Sulfide Control In Sewers Research Articles

Efficient electrosynthesis of HO2- from air for sulfide control in sewers

Electrochemically in-situ generation of oxygen and caustic soda is promising for sulfide management while suffers from scaling, poor inactivating capacity, hydrogen release and ammonia escape. In this study, the four-compartment electrochemical cell efficiently captured oxygen molecules from the air chamber to produce HO2- without generating toxic by-products. Meanwhile, the catalyst layer surface of PTFE/CB-GDE maintained a relatively balanced gas-liquid micro-environment, enabling the formation of enduring solid-liquid-gas interfaces for efficient HO2- electrosynthesis. A dramatic increase in HO2- generation rate from 453.3 mg L-1 h-1 to 575.4 mg L-1 h-1 was attained by advancement in operation parameters design (flow channels, electrolyte types, flow rates and circulation types). Stability testing resulted in the HO2- generation rate over 15 g L-1 and the current efficiency (CE) exceeding 85%, indicating a robust stable operational capacity. Furthermore, after 120 mg L-1 HO2- treatment, an increase of 11.1% in necrotic and apoptotic cells in the sewer biofilm was observed, higher than that achieved with the addition of NaOH, H2O2 method. The in-situ electrosynthesis strategy for HO2- represents a significance toward the practical implementation of sulfide abatement in sewers, holding the potential to treat various sulfide-containing wastewater.

In-situ oxygen generation using Titanium foam/IrO2 electrode for efficient sulfide control in sewers

Hydrogen sulfide (H2S) causes corrosion of pipeline network and endangers sewer’s normal operation, while the rehabilitation or replacement of sewer network incurs expensive manpower and material costs. Conventional chemical dosing strategies such as oxygen injection and caustic soda addition, confront issues with transport and storage of chemical reagents as well as dose control. In contrast, electrochemical treatment techniques have evoked growing attention in sewer management due to its capacity of in-situ effective agent production, real-time dose control and amenability to automation. In this study, the performances of anodic oxygen production using Ti/IrO2 and Titanium foam/IrO2 as electrodes were investigated. Titanium foam/IrO2 electrode exhibited the superior oxygen evolution capacity, with the dissolved oxygen (DO) level attained in the range 8–9 mg L-1 (i.e., higher than that of 5–6 mg L-1 by Ti/IrO2 electrode). It was also indicated that a decrease in flow rate or an increase in current density would accelerate its in-situ oxygen production. DO production with the presence of COD, NH4+, HCO3−, H2PO4−/HPO42− was approximately 6.78–8.37 mg L-1. Moreover, this electrochemical method was illustrated to successfully achieve sulfide removal efficiency of 90 % in the simulated sewage experiment. The cathode compartment simultaneously produced caustic soda (∼0.4 wt% in 210 min), thereby contributing to further sulfide removal. Additionally, the effect of real sewage with different conductivities on sulfide removal proved the wide applicability of electrochemical technology in sewer management. Overall, this study demonstrated the practical potential of concurrent in-situ anodic oxygen production and cathodic caustic soda generation for sulfide control in sewers.

A biological strategy for sulfide control in sewers: Removing sulfide by sulfur-oxidizing bacteria

Sulfide produced from sewers is considered one of the dominant threats to public health and sewer lifespan due to its toxicity and corrosiveness. In this study, we developed an environmentally friendly strategy for gaseous sulfide control by enriching indigenous sulfur-oxidizing bacteria (SOB) from sewer sediment. Ceramics acted as bio-carriers for immobilizing SOB for practical use in a lab-scale sewer reactor. 16 S rRNA gene sequences revealed that the SOB consortium was successfully enriched, with Thiobacillus, Pseudomonas, and Alcaligenes occupying a dominant abundance of 64.7% in the microbial community. Metabolic pathway analysis in different acclimatization stages indicates that microorganisms could convert thiosulfate and sulfide into elemental sulfur after enrichment and immobilization. A continuous experiment in lab-scale sewer reactors confirmed an efficient result for sulfide removal with hydrogen sulfide reduction of 43.9% and 85.1% under high-sulfur load and low-sulfur load conditions, respectively. This study shed light on the promising application for sewer sulfide control by biological sulfur oxidation strategy.

