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ScienceDirect Corrosion and odor management in sewer systems Guangming Jiang1, Jing Sun1,2, Keshab R Sharma1,2 and Zhiguo Yuan1,2 Sewers emit hydrogen sulfide and various volatile organic sulfur and carbon compounds, which require control and mitigation. In the last 5–10 years, extensive research was conducted to optimize existing sulfide abatement technologies based on newly developed in-depth understanding of the in-sewer processes. Recent advances have also led to lowcost novel solutions targeting sewer biofilms. Online control has been demonstrated to greatly reduce the chemical usage. Dynamic models for both the water, air and solid (concrete) phases have been developed and used for the planning and maintenance of sewer systems. Existing technologies primarily focused on ‘hotspots’ in sewers. Future research should aim to achieve network-wide corrosion and emission control and management of sewers as an integrated component of an urban water system. Addresses 1 Advanced Water Management Centre, The University of Queensland, St. Lucia, Queensland 4072, Australia 2 CRC for Water Sensitive Cities, CRC for Water Sensitive Cities, PO Box 8000, Clayton, Victoria 3800, Australia Corresponding author: Yuan, Zhiguo ([email protected]
Current Opinion in Biotechnology 2015, 33:192–197 This review comes from a themed issue on Environmental biotechnology Edited by Spiros N Agathos and Nico Boon
to atmosphere through manholes or pumping stations cause odor nuisance to the nearby residents. In addition, hydrogen sulfide is toxic to human and animals. In this paper, we review the significant progress made to the understanding and mitigation of sewer corrosion and odor, since the publication of a previous comprehensive review in 2008 .
Improved understanding of in-sewer processes The formation of hydrogen sulfide and volatile organic compounds (VOCs) in sewers are well understood. A recent study revealed that the competition of sulfatereducing bacteria (SRB) and methanogenic archaea (MA) resulted in a stratified structure of anaerobic sewer biofilms, with SRB and MA inhabiting in the outer layer (0–300 mm) and inner layer (>200 mm), respectively . This suggests that the interaction between different microorganisms need to be considered when evaluating sulfide production by sewer biofilms. A very recent study showed that MA contribute to the degradation of methanothiol (MT), a dominant volatile organic sulfur compound (VOSC) in sewers , thus reducing accumulation of MT in wastewater. This sheds some light on the largely unknown processes involved in the in-sewer transformation of odorous compounds other than hydrogen sulfide. The understanding of the production and consumption of odorous compounds other than H2S in sewers represent a significant, critical knowledge gap at present.
http://dx.doi.org/10.1016/j.copbio.2015.03.007 0958-1669/# 2015 Elsevier Ltd. All rights reserved.
Introduction Sewer networks are one of the most critical infrastructure assets for modern urban societies. Odor and corrosion problems associated with sewer systems are primarily due to hydrogen sulfide and other organic odorous compounds generated in sewage. Sulfide-induced concrete corrosion causes loss of concrete mass, cracking of the sewer pipes, and ultimately structural collapse. Sewer systems suffering from corrosion often require premature replacement or rehabilitation of damaged pipes, manholes, and pump stations, which involves very high costs at a magnitude of many billion dollars every year. This cost is expected to increase as the aging infrastructure continues to fail. Also, hydrogen sulfide and other odorous compounds released Current Opinion in Biotechnology 2015, 33:192–197
Concrete sewer corrosion induced by H2S is primarily a microbiological process. Most research on concrete corrosion in sewers used pure culture of sulfide-oxidizing bacteria (SOB, e.g. Acidithiobacillus thiooxidans) under simulated sewer environment . This approach neglects the potential interactions between different members of the corrosion-inducing microbial community, and hence could give biased results. Indeed, several recent studies using molecular biological tools revealed a complex microbial community in sewer corrosion biofilms, comprising, among other organisms, SOB, sulfur-oxidizing heterotrophs and fungi [5,6,7,8]. The precise roles of most of these organisms are yet to be fully elucidated. Their interactions between these community members are currently not understood. Several recent sewer corrosion studies have focused on the effects of environmental factors, sewer characteristics, and operational procedures on the corrosion processes and concrete mass loss rate. The rate of pH decrease and the www.sciencedirect.com
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levels of sulfide oxidation on concrete surface increased with temperature, humidity and H2S concentrations at the early stage of sewer corrosion . The active corrosion rate was affected significantly by H2S concentrations but was not affected by temperate between 17 and 30 8C, and by relative humidity only for sewer surfaces in the gasphase, not for the sewer pipe surfaces partially submerged in sewage . It was also found that sulfide oxidation was nearly two orders of magnitude faster on a concrete pipe surface than on a plastic pipe surface . Although the mechanism for acid-induced concrete corrosion was well established , a recent report identified a crucial role of iron in the damage of micro-structure in concrete . The cycle of iron dissolution in the highly acidic corrosion layer and the precipitation at the corrosion front was suggested to be the cause of cracking. This contradicts the traditional thought that the expansive ettringite and gypsum were responsible. Further research on the relationship between chemical reactions and the physical structural changes is needed. An emerging area of importance related to in-sewer processes is the degradation and transformation of illicit drugs, pharmaceuticals and other health-related compounds . Such knowledge can improve the accuracy and reliability of sewage-based epidemiology, which can be used to objectively estimate the health and behavioral status of communities [15,16]. Further investigations such as that by Jelic et al.  in real sewer networks are required to establish a reliable back-estimation model.
