Most likely, not many people have heard of the village of Hünxe in North Rhine-Westphalia. Some 15,000 people live here. And in 2016, precisely 141,437 cubic meters of water were purified for them. This major task falls to the operators of the local sewage treatment plant. Day in, day out, an MBR plant quietly and reliably works to ensure municipal wastewater treatment. MBR stands for membrane bioreactor, a combination of biological wastewater treatment and membrane filtration. Wherever a high level of purification or a small footprint is required, this method is the choice preferred by sewage plant operators. At the Hünxe site, the MBR plant has been running since November 2009.

Special purification method

“A membrane bioreactor is a special method of wastewater purification. Activated sludge, i.e. biomass in the form of microorganisms and bacteria, cleans the wastewater. A membrane then separates this activated sludge from the purified water and retains it in a tank. The result is clean and odorless water,” says Werner Ruppricht, Senior Sales Director at MICRODYN-NADIR, a MANN+HUMMEL subsidiary, whose area of responsibility covers the field of municipal sewage and water treatment. To ensure a reliable process, however, the membrane must be undamaged. Even superficial damage may put a “clean” outcome at risk. “A minor flaw in the membrane and the water is contaminated,” says Ruppricht. “We therefore developed special modules to prevent contamination even if the membrane is damaged.” These modules, which are used in Hünxe, go by the name of BIO-CEL®. And it’s not only Hünxe, but also wastewater plants in Turkey, Israel, South Africa, Turkmenistan, the U.S., Mexico, Aruba, and Asia that have been equipped with the modules.

How a sewage treatment plant works

Exkurs

Once the wastewater has entered the sewage treatment plant, it passes through the first treatment stage, where screens and sieves trap coarse particles. Other large contaminants such as stones, sand, and glass fragments are collected at the subsequent sand trap. The particles that could impair operation of the sewage treatment plant have now been separated out. What remains is sludgy wastewater, which contains a large number of dissolved substances such as urea, organic contaminants, nitrogen compounds, and phosphates. A biological cleaning stage, in the form of an activated sludge process, therefore follows, in which bacteria attend to further cleaning. First they break down dissolved organic carbon compounds such as carbohydrates, proteins, and any remaining lipids. Nitrogen and phosphate compounds are then eliminated and the biomass, consisting of sludge and bacteria, is retained. The purified wastewater then returns to the water cycle via receiving waters such as lakes or rivers. It can now be used to produce drinking water or industrial process water.

Special module design

“There are essentially two types of membrane used in MBR technology for wastewater purification: hollow fiber and flat membranes. Our BIO-CEL® is based on flat membrane technology,” says Ruppricht. In conventional plate modules, the membrane is mounted on a plastic plate and either glued or welded to the edges. Damage to the membrane would allow unfiltered wastewater to escape from the plant. In the case of BIO-CEL®, a special mechanism therefore guarantees the durability and efficiency of the modules. The membrane is laminated onto a spacer material. The resulting sheets are welded on the sides, and the clean filtrate flows out through an opening in the center of the sheet. This has a number of advantages. Thanks to the spacer material, the biomass in the wastewater treatment plant can seal off any damage to the membrane, meaning the bacteria and solid particles are still trapped by the membrane module. “Superficial damage practically ‘heals’ itself,” says Ruppricht. In addition, the membrane module allows for a backflush without damage, despite its strength. As a result, sludge from the activated sludge tank cannot permanently settle on the membrane.

Ultrafiltration membranes in the fight against germs

In an age of micropollutants, microplastics, and multiresistant germs, however, traditional purification methods are insufficient, as Ruppricht knows. He’s been tackling this problem since 2015 and also heads the micropollutants working group at the German Society for Membrane Technology (DGMT). “Through the disposal cycle, multiresistant germs, micro-pollutants, and microparticles enter the wastewater that is treated at sewage plants such as here in Hünxe. For the multiresistant germs, in particular, this is an ideal place to multiply and spread. Conventional biological wastewater purification processes cannot efficiently remove these pollutants.”

Multiresistant Germs and Microplastics

Microplastics is the term for solid, insoluble plastic particles with a size of under 5 millimeters. They are produced naturally during the decomposition of plastic products or are deliberately used as constituents in cosmetic and cleaning agents. Microplastics enter wastewater, where they attract pollutants or are consumed by animals. The particles can no longer be removed from the environment. Micropollutants include residues of medications or pesticides. Then there are multiresistant germs originating in the excessive use of antibiotics in livestock breeding and medicine. They spread and grow in the wastewater, potentially presenting a risk to human health.

Pilot project with germ-free results

Together with the operators of the sewage treatment plant, MICRODYN-NADIR in Hünxe has recently therefore been trialling a new process: a combination of activated carbon and immersed membrane filtration downstream of the conventional wastewater system also installed in Hünxe. It traps trace substances, microplastics, and activated carbon in the purified wastewater from the secondary treatment stage. “We use superfine ultrafiltration membranes whose pores are a thousand times smaller than the diameter of a hair. In addition to trace substances and microplastics, this allows us to separate out germs, too,” explains Ruppricht, adding that “positive results were already seen on the pilot project. The combination is a highly efficient and affordable alternative to conventional processes and provides considerably better separation performance. Our process meets the exacting requirements for wastewater treatment and in particular sets new standards in the separation of multiresistant germs. It means we’re doing something important to protect man and the environment.”

Essence

A membrane bioreactor, a combination of biological wastewater treatment and membrane filtration, is a space-saving and effective solution for treating wastewater. In Hünxe, a pilot project is also investigating the efficacy of a new process using BIO-CEL® modules: a combination of activated carbon and immersed membrane filtration separates trace substances, microplastics, and germs.