Urban stormwater disinfection, quality variability during storage and influence on the freshwater algae: Implications for reuse safety

Abstract

Stormwater reuse is one of the most important ways to mitigate water resource shortage. However, urban stormwater contains many bacteria species, which threaten the reuse safety. Therefore, stormwater disinfection is highly needed. Although disinfection has been widely conducted in the drinking water and reclaimed water, it is rarely carried out for stormwater. This study collected the roof stormwater and undertook chlorination disinfection. Two typical bacteria, Escherichia coli (E. coli) and Staphylococcus aureus (S.aureus) were selected in this study to investigate the disinfection efficiency. It is found that bacteria species present in the stormwater had an important influence on disinfection efficiency while the original stormwater quality did not show an obvious affect. However, when the disinfected stormwater was stored, the stormwater quality was highly variable during its storage process and the variability was affected by bacteria species. The S.aureus containing stormwater showed a high variability of quality and S.aureus significantly regrew. However, the E.coli containing stormwater quality had a relatively low variability and E.coli did not significantly regrew. Additionally, it is noted that after storage, the dissolved form of stormwater was more positive to the freshwater algae's growth while the particulate form (including bacteria and other particulate matters) was less. This implies that a further treatment such as filtration is needed before the stored stormwater is recharged into receiving waters in order to remove particulate forms. These research outcomes can provide useful insight to effective stormwater disinfection and ensure reuse safety.

Introduction

Stormwater reuse is one of the most important ways to mitigate water resource shortage (Goonetilleke et al., 2017; Shen et al., 2020). However, stormwater contains many pollutants which threaten the reuse safety (Liu et al., 2015). Among these pollutants, bacteria/pathogen belong to microbial pollution. A number of microbial types have been found in the stormwater (Ahmed et al., 2019). For example, Sidhu et al. (2012) investigated urban stormwater in Brisbane, Australia and found that the number of Escherichia coli (E.coli) and Enterococcus spp. were up to 34000 CFU/L and 30000 CFU/L, respectively. Additionally, they also detected many other bacteria/pathogen types in the urban stormwater such as Campylobacter jejuni, Campylobacter coli and Salmonella enterica. Simmons et al. (2008) reported a disease outbreak in New Zealand. This disease was caused by Legionella pneumophila present in the roof stormwater which was reused in the residential purposes. These facts mean that bacteria/pathogen are widely present in the stormwater. If they are not properly treated, reusing stormwater would be highly risky. Therefore, before stormwater is reused, the disinfection has to be undertaken (Feng et al., 2018).

Although disinfection has been widely conducted in the drinking water (Du et al., 2021) and even reclaimed water (Du et al., 2017), it is rarely carried out for stormwater. Due to difference on sources and pathways, stormwater might have a very different quality from other conventional water resources. Furthermore, stormwater quality from different sources might significantly vary (Yuan et al., 2019). For example, urban road stormwater generally contains high concentrations of heavy metals and petroleum hydrocarbon due to frequent traffic activities (Hong et al., 2018; Liu et al., 2018; Liu et al., 2019). Roof stormwater is commonly less polluted since they are primarily influenced by atmospheric deposition (Miguntanna et al., 2013; Wang et al., 2010). These could lead to differences on disinfection performance. It is very likely that the disinfection efficiency is highly variable among different water resources, even though the same disinfection approaches are used (Hu et al., 2016). Other than water quality, microorganism types could also have an important influence on disinfection results. For instance, Zou and Tang (2019) developed a ultrasound approach to disinfect water and found that this approach showed a better performance for E.coli inactivation than Staphylococcus aureus (S.aureus) and Bacillus subtilis (B.subtilis). A 2-log inactivation ratio was achieved for E.coli when 17 and 33 kHz of ultrasound frequency were used while the less than 1.5 log inactivation ratio was observed for S.aureus and B.subtilis. They attributed these results to the different cell structures of these bacteria. E.coli’s cell wall is generally thinner than S.aureus and B.subtilis and hence E.coli was easier to be inactivated.

After treatment including disinfection, storage is one of key processes because stormwater might be not able to be reused immediately (Zhang et al., 2018). Previous researchers have noted that stormwater quality changed during storage processes. Zhan et al. (2020) found that dissolved oxygen (DO) concentrations decreased from 6.38 mg/L to 4.36 mg/L due to microorganism growth and organic matter degradation. They also noted that bacteria such as E.coli were the primary contributor of stormwater toxicity to Chinese hamster ovary cells during the whole storage. This study also implies that bacteria can rapidly grow during the storage and hence adversely affect stormwater quality and reuse safety. This raises a question: can bacteria re-grow during stormwater storage when they have been inactivated by disinfection prior to storage? The answer to this question can help on understanding the stormwater storage process and hence ensuring stormwater reuse safety.

Furthermore, after storage, the stormwater can be reused. However, the stormwater quality could vary and bacteria could grow/re-grow during storage. In this context, it is important to know that whether the stormwater going through storage is still safe to be reused or whether the stormwater after storage has human/ecological health risks when they are reused? This is also a key question which needs to be well answered in order to ensure stormwater reuse safety.

This paper focused on stormwater disinfection using a common approach, chlorination. Two typical bacteria were selected in this study, namely E. coli and S.aureus. This study had three objectives. The first objective was to investigate whether stormwater quality is able to significantly influence disinfection efficiency. The second objective was to analyze whether bacteria can re-grow during stormwater storage even though they have been inactivated prior to storage. The third objective was to check whether the stormwater after storage is still safe to be reused due to quality variability and bacteria growth during storage. The innovation of this study was to investigate how the efficiency is if the common disinfection approach (chlorination in this study) is used for stormwater, which has different water quality from conventional water resources such as drinking water. The research outcomes can provide useful insight to effective stormwater disinfection and ensure reuse safety.