WaterSubject

Membrane plants were first introduced for municipal duty in the UK around 13 years ago in Scotland, almost two centuries after John Gibb’s pioneering sand filtration plant introduced in Paisley for potable water treatment. These early membrane plants are characterised by very small flows associated with small isolated communities for whom centralised water treatment and supply would not be cost effective. The membranes used are the more highly permselective nanofiltration (NF) membranes, configured as multitubes (the FYNE process) or spiral-wound modules, rejecting colour as well as providing disinfection. Although providing high-quality water with a relatively simple plant, these membranes are currently considered to operate at too high a pressure to be considered economically viable for large-scale potable water treatment. These processes are also ultimately limited in their application by the production of a retentate waste stream, representing around 25% of the feed stream, which must then be safely discharged.

Two pieces of legislation have effectively promoted the application of membranes for municipal duty in the UK. Treated municipal wastewater discharged to bathing areas is required to meet the European Union Bathing Water Directive. This directive, originally promulgated in 1976 and revised in 2002, stipulated that such waters should meet stringent microbiological guide values of 500/100 mL total coliforms, and 100/100 mL faecal coliforms and faecal streptococci. The first membrane plant to be installed for sewage treatment was the groundbreaking abiotic plant at Aberporth, a Welsh Water site, in 1994. This plant uses enhanced upward flow clarification with lamella plates (the Densadeg process), followed by polishing with hollow fibre (HF) microfiltration (MF) membranes (supplied by USF Memcor, Derby, UK). This process has since been superceded by the membrane bioreactor (MBR) process, whose widespread installation in the south-west of England followed successful pilot trials of the Kubota MBR process in Kingston Seymour, a Wessex Water site, in the mid-1990s.

The more recent rapid increase in membrane implementation for potable water duty from 2000 onwards has arisen from Drinking Water Inspectorate (DWI) rulings regarding cryptosporidium monitoring and control. In a landmark ruling in December 1999, the DWI decreed that plants capable of continuously removing or retaining particles greater than 1 ?m diameter would not be required to monitor cryptosporidia continuously, a procedure that is practically impossible to carry out. This stipulation by the DWI of removal of cryptosporidia by a physical barrier, rather than simple inactivation using ultraviolet irradiation, was critical in establishing membrane processes in the UK. The DWI provided a limited list of membrane products approved for this duty. As the installation and operation of a membrane plant has thus far been considered less costly than continuous cryptosporidium monitoring by the water suppliers, sites identified by the regulator as requiring such monitoring have generally opted for an MF or an ultrafiltration (UF) plant.

The UK currently has around 1100 ML/day membrane capacity (to July 2004) for municipal water supply (around 6% of the water supplied) compared with around one-tenth of this figure for wastewater duty. Installation capacity for potable water has increased sharply in years when large, high-profile schemes such as those of Huntington (1998) – rapidly installed as a direct result of the drought in the early 1990s – and Clay Lane (2001) have come into production, although the total number of installations has increased more steadily.

The vast majority of all other membrane plants are full-flow UF/MF based on hollow fibre membranes, of which there were around 55 by mid-2004. Only a handful of dense membrane (i.e. reverse osmosis/NF, RO/NF) plants, nearly all based on Koch spiral-wound modules, exist outside Scotland: although Scotland has around 40 RO/NF installations, they provide a total of only 15 ML/day installed capacity – 1.4% of the total UK membrane capacity. The constraints imposed by DWI approval mean that the potable market is dominated by Memcor/USF and Norit/X-flow, which, between them, provide almost 1000 ML/day of total UK membrane plant capacity. This balance is likely to shift following the approval of the Pall and Zenon membrane products, as well as the electrodialysis-reversal process.

According to the available data, installed capacity for potable water treatment is reasonably evenly distributed geographically, although two large plants in the United Utilities region are currently on standby. On the other hand, in terms of the number of plants, by far the largest number are located in Scotland. Perhaps somewhat surprisingly, it appears to be the smaller companies that have a disproportionately large share of the installed capacity: the combined operating capacity of Three Valleys Water and Portsmouth Water is about the same as that of the ‘Big 5’: Anglian Water, Severn Trent Water, United Utilities, Thames Water and Yorkshire Water combined (380 ML/day).

Compared with potable water treatment, the increase in installation of wastewater membrane plants has been, if anything, more dramatic and also more regionally based. About half of the municipal wastewater treatment membrane plants on the UK mainland are currently located in the south-west of England. All but one of these are MBRs, and all but three of the 21 MBRs installed (as at July 2004) for sewage treatment are based on flat plate membrane modules.