The saline solution
Anthony Bennett, Technical Director at Clarity, reports on developments in UK desalination, the driving forces behind it, and its growing future
br>There are two main reasons why water companies in the South are actively looking at desalination: climate change (and predicted drier summers), and shifting population demographics, which exacerbate water shortages in areas of
accelerated population growth.
Seawater is an obvious resource to tackle these problems, and desalination is commonly applied in arid and semi-arid locations around the world. But, in the UK, we only use a small percentage of rainfall for drinking water. This can prove difficult to reconcile with applying desalination technology. Consequently, politics have impacted on the procedures for selecting treatment processes to meet future demands.
The Beckton and Newhaven projects described here have made the headlines and become the subject of public debate. We asked Ian Stout, process engineer for Atkins, in light of the recent issues surrounding these projects, whether he thought politics and planning procedures were restricting the UK desalination market. He did not entirely agree and referred us to a Fringe Meeting at the 2005 Labour Party Conference where the need to develop new sustainable resources was addressed. Building new reservoirs and implementing desalination were underlined as important investments for the future.
Stout thinks that actually other socio-economic issues were playing a greater part in directing UK desalination. He says: “The demand for further resources will intensify, and the need to provide clean water will be placed on the custodians, who already face tough water-resource issues, especially in the well publicised periods of drought.”
These issues will potentially be at their most acute in the southern regions of the UK, amplified with London facing a rise in population of up to 800,000 by 2016, when Thames Water has quoted having lower resource availability per capita in comparison with Madrid or Istanbul.
Thermal desalination processes have advantages where waste heat can be utilised but, due to reductions in the capital cost of membranes, and design optimisations resulting in lower operational costs, reverse osmosis (RO) is becoming the preference, and the only method considered in the UK.
To explain how RO works, we start with osmosis – a natural process involving water flow across an ideal semi-permeable membrane. Water passes through the membrane faster than dissolved solids, separating the solids from the water.
The direction of water flow is determined by the chemical potential – a function of pressure, temperature and dissolved solids concentration. When pure water is in contact with both sides of the membrane there is no flow because the chemical potential is equal on both sides. If salt is added to one side, its chemical potential is reduced and osmotic flow from the pure water across the membrane occurs until equilibrium is reached.
Equilibrium results when the hydrostatic pressure differential resulting from volume changes on both sides of the membrane is equal to the osmotic pressure of the salt solution. If we apply external pressure to the salt solution side of the membrane greater than the osmotic pressure, we raise the chemical potential of the water and cause solvent flow in the reverse direction to the pure water side.
This is RO, a cross-flow membrane-separation process providing filtration down to ionic levels for removal of dissolved salts. Permeate is produced from the membrane with the majority of the dissolved content of the feed transferred to the waste-concentrate stream. RO fundamentals are summarised in Table 1. A typical stack of pressure vessels containing RO membranes is shown in Figure 1.
The fact that water and salt have different mass transfer rates through a given membrane results in salt rejection. No membrane is ideal at absolutely rejecting salt but, in desalination, membranes with high salt rejection are utilised so that permeate produced has sufficiently low salinity to meet drinking-water specifications.
To achieve economic levels of permeate production from seawater, the net driving pressure needs to be in the range 80 to 120 bar, and the membrane salt rejection needs to be maximised. This means power consumption becomes the highest operating cost.
Having contacted all water companies in the UK, our findings
fall generally into three geographical categories:
n west and north Britain: high rainfall ensures
surface-water sources are sufficient for water companies not to be considering desalination within 30 years,
n east and southwest England: Desalination is being reviewed and has been investigated (by Anglian Water and Essex and Suffolk Water, for example) but there are no active developments,
n London and southeast England: Desalination is being actively investigated and plans for implementation exist as outlined below.
Dr Ian Pallett, technical director at British Water, confirms the findings but advises that desalination is currently practised on a small scale in the Isles of Scilly and the Channel Islands to satisfy summer demand caused by tourists. RO has advantages in these situations because it is a compact and low-capital cost option which can be turned on easily to meet high demand. James Grinnell, Water Resources Manager at South East
Water, says that by operating “at the margin” RO desalination is an effective technology. “To cope with extended dry periods, RO provides an economic solution because the high operating costs can be offset against low capital costs,” he says.
Thames Water is proposing to build a desalination plant at Beckton in east London, which will be the first to take saline water from the River Thames. Hugh Corrigan, project manager for Atkins at Beckton, says the project is at an advanced design stage and significant enabling works have already been undertaken to facilitate the new works. Construction is planned at the Beckton WwTW site.
