Sir,

I read with interest the article by Wang et al. (2020) on their technology for tetracycline (TC) removal in wastewater, namely a magnetic graphene-anchored zeolitic imidazolate framework. I value the presented work and do not intend to weaken the very well documented idea of absorption of the TC on the newly developed material. However, their conclusions suggesting the use of the Fe3O4/ZIF-8-G composite in wastewater treatment systems are enfeebled by the lack of cost-efficiency analyses and the context of implementing the Fe3O4/ZIF-8-G at a wastewater treatment plant (WWTP). Otherwise, the presented work does not fully match the scope of the Water Science and Technology journal.

Antimicrobial resistance (AMR) in wastewater treatment plants is an emerging problem and research groups focus on fighting AMR in treatment systems as the WWTPs were labelled a ‘hotspot’ for creation and dissemination of antimicrobial resistance genes (ARGs) in the environment. One should remember that conventional wastewater treatment systems were not designed to eliminate a pool of ARGs classified among the contaminants of emerging concern (CECs). The new challenge is to adapt the system at low cost or redesign it to eliminate the human health risk-related issue. The aim is not to remove all the CECs one by one (Rizzo et al. 2013; Do et al. 2018; Manaia et al. 2018; Pärnänen et al. 2019).

Currently, the cost-efficiency of well-organized sanitation is too high if we are aiming at the removal of all pathogens and the elimination of CECs. Moreover, even the bleeding edge technologies, like the Fe3O4/ZIF-8-G composite presented by Wang et al. (2020) removing antibiotics and their by-products may not be sufficient to eradicate the exhaustive AMR problem.

Regarding wastewater treatment, the latest research indicates that claims of WWTPs being the hotspots for ARGs spread were exaggerated. WWTPs are mainly a channel via which the antibiotic-resistant bacteria (ARB) from human faeces enter the environment while the sewage system is enriching them in ARGs. Karkman et al. (2019) showed that the faecal contamination largely explains the ARGs abundance. What is more, their metagenomic investigation has not found proof for AMR selection or dissemination in the environments under anthropogenic pressure such as the presence of antibiotics.

Munck et al. (2015) concluded based on genomic comparison and database search that the core resistome in Danish WWTP rarely exchanges genes with human pathogens and WWTP's ARGs profiles are distinct from other environments’ resistomes. The group also showed that the WWTP's resistome is stable even with diverse, large volumes of wastewater flowing through the plant and the set of ARGs should be considered the core resistome.

I would suggest to Wang et al. to perform the life-cycle assessment analysis of using their TC removing composite at different stages of treatment. The question arises – which points should be protected from the antibiotic pressure? The core resistome at WWTP should not be affected by the TC, but the receiving water biocenosis can be at risk. It may be more important to remove TC from effluent after treatment than to protect activated sludge microbial consortiums from antibiotics. The Fe3O4/ZIF-8-G composites could be installed at various points in a WWTP having different demands for regeneration and generating different operating costs.

The authors of the article commented on in this letter mentioned that they used material with high porosity like biochar to mitigate the ARGs spread. In my opinion, focusing only on the physisorption or chemisorption process is insufficient. The mezzo- and micropores are creating new niches for microorganisms and enrich the biocenosis. Therefore such materials will fight AMR in two ways and work (1) as a scaffold for organisms and (2) as an absorbent for the CECs. In the case of the latter, the regeneration of the absorbent is crucial. However, if the regeneration process removes the biomass it can also lower the efficiency of TC removal. The biomass attached to the material can create a natural barrier for AMR spread owing to high biodiversity (Cui et al. 2016). The additional studies on microbial communities inhabiting the Fe3O4/ZIF-8-G) composite should be performed.

Considering the climate change and a limited pool of resources we need to adapt to new challenges. Dramatic weather phenomena and global warming have an impact on the pool of ARGs and ARB. Additional bacterial selection towards resistance takes place in urban areas in stormwater systems as a result of contamination with heavy metals and faecal and organic molecules like soil particles absorbing antibiotics and other medicines at organic particles (Eggen & Vogelsang 2015; Almakki et al. 2019).

The non-centralized approach, for example using constructed wetlands (CWs) and separation of black and grey water, could be a better solution due to the savings in the limited water resources, cheaper and effective novel methods for greywater treatment etc. CWs use technology based on natural resources and are easy to install and operate. The important aspect is that they are characterized by efficient removal of several CECs. Studies also showed that they are removing up to 99% of ARB from urban wastewater and can provide effluents suitable for water reuse. However, CWs should be used as a secondary wastewater treatment (Chen et al. 2016; Quintela-Baluja et al. 2019). If we separate the faeces and treat only greywater, the CWs can be very effective. Using technology presented by Wang et al. (2020) in this set-up can be much more interesting. The separation could be done as was proposed for LMICs by using Tiger Toilets with Eisenia fetida worms decomposing human faeces, designed with the help of the Bill & Melinda Gates Foundation. It is claimed to remove 99% of the pathogens, reducing waste weight by 85% in the form of compost material. The toilet produces fertilizer, reusable water and carbon dioxide without connections to water supply or sewer systems (Sinha et al. 2010).

This work was supported by the European Structural and Investment Funds, OP RDE-funded project ‘ChemJets’ (No. CZ.02.2.69/0.0/0.0/16_027/0008351).

All relevant data are included in the paper or its Supplementary Information.

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