In-situ advanced oxidation of sediment iron for sulfide control in sewers

Sulfide control is a significant problem in urban sewer management. Although in-sewer dosing of chemicals has been widely applied, it is prone to high chemical consumption and cost. A new approach is proposed in this study for effective sulfide control in sewers. It involves advanced oxidation of ferrous sulfide (FeS) in sewer sediment, to produce hydroxyl radical (·OH) in-situ, leading to simultaneous sulfide oxidation and reduction of microbial sulfate-reducing activity. Long-term operation of three laboratory sewer sediment reactors was used to test the effectiveness of sulfide control. The experimental reactor with the proposed in-situ advanced FeS oxidation substantially reduced sulfide concentration to 3.1 ± 1.8 mg S/L. This compares to 9.2 ± 2.7 mg S/L in a control reactor with sole oxygen supply, and 14.1 ± 4.2 mg S/L in the other control reactor without either iron or oxygen. Mechanistic investigations illustrated the critical role of ·OH, produced from the oxidation of sediment iron, in regulating microbial communities and the chemical sulfide oxidation reaction. Together these results demonstrate that incorporating the advanced FeS oxidation process in sewer sediment enable superior performance of sulfide control at a much lower iron dosage, thereby largely saving chemical use.

Combination of nitrate and sodium nitroprusside dosing for sulfide control with low carbon source loss in sewer biofilm reactors

Nitrate has been widely used in sewer systems for sulfide control. However, significant chemical consumption and the loss of carbon source were observed in previous studies. To find a feasible and cost-effective control strategy of the sulfide control, the effect of nitrate combined with sodium nitroprusside (SNP) dosage strategy was tested in lab-scale sewer biofilm reactors. Results showed that nitrate and SNP were strongly synergistic, with 30 mg N/L nitrate and 20 mg/L SNP being sufficient for sulfide control in this study. While large amount of nitrate alone (100 mg N/L) is required to achieve the same sulfide control effectiveness. Meanwhile, the nitrate combined with SNP could reduce the organic carbon source loss by 80%. Additionally, the high-throughput sequencing results showed that the relative abundance of autotrophic, nitrate reducing-sulfide oxidizing bacteria genera (a-NR-SOB) such as Arcobacter and Sulfurimonas was increased by around 18%, while the heterotrophic, nitrate-reducing bacteria (hNRB) such as Thauera was substantially reduced. It demonstrated that the sulfide control was mainly due to the a-NR-SOB activity under the nitrate and SNP dosing strategy. The microbial functional prediction further revealed that nitrate and SNP promoted the dissimilatory nitrate reduction process which utilizes sulfide as an effective electron donor. Moreover, economic assessment indicated that using the combination of nitrate and SNP for sulfide control in sewers would lower the chemical costs by approximately 35% compared with only nitrate addition.

Simultaneous removal of hydrogen sulfide, phosphate and emerging organic contaminants, and improvement of sludge dewaterability by oxidant dosing in sulfide-iron-laden sludge

Liquid sludge often contains odorous and toxic hydrogen sulfide and high levels of FeII compounds (e.g., iron sulfides), due to the extensive use of iron salts for hydrogen sulfide control in sewers and for enhanced primary treatment and phosphate removal in wastewater treatment plants. We proposed and verified that dosing appropriate chemical oxidants in the sulfide-iron-laden sludge can be a simple and cost-effective strategy to remove hydrogen sulfide, phosphate, and emerging organic contaminants, and to improve sludge dewaterability simultaneously. Among the seven oxidants investigated, H2O2, ClO2 and NaClO2 were the more cost-effective oxidants than others to control hydrogen sulfide release from the liquid sludge. Dosing these three oxidants also improved sludge dewaterability and removed dissolved phosphate from the liquid sludge, with H2O2 performing the best. Hydrogen sulfide was removed via both direct oxidation by the dosed oxidants and indirect oxidation by the FeIII that was in-situ formed from oxidation of the FeII compounds in the sludge. The in-situ formed FeIII also precipitated/adsorbed the soluble phosphate into the solid form (FePO4). Fenton-like reactions occurred between H2O2 and the FeII compounds in the sludge, and hydroxyl radicals (HO•) were generated. HO• oxidized hydrogen sulfide, destructed refractory organic emerging contaminants and sludge extracellular polymeric compounds (EPSs), and improved the sludge dewaterability. The formation of HO• can be enhanced by hydrogen sulfide and the sludge EPSs present in the sludge through providing more available FeII for the Fenton-like reactions. This study demonstrates the importance of selecting and dosing suitable oxidants to the sulfide-iron-laden sludge with due consideration for the multiple benefits in engineering practices. The same principles may be also used in formulating a dual oxidant-iron strategy to treat sulfide-iron-laden sewage, sludge, and sediments for simultaneous abatement of various pollutants.

