Boosting capacity
Membrane bioreactors provide a much-needed increase in sewage treatment capacity

Though the last century saw worldwide acceptance of biological sewage treatment plants, further innovation is necessary. Increasing cost pressure and more stringent discharge limits require new techniques for sewage treatment—and membranes fit the bill.
Michael Lyko
In conventional treatment plants, wastewater is collected, stored in a compensation tank if necessary, preconditioned to remove coarse particles and fibers, and then sent to the biological treatment tanks. Biological treatment begins with denitrification, followed by nitrification through the addition of oxygen. Next, the biomass is separated by sedimentation, and most of it returns to the process; only a small fraction is disposed of as surplus sludge.

The separation of biomass is a critical limitation in conventional wastewater treatment. The maximum economic biomass concentration is 3–5 g/l; increasing this concentration would increase the capacity of the complete plant roughly in proportion. In addition, sedimentation does not provide complete separation of solids, but leaves part of the biomass—including coliform and other bacteria —in the treated effluent.

Both these issues can be tackled effectively by using membranes to separate the biomass. Membranes can be used in new plants, and also retrofitted to old plants to increase both their capacity and the quality of their treated water.

Membrane bioreactors

Like conventional treatment plants, membrane bioreactors (MBRs) rely on biology to purify wastewater. Unlike conventional plants, however, they use a physical barrier to separate clear water from bacteria, biomass and other solids. The result is higher effluent quality and greater capacity in the same space, or the same capacity from a more compact plant. Membranes can be used externally, mostly in the form of tubes, or submerged directly in the wastewater to be treated. At least for aerobic treatment, submerged membranes have become the more widely accepted. Here, specially-designed membrane modules are placed in either the nitrification cell or an extra filtration container. Air blown across the outside of the membrane creates a crossflow that largely stops solids sticking to the membrane surface, while a gentle vacuum on the other side of the membrane draws through clean water. The practical limit on biomass concentration is set by the fluid viscosity, which in most wastewater streams rises rapidly above a concentration of around 12 g/l. This increases the cost of oxygenating the system during nitrification, and of creating the crossflow needed for membrane filtration. As a result, the maximum practical biomass concentration in MBR plants is generally 10–12 g/l—enough to increase the capacity of a new plant by a factor of three. For retrofits, the modular nature of MBR systems makes it possible to increase capacity gradually, in step with rising demand.

Thousands of MBR plants, large and small, are now in operation for sewage treatment. The differences in economics are determined by the construction of the different membrane modules. As with every other filtration process, the membrane surface becomes fouled during operation. For economic efficiency it is important to design the MBR so that fouling is kept to a minimum and cleaning is needed only infrequently.

Capillaries versus plates

At present, several different types of membrane and membrane module are available. All of them will separate clean water from biomass, but their economics vary in practice.

Micro- and ultrafiltration membranes both separate bacteria; ultrafiltration membranes also separate viruses, and have smoother surfaces, so they are much easier to clean. Both types have pressure drops low enough to keep energy demands modest.

Submerged MBR membrane modules are currently available in two basic forms: capillary and plate. In both types, coarse air bubbles rising from the bottom to the top of the module increase mass transfer rates and reduce fouling.

Capillary membranes take the form of tiny tubes around 2 m long, with flow from outside to inside. Capillary modules consist of bundles of these tubes, with a high packing density. Depending on the manufacturer, the capillaries are attached at both ends, or only at the bottom.

The main difficulties with capillary modules are silting and braiding. Silting appears at the bottom end of the membrane module, where the fibers are fixed. Falling particles trapped between the fibers and unable to leave the module reduce the filtration area and restrict crossflow (Figure 1).

Braiding arises from hairs and fibers which twist around the membrane fibers and are difficult to remove. Braiding starts at the top and grows towards the bottom, again restricting membrane area and crossflow performance.

A screen with a mesh size of 0.5 mm or below placed in the raw water stream can reduce both silting and braiding, but cannot eliminate them; even if all fibers are removed, thread-forming bacteria can still cause braiding. The material removed by the screen also needs to be disposed of.

Plate membrane modules, in contrast, use flat membranes stacked or suspended in parallel sets. A plate separates each pair of membranes, and the edges of the package are glued or welded together. The packing density of plate modules is lower than that of capillary modules.

Plate modules eliminate both silting and braiding. Most plate modules are open at the bottom, so solid particles do not become trapped, and fibers cannot become entangled on the flat membrane surfaces.

On the other hand, the lower fluid velocities near the edges mean that the edges of the membranes can become blocked, while internal tension can narrow the gaps between the plates, again causing blocking. More aeration can solve both these problems, but at an increased energy cost. Another difficulty with plate modules is that in general they cannot be cleaned by backwashing.

Best of both worlds

So is there a better solution? Yes, in the form of the patented Bio-Cel module developed by Microdyn-Nadir of Wiesbaden/Germany. Although the module design itself is relatively new, the membranes on which it depends have been proved in the market for more then ten years (Figure 2).

Bio-Cel combines the advantages of capillary and plate modules in a new type of submerged membrane module. The key is to sandwich two membranes around a spacer material, creating a plate module without a plate. The edges are welded and the filtrate is extracted through a hole in the middle, which keeps pressure drops to a minimum. A series of these individual membrane bags are joined vertically to form a module.

The Bio-Cel module does not suffer from braiding, because it uses flat membranes, or silting, because the module is open at the bottom. Frameless fixing of the membranes means no edge blocking. Since the membrane bags are flexible, internal tension is not a problem, and the gaps between the membranes do not become blocked. The absence of plates allows a higher packing density. Bio-Cel modules can also be backwashed, and due to the better hydraulic conditions this works considerably better than it does with capillary membranes.

Since the beginning of 2008, Bio-Cel modules have also been available in cassettes, making the system even easier to install and service. Particularly in large plants, the ability to separate the aeration and membrane systems is a big step forward. The latest version, the Bio-Cel BC 400 cassette system, will be introduced to the market in May the IFAT fair in Munich. This new module is designed especially for large plants and has a membrane area of 400 m2. Its high packing density, comparable to that of capillary modules, makes the BC 400 ideal for plants that must handle large volumes in small areas.

Several years of experience in wastewater treatment plants around the world have proved the success of MBR technology based on submerged modules. However, this experience has also shown a need for improvement in module construction to overcome operational problems like silting, braiding, and edge and gap blocking. The new Bio-Cel module from Microdyn-Nadir combines the advantages of existing capillary and plate systems, without their disadvantages, by using a patented flexible membrane bag. The system has been tested on both industrial wastewater and municipal sewage treatment. n