Reverse Osmosis Lecture 17
Reverse osmosis Reverse osmosis is a membrane technology used for separation also reffered as Hyperfiltration. In a typical RO system the solution is first filtered through a rough filter like sand or active carbon, or dual filter etc. If solution contains Ca ++, Mg salts, iron, carbonates, then acid dosing system is introduced. The ph is adjusted and the solution is then filter through micro cartridge filter (5 10 micron). The pretreated water is then pumped in to the RO tank with a high pressure pump. The membrane separates the pollutants in concentrated form in the reject stream and the pure water is collected as a permeate.
Why RO is the best If other variables are kept constant, the water flow rate (flux) is proportional to the net pressure. The pressure range for RO systems varies from 10 Kg/cm 2 to 65 Kg/cm 2 Thus it is evident that RO can be advantageously used for the treatment of textile waste water provided the design, pretreatment stringency, operating parameters are strictly adhered to avoid fouling of membrane modules
Applications RO is more useful to separate salts and organic compounds from textile effluents. Some of the wastewater varieties from textile industry that can be treated by RO for recovery of reusable water such as: 1. Rayon industry process wastewater. 2. Textile dyes house effluent. Up to 80% of warm dye house wastewater can be recovered for recycle by RO membrane.
Criteria for designing RO treatment System Normally four parameters are considered for designing the systems: They are 1. Solution based variables such as: suspended solids, dissolved solids inorganic and organic, microorganisms, sparingly soluble materials, oxidizing chemicals, organic solvents ( nature and concentration), temperature and ph 2. Minimum pre treatment requirement: Acid or alkali dosing for ph adjustment, filtration to obtain required salt density index, anti scalant and heat exchange for reducing the higher temperature of the solution
Parameters for design of RO 3. Membrane variable: Polymer type and module geometry, pressure, flow rate, pressure loss. Water recovery and concentration levels, minimum tolerable flux and desired flux levels, module arrangements. 4. Cleaning requirement: The fouling of membrane depend on the extend of pretreatment and module type chosen. The flux loss due to particulate or bacterial adhesion generally increases in the following order tubular>plate and frame>spiral wound> hollow fine fibre
RO Separation of Organic Pollutants from Wastewater Studies have been performed on the separation of organics and organic pollutants by RO membranes, and these studies have identified some of the unique aspects associated with organic separation. Sourirajan (1970) and Sourirajan and Matsuura (1985) have compiled separation and flux data of cellulose acetate membranes for a large number of organic compounds, including many organic pollutants. They found that organic separation can vary widely (from <0% to 100%) depending on the characteristics of the organic (polarity, size, charge, etc.) and operating conditions (such as feed ph, operating pressure, etc.).
Design speciality RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling The RO technique is a highly efficient process, in terms of high recovery, low operating cost RO membranes have a retention rate of 90% or more for most types of ionic compounds and they produce a high quality of permeate RO permeates the removal of all mineral salts, hydrolyzed reactive dyes and chemical auxiliaries, but the problem involved is that the higher the concentration of salts, the more important the osmotic pressure becomes and consequently, the greater the energy required.