Recovery of in-sewer dosed iron from digested sludge at downstream treatment plants and its reuse potential

Iron-based coagulants are dosed in enormous amounts and play an essential role in various segments of our urban water infrastructure. In order for the water industry to become circular, a closed-loop management strategy for iron needs to be developed. In this study, we have demonstrated for the first time that in-sewer dosed iron, either in the form of FeCl3 or ferric-based drinking water sludge (Fe-DWS) as a means to combat sewer corrosion and odour, can be recovered in the form of vivianite in digested sludge in down-stream wastewater treatment plants. Importantly, about 92 ± 2% of the in-sewer dosed Fe was estimated to be bound in vivianite in digested sludge. A simple insertion of Neodymium magnet allowed to recover 11 ± 0.2% and 15.3 ± 0.08% of the vivianite formed in the digested sludge of the in-sewer dosed iron in the form of FeCl3 and Fe-DWS, respectively. The purity of recovered vivianite ranged between 70 ± 5% and 49 ± 3% for in-sewer dosed FeCl3 and Fe-DWS, respectively. Almost complete (i.e. 98 ± 0.3%) separation of Fe in the form of ferrihydrite was achieved from vivianite after alkaline washing. Subsequent batch experiments demonstrated that the recovered ferrihydrite can be directly reused for efficient sulfide control in sewers. At a ferrihydrite-Fe:S molar ratio of 1.2:1, sewage dissolved sulfide concentrations was reduced from 15 mgS/L to below 0.5 mgS/L within 1 h of reaction. Overall, the results obtained in our study flag a first step for utilities towards a closed-loop iron-based coagulant management approach.

Full-scale investigation of in-situ iron and alkalinity generation for efficient sulfide control

Hydrogen sulfide induced corrosion of concrete sewer pipes is a major issue for wastewater utilities globally. One of the most commonly used methods to combat hydrogen sulfide is the addition of ferric chloride. While a reliable and effective method, ferric chloride is acidic causing OH&S concerns as well as alkalinity consumption in sewage. This study investigates, under full-scale field conditions, an alternative method for sulfide control by in-situ electrochemical generation of iron ions using sacrificial iron electrodes. This method concomitantly produces alkalinity through cathodic OH− generation, rather than consumption. The gaseous hydrogen sulfide concentrations at the discharge wet well of a real-life rising main (length: ∼1 km in, diameter: 150 mm) decreased from 173 ppm to 43 ppm (90 percentile of peak values), when a current of 0.86 A/m3 of sewage was applied. The 90 percentile peak H2S value was further reduced to 6.6 ppm when the applied current was increased to 1.14 A/m3 sewage. Moreover, methane generation was almost completely inhibited from 25.3 ± 1.46 mg COD/L to 0.06 ± 0.04 mg COD/L. The overall cell voltage remained constant throughout the experimental period clearly showing the stability of the process. Detailed characterization of the down-stream sewer pipe biofilm revealed the complexity of the iron chemistry as the in-situ produced iron ions undergo transformation into a variety of iron species. Overall, this study demonstrates that in-situ generation of iron and alkalinity is an effective alternative method for hydrogen sulfide control in sewers.

Opportunities for reducing coagulants usage in urban water management: The Oxley Creek Sewage Collection and Treatment System as an example