In-depth understanding and optimization of chemical dosing to control sulfide in sewers In the last five years, many of the commonly used chemicals in sewers have been evaluated and optimized systematically while new technologies have also been developed [18,19]. Oxygen, which is an important chemical for sulfide control, does not have a long-lasting inhibitory effects on sulfide production by SRB , with SRB activity resuming immediately after oxygen depletion. Further, oxygen injection stimulates SRB growth and activity in downstream pipe sections due to increased availability of sulfate. Nitrate behaves similarly [21–23]. Therefore, in practical applications, oxygen or nitrate should be added shortly before the point of sulfide control with an HRT of, for example, 1–2 h. For iron salts, a recent study showed that, in addition to precipitating sulfide in sewage, Fe3+ significantly inhibits sulfide and methane production by sewer biofilms [24–26], leading to reduced demand for iron salts. Therefore, iron salts should be added at upstream locations of a sewer network. It was also shown that iron-rich drinking water treatment sludge is a feasible source of iron for sulfide removal in sewers . www.sciencedirect.com
Mg(OH)2 is commonly used to elevate sewage pH to 8.5–9.0 to reduce the transfer of hydrogen sulfide from liquid to air. It was found recently pH in this range reduces SRB activity in sewer biofilm by 30–50%, while completely suppressing methane production . Hence, Mg(OH)2 should be added at upstream locations in practical applications. Intermittent dosing of NaOH is often used to raise sewage to above 11.0, thereby inactivating sewer biofilms. A recent study showed that this method does not completely inactivate sewer biofilm, and therefore weekly or more frequent dosage, each lasting for a few hours, is needed . It is anticipated that on-line dosing control, which takes the dynamics of sewage flow and sulfide production into consideration, will replace the currently used constant or flow-paced dosing regimes with significant cost savings. The control system should consider the temporal and special variations of sewage flow, wastewater characteristics and sulfide production. Considering the fact that sewer networks are distributed systems with a long transport time, feed-forward control algorithms will probably play a dominating role in determining the dosing rate, with feedback providing further correction. Three field studies have recently been carried out by the authors’ group (results to be published), which achieved 15–45% savings in chemical consumption, while ensuring better sulfide control performance. A hybrid automata control strategy was proposed to coordinate the operation of sewage pumping stations in a network along with chemical dosing control to ensure consistent effectiveness in a network . Most of the conventional chemical dosing strategies require continuous chemical addition in order to achieve effective sulfide control at all times, incurring high chemical consumption and operational costs. Jiang et al. revealed that free nitrous acid (FNA, i.e. protonated nitrite) is a strong biocide for sewer biofilms [31,32]. The findings were successfully translated into a practical technology that involves intermittent (8–24 h every 1–2 weeks) dosing of nitrite and acid. A 6-month field trial showed excellent sulfide control performance with substantially reduced chemical consumption costs [33,34]. An alternative approach to chemical dosing is the electrochemical oxidation of sulfide to remove hydrogen sulfide from wastewater [35,36]. The same method was also demonstrated to produce caustic (NaOH), which can be dosed to sewer pipes to inactivate sewer biofilm [37,38]. This method avoids the need for the transport, handling and storage of concentrated caustic solutions.
Sewer gas treatment for odor abatement Gas phase odor treatment technologies currently applied in sewer systems primarily include chemical scrubbing, activated carbon adsorption, biofiltration and biotrickling Current Opinion in Biotechnology 2015, 33:192–197
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The traditional H2S-based method to assess the performance of sewer odor treatment units was found to be not always effective, as some non-H2S odorants such as VOCs and VOSCs also contribute to sewer odor, but may be less efficiently removed by the treatment units . However, methods for the monitoring of non-H2S odorous compounds are still under development [43–45]. The empty bed retention time (EBRT) and mass transfer in biotrickling filtration systems have been found to have significant effects on MT and hydrophobic VOCs removal [46,47]. Also, high load of H2S was found to negatively affect the VOSCs removal in these systems [47,48]. Photocatalytic systems, non-thermal plasmas and membrane bioreactors are being investigated in laboratory studies as emerging technologies for the removal of both H2S and non-H2S odorants [49–51], with however full-scale applications to be carried out.
