Aeration in the biological stage of a wastewater treatment plant uses a great deal of electricity, and the condition of the diffuser membranes mounted on the pipework at the bottom of the basin largely determines how high the power bill ends up. Gergely Halász, lead procurement engineer at Water4All Zrt., explains what makes a good membrane, why it pays to replace them every seven years, and why he considers one Chinese-developed product unbeatable.
For readers less familiar with the field: what is a membrane for at a wastewater treatment plant?
Blowers and membranes together are perhaps the most important equipment in a biological wastewater treatment plant. They are what force air into the wastewater, allowing the aerobic microorganisms that multiply there and break down the organic matter to survive.
So should we picture membranes roughly as shower heads lined up on the bottom of the basin, facing upwards and releasing air bubbles into the water through tiny holes?
Essentially, yes. In the industry we simply call these “shower heads” membranes, and they are usually installed on plastic or metal pipework running along the bottom of the basin. The microbubbles they release rise through the water and supply oxygen to the mixture of wastewater and activated sludge. To do this, the blower compresses ambient air, typically to the pressure of a 5–6 metre water column. This is needed so that the air can overcome the water pressure and pass through the membranes as fine bubbles into the mixture of wastewater and activated sludge.
What distinguishes a good membrane from a less good one?
For the operator of a wastewater treatment plant, what matters most is that the aeration system runs as reliably and for as long as possible, with the lowest possible energy use. Besides service life, controllability is also important, since the aeration rate has to follow the plant’s fluctuating load. For efficient aeration, we want the bubbles to be as small as possible, because the smaller they are, the better the oxygen dissolves into the wastewater–sludge mixture. But that is not the only factor: basin depth, which is decided at the design stage, matters too. It makes a difference whether the oxygen has to dissolve in a 5- or 6-metre water column, or an even taller one, and equally how much blower capacity has to be assigned to it, since this determines power consumption. And if that is not complex enough, add the quality of the membrane itself.
Let’s look at that last point. What makes a good membrane?
In municipal wastewater treatment plants, membranes are usually made of EPDM rubber. The drawback is that, over time, the wastewater leaches certain cross-linking agents out of the rubber, so the membranes weaken and stretch. As they stretch, the bubbles blown through them grow larger and larger, and aeration efficiency drops.
How long does it take for this to become critical?
The optimal replacement interval for membranes is seven years — at least, that is the generally accepted view in the industry. Put another way, this is the point at which the energy saving gained by replacing them exceeds the cost of the replacement.
So if I understand correctly, softened, stretched membranes mean less oxygen in the wastewater, which is therefore treated more slowly; the wastewater stays in the basin longer, and the blowers have to run for longer.
For longer, and possibly at higher output…
Do membranes also differ in service life?
They certainly do. PTFE-coated EPDM membranes are slightly more expensive, but in return they are more resistant, and their cross-linked structure leaches out more slowly. There are also silicone membranes, which are even more durable — and, of course, considerably more expensive. Because of the higher price, silicone is not normally used in municipal plants, but some industrial wastewaters are of a kind that justifies working with such resistant membranes from the outset.
Is there a membrane you would wholeheartedly recommend to your partners?
In terms of value for money, of the products we know, the Chinese-developed membranes from Aquasust are currently unbeatable, for three reasons. They are more durable than most competitors, they provide excellent oxygen transfer, and they are also cheaper than comparable rivals. Most importantly, though, their perforations have a special shape: even after long use, they do not tear into one another. As a result, the bubbles stay small for much longer. It is a truly unique and ingenious solution. We have already used them at many of our partners’ plants in Hungary, and the operators are very satisfied with them.
So these Chinese products are competitive in technical innovation too, not just on price?
As far as I know, they spent a very long time experimenting to find the optimal design for the holes in the membrane. They make the membrane itself from a premium American raw material, which ensures a long design life and efficient operation. And since it is made in China and comes directly from the factory, without intermediaries, the competitive price is no surprise.