Wastewater Treatment Using Staged Moving Bed Biofilm Reactor (MBBR) - Biological Transformation Products and Toxicity

Gordon Tze Hoong Ooi

Research output: Book/ReportPh.D. thesisResearch

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Abstract

The detection of pharmaceuticals and personal care products (PPCPs) in wastewater has caught the awareness of many societies, due to their impacts on the environment. The presence of these compounds in wastewater ranging from nanograms per liter to milligrams per liter is described as micropollutants. Since micropollutants are only removed partially by conventional wastewater treatment plants (WWTPs) as these treatment systems are not designed to remove pharmaceuticals, they can be detected in receiving water. These micropollutants, even at a very low concentration, were reported to pose acute and/or chronic toxic effects on aquatic organisms. Therefore, removal of these compounds is crucial in shaping a sustainable environment.
Biofilm-based processes, such as Moving Bed Biofilm Reactor (MBBR), have shown to be effective in removing a number of micropollutants, despite its relatively small footprint. MBBR consists of biofilm grown on plastic carriers, which are suspended in a reactor via mechanical mixing or aeration. This offers a compact and resilient technology to treat wastewater. Hence, technical and operational enhancements to boost micropollutant removal in this treatment are extremely attractive. Other than micropollutant removal, investigations on their fate and associated toxicity are also of importance.
In previous studies, the highest degradation rate constant (k) for the removal of 22 pharmaceuticals from diverse categories, was primarily attributed to the first reactor in a staged MBBR. However, when these degradation rate constants (k) were normalized with the respective biofilm in each reactor (kbio), the last reactor in a staged MBBR presented the most competent biofilm. In other words, biofilm in the last reactor exhibited the potential to degrade persistent pharmaceuticals, i.e. diclofenac, and a higher ratio of this biomass was involved in the degradation.
Despite the fact that the last reactor presented the most competent biofilm, biomass abundance in this reactor was too low to perform sufficient pharmaceutical removal. Therefore, intermittent feeding was designed to boost the biomass abundance while retaining the competency. This novel concept was implemented to a laboratory-scale MBBR operated on effluent wastewater intermittently fed with settled raw wastewater. Results showed a relatively higher kbio, especially for diclofenac, atenolol and metoprolol, when comparing with other studies on activated sludge and suspended biofilm. In fact, diclofenac was removed completely in the batch experiment with half-life of 2.1 hours.
Hospital wastewater is one of the main sources where high concentrations of pharmaceuticals are presence. Hence, an on-site treatment of hospital wastewater has been considered, in order to decrease pharmaceutical input into municipal wastewater treatment plants (WWTPs). A pilot-scale MBBR consisting of six reactors in series was built with integration of intermittent feeding of the latter reactors aimed for a more efficient micropollutant removal. Based on the results, the nitrifying MBBR achieved higher removal compared with denitrifying MBBRs, except for azithromycin, clarithromycin, diatrizoic acid, propranolol and trimethoprim. In the batch experiment, nitrifying MBBRs have the ability to remove most of the analyzed pharmaceuticals except for carbamazepine, diclofenac and iopamidol, with degradation rate constants ranging from 5.0 × 10-3 h-1 to 2.6 h-1. In general, the highest degradation rate constants (k) are from the nitrifying MBBRs. However, when the degradation rate constants were normalised to the respective biomass (kbio), the intermittently fed reactors presented the highest specific activity. Out of the 22 compounds studied, 17 compounds were removed with more than 20% through this pilot plant. Moreover, based on a model of the MBBR system, 8 of these compounds were estimated to be almost completely removed after MBBR treatment. This includes 3 of the 4 iodinated X-ray contrast media, which are used extensively in hospitals.
During biological removal of pharmaceuticals, total mineralization into carbon dioxide and water, is not often achieved. In many circumstances, formation of transformation products that are possibly persistent can occur. In this research study, the antibiotics, clindamycin and fusidic acid, were tested to investigate the pathways following MBBR treatment. Clindamycin was removed via biological treatment, forming transformation products, clindamycin sulfoxide and N-desmethyl clindamycin as well as 3 other mono-oxygenated products. Fusidic acid, on the other hand, formed transformation products that were removed over time, achieving no visible chromatographic peak after 72.5 hours of treatment. Furthermore, bioassay showed no significant antibacterial activities after fusidic acid has been degraded. A reduced Microtox® inhibition was also observed following fusidic acid biodegradation. In contrast, results showed that freshwater crustacean, Daphnia magna, is not affected by fusidic acid, even at concentrations as high as 250 mg L-1.
In conclusion, MBBR could apply as a substitute for conventional activated sludge (CAS) to treat hospital/municipal wastewater, and as a polishing step after WWTP to lower pharmaceutical concentrations before discharge. The application to treat industrial wastewater was also proved feasible.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages67
Publication statusPublished - 2018

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