would be manned for 24 hours, using a 3 shift pattern by at least 2 persons with a total employment allowance of 10 people.
Through the requirement of continual 24 hour manning, the level of plant automation may be tailored to reflect this, to provide a reduced level of cost associated with plant automation.
Operating costs
An overall summary of the desalination plant operating costs and cost assumptions are presented in Table 4.
Capital costs
The estimate has been calculated from first principles based on the concept design presented in this report. The estimate includes allowances for inherent (rate & quantity) risks, contingent risks, overheads and Client costs. These costs are presented in Table 5.
Acknowledgements
The authors would like to thank both Wyong and Gosford councils' for their permission to publish parts of the work conducted on their behalf. MWH would also like to thank all the designers who worked tirelessly to develop the plant concept including Clive Hare, Martin Towers and Dale Rohe.
References
1. Dow Chemical Company. (2004) FILMTEC Reverse Osmosis Membranes Technical Manual. DOW Liquid Separations.
2. Voutchkov, N. (Date Unknowm) Thorough study is the key to large beach-well intakes. J. Desalination and Water Reuse., 14/1, 16 - 20.
3. Blazevski, M, Singh, M, Greenwood, E, . (2001) Operating Cost Comparisons between immersed Ultrafiltration and Conventional Pre-treatment for Seawater RO International Desalination Association World Congress: SP05-239
4. Blazevski, M, Singh, M, Greenwood, E, . (2001) Operating Cost Comparisons between immersed Ultrafiltration and Conventional Pre-treatment for Seawater RO International Desalination Association World Congress: SP05-239
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