Membrane process utilizes Reverse Osmosis technology which had become popular and continued to gain market share in the desalination market because of the technology’s improved reliability and reduced energy requirements. Even in seawater application, where high feed salinity requires high-pressure RO operation, energy requirements for RO processes are lower than the combined energy requirements of distillation processes.
The SBR system can be designed with the ability to treat a wide range of influent volumes whereas the continuous system is based upon a fixed influent flow rate. Thus, there is a degree of flexibility associated with working in a time rather than in a space sequence. Advantages includes easy and compact construction, control in time, selection of well settling sludge (‹integrated select.
The process is similar to membrane filtration. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure and water flux rate.
Forward Osmosis is an emerging membrane desalination technology which uses a semi permeable membrane to separate water from dissolved solutes wherein it blocks higher molecules utilizing the osmotic pressure gradient. It is very effective in rejecting wide range of contaminants and is very resistant to fouling. It gives us the advantages of having low energy cost with high water recovery.
Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g., ethanol).
Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.
Ultra filtration, like reverse osmosis, is a cross-flow separation process. It is a separation process using membranes with pore sizes in the range of 0.1 to 0.001 micron.
It will remove high molecular-weight substances, colloidal materials, and organic and inorganic polymeric molecules. Here liquid stream to be treated (feed) flows tangentially along the membrane surface, thereby producing two streams. The stream of liquid that comes through the membrane is called permeate.
The type and amount of species left in the permeate will depend on the characteristics of the membrane, the operating conditions, and the quality of feed. The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.
Substances that are larger than the pores in the membranes are fully removed. Substances that are smaller than the pores of the membranes are partially removed, depending on the construction of a refuse layer on the membrane.
Micro filtration is pressure-dependent processes, which remove dissolved solids and other substances from water to a lesser extent than nano filtration and Reverse Osmosis.
Membranes with a pore size of 0.1 – 10 µm perform micro filtration. Microfiltration membranes remove all bacteria. Only part of the viral contamination is caught up in the process, even though viruses are smaller than the pores of a micro filtration membrane. This is because viruses can attach themselves to bacterial biofilm.
Nanofiltration is a low to moderately high pressure (typically 50 - 450 psig) process in which monovalent ions will pass freely through the membrane but highly charged, multivalent salts and low molecular weight organics will be rejected to a much greater degree. Typical NF applications include water softening, desalination of dyestuffs, acid and caustic recovery and color removal.
This process is carried out by having a process solution flow along a membrane surface under pressure. Cross flow membrane filtration uses a high cross flow rate to enhance permeate passage and reduce membrane fouling. Retained solutes (such as dissolved salts) leave with the flowing process stream and do not accumulate on the membrane surface.
