Modeling of hydrogen sulfide oxidation in concrete corrosion products from sewer pipes

Rebar corrosion and its interaction with concrete degradation in reinforced concrete sewers

Concrete corrosion, as a major issue in sewer management, has attracted considerable research. In comparison, the corrosion of reinforcing steel bar (rebar) is not well understood. Particularly, fundamental knowledge of rebar corrosion and its interactions with concrete corrosion/cracking is largely lacking. This study investigated rebar corrosion and concrete degradation using reinforced concrete coupons exposed in a pilot sewer system. The physical-chemical corrosion characteristics were investigated in local regions; the nature of rebar rusts was analyzed using the advanced mineral analytical techniques, including Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD); further, the interactions between rebar corrosion and concrete corrosion/cracking were elucidated by characterizing the microstructure and element distribution in interfacial areas using Mineral Liberation Analysis (MLA). The rebar corrosion products were found to be iron oxides, oxyhydroxides, chlorides, sulfides and sulfates. The predominant rebar corrosion reactions varied with exposure time and the development of concrete corrosion. When concrete corrosion reached rebar surface, the cracking of the concrete cover was influenced by multiple effects, including the macro-cracking induced by the corrosion products expansion, and the micro-cracking accelerated by the dissolution, diffusion and deposition of Fe derived from rebar rusts at the concrete corrosion front. A conceptual model elucidating rebar corrosion and the complex interactions between rebar corrosion and concrete degradation is proposed to support the development of corrosion prevention and refurbishment strategies for reinforced concrete sewers.

The rapid chemically induced corrosion of concrete sewers at high H2S concentration

Concrete corrosion in sewers is primarily caused by H₂S in sewer atmosphere. H₂S concentration can vary from several ppm to hundreds of ppm in real sewers. Our understanding of sewer corrosion has increased dramatically in recent years, however, there is limited knowledge of the concrete corrosion at high H₂S levels. This study examined the corrosion development in sewers with high H₂S concentrations. Fresh concrete coupons, manufactured according to sewer pipe standards, were exposed to corrosive conditions in a pilot-scale gravity sewer system with gaseous H₂S at 1100 ± 100 ppm. The corrosion process was continuously monitored by measuring the surface pH, corrosion product composition, corrosion loss and the microbial community. The surface pH of concrete was reduced from 10.5 ± 0.3 to 3.1 ± 0.5 within 20 days and this coincided with a rapid corrosion rate of 3.5 ± 0.3 mm year ^-1. Microbial community analysis based on 16S rRNA gene sequencing indicated the absence of sulfide-oxidizing microorganisms in the corrosion layer. The chemical analysis of corrosion products supported the reaction of cement with sulfuric acid formed by the chemical oxidation of H₂S. The rapid corrosion of concrete in the gravity pipe was confirmed to be caused by the chemical oxidation of hydrogen sulfide at high concentrations. This is in contrast to the conventional knowledge that is focused on microbially induced corrosion. This first-ever systematic investigation shows that chemically induced oxidation of H₂S leads to the rapid corrosion of new concrete sewers within a few weeks. These findings contribute novel understanding of in-sewer corrosion processes and hold profound implications for sewer operation and corrosion management.

Influence of Dissolved-Aluminum Concentration on Sulfur-Oxidizing Bacterial Activity in the Biodeterioration of Concrete