Control and mitigation of microbially induced concrete corrosion The development and application of corrosion-proof concrete for the construction of new sewers can proactively prevent the occurrence of corrosion. This approach includes applications of admixtures, protective coatings, protective biofilms, and acid-resistant cement to prevent chemical attack by sulfuric acid [52,53]. Antimicrobial coatings such as silver/copper zeolites have shown some effects in reducing the microbial growth and activity of a pure culture of SOB . Another study using chemical exposure test found that epoxy coating and polyuria lining resulted in corrosion protection, while the addition of hydrous silicates or antimicrobial compounds only inhibited SOB without improving performance towards degradation . It needs to be highlighted that both accelerated simulation tests and incubation tests may not be an appropriate procedure for the evaluation of antimicrobial concrete formulations or coating materials. Further long-term in situ tests under real sewer conditions are required for the evaluation of any materials.
Modelling and monitoring of sewer processes and concrete corrosion Dynamic models have been developed to predict hydrogen sulfide generation in sewers with dynamic sewage flow , which was recently enhanced to predict pH variations in sewers . The effects of pH on sulfide generation by sewer biofilms were adequately modelled as inhibition of free ammonia at pH > 6.75 and pH itself Current Opinion in Biotechnology 2015, 33:192–197
Dissolved H2S (mg S/L)
filtration . Activated carbon adsorption and chemical scrubbing present higher environmental impacts and higher operation cost than their biological counterparts (biofiltration and biotrickling filtration) . Therefore, physical/chemical treatment systems are being gradually replaced by biological treatment systems [39,41].
Without chemical dosing
Mg(OH)2 Dosing FeCl3 Dosing
Time (Hours) Current Opinion in Biotechnology
A model-based comparison of dissolved H2S levels with the dosing of different chemicals in a sewer system in Gold Coast, Australia. In this particular case, FeCl3 is revealed to be the most effective chemical.
at pH < 1.30 . In recent years, sewer modelling has been employed as a tool to investigate the impacts of chemical dosing on sulfide production and to develop appropriate strategies for sulfide control [58,59]. An example of such an application is presented in Figure 1. Dynamic ventilation models have been developed and evaluated to predict airflow in gravity sewers under natural or forced ventilation conditions [60,61]. These models, though based upon fundamentals, are empirical in nature as a number of factors affecting the airflow are lumped together as the coefficients in the model. Moreover, the model requires solving complex partial differential equations for the prediction of the dynamics of airflow . Integration of such a model with a sewer model therefore poses significant challenges. Further work needs to be done towards developing a simplified approach for modelling the airflow, yet taking into account all the influencing factors. Also, the effects of airflow velocity on the hydrogen sulfide adsorption and oxidation kinetics by corroding concrete surface has been modelled empirically with a power function of Reynolds number . The uptake and oxidation of hydrogen sulfide in corroding sewers were investigated and used to predict the sewer corrosion rate . Another study proposed a bilinear corrosion loss model based upon observed corrosion rates in field sewers . The sewer service life was also analyzed using safety and risk assessment to determine the probability of failure to assist sewer maintenance and planning . Conventionally, the degradation of concrete can be theoretically modelled considering the chemical and physical reactions involved in the corrosion process . However, there is still a lack of reliable www.sciencedirect.com
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model designed specifically for sewer conditions to predict service life of sewers, which would be a crucial tool to prioritize resources for maintenance and rehabilitation.
impact of global warming and extreme weather on sewer corrosion and odor management is also required.
Acknowledgements The monitoring of H2S levels in sewers was widely achieved using Odalog H2S sensors. Recent development of the UV–vis spectroscopy technology allows the simultaneous monitoring of dissolved sulfide and nitrate [67,68]. Another sensor has been developed more recently, which is able to measure dissolved sulfide and dissolved methane simultaneously with the use of an automated stripping chamber and gas-phase hydrogen sulfide and methane sensors . However, no online monitoring of odor levels can be achieved at present due to the various compounds involved.
We acknowledge the support by the Australian Research Council, DC Water, Gold Coast City Council, Melbourne Water Corporation, South East Water P/L and Western Australia Water Corporation through Project LP110201095, and by the CRC for Water Sensitive Cities through Project C3.1. Dr Guangming Jiang is the recipient of a Queensland State Government’s Early Career Accelerate Fellowship.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
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Conclusions For H2S control at corrosion and odor ‘hot spots’, both liquid-phase and gas-phase technologies are suitable and effective, if implemented appropriately by carefully choosing dosing locations and using on-line control. However, it is cost-prohibitive to achieve network-wide corrosion control using these technologies. The nextgeneration technology should achieve network-wide corrosion and odor control with the assistance of advanced sewer model. Corrosion and odor control in sewers are experiencing significant new challenges. Reduced sewage flow due to restricted water use and decentralized water reuse/recycling results in not only more concentrated sewage but also increased hydraulic retention time of sewage in sewers; both are expected to substantially enhance H2S and VOC production. Significant research is needed to investigate the integrated management of sewers as part of an urban water system. New sewers are built at a high rate accompanying the population growth and urbanization. Water industry is seeking for new materials to construct corrosion resistant sewers with a long service life. An understanding of the www.sciencedirect.com
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