22 Dec FILTRATION SYSTEMS IN DESALINATION PLANTS
Water desalination is a process that converts seawater or brackish (high salinity) water into fresh water suitable for drinking and irrigation.
There are several desalination methods such as multi-stage flash evaporation, multi-effect distillation, freezing, electrodialysis, among others, but the method used most widely worldwide, and for the uses described above, is reverse osmosis.
In general terms, in reverse osmosis, pressure is applied to the salt water to force it through a semipermeable membrane that allows the water (solvent) to pass through but rejects the dissolved salts (solute). The pressure that is applied must be sufficient to overcome the osmotic pressure of the treated water. This generates two water flows: one with desalinated water and the other with a high salt concentration.
Reverse osmosis is the most widely used method worldwide. The osmosis membranes have very demanding water quality requirements in terms of suspended solids because a large load of suspended solids significantly reduces their efficiency and could damage them. Sand filtration supplemented by microfiltration is used to reduce the solids load.
The variable solids load in seawater, as well as the large decrease in filtration speed required by sand filters, means that these units are generally very large. These plants often have numerous horizontal sand filters more than 3.5 metres in diameter and more than 10 metres long. The most widespread solution is the use of ebonite-lined carbon steel filters (sea water is corrosive for carbon steel). PVC and GRP cartridges holders are installed downstream, with cartridges (generally 5 microns, they may be up to 1 micron) that guarantee proper solid load levels and low turbidity when entering the reverse osmosis membranes.
As a result of the evolution of manufacturing techniques and the drop in production costs, installations began to be designed with a double membrane process. These systems have the usual osmosis membranes, with ultrafiltration membranes installed before them. Ultrafiltration membranes are less sensitive to turbidity and suspended solids loads. They can generally operate optimally with solids loads with particles smaller than 100 microns, always taking into account that the bed filters and microfiltration are properly sized.
In order to supply the large flows of water that these plants require (estimated at roughly 1.5 times the production needs), the water must pass through pre-filtration at between 100-300 microns (depending on the manufacturer of ultrafiltration membranes). Screen and disc systems perfectly meet the requirements for this, with low space requirements, filtration at the pipe speed and chemical resistance. With the progress in manufacturing techniques, glass-fibre reinforced polyamide disc filters like the Autosenior Lama are also used.
Currently, depending on the flow to be produced and the intake conditions, the leading water engineering companies (with Spain’s companies leading the industry worldwide) that undertake this type of project will use one system or another.
At LAMA SISTEMAS DE FILTRADO, we participate in the preparation of studies and projects with specific needs of projects involving salt water. Thanks to our ebonite-coated screen and sand filters, as well as our specific disc systems for seawater, we can design the solutions demanded by our customers and work with them to size and install the filtration systems for this use.