A membrane filter is a thin layer of semi-permeable material. Semi-permeable means it only allows materials to pass up to a certain size, or of a specific shape or character. In pressure-driven membrane filtration, a liquid is forced through a membrane, which separates the water into two parts: the retentate, or concentrate, and the permeate or filtrate. In wastewater treatment, membranes are used to trap unwanted materials in the retentate.

Types Of Membrane Filtration

Different types of membranes are used for different purposes, such as desalinating, disinfecting, and removing organics from water and wastewater. The four pressure-driven membrane filtration processes used for liquid separations are (from smallest to largest pore size): reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF).

RO membranes have the smallest pores. RO depends on ionic diffusion to reverse the solution’s osmotic process in order to drive water away from dissolved molecules. RO can effectively remove radium, natural organic substances, pesticides, and pathogens. RO is the type of membrane filtration most commonly used by ProAct.

NF is similar to RO but is generally used to remove divalent and larger ions. Monovalent ions such as sodium and chloride will pass through an NF membrane. In water treatment, NF membranes are used to reduce or remove color, hardness, and pesticides.

UF membranes are characterized by their molecular weight cut‐off. UF removes colloids, emulsified oils, emulsions, metal hydroxides, suspended solids, and other large molecular weight materials from water. UF is mostly used for clarifying solutions with suspended solids, viruses and bacteria, or high concentrations of macromolecules. In water treatment, this includes oil/water separation.

MF is more frequently used for bottling and pharmaceuticals than water treatment. MF systems operate at relatively low pressures.

In addition to the four membrane filtration pore sizes, membranes can be packaged in a variety of configurations. These include hollow fiber, spiral, and tubular shapes. Each provides different advantages and disadvantages. The membrane, packaged in a specific shape, is fitted inside a rigid housing, or in disposable elements. The membrane configurations are described below.

Hollow fiber membranes are used mostly in UF and MF filters. Source water flows through the center, and treated water passes through the fiber wall to the outside of the membrane. Hollow fiber membranes are tough, which allows backwashing, to extend the time between chemical cleaning cycles. Hollow fiber membranes are used in municipal water and wastewater treatment, and in food and beverage production, among other uses.

Spirals are the most common configuration and are used in all four filter types. Spiral filter elements are made of layers of flat membrane sheets and feed separators wound around a hollow core. Source water enters the filter element and water flows through the membrane into channels, leaving the contaminants behind. The channels spiral to the central core, where treated water exits. RO and NF spiral filters are widely used for water and wastewater treatment. UF and MF spirals are often used in the industrial and life sciences industries.

Tubular membranes are mostly used for UF and MF filters. They are specifically designed for streams with high solids and viscous fluids. Source water is pumped along the membrane surface in a sweeping “cross-flow” action. Tubular membranes are rugged and
handle a wide pH and temperature range without plugging. Tubular membranes are ideal for wastewater contaminated with oil, grease, heavy metals, and suspended solids.

Pre-filtration

With membrane filters, pre-filtration is usually needed because the membrane’s small pore spaces can easily become blocked by suspended solids. Membrane filters effectively remove contaminants from waste streams, so the water can be recycled, discharged, or further treated.

ProAct Membrane Filtration Services

ProAct’s design engineers can develop a system tailored to your project. ProAct’s specialists evaluate whether membranes are the best technology, and, if so, choose the best membrane type. The membrane material, pore size, configuration, and housing are all considered. Call on ProAct today!