Enhancing sulfide mitigation via the sustainable supply of oxygen from air-nanobubbles in gravity sewers
Graphical abstract
Introduction
Sewer networks are critical urban water infrastructures for collecting and transporting household and industrial sewage to wastewater treatment plants (WWTPs) (Jin et al., 2018; Yan et al., 2020). Generally, some urban sewer networks are anaerobic environments due to being completely sealed (pressure sewers) or poorly ventilated (gravity sewers). These environments induce the formation of anaerobic sewer biofilms/sediments (Li et al., 2019; Sun et al., 2014), which include an abundance of anaerobic microorganisms such as fermentation bacteria (FB), hydrogen-reducing acetogen (HPA), sulfate-reducing bacteria (SRB), and methanogenic archaea (MA), thereby leading to complex transformation processes (Hvitved-Jacobsen et al., 2013; Jin et al., 2018; Li et al., 2019; Shammay et al., 2016; Wu et al., 2020).
Hydrogen sulfide (H2S), is the most ubiquitous harmful gas pollutant emitted from sewer systems (Shammay et al., 2016; Wu et al., 2020; Zuo et al., 2019). Generally, the SO42− is reduced into S2− by SRB under anaerobic conditions, and the S2− is further converted to HS− and H2S. Previous investigations have shown that the overall conversion rates from SO42− to H2S are approximately 50% and 20% in pressure and gravity sewers, respectively (Auguet et al., 2015; Mohanakrishnan et al., 2009; Zuo et al., 2019). The dissolved sulfide in wastewater of sewers contains three species (S2−, HS−, H2S), and their relative concentrations are controlled by the equilibrium (Eqs. (1), (2)) and water pH, but only the H2S species can be emitted into the headspace (Rathnayake et al., 2019). H2S emits into the headspace of sewers may cause severe concrete pipeline corrosion, lead to malodour, and pose health hazards to human beings (Jiang et al., 2015; Liang et al., 2019; Pikaar et al., 2014). It has been reported that sulfide concentrations lower than 0.5 mg S/L induce little corrosion, while sulfide concentrations higher than 2.0 mg S/L trigger severe concrete pipe corrosion (Hvitved-Jacobsen et al., 2002).where pKa,1 and pKa,2 are 7.04 and 12.89, respectively for typical wastewater conditions.
The production and emission of H2S can be controlled by dosing chemicals, including alkali, nitrate, iron salts, free nitrous acid and free ammonia (Cao et al., 2019; Lin et al., 2017; Liu et al., 2015; Lu et al., 2018; Zhang et al., 2008). Alkali addition can suppress the SRB activity and decrease the fraction of dissolved H2S species at high pH values to control the sulfide emission (Rathnayake et al., 2019). Nitrate dosing can also inhibit the conversion of SO42− to S2− by suppressing the SRB activity while reduce the sulfide concentration through anoxic sulfide oxidation (Zhang et al., 2021). Recently, it is reported that ferrate, free nitrous acid and free ammonia showed strong biocidal effects on SRB (Zuo et al., 2020a, Zuo et al., 2020b; Yan et al., 2020), suggesting that dosing of biocides can be an appealing option for sewer sulfide mitigation. Nevertheless, the continuous dosing of these chemicals in sewer systems is not cost-effective for practice, and creates negative effects on the downstream WWTPs (Zhang et al., 2008).
Air or oxygen injection is therefore an economical and environmentally friendly method for controlling sulfide in pressure sewers (Ganigué and Yuan, 2014; Zhang et al., 2008). In practice, a DO concentration exceeding 0.5 mg/L is generally sufficient to inhibit the production of sulfide (Hvitved-Jacobsen et al., 2013). Air or oxygen injection can effectively increase the DO concentration and the oxidation reduction potential (ORP) of sewage, resulting in aerobic upper layer formation in biofilms (Hvitved-Jacobsen et al., 2013). The sulfide generated in the deeper layer of the biofilm and/or in the sediments that might diffuse into the wastewater phase will be oxidized by chemical and biological oxidation (Nielsen et al., 2005; Nielsen and Hvitved-Jacobsen, 2006; Rathnayake et al., 2019). In addition, the sulfate reducing activity of SRB that inhabits the upper layer biofilm will be inhibited, thus decreasing the production of sulfide. Gutierrez et al. (2008) evaluated the effectiveness of oxygen injection on sulfide production in a simulated sewer reactor, and the results showed a 65% reduction in the overall sulfide discharge. However, because the solubility of oxygen in water is relatively low under normal atmospheric conditions, the air or oxygen injection method is not suitable for application in gravity sewers (Zhang et al., 2008). Additionally, oxygen cannot inactivate SRB, and sulfide accumulation resumes instantaneously upon the depletion of oxygen (Gutierrez et al., 2008). In China, gravity sewers are being constructed as the main sewer systems (Gao et al., 2020). Therefore, a new-type air or oxygen injection method that can increase the oxygen solubility and enhance the sulfide controlling efficiency in gravity sewers is needs to be developed.
