First dedicated seawater pre-treatment membrane
Seawater reverse osmosis (SWRO) is fast becoming the most popular technology for producing potable water from seawater. However, to date, SWRO plants have not always met expectations for factors such as capacity, recovery and downtime. Frans Knops, product manager for Norit's Dutch subsidiary X-Flow, explains how the development of a new dedicated ultrafiltration (UF) membrane for seawater has achieved the lowest total costs of ownership (TCO) on the market.
TCO is influenced by many parameters so, to achieve the lowest TCO in the development process, X-Flow addressed the following specific parameters:
SDI and turbidity: apart from SDI <3, manufacturers of SWRO membranes require a feed turbidity <0.1 NTU in order to guarantee membrane life time.
Microbiology: a high level of retention of larger micro-organisms (bacteria and algae) is desirable, while high virus retention is not critical for a pretreatment membrane.
The membranes should operate at average transmembrane pressures of 0.2 bar and a maximum transmembrane pressure (fouled condition) of 0.5 bar.
Chemicals for cleaning and pretreatment
The membranes should only use non-proprietary chemicals that are either being used in upstream or downstream processes or that are available worldwide in bulk shipments. Chemicals of choice are: hydrochloric or sulphuric acid and sodium hypochloride.
The membranes should not only be able to use a direct intake, but also the cooling tower seawater intake, in case the plant is being built in combination with a power plant.
The membrane should be able to cope with varying water qualities which could be caused by factors such as storms, river outfalls and wastewater treatment outfalls. This will allow for SWRO construction close to centers of population, thus minimising the investment and operating costs for transfer of potable water.
Goal was achieving a minimum flux of 100 Lmh at 0.2 bar average operating pressure. The high flux rate ensures that the footprint of the UF system is small enough to be combined in a common UF & SWRO building.
The low operating pressure will allow for feeding the UF directly with the SWRO transfer pumps while maintaining sufficient UF permeate pressure to feed the SWRO high pressure pumps.
One of the most significant factors for a cost-effective operation is the ability of the pretreatment system to produce a constant quality level of SWRO influent at an adequate cost level. The specialised chemistry of the Seaguard pretreatment membrane continuously offers the highest quality RO feed water at a TCO lower than existing technologies.
Rick Rosberg, X-Flow's global director of sales and marketing says, "The introduction of the Seaguard is an important step towards the standardisation of dual membrane treatment for desalination of seawater. Back in 1997 we presented our first paper about this topic at a seawater pre-treatment conference in Kuwait and one year later in 1998 we completed our first pilot trails on Seawater.
"Now almost eight years later the performance and TCO are at such levels that membrane pre-treatment outperforms conventional technologies."