Several studies undertaken on the biodeterioration of concrete sewer infrastructures have highlighted the better durability of aluminate-based materials. The bacteriostatic effect of aluminum has been suggested to explain the increase in durability of these materials. However, no clear demonstration of the negative effect of aluminum on cell growth has been yet provided in the literature. In the present study, we sought to investigate the inhibitory potential of dissolved aluminum on nonsterile microbial cultures containing sulfur-oxidizing microorganisms. Both kinetic (maximum specific growth rate) and stoichiometric (oxygen consumption yield) parameters describing cells activity were accurately determined by using respirometry measurements coupled with modeled data obtained from fed-batch cultures run for several days at pH below 4 and with increasing total aluminum (Al_tot) concentrations from 0 to 100 mM. Short-term inhibition was observed for cells poorly acclimated to high salinity. However, inhibition was significantly attenuated for cells grown on mortar substrate. Moreover, after a rapid adaptation, and for an Al_tot concentration up to 100 mM, both kinetic and stoichiometric growth parameters remained similar to those obtained in control culture conditions where no aluminum was added. This argued in favor of the impact of ionic strength change on the growth of sulfur-oxidizing microorganism rather than an inhibitory effect of dissolved aluminum. Other assumptions must therefore be put forward in order to explain the better durability of cement containing aluminate-based materials in sewer networks. Among these assumptions, the influence of physical or chemical properties of the material (phase reactivity, porosity, etc.) might be proposed.IMPORTANCE Biodeterioration of cement infrastructures represents 5 to 20% of observed deteriorations within the sewer network. Such biodeterioration events are mainly due to microbial sulfur-oxidizing activity which produces sulfuric acid able to dissolve cementitious material. Calcium aluminate cement materials are more resistant to biodeterioration compared to the commonly used Portland cement. Several theories have been suggested to describe this resistance, and the bacteriostatic effect of aluminum seems to be the most plausible explanation. However, results reported by the several studies on this exact topic are highly controversial. This present study provides a comprehensive analysis of the influence of dissolved aluminum on growth parameters of long-term cultures of sulfur-oxidizing bacterial consortia sampled from different origins. Kinetic and stoichiometric parameters estimated by respirometry measurements and modeling showed that total dissolved-aluminum concentrations up to 100 mM were not inhibitory, but it is more likely that a sudden increase in the ionic strength affects cell growth. Therefore, it appears that the bacteriostatic effect of aluminum on microbial growth cannot explain the better durability of aluminate based cementitious materials.

Periodic deprivation of gaseous hydrogen sulfide affects the activity of the concrete corrosion layer in sewers

Sulfide induced concrete corrosion significantly reduces the service life of the sewer systems. Gaseous hydrogen sulfide (H₂S) levels are a key factor affecting the corrosion rate and these fluctuate due to the diurnal flow pattern of sewers. Currently, there is little known about how such fluctuations, in particular the periodic deprivation of H₂S, may affect the corrosion activity. This study investigated the impact of the deprivation of H₂S on the sulfide uptake rate (SUR) of concrete coupons incubated in laboratory corrosion chambers. After systematic evaluation of the gaseous H₂S concentration profiles of two sewer systems, two types of profiles, i.e. short- (1 h) and long- (12 h) term deprivation of H₂S, were applied to the concrete coupons. In comparison to the baseline SUR, exposing the concrete coupon to 0 ppm of H₂S for 1 h consistently caused a temporary increase of the SUR (i.e. 3.2%-12.5%) following re-supply of H₂S at baseline levels. With the continuous re-supply of H₂S, there was gradual and steady decrease of SUR to the level close to the baseline SUR. However, for the case after deprivation of H₂S for 12 h, the SUR was 5.1% lower than baseline SUR and gradually increased to a level similar to the baseline SUR during the 20-30 min of continuous re-supply of H₂S. In addition, the simultaneous deprivation of H₂S and O₂ for 1 h had negligible impact on the SUR. Further analysis suggests that the historically accumulated intermediates of sulfide oxidation could act as electron donors for sulfide oxidizing bacteria (SOB). The replenishment of the intermediates upon the re-supply of H₂S could play a key role in the increase of SUR after short-term deprivation of H₂S. However, the activity of SOB could be diminished after long-term deprivation of H₂S, although the sulfur intermediates still could be available. Estimating the sulfide uptake by concrete using the SUR of the average H₂S concentration could lead to overestimation of the sulfide uptake. There could be more significant overestimation for the case with longer deprivation of H₂S.

