Membrane Filtration (Micro and Ultrafiltration) in Water Purification

Abstract

Membrane filtration technology has revolutionized particle filtration in the water and waste water treatment sector. In contrast to classical fixed bed media filtration (see chapter “Filtration in Water and Used Water Purification”), membrane technology provides complete particle retention, regardless of the applied feed quality. In addition, mechanical disinfection is achieved by retention of germs and viruses to a great extent, depending on the applied membrane pore size. Membrane filtration may be used for all sorts of water qualities.

Several system types are readily available on the market. For MF and UF, various membrane types such as flat sheet, mainly used in MBR application or combined flat sheet-hollow fiber, are being used. An increasing demand can be seen in the use of ceramic membranes for heavy duty applications. The most widely spread membrane type are hollow fibers. Those may be used in two different ways: in pressure driven membrane systems (filtration mostly inside-out) and in submerged membrane systems (filtration outside-in). Some products on the market also combine features of both systems.

The choice of the most suitable membrane filtration type should always be made based on comprehensive consideration of reigning hydraulic and spatial boundary conditions, as well as the membrane performance itself in respect to the water type to be treated.

While a submerged membrane system may be integrated in an existing tank or basin and therefore economize space, a pressure-driven membrane system may make use of available upstream pressure for the filtration as a driving force.

Membrane material and structure is critical for its chemical and mechanical resistance. A membrane fiber basing on acetyl cellulose may not tolerate severe chemical cleaning. A thin membrane capillary may eventually break and ask for repair intervention.

Therefore, in projects with strong emphasis on total hydraulic yield, achievable peak performance, minimized chemical consumption and liable process stability, piloting of different membrane filtration systems may be essential for a successful project outcome.

The key for stable operation is, besides the chosen membrane type and system, the applied cleaning strategy. Cleaning may be triggered by timer, volume treated, TMP, or permeability. It is done by a combination of water backwash (BW), process air, and backwash with or circulation of chemical agents. Those are either dosed inline or else prepared in a stirred tank. Manifold chemical agents are used, such as bases, acids, and/or oxidants. CIPs are performed once to twice a year, in order to prevent irreversible fouling of the membranes. During CIP, membranes are exposed to higher chemical concentrations and longer soaking times than during normal cleaning. Membrane cleaning may take from 30 s (e.g., BW) up to 30 min (CEB) or 4–12 h (CIP).

Because of the frequent cleaning intervals, membrane filtration requires a high grade of process automation. Accordingly, treatment steps have to be designed with enough redundancies in order to be able to compensate nonproduction time of trains being cleaned.