How to separate micropollutants with an innovative membrane process
by Werner Ruppricht

Elimination of micropollutants in biologically treated wastewater has become increasingly important. One reason for this is that contamination of the environment by these substances has now been clearly proven. The regulatory framework in different jurisdictions is expected to be tightened—and threshold values increased—for micropollutants in cleaned wastewater that is discharged into the aquatic environment.

Most of the currently applied process combinations do not include membrane filtration, although this process is often advantageous, in particular with respect to the issues of multi-resistant germs, antibiotic drugs and microplastic particles that can come from a wide variety of sources.

Today, a combination of powdered activated carbon and subsequent sand filtration is mainly used to eliminate micropollutants. This process can reduce trace substances like drug residues in the effluent of a wastewater treatment plant.

More to Consider as Discarded Medicine Causes New Issues

However, it is disadvantageous in that the sand filter does not ensure complete separation of activated carbon particles. Furthermore, sand filters do not function as a barrier for microplastics and multidrug-resistant germs. An increasing use of antibiotics in human and veterinary medicine has led to a steady increase of these germs in the environment.

Since multidrug-resistant microorganisms are able to transfer resistance genes to other bacterial strains, a health risk will emerge.

Due to the high concentration of microorganisms and the contamination by antibiotics coming into the water supply, wastewater treatment plants are considered to be a source for storage and distribution of multidrug-resistant microorganisms.

These problems are expected to bring stricter measures and regulations from places such as within the European Wastewater Directive. It is apparent that besides treating wastewater by activated carbon adsorption and sand filtration, further treatment steps must be implemented.

water plantImage 1. A municipal wastewater treatment plant in Hünxe, Germany, was the site of a test trial to remove micropollutants using ultrafiltration membranes. (Courtesy of MICRODYN-NADIR)

Test Trial Created by Multi-Level Partnership

A test trial to remove micropollutants was recently conducted by the Essen, Germany-based water management association Emschergenossenschaft and academic/scientific partner Technical University Dresden. A municipal wastewater treatment plant in Hünxe, Germany, operating with a capacity of 17,000 population equivalents (PE) was chosen for the trial.

This plant is divided into a membrane bioreactor (MBR) plant and a conventional biological treatment plant, each with a capacity of 8,500 PE. Tests were carried out with the effluent of the conventional treatment plant.

The test plant consisted of a filtration tank equipped with a module that contains two ultrafiltration cassettes, each with 25 square meters (m2) of membrane area and the corresponding peripheral equipment such controls, blowers and pumps.

The test process combined adsorption of micropollutants on activated carbon and membrane filtration. The membrane filtration step takes over the job of separating powdered activated carbon, microplastics and multidrug-resistant germs.

This process ensures not only compliance to threshold values for drug residues and other trace substances, but also compliance to expected concentration limits for multidrug-resistant germs and microplastics in effluents of wastewater treatment plants.

The filtration tank of the test stand was fed with the effluent of the sedimentation tank. Activated carbon is then added from a receiver tank and concentrated in the filtration tank. The activated carbon is separated by the membrane module, and the permeate is transferred into the effluent of the sedimentation tank again.

Ultrafiltration Membranes Subjected to Harsh Conditions

The module was operated in cycles comprising a filtration cycle, a backwash cycle and a phase in which the membranes were only aerated. Chemical cleanings by backwashing with chlorine or citric acid solutions were carried out as needed.

The ultrafiltration membranes being subjected to harsh conditions, including fouling, was not a concern. Earlier laboratory tests had proven that the membrane laminate “heals” itself in less than two minutes even if there is the unlikely case of a damaged membrane.

Besides the standard parameters, such as the concentration of solids in the filtration tank, pressure, air consumption, temperature and permeability, the separation efficiency with respect to micropollutants was also measured.

Positive Results Promise Future Implementations

The experiments showed that the combination of activated carbon adsorption and carbon separation by an immersed ultrafiltration module works well and activated carbon is reliably separated with MBR technology.

The increasing concentration of activated carbon had no impact on the membrane performance, and also no change of retention capability could be detected.

Fouling layers that built up during the filtration operation could be removed almost completely by backwashing the module with permeate. The proportion of fouling that could only be removed by chemical cleaning was very small.

No performance decline of the module was detected at all, and the combination of the ultrafiltration module and carbon dosage has an elimination rate for micropollutants that was by 90 percent higher than that of the site’s conventional plant.