Основы процессов мембранного фильтрования

Принципиальная сущность методов мембранного фильтрования заключается в фильтровании растворов од давлением через специальные полупроницаемые мембраны, пропускающие молекулы воды и полностью или частично задерживающие молекулы растворенных веществ.

Схема единичного мембранного элемента приведена на рис. 1

Basic Terminology

Some basic terms need to be defined to further understand RO

and NF.

Recovery is defined as the amount of feedwater that is recovered as pure product or “permeate.” It is expressed in terms of percent of feedwater that becomes product water. The higher the recovery of the system, the lower the amount of wastewater generated. Recovery generally ranges from about 50% to as high as 90%, depending on the application. The most common recovery is 75% for most industrial and commercial applications. Recovery is set by the RO system designer and is a balance between the amount of wastewater generated and the degree to which feedwater becomes concentrated as more water is recovered.

Rejection (or salt rejection) is defined as the degree to which a given species is retained by the membrane. It is expressed in terms of percent of feed concentration. The higher the rejection, the more concentrated the product and concentrate streams become. Rejection is typically dependent on the specific species and the specific membrane. Rejection is a property of the membrane; it is not set by the designer of the RO system. Salt passage is opposite of rejection, such that 98% rejection corresponds to 2% passage of the solute in question.

Concentration factor is a term used to determine the relative concentration of the reject stream to the feed stream. Concentration factor is calculated by taking the feed flow rate and dividing it by the reject flow rate. This factor is then used to multiply the feed concentration to determine the approximate reject concentration of a species of interest.

Flux is the amount of water that passes through a given membrane area during a given unit of time. Flux is expressed in gallons per square foot per day (gfd) or liters per square meter per hour (L/[m² · h]). Flux is an important design variable, as the flux rate can have a direct impact on the degree and rate of fouling and scaling of the membrane. Flux is a direct measure of the rate at which material is transported to the surface of the membrane. Material that does not pass through the membrane is then trapped in the boundary layer, where the slow rate of diffusion away from the membrane gives solutes and scale sufficient time to foul and scale the membrane. The lower the feedwater quality, the greater the potential for fouling and scaling the membrane. Table 8.2 lists the recommended flux rates for feedwater as a function of the feedwater quality. As the table shows, higher feedwater quality can result in higher flux rates. The advantage of higher flux rates is that less membrane area, hence, smaller footprint and, more importantly, lower capital cost, are required.