In this feature article, Anthony Bennett, Technical Director at Clarity, reports on developments in the application of desalination technology in the UK market.He identifies the driving forces for using desalination and describes briefly how reverse osmosis (RO) operates.Having contacted all UK water companies, he reports on planned future projects.Two case studies are summarised – in London and in the South of England.Anthony looks at how advances in RO membrane technology can be used to move the implementation of these projects from design to operating reality.
There are two primary reasons why water companies in the South of England are actively looking at desalination technology: climate change (and predictions for drier summers), and shifting population demographics, which exacerbate water shortages in certain areas of accelerated population growth.
Seawater provides an obvious resource to tackle these problems, and desalination is commonly applied in various arid and semi-arid locations around the world.But in the UK, we only utilise a small percentage of our annual rainfall for the production of drinking water.This can prove difficult to reconcile with applying desalination technology.Consequently politics has impacted on the procedures for selecting treatment processes to meet future demands.
The Beckton and Newhaven projects described below have reached the media 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 requirement to develop new sustainable resources was addressed. The need to meet future demands by potentially building new reservoirs and implementing desalination technology were underlined as important investments for the future.
Stout thought that actually other socio-economic issues were playing a greater part in directing the UK desalination market. He said, “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 have quoted having lower resource availability per capita in comparison to 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, RO is becoming the preferred desalination technology, 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 at a faster rate than dissolved solids.The difference of passage rate results in separation of solids from 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 of the membrane.If salt is added to one side of the membrane, the chemical potential of the resulting solution 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 an 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 a level of 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.
Table 1 – RO Fundamentals
RO membrane permeate production is proportional to net driving pressure (NDP) across the membrane
salt flow is proportional to concentration differential across the membrane and is independent of applied pressure
permeate salinity depends on relative mass transfer rates of water and salt through the membrane
chemical and physical membrane properties determine its ability to allow preferential transport of water over salt
The fact that water and salt have different mass transfer rates through a given membrane results in the phenomenon of salt rejection.No membrane is ideal in the sense that it absolutely rejects 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 NDP needs to be in the range 80 to 120 bar, and the membrane salt rejection needs to be maximised.This means that power consumption becomes the primary component of the overall operating cost.
Having contacted all water companies in the UK, our findings fall generally into three geographical categories:-
- West and North Britain – high rainfall ensures surface water sources are sufficient enough for water companies not to be considering desalination within 30 years;
- East & South West England – here desalination is being reviewed and has been investigated (by Anglian Water and Essex and Suffolk Water, for example) but there are no active developments;
- London and South East England – here desalination is being actively investigated and plans for implementation exist as outlined below.
We also spoke to Dr Ian Pallett, Technical Director at British Water, who confirmed our findings but advised that desalination is currently practiced on a small scale in the Isles of Scilly and the Channel Islands to satisfy summer demand caused by transient tourist populations.RO has advantages in these situations because it is a compact and low capital cost option that can be turned on easily to meet high demand.
James Grinnell, Water Resources Manager at South East Water, told us that by operating “at the margin” RO desalination is an effective technology.He said, “To cope with extended dry periods, RO provides an economic solution because the high operating costs can be offset against low capital costs.”
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. Mr. Hugh Corrigan, Project Manager for Atkins at Beckton, told us that 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 Sewage Treatment Works site.
The Beckton design is for a capacity of 150 MLD that will supply 400,000 households.15 km 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 £200 Million.
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 told us that the need for the new plant was 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.Corrigan said, “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 added, “The availability of existing surplus land next to the River Thames was also a consideration in the site selection process.Despite the high technology to be invested in the plant, the project still represents an economically favourable option.”
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 6 hours per day, drawing off at the rate of 600 MLD 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 to the treatment process.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 told us that the application was refused because Mr. Livingstone had 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 experienced in long, dry periods cannot easily be obtained from these existing supplies without detriment to the environment, hence the need for an alternative was required.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 9.5 MLD.South East Water supplies a total of 400 MLD on average, so it would represent only 2 % of the overall supply.The Company operates two geographically separate regions.400 MLD is the total input across both regions but approx 180 MLD 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 to sand filtration but, because of the considerably finer level of filtration, it affords various advantages that reduce RO operating and capital costs.These include reduced membrane fouling and the diminished requirement for membrane cleaning coupled with the ability to operate at higher recoveries and flux rates resulting in less RO membrane area required.
Grinnell told us that the construction contact should be completed in 2007 and that the design of the full-scale plant will be developed as part of an ongoing evaluation of the pilot test data.
Stout told us that for RO desalination to become a more economic technology 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 said, “The global desalination market 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 explained that advances have been made in the drive for energy and process efficiencies in desalination processes. 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. Stout said, “These energy saving devices have made desalination more efficient, with figures of 3 to 4 kWh/m3 being quoted for seawater RO plants, by the International Desalination Association.”
Advances in membrane manufacturing techniques and optimisation methods for RO systems mean that energy consumption can only reduce over coming years and hence, the case for utilising RO technology will become ever stronger.
The other challenge is the environmental impact of RO concentrate.Discharges into receiving waters can contain high and variable salinity levels.Preservatives, biocides and low oxygen concentrations can potentially cause adverse impacts on the immediate ecology. Stout said, “The actual extent of these impacts and consensus in academia is unresolved. Research continues but it is evident that the potential environmental impacts are being addressed in a positive manner. Impacts on ecology from many of the outputs can be assessed when good baseline information is collected and careful reviews are undertaken”
As developments in RO technology continue and the effects of climate change and shifting population demographics intensifies it is hard to imagine a future in the UK without the increased use of desalination technology.
About the Author
Anthony Bennett is Technical Director at Clarity in the UK, who provide a range of technical authoring services.Contact firstname.lastname@example.org, or visit www.clarityauthoring.co.uk.
Figure 1 - Typical stack of RO
high pressures tubes
Figure 2 - Cartridge pre-filtration on
Beckton pilot plant
Figure 3 - Reverse osmosis pilot
plant at Beckton