The new desalination plant in Singapore is a first in many ways. Frank Rogalla and Black & Veatch colleagues William Young and Don Ratnayaka discuss the plant
In April this year, the Mayor of London, Ken Livingstone, blocked proposals by Thames Water to build the UK’s first desalination plant.
Planned on the Beckton site near Barking in east London, it was deemed too energy intensive, and not in line with the “sustainable management of water supply resources”.
Singapore has just opened its first seawater-desalination plant, with a net output of 136,000m3/d. Its design-specific energy consumption of 4.35kWh/m3 is arguably the lowest in the world. In the first year, the private owner of the desalination facility will sell the water to the Public Utility Board (PUB) at around £0.26/m3.
Singapore, with a population of over 4M, has been relying on surface water, mainly imported from Malaysia, to meet water demands. One alternate source is the
production of ‘Newater’, primarily used for industrial applications which are otherwise fed by potable water mains.
This water is reclaimed from secondary effluent at three of the existing six WwTWs. A fourth plant is expected to be operational by the end of 2006, and will be able to produce 116,000m3/d of Newater, at a first-year price of £0.10/m3 after reverse osmosis (RO). In addition, after micro-filtration (MF), 46,000m3/d of industrial water is produced, sold at half the price.
Both projects are the first public-private partnership (PPP) projects undertaken by the Singapore Government, awarding a 20-year contract to a private provider of a complete service, including design, build, ownership and operations.
The desalination project is provided by SingSpring, a fully owned subsidiary of Hyflux, a prominent water treatment contractor in Singapore. Its subsidiary, Hydrochem, provided engineering, procurement and construction (EPC) of the desalination plant, and commissioned Black & Veatch to carry out the design of the plant between August 2003 and commissioning in June 2005.
This desalination plant is the largest in this region and one of the largest seawater RO (SWRO) plants in the world, with the least energy-intensive design. The plant is located in an area of 61,500m2 on the beachfront at Tuas, at the western end of Singapore.
It has an open seawater intake, dissolved-air flotation-filtration for pre-treatment, two passes of RO membranes with energy recovery for the first pass, chemical post-treatment, treated water storage and pumping. The brine waste from the process is discharged into the sea.
The major challenges in the plant design were the removal of boron, meeting the stringent specific energy-consumption guarantee, minimising the plant footprint, designing for a 23% turndown of capacity, and ensuring the reliable operation of the plant.
Two options were considered for seawater intake – beach-well infiltration and a conventional intake. Beach wells would produce a better and consistent quality of raw water, less affected by chemical or oil pollution than an open intake, limiting the pre-treatment needs to coagulation and filtration only. Trials, however, indicated it was not feasible to provide the required space for achieving sufficient yield from the beach wells.
As a result, a conventional seawater intake was adopted. To achieve a design output of 136,000m3/d of treated water, the seawater intake has to have a capacity about 2.5 times higher (354,500 m3/d). The intake system consists of: a pair (one duty, one standby) of coarse bar screens (with 30mm spacing between bars); a pair (one duty, one standby) of travelling band screens (with 3mm openings); and four (three duty, one standby) submersible-type seawater-intake pumps.
The seawater intake is provided with a chlorination facility to introduce shock doses of chlorine for the prevention of biological growth in the intake system. The facility is designed to provide dosages up to 10mg/l of chlorine from liquid chlorine supplied in one-tonne drums.
It is proposed to introduce shock chlorination once a week for 30min each time, and adjust the frequency, duration and dosage based on
experience gained during the initial operation of the plant.
The raw seawater can have an oil content of up to 10mg/l and a silt density index (SDI) as high as seven, due primarily to suspended solids, iron and organic carbon. For the RO membranes to operate effectively and efficiently, any oil in the water has to be removed and the SDI has to be less than three. To achieve these objectives, the seawater will be treated by coagulation followed by a combined dissolved-air flotation-filtration process, known as the “in-filter DAF”.
Pre-treatment includes chlorination at the works inlet to prevent bacterial fouling of membranes and favour coagulation for suspended solids, iron and organic-matter removal. Coagulation is by ferric chloride at a pH of about 6 to 6.5, as higher pH values in the range 8 to 8.2 (the optimum pH for boron removal in the first pass RO) were not successful. Sulphuric acid is used for coagulation pH correction. Storage and dosing facilities for sulphuric acid (98% w/w H2SO4) and ferric chloride (40% w/w FeCl3) are provided at the plant.
