Intensified Disinfection Amid COVID-19 Pandemic Poses Potential Risks to Water Quality and Safety.

The highly infectious coronavirus, COVID-19, caused by the severe acute-respiratory syndrome coronavirus 2 (SARS-CoV-2), has had a major impact on global health with over 42 745 212 cases of infection and more than 1 million deaths globally since late 2019 (https://covid19.who.int/). The rapid transmission of SARS-CoV-2 occurs principally through contact or inhalation of viral droplets and aerosols. Concerns of infectious aerosol formation from wastewaters carrying virus RNA from human feces have also been raised. In response, intensified disinfection procedures have been undertaken in indoor and outdoor settings and during wastewater treatment, using chlorine-based disinfectants. As well as the immediate risks to workers applying the high rates of disinfectant, the high-chlorine residues pose a new and significant challenge to environmental water quality and drinking water safety.

For example, widespread and heavy sanitation of streets, buildings, and even beaches with chlorine-based disinfectants is a common practice to disinfect SARS-CoV-2 contaminated surfaces in many countries. Although the high-pressure spray may reduce viral spread, high concentrations of residual chlorine may remain on solid surfaces. Subsequent water rinsing or natural precipitation then flushes high concentrations of chlorine residues into the environment, including soils, surface water, shallow groundwater, and stormwater drains.

A high dose of chlorine is also routinely applied to disinfect hospital and municipal wastewaters to ensure inactivation of the virus. For example, in China an effective chlorine dose of 50 mg/L and ≥1.5 h contact time are employed for wastewater from COVID-19 designated hospitals to ensure a remaining residual of chlorine of over 6.5 mg/L, a low level of chlorine after initial disinfection for preventing pathogenic regrowth (http://www.gov.cn/zhengce/zhengceku/2020-02/02/content_5473898.htm). Greater chlorine doses are used for <1 h contact time. The minimum chlorine exposure (i.e., Ct value that is the product of disinfectant concentration in mg/L and contact time in minutes) of 585 mg·min/L is similar to that adopted for wastewater reuse in California, which is ensured to achieve 4 log inactivation of enteric viruses (https://nepis.epa.gov/Adobe/PDF/P100FS7K.pdf). However, because SARS-Cov-2 is more susceptible to chlorine than enteric viruses, the infectious risk of chlorinated effluents is assumed to be low.[1] The operation of disinfection facilities has been surveyed at 56 municipal wastewater treatment plants (WWTPs) in China during the pandemic. Some WWTPs using chlorine disinfection purposefully increased chlorine dose from 1.5 to 4–5 mg/L for precaution.[2] Moreover, chlorination was additionally deployed as an extra safeguard at 14 WWTPs originally using UV disinfection only.[2] In the same survey, chlorine residuals in treated wastewater of 24 WWTPs ranged within 0.09–8.5 mg/L, with an average of 1.12 mg/L.[2]

This increased influx of chlorinated disinfectant residues from point and nonpoint sources into natural and wastewaters poses lethal and sublethal risks to aquatic organisms. For example, during February and March 2020, concentrations of residual chlorine in some lakes of China remained mostly undetectable, but increases of up to 0.4 mg/L were also reported,[3] exceeding the level of chlorine (0.019 mg/L) with an expected acute toxicity effect on freshwater organisms.[4] Associated acute toxicity effects on freshwater organisms include damage to cell membranes, proteins and nucleic acids, resulting in impacts to species diversity. Residual disinfectants can also inactivate bacteria involved in the continual transformation of nitrogenous compounds and disrupt the nitrogen cycle in aquatic ecosystems. Moreover, chlorine can also transform environmentally available ammonium to less reactive, but more stable chloramines with the similar toxic implications.

In addition to the presence of chlorine itself at concentrations endangering aquatic life, the unintended formation of harmful disinfection byproducts (DBPs) in water bodies receiving treated wastewaters is a secondary, but pressing issue. Residual chlorine can react with dissolved organic matter forming chlorinated DBPs (e.g., chloroform), many of which display biological toxicity. For example, trihalomethanes (THMs) and haloacetic acids (HAAs) could exert acute genotoxicity to bacteria.[5] Halophenolic DBPs, especially iodinated forms, induce developmental toxicity to polychaete worm embryos and can inhibit the growth of algae.[6] Epidemiological studies have also demonstrated increased risk of bladder cancer, birth defects, and miscarriage from human exposure to DBPs.[5] In addition, the combination of a disinfectant residual further compromises local microbial communities to disable the biochemical degradation of DBPs.[7] Ammonia of raw water at six China drinking water treatment plants (DWTPs) using the Yangtze River as water sources was found to have a 25–67% decrease during the COVID-19 outbreak, in comparison to that of the corresponding period of previous years. This indicates the possible in situ formed chloramines, which can facilitate the production of nitrogenous and iodinated DBPs. However, conventional water treatment processes provide inadequate removal and mitigation of DBPs.[8] This issue is of particular concern for de facto reuse where treated wastewater at upstream communities contributes a major fraction of raw water at downstream DWTPs in a low-flow season. Fortunately, many DWTPs along the Yangtze river equip with advanced treatments units (e.g., granular activated carbon filtration), thus lessening the concerns over DBPs.

To minimize the loading of chlorine-based disinfectants into the aquatic environment, multiple mitigation strategies can be applied. The optimization of conventional treatment processes (i.e., coagulation, flocculation, sedimentation, and rapid sand filtration) early in the treatment procedure is important to reduce the virus load prior to the disinfection step (https://www.epa.gov/sites/production/files/documents/SWTR_Fact_Sheet.pdf). An activated sludge or lagoon WWTP system without disinfection can typically accomplish 1–3 log reduction for pathogen indicators in wastewater.[9] For water or wastewater, Ct values should be properly determined for downstream disinfection. Furthermore, dechlorination in combination with reducing agents (e.g., sulfur dioxide and sulfite compounds) should be undertaken before discharge to attenuate the toxicity and mutagenicity of chlorinated effluents. In addition, the quality of receiving water bodies should be tightly monitored. Chlorine residuals and non- or semivolatile DBPs (e.g., HAAs and haloacetamides), can be used as reliable indicators for assessment of intensified disinfection-related water pollution, while total organic halogen can provide quantification of overall halogenated DBPs.

In summary, the upsurge and overuse of chlorine-based disinfectants during the COVID-19 pandemic pose a threat to ecological and human health by impacting water quality. To mitigate high levels of chlorine and DBPs in wastewater discharges and drinking waters, various mitigation strategies should be employed simultaneously to protect water quality in these unprecedented times. Approaches include strengthened water quality monitoring for the receiving water bodies and multiple barriers for minimization of chlorine loadings to the environment from wastewater release. This calls for strong and global collaborations of industry, academia, and government.