Black & Veatch's Frank Rogalla looks at the quality of desalination processes
In the UK, water demand is increasing and fresh water resources are dwindling - Thames Water, for instance, is now considering desalination of water sources with marginal quality, such as the brackish River Thames estuary. Up to now, desalination was limited to arid and coastal regions, especially those with high population growth and limited fresh water resources, such as the Middle East and some Mediterranean countries such as Algeria, Italy, Israel, Malta and Spain.
To alleviate the water shortage on the arid south-eastern coast of Spain, the new national government, elected in March 2004, decided to replace the project of the previous government to pump the river Ebro from the north to the south. The new concept comprises planned expenditures of close to e4B, with total desalination capacity of 1,700Ml/d in 15 plants, in addition to nine new effluent reuse plants of a total capacity of 500Ml/d. Currently Spain's largest plant, with a capacity of 120Ml/d, is at Carboneras and has an investment cost of e95M, with two larger plants currently being built. In the US, with the exception of brackish water desalination in Florida, ocean desalination facilities were nearly unheard of.
A landmark installation was the 11,400m3/d plant at Key West, Florida in 1980. At the time, it was the world's largest single pass seawater reverse osmosis (SWRO) plant and the first to demonstrate energy recovery on a large plant from high-pressure brine. Recently refurbished, and currently on standby service, most of the original membranes are still in use. Only recently the Tampa Bay SWRO plant was designed to produce an initial 95,000m3/d of water, with planned expansion to add a further 37,000m3/day in the future. Located adjacent to Tampa Electric's Big Bend 2,000mW power station, it is currently the largest of its kind in the US. In July 1999, a contract on a design, build, own, operate and transfer (DBOOT) basis was awarded and after thorough evaluation by state and federal authorities, all necessary approvals were granted by late spring 2001. Construction began in August 2001 and the first 20,000m3 of water was produced in March 2003. The project involved the plant itself, a seawater intake, concentrate discharge system, various chemical storage and dosing facilities and 24 miles of product water transmission main.
The overall cost was around £57M. However, subsequently, the plant has run only sporadically, producing far short of its intended output. Currently, a proposal by affiliated companies owned by Thames Water Aqua Holdings is being implemented to upgrade the plant.
This project includes significant modifications to the desalination plant's intake system, headworks, pretreatment process, membrane cleaning process, post treatment and other plant improvements with an additional capital cost of around £15M. Wholesale cost of the product water had been projected at an average £0.34/m3 over the next 30 years, dropping to £0.26/m3 once the plant is successfully completed and £44M in co-funding from the south-west Florida water management district are expected to permanently reduce the debt cost of the facility.
The new modifications are expected to increase the previously contracted costs for desalinated water to £0.37/month on the average household's water bill. Now, three desalination projects are being planned on the west coast of the US, as well as several other desalination facilities in various other geographic locations such as Texas. One of the major obstacles of in implementing desalination processes is its cost - both capital and operating cost. The operating cost is typically dominated by energy consumption.
While the theoretical minimum energy required to overcome the seawater osmotic pressure is 0.7kWh/m3, typical SWRO systems currently in operation require around five times more. This difference is due to many factors, including membrane array (accounts for 70% including recovery, membrane resistance, system design), pump and motor (20%) and energy recovery device (10%). Typically, the energy consumption of a seawater reverse osmosis (RO) plant is in the range of 3-4kW/hr/m3 depending on the salinity and temperature. Even for brackish water desalination applications the energy cost accounts for a significant part of the operations budget and is highly dependent on salinity, system design and membranes used. To improve the energy efficiency of a system, various design alternatives for the membrane system (as well as novel uses of energy recovery devices to account for daily and seasonal water quality changes, particularly temperature and/or salinity) are being evaluated.
Singapore is building new desalination units to improve its water independence, in addition to reusing wastewater with RO treatment. To meet water consumption demands, Singapore has been relying on surface water and water imported from Malaysia. As water consumption increased over the years and as the first of two water purchase agreements with Malaysia will end in 2011, Singapore has been looking into alternate sources of water to ensure unhindered supply to its population.
One such initiative was the production of NEWater reclaimed from secondary effluent from the municipal WwTWs. This has been successfully undertaken at three of the existing six WwTWs and the fourth is under implementation. The NEWater produced from these plants is primarily used for applications in industries which are otherwise fed by potable water. Another initiative was to desalinate seawater. In 2002, the Public Utilities Board (PUB) embarked on a programme to build the first seawater desalination plant in Singapore using a public-private-partnership model. A design, build, own, operate 20-year concession was awarded to SingSpring, a fully owned subsidiary of Hyflux, a prominent water treatment contractor in Singapore. As membrane technology has become more advanced in the last decade, desalination plants using RO membranes for salt removal have become increasingly popular. On completion, the Singapore plant will have the following firsts to its credit:
Boron poses reproductive dangers in humans and is suspected to have other health-averse properties. The level of boron in the seawater and the required level in the drinking water pose a particular challenge to the design of the desalination plant in this part of the world where the seawater temperatures are relatively high. The PUB has adopted WHO's 1998 guidelines for drinking water quality, and required the desalination plant to lower boron concentrations in the seawater from a maximum of 5mg/l to below 0.5mg/l. A number of pre-treatment options were considered for the quality of the seawater to be treated. Beach wells were ruled out for practical reasons.
The original tender proposal included DAF for particulates, oil and grease removal, used in conjunction with gravity and/or pressure sand filtration for a higher degree of particle removal was originally proposed. This arrangement was considered appropriate on both technical and economic grounds. The in-filter DAF option eventually chosen for this project combines and integrates the traditional separate DAF units and sand filtration units resulting in a smaller footprint, thus saving land. Pilot trials have shown that a silt density index (SDI) of three can be consistently achieved. A two-pass RO system is provided:
Brine from RO1 would be rejected from the membranes at a high pressure. In order to provide an energy efficient system and use the residual energy in the reject water, a state-of-the-art energy recovery system (ERS) is employed on RO1. The provision of an ERS reduces the energy consumption of the whole RO process substantially. The RO permeate is then stabilised with chemicals to make it compatible with regular PUB water before finally being stored and pumped into the distribution system. The reject from the treatment process is discharged back to the sea, together with other cleaning streams, which are neutralised and pre-treated before discharge.
The Singapore desalination tariff of £0.23/m3 at current exchange rates is currently the lowest in the world for an RO desalination plant, by combining a design optimisation to reduce both construction and operating costs with a local cost base and a competitive debt package. The tariff structure, similar to many Middle East IWPP projects, comprises capacity payments and variable (output) payments for actual dispatch. The capacity payments provide for debt service, fixed costs and equity returns.
The output payments provide for recovery of variable operating costs. Both capacity and output payments are indexed to inflation and foreign exchange rates to provide protection against fluctuations in these variables. Detailed design of the desalination plant commenced in August 2003 and the first construction contract (piling) was awarded in early January 2004.Piling work began in mid-January
2004. Mechanical completion of the plant was scheduled for completion in February 2005 with the plant commercial operation targeted for June 2005, in time for the next World Congress of the International on Desalination Society (IDS) from September 11-16, 2005 in Singapore