Ceramic Membranes for Wastewater Filtration – Part 1
Figure 1. The range of filtration separation processes and an overview of their use in specific applications.
Meanwhile, many wide-ranging applications of tubular polymer or ceramic membranes and high-quality porous ceramic membranes are being developed as part of efficient filtration systems to improve filtration efficiency, reduce maintenance downtime and lower energy use. Such operational benefits can reduce the cost of water treatment.
Fundamentals of filtration systems
Polymer and ceramic membranes are currently used to clean, conserve and reuse water in a number of industrial applications and in municipal wastewater recycling, including wastewater from plants, buildings and even ships. Polymer membranes currently account for about 75% of the membrane market, with the remaining 25% held by ceramic and metal membranes. Figure 1 shows the range of filtration separation processes and an overview of their use in specific applications.
Ceramic membranes are commonly used in dairy production and fruit juice filtration, whereas polymer membranes currently dominate wastewater treatment. Because of their low cost, polymer membrane systems are widely used in the wastewater filtration market. However, while polymer membranes are cheaper than equivalent ceramic membranes, polymer filtration systems require more frequent filter replacement.
Compared with ceramic membranes, the low durability of polymers limits their range of applications. In general, ceramic membrane modules usually have metal housings, which are more limited in aggressive environments than their ceramic filter elements. Conversely, when polymer membranes are used in harsh aggressive environments, the membranes themselves are attacked first. In addition, flow rates through ceramic membranes can be higher than through polymer membranes with the same pore diameter. Ceramic filters also require lower pressure (and less energy) to recirculate liquids.
As water reuse becomes more important, some demanding industrial applications face increasing needs to filter harsh media. Because ceramic membranes are particularly resistant to chemicals and abrasion, they have advantages over polymer membranes in certain applications. One example of a harsh environment is the separation of oil, water and sand in the oil industry – a process involving both chemical attack and abrasion.
While ceramic membranes can withstand pH values from 0 to 14, polymer membranes are stable only within a narrow pH range; polymer membranes can be selected to resist neutral, acidic or low-alkaline pH, but generally a single polymer material cannot withstand all three. Ceramic membranes can also be used in high-temperature applications. They can be sterilised or steam-cleaned in special applications, such as in the medical industry, where polymers cannot be used. Moreover, ceramic membranes are stronger and stiffer, making them more dimensionally stable under pressure than polymer materials.
When filtering water containing oil or fatty acids (surfactants), chemical dispersants (emulsion breakers) are often used to limit the formation of organic fouling on the surface of organic membranes. High-quality ceramic membranes are more resistant to fouling without the use of dispersant chemicals.
While fouling of both polymer and ceramic membranes is inevitable, ceramic membranes can serve more applications because they are suitable for clean-in-place (CIP) systems. In some applications, membranes must be cleaned with aggressive chemicals and withstand high pressure from both sides. This is especially necessary during backflushing to prevent fouling on the membrane surface or in the pores, which reduces the membrane’s filtration capacity.
For example, cleaning with peroxide-based chemical cleaners or high-temperature steam is acceptable for ceramics but not for polymers. Cleaning by burning off with high-temperature air can be applied to ceramic membranes but would melt polymer membranes. Clean-in-place (CIP) can be performed continuously and automatically during filtration with ceramic membranes, but is more difficult with polymer membranes.
At the same time, as industries adopt water reuse processes and water retention methods, they are also reducing the aggressive impact of production on their equipment. Membrane selection and filtration system design aim to reduce the impact on the equipment used in filtration systems. Recently developed ceramic membranes can achieve compactness (greater membrane surface area per unit volume) thanks to their design flexibility and geometry. The compactness and improved design of ceramic filtration modules also contribute to the energy efficiency of filtration systems.
