Antibiotics are released into sewage systems via human excretions. Since most of them are recalcitrant to conventional biological treatment in urban wastewater treatment plants (WWTPs), there is an increased possibility of transmission to the receiving environment and crops irrigated with treated wastewater, either in the form of the parent compounds or transformation products. WWTPs are hubs for the proliferation of antibiotic-resistant bacteria (ARB) and transmission of antibiotic resistance genes (ARGs) and other mobile genetic elements. However, their potential uptake by treated wastewater-irrigated crops is still largely unexplored.
Evaluating ARB/ARGs levels in treated wastewater and receiving environments
With the support of the Marie Skłodowska-Curie programme, the ANSWER project investigated the risks associated with urban wastewater reuse in agriculture. This was achieved by a multidisciplinary consortium of experienced academic/non-academic researchers and early-stage researchers (ESRs). Partners investigated the entire wastewater reuse cycle starting from the wastewater treatment, by utilising existing and developing new microbiological protocols and biodetection systems for monitoring ARB/ARGs. Also, they assessed their fate in soil/crops and water resources by advanced chemical analysis and testing of new toxicological endpoints. “Our work led to various interesting findings including the fact that exogenous bacteria thriving in wastewater can reside in soil for a long time,” emphasises project coordinator Despo Fatta-Kassinos of the University of Cyprus. “Specific mobile ARGs are strongly associated with wastewater effluents which can be targeted to track dynamics along the wastewater-soil-crop continua,” she continues. Employing experiment- and modelling-based approaches as well as advanced chemical, microbiological and molecular analytical tools, the ESRs monitored the transmission of antibiotics, ARB and ARGs in controlled and real environmental conditions in Cyprus, Spain and Portugal. To determine their uptake, ESRs quantified these levels in different crops.
Minimisation of ARB and ARGs
Partners evaluated various conventional and advanced wastewater treatment technologies for removing antibiotics, ARB and ARGs, including conventional activated sludge, CAS, ozonation, membrane filtration and photocatalysis. Among other factors, they found the efficacy of CAS depended on operating conditions, microbial activity and selection pressures. Concerning filtration, membranes with a molecular weight cut-off of 5 kDa were efficient in reducing ARGs.
Uptake by crops
Regarding ARG content in crops, ANSWER partners found that manure fertilisation (rather than irrigation water) was responsible. The project's scientific manager Irene Michael-Kordatou notes: “One of the significant achievements of the project was the development of a human stomach model, which can help assess the possible risks of ARB/ARGs transmission by consumption of wastewater-irrigated crops.” This led to the discovery that ARB/ARGs persist for long periods, confirming the hypothesis of potential transmission to humans, a topic that deserves further investigation.
ANSWER significance and future directions
To advance knowledge on the related environmental and health impact, ANSWER developed a database (hosted by the Norman Network) for information exchange on antibiotics, ARB and ARGs. Partners proposed recommendations to further support decision-making in relevant legislation on irrigation use of treated wastewater. A children's educational book, ‘The Secret Handbook of the Blue Circle’, raised awareness of the water cycle and wastewater reuse. With a view to the future, Fatta-Kassinos is confident that “the ANSWER network can contribute substantially to the development of wastewater reuse policies, the aim being the protection of ecological and human health.”
ANSWER, antibiotics, antibiotic-resistant bacteria (ARB), antibiotic-resistance genes (ARGs), wastewater treatment, plants, crops, transmission, treated wastewater