Second life of water and wastewater in the context of circular economy - Do the membrane bioreactor technology and storage reservoirs make the recycled water safe for further use?

In recent years water demand drastically increased which is particularly evident in tourism-burdened mountain regions. In these areas, climate neutral circular economy strategies to minimize human impact on the environment can be successfully applied. Among these strategies, treated wastewater reuse and retaining water in storage reservoirs deserve particular attention. This study aimed to determine if recycled water produced with two circular economy systems, namely membrane bioreactor treatment plant (MBR) with UV-light effluent disinfection and a storage reservoir, is safe enough for further use in green areas irrigation in summer and artificial snow production in winter. The assessment was based on the presence and concentration of antimicrobial agents, antibiotic resistant bacteria, antibiotic resistance genes, bacterial community composition and diversity. The treated water and wastewater was compared with natural water in their vicinity. Both systems fulfill the criteria set by the European Union in terms of reclaimed water suitable for reuse. Although the MBR/UV light wastewater treatment substantially reduced the numbers of E. coli and E. faecalis (from e.g. 32,000 CFU/100 ml to 20 CFU/100 ml and 15,000 CFU/100 ml to nearly 0 CFU/ml), bacteria resistant to ampicillin, aztreonam, cefepime, ceftazidime, ertapenem and tigecycline, as well as ESBL-positive and multidrug resistant E. coli were highly prevalent in MBR-treated wastewater (88.9 %, 55.6 %, 33.3 %, 22.2 % and 11.1 % and 44.4 and 55.6 %, respectively). Applying additional tertiary treatment technology is recommended. Retaining water in storage reservoirs nearly eliminated bacterial contaminants (e.g. E. coli dropped from 350 CFU/100 ml to 10 CFU/100 ml), antibiotic resistant bacteria, resistance genes (none detected in the storage reservoir) and antibiotics (only enrofloxacin detected once in the concentration of 3.20 ng/l). Findings of this study point to the limitations of solely culture-based assessment of reclaimed water and wastewater while they may prove useful in risk management and prevention in wastewater reuse.