Iron and aluminium based coagulants are used in enormous amounts and play an essential role in urban water management globally. They are dosed at drinking water production facilities for the removal of natural organic matter. Iron salts are also dosed to sewers for corrosion and odour control, and at wastewater treatment plants (WWTPs) for phosphate removal from wastewater and hydrogen sulfide removal from biogas. A recent laboratory study revealed that iron dosed to sewers is available for phosphate and hydrogen sulfide removal in the downstream WWTP. This study demonstrates for the first time under real-life conditions the practical feasibility and effectiveness of the strategy through a year-long full-scale investigation. Over a period of 5 months, alum dosing at ∼190 kg Al/day to the bioreactor in a full-scale WWTP was stopped, while FeCl2 dosing at ∼160 kg Fe/day in the upstream network was commenced. Extensive sampling campaigns were conducted over the baseline, trial and recovery periods to investigate sulfide control in sewers and its flow-on effects on phosphate in WWTP effluent, H2S in biogas, as well as on the WWTP effluent hypochlorite disinfection process. A plant-wide mass balance analysis showed that the Fe2+ dosed upstream was effectively used for P removal in the activated sludge tanks, with an effluent phosphate concentration comparable to that in the baseline period (i.e. with alum dosing to the bioreactor). Simultaneously, hydrogen sulfide concentration in biogas decreased ∼43%, from 495 ± 10 to 283 ± 4 ppm. No effects on biological nitrogen removal and disinfection processes were observed. Both effluent phosphate and H2S in biogas increased in the recovery period, when in-sewer dosing of FeCl2 was stopped. X-ray diffraction failed to reveal the presence of vivianite in the digested sludge, providing strong evidence that thermal hydrolysis prevented the formation of vivianite during anaerobic digestion. The latter limits the potential for selective recovery of Fe and P through magnetic separation. Overall, our study clearly demonstrates the multiple beneficial reuse of iron in a real urban wastewater system and urges water utilities to adopt an integrated approach to coagulant use in urban water management.

Electrochemical Production of Magnetite Nanoparticles for Sulfide Control in Sewers.

Recently, naturally occurring magnetite (Fe3O4) has emerged as a new material for sulfide control in sewers. However, unrefined magnetite could have high heavy metal contents (e.g., Cr, Zn, Ni, Sn, etc.) and the capacity to remove dissolved sulfide is reasonably limited due to relatively large particle sizes. To overcome the drawbacks of unrefined magnetite we used an electrochemical system with mild steel as sacrificial electrodes to in-situ generate high strength solutions of plate-like magnetite nanoparticles (MNP). MNP with a size range between 120 and 160 nm were electrochemically generated at 9.35 ± 0.28 g Fe3O4-Fe/L, resulting in a Coulombic efficiency (CE) for iron oxidation of 93.5 ± 2.8%. The produced MNP were found to effectively reduce sulfide levels in sewage from 12.7 ± 0.3 to 0.2 ± 0.0 mg S/L at a sulfide-to-MNP ratio of 0.26 g S/g Fe3O4-Fe. Subsequently, MNP were continuously generated with polarity switching at stable cell voltage for 31 days at 4.53 ± 0.35 g Fe3O4-Fe/L with a CE for iron oxidation of 92.4 ± 7.2%. The continuously produced MNP reduced sulfide at similar levels to around 0.2 mg S/L at a ratio of 0.28 g S/g Fe3O4-Fe.

Simultaneous use of caustic and oxygen for efficient sulfide control in sewers

Periodic caustic shock-loading is a commonly used method for sulfide control in sewers. Caustic shock-loading relies on the elevation of the sewage pH to ≥10.5 for several hours, thereby removing sewer pipe biofilms as well as deactivating SRB activity in the remaining biofilm. Although a widely used method, SRB activity is often not completely inhibited, and as such sulfide is still being generated. Here, we propose and experimentally demonstrate an innovative approach which combines caustic with oxygen, another commonly used method, as a dosing strategy for overcoming the drawbacks of caustic shock-loading. Six laboratory-scale rising main reactors were subjected to three dosing schemes over a period of three months, namely (i) simultaneous caustic and oxygen addition, (ii) caustic addition and (iii) no chemical addition. Our results showed that the combination of caustic and oxygen achieved efficient sulfide control, leading to a prolonged biofilm recovery period in between caustic shocks. In addition, methane emissions were reduced to a negligible level compared to caustic treatment only. To translate the findings to real-life application, the key parameters obtained during the long-term lab-scale experiments were subjected to extensive simulation studies using the SeweX model under a wide range of conditions commonly found in sewers. Overall, this study highlights the potential of periodic shock-loading and intermittent oxygen injection as combined dosing strategy for efficient sulfide control in sewers.