Recently, nanobubble (NB) technology, as a less chemical consuming and sustainable method for environmental remediation and water treatment, has been gaining the attention of researchers. The most commonly processes applied in water treatment utilizing NBs technology are aeration, floatation, disinfection and ozone advanced oxidation (Atkinson et al., 2019; Temesgen et al., 2017). Compared to coarse bubbles, NBs exhibit extraordinary stability and dispersibility in bulk water because of their ultrafine size and negative surface charge. Thus, NBs containing air can sustainably supply oxygen to the surrounding water to increase the DO concentration (Azevedo et al., 2019; Li et al., 2016). It has been reported that after oxygen and air NB aeration, the DO level stabilizes to average concentrations of 42.0 mg/L and 13.5 mg/L, respectively (Tekile et al., 2016), which is benefit to enhance aeration efficiency and reduce operational cost (Temesgen et al., 2017). Additionally, because of their high surface tension, NBs tend to attach to a liquid-solid interface, and the shortening the oxygen diffusion distance, enhancing oxygen penetration into the deeper layer of the biofilm. Xiao and Xu (2020) reported that NBs aeration can offer a superior oxygen supply capacity and 1.5 times higher oxygen transfer efficiency than traditional aeration, resulting in the changing of microbial community composition and metabolic pathways of biofilms.
Considering the extraordinary oxygen supply capacity and transfer efficiency properties of NBs, it can be inferred that the shortcomings of traditional air or oxygen injection method mentioned above would be overcome if NB technology is used as a new-type air or oxygen injection method, and a better controlling efficiency of sulfide mitigation for gravity sewers might be obtained. Therefore, the objectives of this work are to investigate the effectiveness of air-containing nanobubbles (ANB) injection on sulfide control in gravity sewers to evaluate whether it is feasible for practical application. Four laboratory simulated sewer reactors were set and the sulfide production rates were studied under traditional air and ANB injection conditions. The microbial community characteristics of the biofilm were analyzed by high-throughput sequencing. In addition, the potential impacts of ANBs injection on wastewater characteristics were also evaluated. This study may provide a new-type of air injection method, that is, ANB injection for the management of severe concrete pipe corrosion and malodour issues in gravity sewers with poor ventilation.
Section snippets
Simulated sewer reactor setup and operation
Four simulated laboratory-scale gravity sewer reactors, which are named RC, ER1, ER2, and ER3, were made at a working volume of 6.5 L each with an internal diameter of 160 mm and height of 300 mm (Figs. 1 and S1(a)). RC was set as a control reactor, ER1 was set as an experimental reactor with traditional air injection using an air pump (power 58 W, air flow rate 75 L/min), and ER2 and ER3 were set as the experimental reactors with ANB injection using an NBs generator (NANO-LF1500, power 90 W,
Sulfide productions of the simulated sewer reactors
To evaluate the suppressive effect on the sulfide production of ANB injection, both the concentrations of gaseous H2S in the reactor airspace and the dissolved sulfide in the effluent were measured. Fig. 2(a) and (b) illustrate the concentration variations of gaseous H2S and dissolved sulfide in the sewer reactors as a function of operational time, respectively. It can be seen in period 1 that the sulfide concentration profiles continuously increased during operating days 0 to 30, and
Conclusions
This study quantified and evaluated an environmental method of ANB injection for sulfide mitigation in gravity sewers for the first time. The results showed that the ANB injection method exhibited a significant suppressive effect on sulfide production, and an average inhibition rate of 45.36% was obtained, this rate was 3.75 times higher than that of the traditional air injection method. The ANBs provided more oxygen to biofilms, offering superior oxygen supply capacity and 5.04 times of DO
CRediT authorship contribution statement
Zhiqiang Zhang: Conceptualization, Supervision, Investigation, Writing – review & editing, Project administration. Na Chang: Investigation, Data curation, Visualization. Sheping Wang: Supervision, Writing – review & editing. Jinsuo Lu: Conceptualization, Writing – review & editing, Project administration, Supervision. Kexin Li: Investigation, Data curation. Cailin Zheng: Investigation, Data curation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 52000146, 51778523), the China Postdoctoral Science Foundation (grant no. 2020M673351), and the Key Research and Development Program of Shaanxi Province (grant no. 2019ZDLSF06-04).
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