Einfeldt, Jörn

Micropollutants are present in low concentration in water bodies. Due to an inadequate elimination in municipal, mechanical-biological wastewater treatment plants, they represent a main path of entry. In addition to the establishment of micropollutant elimination, e.g. as a fourth treatment step, increasing the energy efficiency of wastewater treatment plants is a current issue in urban water management. The alternative wastewater treatment plant concept MicroStop addresses both issues. In addition to an all-encompassing reduction of particles, nutrients, and multiresistant germs, micropollutants are eliminated by a combination of biological degradation in fixed-bed reactors with physical retention by nanofiltration. The combination process is preceded by enhanced carbon extraction in the pre-treatment stage by coagulation/flocculation. To assess the feasibility of the concept, tests were carried out for enhanced carbon extraction and the biological degradation of micropollutants under nitrification and denitrification in the fixed-bed reactors. Using the coagulation/flocculation chemical Sachtofloc 46.12 at a dosage of 0.2 mmol Al/L wastewater, a further reduction in total organic carbon (TOCN) in the primary clarification effluent of 49 % (wastewater treatment plant A) and 30 % (wastewater treatment plant B) was achieved in 1 Liter jar tests. The reduction was limited by the proportion of dissolved organic carbon (DOCN) in the wastewater, which was 52 % (wastewater treatment plant A) and 68 % (wastewater treatment plant B). A combination of the MicroStop concept with other processes for enhanced carbon extraction, such as POWERSTEP, is possible based on the overall results. In the biological fixed-bed reactors, nitrification with > 99 % reduction and denitrification with 94 – 99 % could be established. In one test phase, nitrite accumulation occurred, which was prevented by adjusting the acetic acid dosage. Three test phases (purely aerobic operation and nitrification/denitrification without/with flocculation supernatant) demonstrated further potential for the biodegradation of some of the pollutants tested. For gabapentin (> 93 to > 98 %), iomeprol (> 69 to > 96 %) and metoprolol (> 62 to > 76 %), very good elimination rates below the limit of quantification were achieved in all test phases under aerobic conditions (with/without nitrification). Diclofenac was reduced with 47 to 67 %, better than known from literature, but never below a threshold concentration of 1.8 – 2.6 µg/L. A longer retention time showed a positive influence. Sulfamethoxazole showed fluctuating results with degrees of elimination between – 42 and 87 %. A reduction only occurred under denitrification and became more stable under increased acetic acid dosing during a longer retention time. Under aerobic conditions, there was an increase in pollutants. A retransformation of the main metabolite N4-acetylsulfamethoxazole is assumed to be the cause. Carbamazepine and its metabolite carbamazepine 10,11-epoxide showed persistent behaviour. The sweetener cyclamate was only detected in one test phase and was very well eliminated with 97 %.

Current wastewater treatment is not designed to eliminate micropollutants (MP) such as pharmaceuticals, pesticides, and microplastics, which is why they enter surface waters via municipal wastewater. A combination of a fixed-bed reactor (FBR) and nanofiltration (NF) was implemented to eliminate MP. The NF acts as a barrier to MP, while biodegradation of the pollutants is induced in the FBR. The experiments show that NF can separate on average more than 93% of MP. Except for two problematic substances, the average biodegradation rate in the FBR was 78%. With a combination plant of NF and FBR, the spectral absorption coefficient SAK270 could be eliminated to 89 %, and this parameter is considered to be of great significance with regard to the amount of MP and their metabolites.

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