Because of the possibility of high suspended solids in the water, a clarification stage was necessary and direct media filtration inadequate. Dissolved-air flotation was considered to be a good option because it would help to remove the insoluble fraction of oil. Three options were considered in pilot plant scale for solid-liquid
The first option of using separate modules for dissolved-air flotation followed by filtration allows higher flotation rates as the latter is not tied down to the filtration rates but would require a substantial footprint as the two processes require independent structures.
The in-filter DAF design, where each flotation unit sits on top of a
filtration unit, gave a space advantage. While MF membranes could perhaps lower SDI values in the treated water, there are not many operating plants in the world with MF membranes as pre-treatment for SWRO. Furthermore, the lifecycle cost of MF membranes was higher compared with in-filter DAF.
A total of 20 DAF units are arranged to allow up to two units to be taken out of service for filter washing and cleaning/maintenance. A roof is provided over the units to prevent disturbance to the float by wind and rain. Each of the DAF units is provided with two mechanical flocculators within the same module.
The filters utilise mono-grade sand. The air required for the flotation process is added to the recycle water (up to 12% of the flow) drawn from the filtrate.
As confirmed by pilot-plant trials, the water quality at the outlet of the DAF is oil-free with an SDI of 3 or less. To avoid membrane degradation, a dechlorination stage using sodium metabisulphite is provided prior to a filtered water storage tank. The pH of the RO feed is increased by dosing caustic soda (50% w/w NaOH) to about 8.2 to achieve a degree of boron removal in the first pass.
The pre-treated, dechlorinated and pH-adjusted seawater is collected and pumped through the RO membranes at high pressure, generated by means of two pumps operating in series to develop a maximum pressure of 66bar. Because there is a high residual pressure in the RO reject water, energy can be recovered and reused in the system in order to minimise operating costs.
The dual work exchange energy recovery (DWEER) concept provided by Calder, is utilised for this purpose, and is more efficient compared with conventional Pelton turbines, which have been used extensively in the past for energy recovery.
On each of the streams, 5µm cartridge
filters are provided as safety filters for the RO units to take out any fine particulate matter that are likely to be present in the filtered water and can cause membrane fouling.
The first pass RO membranes are fed at
a pressure range of 50.6bar/g to 58.1bar/g depending primarily on the salinity of
feed water, operating temperature and age
To achieve this variation in pressure, the first set of pumps is provided with variable speed drives, designed for an operating range of 4-20.5bar/g. The second set of pumps in series is equipped with fixed-speed drives, rated at a pressure of 45.5bar/g.
Lower pressures, better salt rejection and
lesser fouling characteristics have made
SWRO more cost effective. The level of boron at 5mg/l in the seawater pose a particular challenge to RO technology with relatively high seawater temperatures.
Boron poses reproductive dangers in humans, and the World Health Organisation’s 1993 guidelines for drinking-water quality require a maximum of less than 0.5mg/l.
For effective boron removal, the plant relies on a two pass RO, which would be operated at high pH.
First pass RO is a single-stage process comprising ten trains operating at a pH of 8.2, each train having 180 pressure vessels within each vessel seven RO membrane elements of Toray model TM820H-400B. The recovery rate is 45%.
The feed water total dissolved solids (TDS) concentrations of up to 35,000mg/l
are reduced to less than 500mg/l and
boron concentrations of up to 5mg/l to around 0.8mg/l.
The reject is disposed of after energy recovery, while a proportion of the permeate would collected in permeate break
tanks and will be pumped through the second pass RO.
RO2 is a two-stage process, with each having 130 pressure vessels times seven RO membrane elements of Toray TM720. The pH is adjusted to 10 using 50% w/w caustic soda for boron reduction from 0.8mg/l to 0.5mg/l. The overall recovery rate is 90%.
The permeate from the first and second stages of RO2 is mixed with the permeate from RO1 which bypasses RO2 to the post-chemical treatment process. The second stage RO2 reject is returned to RO1 for re-treatment.
Anti-scalant chemicals will be injected into the common ERS header to prevent carbonate and sulphate scaling of the membranes. There are no facilities for degassing the RO2 feed.
Instead the feed is dosed with caustic soda, which neutralises the free CO2 and raise the pH to 10. At pH 10.5 magnesium hydroxide would precipitate. Therefore strict control of pH is necessary.
The final water quality is TDS concentrations of 50mg/l and boron concentrations
of 0.5mg/l or better. With low TDS and low alkalinity, the RO permeate water is both chemically aggressive and corrosive, and
is stabilised by re-mineralisation with lime and CO2 dosing in the post chemical
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