In the early days of seawater reverse osmosis (SWRO), there was a tremendous need for reliable pretreatment but only trial and error taught appropriate solutions, such as well-optimized coagulation schemes followed by dual-stage, dual-media filtration. Nowadays ultrafiltration technology is also available,but to what extent and how cost effectively can it address SWRO pretreatment needs? And to what extent do market needs and technology capabilities match?
|Figure 1: The installed capacity plotted versus the installation year for dual membrane system references forseawater |
|desalination that have been built, are being built, or are planned, based on the authors’ industry knowledge. |
To address the latter question, one must look at market needs and trends. Two market needs, in particular, will be examined: minimum primary pretreatment schemes and low environmental impact schemes.
Adoption of Ultrafiltration Technology in SWRODesalination
In the last ten years, ultrafiltration (UF) or microfiltration (MF) pretreatment has gained widespread attention as potential pretreatment to seawater desalination by SWRO. Systems composed of UF or MF pretreatment for SWRO desalination are often termed “integrated membrane system” or “dual membrane system.”
Between 1995 and 2005, many pilot studies were undertaken. In recent years, anumber of SWRO plants — some very large (>100,000 m³/d) — have implemented ultrafiltration or microfiltration pretreatment.
The data on adoption of ultrafiltration for SWRO pretreatment is actually very promising with regard to the speed of adoption: already 42 larger scale seawater desalination plants, with a joint capacity of about 3.4 million m³/d of SWRO desalinated water, are using or areplanning to use ultrafiltration as pretreatment. For the 2010-2011 period, the start-up of a multitude of large plants is planned.
Market Needs Driving Adoption
There have been various market needs that have emphasized considering ultrafiltration pretreatment over conventional pretreatment:
• Ability to cope with difficult waters: Some feed waters from the more contaminated seas have posedsignificant challenges to SWRO systems in the past (e.g., Persian Gulf, Yellow Sea, Gulf of Mexico). Sea bodies with medium quality can pose challenges as well if captured with low sophistication intake systems (e.g., channel, short pipe) because algae blooms at the top surface or suspended solids from the sea ground created during storms will influence the intake quality.
• Reliability: Highvalue is given to reliably reaching maximum plant capacity. This is the case in industrial plants, for example, where the cost of downtime due to lack of water is much larger than the water production cost.
• Plant footprint: UF pretreatment systems generally require a lower footprint than media filtration systems, especially where dual-stage media filters in combination with sedimentation orflotation are used. Site-specific conditions, such as size limitations or buildings with high overhead costs, will increase focus on this aspect.
• Ease of design and operation: Despite requiring more focus on sustained permeability and productivity, ultrafiltration systems provide much more stable water quality than a multimedia filtration system does, without the need to monitor filterripening time or breakthrough, or the need to ensure appropriate layering of multimedia after backwash. Therefore, process design and control is easier and more automated than with conventional pretreatment, and this enables a wider market group access to reliable SWRO operation, especially participants with less know-how in multimedia filtration, or in smaller plants that do not have the resources...