Impact of fluctuations in gaseous H2S concentrations on sulfide uptake by sewer concrete: The effect of high H2S loads

The acid production from the oxidation of hydrogen sulfide (H2S) in sewer air results in serious corrosion of exposed concrete surfaces in sewers. Large fluctuations of gaseous H2S concentrations occur in sewers due to the diurnal profiles of sewage flow and retention times and the necessity of intermittent pumping of sewage from pressure pipes into gravity pipes. How the high concentrations of H2S due to these events may affect H2S uptake and subsequent corrosion by concrete sewers is largely unknown. This study determined the effect of short- and long-term increases in H2S levels on the sulfide uptake rate (SUR) of concrete surfaces with an active corrosion layer. The results showed that during the high load situation the SUR increased significantly but then decreased (compared to the baseline SUR) by about 7-14% and 41-50% immediately after short- and long-term H2S high-load periods, respectively. For both exposure conditions, the SUR gradually (over several hours) recovered to approximately 90% of the baseline SUR. Further tests suggest multiple factors may contribute to the observed decrease of SUR directly after the high H2S load. This includes the temporary storage of elemental sulfur in the corrosion layer and inhibition of sulfide oxidizing bacteria (SOB) due to high H2S level and temporary acid surge. Additionally, the delay of the corrosion layer to fully recover the SUR after the high H2S load suggests that there is a longer-term inhibitive effect of the high H2S levels on the activity of the SOB in the corrosion layer. Due to the observed activity reductions, concrete exposed to occasional short-term high H2S load periods had an overall lower H2S uptake compared to concrete exposed to constant H2S levels at the same average concentration. To accurately predict H2S uptake by sewer concrete and hence the likely maximum corrosion rates, a correction factor should be adopted for the H2S fluctuations when average H2S levels are used in the prediction.

Identification of controlling factors for the initiation of corrosion of fresh concrete sewers

The development of concrete corrosion in new sewer pipes undergoes an initiation process before reaching an active corrosion stage. This initiation period is assumed to last several months to years but the key factors affecting the process, and its duration, are not well understood. This study is therefore focused on this initial stage of the corrosion process and the effect of key environmental factors. Such knowledge is important for the effective management of corrosion in new sewers, as every year of life extension of such systems has a very high financial benefit. This long-term (4.5 year) study has been conducted in purpose-built corrosion chambers that closely simulated the sewer environment, but with control of three key environmental factors being hydrogen sulfide (H2S) gas phase concentration, relative humidity and air temperature. Fresh concrete coupons, cut from an industry-standard sewer pipe, were exposed to the corrosive conditions in the chambers, both in the gas phase and partially submerged in wastewater. A total of 36 exposure conditions were investigated to determine the controlling factors by regular retrieval of concrete coupons for detailed analysis of surface pH, sulfur compounds (elemental sulfur and sulfate) and concrete mass loss. Corrosion initiation times were thus determined for different exposure conditions. It was found that the corrosion initiation time of both gas-phase and partially-submerged coupons was positively correlated with the gas phase H2S concentration, but only at levels of 10 ppm or below, indicating that sulfide oxidation rate rather than the H2S concentration was the limiting factor during the initiation stage. Relative humidity also played a role for the corrosion initiation of the gas-phase coupons. However, the partially-submerged coupons were not affected by humidity as these coupons were in direct contact with the sewage and hence did have sufficient moisture to enable the microbial processes to proceed. The corrosion initiation time was also shortened by higher gas temperature due to its positive impact on reaction kinetics. These findings provide real opportunities for pro-active sewer asset management with the aim to delay the on-set of the corrosion processes, and hence extend the service life of sewers, through improved prediction and optimization capacity.

