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Review
. 2020 Dec 3;21(23):9241.
doi: 10.3390/ijms21239241.

A Review on the Fate of Legacy and Alternative Antimicrobials and Their Metabolites during Wastewater and Sludge Treatment

Timothy Abbott  1 , Gokce Kor-Bicakci  1   2 , Mohammad S Islam  1 , Cigdem Eskicioglu  1
Affiliations
Review

A Review on the Fate of Legacy and Alternative Antimicrobials and Their Metabolites during Wastewater and Sludge Treatment

Timothy Abbott et al. Int J Mol Sci. .

Abstract

Antimicrobial compounds are used in a broad range of personal care, consumer and healthcare products and are frequently encountered in modern life. The use of these compounds is being reexamined as their safety, effectiveness and necessity are increasingly being questioned by regulators and consumers alike. Wastewater often contains significant amounts of these chemicals, much of which ends up being released into the environment as existing wastewater and sludge treatment processes are simply not designed to treat many of these contaminants. Furthermore, many biotic and abiotic processes during wastewater treatment can generate significant quantities of potentially toxic and persistent antimicrobial metabolites and byproducts, many of which may be even more concerning than their parent antimicrobials. This review article explores the occurrence and fate of two of the most common legacy antimicrobials, triclosan and triclocarban, their metabolites/byproducts during wastewater and sludge treatment and their potential impacts on the environment. This article also explores the fate and transformation of emerging alternative antimicrobials and addresses some of the growing concerns regarding these compounds. This is becoming increasingly important as consumers and regulators alike shift away from legacy antimicrobials to alternative chemicals which may have similar environmental and human health concerns.

Keywords: antimicrobial metabolites; antimicrobials; benzalkonium chlorides; emerging alternative antimicrobials; quaternary ammonium compounds; sludge digestion; transformation products; triclocarban; triclosan; wastewater treatment.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Occurrence of different classes of pharmaceuticals and personal care products in primary, secondary and digested sludges (the number of compounds for each class is shown in brackets). Adapted from reference [23].
Figure 2
Figure 2
Flow of antimicrobials and their metabolites from products to the environment.
Figure 3
Figure 3
Number of scientific documents on the topic of “triclosan” published for the time period from 1975 to 2019 (data generated from reference [74]).
Figure 4
Figure 4
Common pathways for the transformation and degradation of triclosan. The primary reactions above are identified as follows: [1] glucuronidation; [2] methylation; [3] sulfation; [4] hydroxylation; [5] meta-cleavage of C-O bond; [6] chlorine substitution; [7] photolysis; [8] reductive dechlorination.
Figure 5
Figure 5
Number of scientific documents on the topic of “triclocarban” published for the time period from 1975 to 2019 (data generated from reference [74]).
Figure 6
Figure 6
Proposed pathways for the potential transformation/degradation of triclocarban by microorganisms. The reactions are denoted as follows: [1–3] reductive dechlorination; [4] hydrolysis; [5] dechlorination; [6] deamination and hydroxylation; [7] meta-cleavage; [8] ortho-cleavage; [9] dechlorination; [10] deamination and hydroxylation; [11] ortho-cleavage (adapted from references [170,173,179,180]).
Figure 7
Figure 7
Number of scientific documents on the topic of “benzalkonium chloride” published for the time period from 1975 to 2019 (data generated from reference [74]).
Figure 8
Figure 8
Benzalkonium chloride (n = 6, 8 and 10) metabolism scheme (adapted from references [285,288,290]).
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