Membrane
However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited.
Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, several experienced companies serve the market.
Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes.
minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness ions, (3) lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration.
The corresponding mass balance equations are: To control the operation of a membrane process, two modes, concerning the flux and the TMP, can be used.
In the case of the dead-end filtration process, the resistance increases according to the thickness of the cake formed on the membrane.
As a consequence, the permeability (k) and the flux rapidly decrease, proportionally to the solids concentration [1] and, thus, requiring periodic cleaning.
The loss of RO performance can result from irreversible organic and/or inorganic fouling and chemical degradation of the active membrane layer.
A variety of oxidative solutions, cleaning and anti-fouling agents is widely used in desalination plants, and their repetitive and incidental exposure can adversely affect the membranes, generally through the decrease of their rejection efficiencies.
[12] Fouling can take place through several physicochemical and biological mechanisms which are related to the increased deposition of solid material onto the membrane surface.
Relaxation and backwashing effectiveness will decrease with operation time as more irreversible fouling accumulates on the membrane surface.
One drawback to using modification techniques is that, in some cases, the flux rate and selectivity of the membrane process can be negatively impacted.
Discarded RO membrane modules are currently classified worldwide as inert solid waste and are often disposed of in landfills; although they can also be energetically recovered.
However, various efforts have been made over the past decades to avoid this, such as waste prevention, direct reapplication, and ways of recycling.
The size of these RO plants has also increased significantly, with some reaching a production capacity exceeding 600,000 m3 of water per day.
[22] Four types of fouling are found on RO membranes: (i) Inorganic (salt precipitation), (ii) Organic, (iii) Colloidal (particle deposition in the suspension) (iv) Microbiological (bacteria and fungi).
Thereby, an appropriate combination of pre-treatment procedures and chemical dosing, as well as an efficient cleaning plan that tackle these types of fouling, should enable the development of an effective anti-fouling technique.
Reuse of RO membranes include the direct reapplication of modules in other separation processes with less stringent specifications.
Studies shows that hydraulic permeability, salt rejection, morphological and topographical characteristics, and field emission scanning electron and atomic force microscopy were used in an autopsy investigation conducted.
The old RO element's performance resembled that of nanofiltration (NF) membranes, thus it was not surprising to see the permeability increase from 1.0 to 2.1 L m-2 h-1 bar-1 and the drop in NaCl rejection from >90% to 35-50%.
It is proposed to adapt this original concept, by internally reusing older RO membranes within the same pressure vessel.