The Beckton design is for a capacity of 150Ml/d that will supply 400,000 households. Fifteen kilometres of pipeline will be laid to transfer desalinated water to Woodford Reservoir where it will be blended with existing supplies. The scheme is estimated to cost £200M.
Extensive pilot work has been carried out on site. Figure 2
shows cartridge filters used as part of the pre-treatment to the RO
membranes, which were installed in pressure vessels on the skid shown in Figure 3.
Corrigan says the need for the new plant is driven by the predicted rise in London’s population and, despite Thames Water’s significant investment in replacing leaking water mains, there is currently a shortfall in resources to meet the requirement of a drought year.
“The selection of the Beckton site will mean the plant is strategically located to serve the major growth areas in east London including the Thames Gateway Zone,” he says. “The availability of existing surplus land next to the River Thames was also a consideration in site selection. Despite the high technology to be invested in the plant, the project still represents an economically favourable option,” he adds.
The plans include new pumped river water abstraction. Raw water will discharge to a buffer tank prior to clarification and filtration and treatment by RO membrane technology.
While the main plant is intended to operate continuously, the saline river-water intake will operate for six hours per day, drawing off at the rate of 600Ml/d of highest-quality water to optimise membrane loading. The buffer tank will provide flow balancing with the contents mechanically mixed to achieve a uniform supply. Membrane pre-filtration will include pressure sand filters, cartridge filters and diatomaceous earth filters. The RO process will operate at feed pressures in excess of 80 bar. Permeate will be re-mineralised and chlorinated.
Ken Livingstone, the Mayor of London, has declined the latest planning application for the project. A spokesman for the Mayor says the application was refused because Livingstone decided the current proposal did not deliver “sustainable and efficient management of water supplies in London”. Thames Water is appealing against the decision.
Mid-Sussex is an area where water resources are particularly stretched, and South East Water already abstracts the majority of its supplies from surface- and ground-water supplies. The extra demand in long, dry periods cannot easily be obtained from these existing supplies without detriment to the environment, hence the need for an alternative. Newhaven is a suitable location from which to deliver water into Mid-Sussex. This is where a desalination pilot trial is ongoing and tenders for a full-scale system are currently being sought.
The results from the pilot plant are being used to design a full-scale plant rated at 9.5Ml/d. South East Water supplies a total of 400Ml/d on average, so it would represent only 2% of the overall supply. The company operates two geographically separate regions. The total input across both regions is 400Ml/d but about 180M/ld is supplied in the southern region.
The desalination process trialled included RO but two pre-treatment technologies were tested: sand filtration versus ultrafiltration (UF). UF has higher capital costs compared with sand filtration but, because of the finer level of filtration, it has advantages that reduce RO operating and capital costs. These include reduced membrane fouling and the lessened need for membrane cleaning, coupled with the ability to operate at higher recoveries and flux rates resulting in less RO membrane area required. Grinnell says that the construction contact should be completed in 2007 and the design of the full-scale plant will be developed as part of an ongoing evaluation of the pilot test data.
Stout says that, for RO desalination to become more economical, it will need to be seen as the appropriate technology in resource-scarce areas, energy efficiency will need to be maximised, and the perceived and actual environmental impact of waste concentrate investigated.
But is RO the right technology? Stout says: “Global desalination has been building rapidly, with a current predicted potable-water market share of 59%. Coupled with the current level of membrane knowledge in the UK, it is difficult to see if any other technology, apart from co-generation, would be both viable and, more importantly, trusted.”
Stout also explains that advances have been made in the drive for energy and process efficiencies. Utilisation of exhaust steam from power-generation plants could help both thermal and RO systems. Energy recovery from waste RO concentrate, hybrid technologies and the use of renewable energy are being considered. “These energy saving devices have made desalination more efficient, with figures of 3-4kWh/m3 being quoted for seawater RO plants, by the International Desalination Association,” he says.
Advances in membrane manufacturing and optimisation methods for RO systems mean energy consumption can only reduce over coming years and hence, the case for using RO will become stronger.
The other challenge is the environmental impact of RO concentrate. Discharges into receiving waters can contain high and variable salinity. Preservatives, biocides and low oxygen concentrations can adversely impact on ecology. “The actual extent of these impacts is unresolved. Impacts on ecology from many of the outputs can be assessed when good baseline information is collected and careful reviews are undertaken,” Stout says.
As developments in RO technology continue, and the effects of climate change and shifting demographics intensify, it is hard to imagine a future in the UK without the increased use of desalination.
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