Determining the long-term effects of H₂S concentration, relative humidity and air temperature on concrete sewer corrosion

Many studies of sewer corrosion are performed in accelerated conditions that are not representing the actual corrosion processes. This study investigated the effects of various factors over 3.5 years under controlled conditions simulating the sewer environment. Concrete coupons prepared from precorroded sewers were exposed, both in the gas phase and partially submerged in wastewater, in laboratory controlled corrosion chambers. Over the 45 month exposure period, three environmental factors of H2S concentration, relative humidity and air temperature were controlled at different levels in the corrosion chambers. A total of 36 exposure conditions were investigated to determine the long term effects of these factors by regular retrieval of concrete coupons for detailed analysis of surface pH, corrosion layer sulfate levels and concrete loss. Corrosion rates were also determined for different exposure periods. It was found that the corrosion rate of both gas-phase and partially-submerged coupons was positively correlated with the H2S concentration in the gas phase. Relative humidity played also a role for the corrosion activity of the gas-phase coupons. However, the partially-submerged coupons were not affected by humidity as the surfaces of these coupons were saturated due to capillary suction of sewage on the coupon surface. The effect of temperature on corrosion activity varied and possibly the acclimation of corrosion-inducing microbes to temperature mitigated effects of that factor. It was apparent that biological sulfide oxidation was not the limiting step of the overall corrosion process. These findings provide real insights into the long-term effects of these key environmental factors on the sewer corrosion processes.

Prediction of sulphide build-up in filled sewer pipes

Millions of dollars are being spent worldwide on the repair and maintenance of sewer networks and wastewater treatment plants. The production and emission of hydrogen sulphide has been identified as a major cause of corrosion and odour problems in sewer networks. Accurate prediction of sulphide build-up in a sewer system helps engineers and asset managers to appropriately formulate strategies for optimal sewer management and reliability analysis. This paper presents a novel methodology to model and predict the sulphide build-up for steady state condition in filled sewer pipes. The proposed model is developed using a novel data-driven technique called evolutionary polynomial regression (EPR) and it involves the most effective parameters in the sulphide build-up problem. EPR is a hybrid technique, combining genetic algorithm and least square. It is shown that the proposed model can provide a better prediction for the sulphide build-up as compared with conventional models.

A rapid, non-destructive methodology to monitor activity of sulfide-induced corrosion of concrete based on H2S uptake rate

Many existing methods to monitor the corrosion of concrete in sewers are either very slow or destructive measurements. To overcome these limitations, a rapid, non-invasive methodology was developed to monitor the sulfide-induced corrosion process on concrete through the measurement of the H2S uptake rates of concrete at various corrosion stages. The H2S uptake rate for a concrete coupon was determined by measuring the gaseous H2S concentrations over time in a temperature- and humidity-controlled gas-tight reactor. The reliability of this method was evaluated by carrying out repeated tests on different concrete coupons previously exposed to 50 ppm of H2S, at 30 °C and 100% relative humidity for over 32 months. The H2S uptake measurements showed good reproducibility. It was also shown that a severely corroded coupon exhibited higher sulfide uptake rates than a less corroded coupon. This could be explained by the corrosion layer in the more corroded coupon having a higher biological sulfide oxidation activity than the less corroded coupon. Additionally, temperature changes had a stronger effect on the uptake rate of the heavily corroded coupon compared to the less corroded coupon. A corrosion rate of 8.9 ± 0.5 mm/year, estimated from the H2S uptake results, agreed well with the corrosion rate observed in real sewers under similar conditions. The method could be applied to investigate important factors affecting sulfide-induced concrete corrosion, particularly temperature, fluctuating gaseous H2S concentrations, oxygen concentrations, surface pH and relative humidity.

pH dynamics in sewers and its modeling

pH variation in sewers has a significant effect on hydrogen sulfide production and emissions, and hence its accurate prediction is critical for the optimization of mitigation strategies. In this study, the nature and dynamics of pH variation in a sewer system is examined. Three sewer systems collecting domestic wastewater were monitored, with pH in all cases showing large diurnal variations. pH in fresh sewage in all three cases had a very similar trend with maximum pH in the range of 8.5-8.7. pH variation in fresh sewage followed the same pattern as the sewage flow rate, suggesting that sewage pH is influenced by household water use. Nitrogen content of the wastewater was found to be the most influential factor causing pH variation in fresh sewage, with the total ammonium concentration variation well correlated with the pH variation. A methodology for predicting pH variation in sewers is developed and calibration protocols proposed. The methodology, which is based on the concept of charge balance, was validated using titration curves and field pH data. Measurement of the total ammonium concentration in fresh sewage was found necessary and adequate for the calibration of the charge balance-based pH model.

