Still open research questions on technologies, microplastics, and antibiotic resistance

Abstract

In this final chapter, a summary of the still open questions to be solved for the scale-up of the technologies described in the previous chapters (including post-treatment options), for application in cold and temperate climate areas is provided. Special attention is dedicated to upflow anaerobic sludge blanket and anaerobic membrane bioreactors (AnMBRs), which are, even if with different peculiarities, the most promising solutions. In addition to anaerobic mainstream treatment technologies, recent research demand focuses also on two important emerging topics, which are of interest not only for anaerobic wastewater treatment, but also for conventional activated sludge plants, that is, the fate of microplastics (MPs) in plants and in receiving water bodies and the antibiotic resistance spreading. For these two topics, which were not analyzed in detail in the previous chapters, we propose in this chapter a general presentation of the state of knowledge in relation to wastewater treatment plants (WWTPs) with particular focus on anaerobic systems and related research needs. Concerning MPs, in spite they are recognized as ubiquitous pollutants, there is a strong research demand on the standardization of sampling and analytical protocols and on powerful technologies able to improve their removal. Even though information about the fate and effect of MPs in WWTPs with anaerobic treatments is scarce, AnMBRs have demonstrated high removal rates, which suggest them as a promising technology. As regard to antibiotic resistance, WWTPs are one of the main sources of dissemination of antibiotic-resistant bacteria (ARBs), antibiotic-resistant genes (ARGs), and antibiotic residues into the environment. Even if the available data are referring mainly to warm climate regions, it is worth noting that anaerobic reactors, alone or in combination with aerobic post-treatment, can remove from 0.5 to 3.0 log units of ARBs and ARGs depending on the resistance bacteria or gene. Also in this case AnMBRs achieved the best performance while good results have been also obtained with combined treatment options (i.e., anaerobic–aerobic and anaerobic post-treatment). Research demand on antibiotic resistance is driven by the need of clarifying the fate of ARBs, ARGs, antibiotics, pathogens in the treatment line, and the effects of wastewater characteristics on the plant performance. Important challenge is the development and evaluation of more effective disinfection and treatment methods (such as ultrafiltration and advanced oxidative processes) able to reduce the antibiotic resistance spreading from WWTPs.