Many ion exchange processes exist for a variety of industrial water and wastewater applications.
The ion exchange process consumes large quantities of regeneration chemicals, such as brine, acid, and caustic materials that can present significant handling and disposal problems.
In recent years, membrane processes have been used increasingly for the production of “pure” waters from fresh water and seawater.
Membrane processes are also being applied in process for industrial uses and in waste water systems.
Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electrodeionization (EDI). These processes (with the exception of UF) reduce most ions; RO and UF systems also provide efficient reduction of non-ionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to reduce contaminants, and suspended solids.
In many process and wastewater applications, reduction of dissolved ions is not required but efficient reduction of colloidal inorganic or organic molecules is. Ultrafiltration (UF) membrane configurations and system designs are similar to those used in the single-stage RO process. Because the large molecules removed by UF exhibit negligible osmotic pressure, operating pressures are usually much lower than in RO systems. Below figures illustrates the performance of ultrafiltration membranes.
Processes that rely on microporous membranes must be protected from fouling. Membrane foul-ing causes a loss of water production (flux), reduced permeate quality, and increased trans-membrane pressure drop.
Membrane fouling is typically caused by precipitation of inorganic salts, particulates of metal oxides, colloidal silt, and the accumulation or growth of microbiological organisms on the membrane surface. These fouling problems can lead to serious damage and necessitate more frequent replacement of membranes.
Membrane feedwater should be relatively free from colloidal particulates. The most common particulates encountered in industrial membrane systems are silt, iron oxides, and manganese oxides.
Silt Density Index (SDI) testing should be used to confirm sufficient water quality for the specific membrane system employed. SDI evaluates the potential of feedwater to foul a 0.45 µm filter. Unacceptable SDI measurements can be produced even when water quality is relatively high by most industrial water treatment standards. Where pretreatment is inadequate or ineffective, chemical dispersants may be used to permit operation at higher-than-recommended SDI values. RO systems are highly susceptible to particulate fouling, EDI systems are more forgiving, and UF systems are designed to handle dirty waters.
Membrane processes produce a concentration gradient of dissolved salts approaching the membrane surfaces. The concentration at the membrane may exceed the solubility limits of certain species. Calcium carbonate (CaCO3) and calcium sulfate (CaSO4) are typical precipitates formed. Silica, barium, and strontium salts are also frequently identified in membrane deposits. Because of their low solubility, very low levels of feedwater barium or strontium can cause membrane fouling.
Various saturation indexes, such as the Stiff-Davis and Langelier, should be maintained below precipitating values in the brine (through pH control or deposit control agents) to prevent calcium carbonate fouling. Other precipitates may be controlled by the proper application of deposit control agents.
Cellulose acetate membranes can be degraded by microbiological activity. Proper maintenance of chlorine residuals can prevent microbiological attack of these membranes.
Polyacrylamide membranes are resistant to microbiological degradation; however, they are susceptible to chemical oxidation. Therefore, chlorination is not an acceptable treatment. If inoculation occurs, microbiological fouling can become a problem. Nonoxidizing antimicrobials and biodispersants should be used if serious microbiological fouling potential exists.