Growth kinetics of hydrogen sulfide oxidizing bacteria in corroded concrete from sewers

Hydrogen sulfide oxidation by microbes present on concrete surfaces of sewer pipes is a key process in sewer corrosion. The growth of aerobic sulfur oxidizing bacteria from corroded concrete surfaces was studied in a batch reactor. Samples of corrosion products, containing sulfur oxidizing bacteria, were suspended in aqueous solution at pH similar to that of corroded concrete. Hydrogen sulfide was supplied to the reactor to provide the source of reduced sulfur. The removal of hydrogen sulfide and oxygen was monitored. The utilization rates of both hydrogen sulfide and oxygen suggested exponential bacterial growth with median growth rates of 1.25 d(-1) and 1.33 d(-1) as determined from the utilization rates of hydrogen sulfide and oxygen, respectively. Elemental sulfur was found to be the immediate product of the hydrogen sulfide oxidation. When exponential growth had been achieved, the addition of hydrogen sulfide was terminated leading to elemental sulfur oxidation. The ratio of consumed sulfur to consumed oxygen suggested that sulfuric acid was the ultimate oxidation product. To the knowledge of the authors, this is the first study to determine the growth rate of bacteria involved in concrete corrosion with hydrogen sulfide as source of reduced sulfur.

Microbial community structures and in situ sulfate-reducing and sulfur-oxidizing activities in biofilms developed on mortar specimens in a corroded sewer system

Microbially induced concrete corrosion (MICC) caused by sulfuric acid attack in sewer systems has been a serious problem for a long time. A better understanding of microbial community structures of sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) and their in situ activities is essential for the efficient control of MICC. In this study, the microbial community structures and the in situ hydrogen sulfide production and consumption rates within biofilms and corroded materials developed on mortar specimens placed in a corroded manhole was investigated by culture-independent 16S rRNA gene-based molecular techniques and microsensors for hydrogen sulfide, oxygen, pH and the oxidation-reduction potential. The dark-gray gel-like biofilm was developed in the bottom (from the bottom to 4 cm) and the middle (4-20 cm from the bottom of the manhole) parts of the mortar specimens. White filamentous biofilms covered the gel-like biofilm in the middle part. The mortar specimens placed in the upper part (30 cm above the bottom of the manhole) were corroded. The 16S rRNA gene-cloning analysis revealed that one clone retrieved from the bottom biofilm sample was related to an SRB, 12 clones and 6 clones retrieved from the middle biofilm and the corroded material samples, respectively, were related to SOB. In situ hybridization results showed that the SRB were detected throughout the bottom biofilm and filamentous SOB cells were mainly detected in the upper oxic layer of the middle biofilm. Microsensor measurements demonstrated that hydrogen sulfide was produced in and diffused out of the bottom biofilms. In contrast, in the middle biofilm the hydrogen sulfide produced in the deeper parts of the biofilm was oxidized in the upper filamentous biofilm. pH was around 3 in the corroded materials developed in the upper part of the mortar specimens. Therefore, it can be concluded that hydrogen sulfide provided from the bottom biofilms and the sludge settling tank was emitted to the sewer atmosphere, then oxidized to corrosive compounds in the upper and middle parts of the manhole, and only the upper part of the mortar specimens were corroded, because in the middle part of the manhole the generated corrosive compounds (e.g., sulfuric acid) was reduced in the deeper parts of the biofilm.