Reverse osmosis pretreatment steps

Feed characteri2ation, particularly for nondesalination appHcatioas, should be the first and foremost objective in the design of a reverse osmosis plant. This involves the determination of the type and concentration of the main solutes and foulants in the stream, temperature, pH, osmotic pressure, etc. Once the feed has been characteri2ed, a reaHstic process objective can be defined. In most cases, some level of pretreatment is needed to reduce the number and concentration of foulants present in the feed stream. Pretreatment necessitates the design of processes other than the RO module, thus the overaH process design should use the minimum pretreatment necessary to meet the process objective. Once the pretreatment steps have been determined and the final feed stream defined, the RO module can be selected. 

Figure 5.22 Flow scheme showing the pretreatment steps in a typical seawater reverse osmosis system [50]...

Electrodeionization systems were first suggested to remove small amounts of radioactive elements from contaminated waters [27], but the principal current application is the preparation of ultrapure water for the electronics and pharmaceutical industries [28], The process is sometimes used as a polishing step after the water has been pretreated with a reverse osmosis unit. 

A variety of liquids have been treated with reverse osmosis and ultrafiltration membranes ranging from seawater, to wastewater, to milk and yeast suspensions. Each liquid varies in composition and in the type and fraction of the solute(s) to be retained by the membrane. Complicating factors include the presence of substances such as oil in seawater and wastewater [12-15]. The presence of the oil normally necessitates an additional pretreatment step further complicating the fouling process. The presence of humic acids in surface water and wastewater also needs special attention [16,17]. The fouling phenomena, the preventive means (i.e., pretreatment), and the frequency and type of membrane cleaning cycle are all dependent on the type of liquid being treated. 

Membrane filters are made in a wide variety of pore sizes (Fig. 1). The effective pore size for membranes vary, and membranes can be used in reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). RO membranes are widely used in water treatment to remove ionic contaminations from the water. These membranes have an extreme small pore size and, therefore, require excellent pretreatment steps to reduce any fouling or scaling of the membrane, which would reduce the service lifetime. RO membranes are used by extensive pressures on the upstream side of the filter membrane to force the liquids through the pores. 

The equipment required for water treatment will be determined by the quality of the incoming water. Typically, a USP pharmaceutical grade system will require pretreatment (filters), deionization, reverse osmosis, and potentially a polishing step such as continuous deionization. Many systems now incorporate UV filters for sanitization, which kill micro-bials and also eliminate ozone. 

Sea water usually contains total dissolved solids (TDS) at the level of 35 000 ppm around the Middle East, this can go up to 40 000-50 000 ppm. Brackish waters can have TDS as low as 700-1000 ppm, going all the way up to 10 000 ppm plus. Desalination of sea water and brackish water involves a significant number of pretreatment steps, which are essential to the successful long-term performance of the reverse osmosis (RO) membranes incretisingly and invariably employed in new desalination plants. There are also a few post-treatment steps afl er desalination. The exact nature and details of the pretreatment steps are influenced by the source water for the plant As shown in Figure 11.2.1, typical pretreatment steps consist of a variety of separation... 

Post-treatment of the desalted permeate from the reverse osmosis unit (Section 7.2.1.2) consists of the addition of CI2 and Ume for disinfection and corrosion protection. If H2S is present, it is eliminated by air stripping. For brackish-water feeds containing hydrocarbons, an activated carbon adsorber is used to remove dissolved hydrocarbons prior to microfiltration, which is followed by steps needed during the pretreatment and post-treatment processes dechlorination is required as a pretreatment if the RO membrane for desalination cannot tolerate residual chlorine dissolved otcygen is often removed to avoid damaging the RO membrane via vacuum based deaeration or addition of sodium bisuMte. An introduction to the pretreatment and post-treatment processes for membrane based sea-water and brackish-water desalination has been provided by Williams et al. (2001). The scale of such desalination plants is quite large, as much as 87 million gallons per day at Ashkelon, Israel, for example. 

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