Nitrite production from urine for sulfide control in sewers

Most commonly used methods for sewer sulfide control involves dosing chemical agents to wastewater, which incurs high operational costs. Here, we propose and demonstrate a cost-effective and environmentally attractive approach to sewer sulfide control through urine separation and its subsequent conversion to nitrite prior to intermittent dosage to sewers. Urine collected from a male toilet urinal was fed to laboratory-scale sequencing batch reactors. The reactors stably converted roughly 50% of the nitrogen in urine to nitrite, with high abundance (at 17.46%) of known ammonia-oxidizing bacteria (AOB) of the genus Nitrosomonas, and absence (below detection level) of typical nitrite-oxidizing bacteria of the genus Nitrospira, according to 454 pyrosequencing analysis. The stable nitrite production was achieved at both relatively high (1.0–2.0 mg/L) and low (0.2–0.3 mg/L) dissolved oxygen concentrations. Dosing tests in laboratory-scale sewer systems confirmed the sulfide control effectiveness of free nitrous acid generated from urine. Life cycle assessment indicated that, compared with commodity chemicals, nitrite/free nitrous acid (FNA) production from urine for sulfide control in sewers would lower the operational costs by approximately 2/3 and greenhouse gas (GHG) emissions by 1/3 in 20 years.

Electrochemical oxidation of iron and alkalinity generation for efficient sulfide control in sewers

The addition of iron salts is one of the most commonly used dosing strategies for sulfide control in sewers. However, iron salts decrease the sewage pH which not only reduces the effectiveness of sulfide precipitation but also enhances the release of residual sulfide to the sewer atmosphere. Equally important, concentrated iron salt solutions are corrosive and their frequent transport, handling, and on-site storage often come with Occupational Health and Safety (OH&S) concerns. Here, we experimentally demonstrated a novel sulfide control approach using electrochemical systems with parallel placed iron electrodes. This enabled combining anodic dissolved iron species release with cathodic hydroxyl anion production, which alleviates all the aforementioned concerns. A long-term experiment was successfully carried out achieving an average sulfide removal efficiency of 95.4 ± 4.4% at low voltage input of 2.90 ± 0.54 V over the course of 8 weeks. This electrochemical method was demonstrated to successfully achieve efficient sulfide control. In addition, it increases the sewage pH, thereby overcoming the drawbacks associated with the pH decrease in the case of conventional iron salt dosing. Ferrous ions were produced at an overall coulombic efficiency (CE) of 98.2 ± 1.2%, whereas oxygen evolution and direct sulfide oxidation were not observed. Short-term experiments showed that increasing either inter-electrode gap or current density increased the cell voltage associated with the increase in the ohmic drop of the system. Overall, this study highlights the practical potential of in-situ generation of dissolved iron species and simultaneous hydroxyl anion generation for efficient sulfide control in sewers.

Indirect sulfur reduction via polysulfide contributes to serious odor problem in a sewer receiving nitrate dosage

Nitrate dosing is commonly used to control hydrogen sulfide production in sewer systems. However, quick rebound of the sulfide concentration after nitrate depletion has been observed and results in more serious odor and corrosion problem. To investigate the mechanism of sulfide regeneration in the nitrate-free period, a laboratory-scale sewer reactor was run for 30 days to simulate sulfide production and oxidation with intermittent nitrate addition. The results show that nitrate addition substantially reduced the sulfide concentration, but the produced elemental sulfur was then quickly reduced back to sulfide in nitrate-free periods. This induced more and more sulfide production in the sewer reactor. Elemental sulfur and polysulfide reductions were found in the sewage in nitrate-free periods, showing their contributions to the sulfide regeneration. Through batch tests, polysulfide was confirmed as the key intermediate for accelerating sulfur reduction during the nitrate-free period in the sewer. Sulfide production rates significantly increased by 65% and 59% in the presences of tetrasulfide and sulfur with sulfide, respectively, at the beginning of the test. While polysulfide formation was prevented by the ferrous chloride addition, the sulfur reduction rate remarkably decreased from 12.8 mgS/L-h to 1.8 mgS/L-h. This indicates that direct sulfur reduction was significantly slower than the indirect sulfur reduction via polysulfide; the latter process could be the cause for the quick rebound of the sulfide concentration in the sewer with intermittent nitrate dosing. Thus, the pathways of sulfur transformations in a sewer, both in the presence and absence of nitrate, were proposed. Microbial community analysis results reveal that some common sulfate-reducing bacteria (SRB) genera in sewer sediment were possible sulfur reducers. According to this finding, the effect and strategy of nitrate dosing for hydrogen sulfide control in sewers should be re-evaluated and re-considered.

Scaling-Free Electrochemical Production of Caustic and Oxygen for Sulfide Control in Sewers.

Caustic shock-loading and oxygen injection are commonly used by the water industry for biofilm and sulfide control in sewers. Caustic can be produced onsite from wastewater using a two-compartment electrochemical cell. This avoids the need for import and storage of caustic soda, which typically represents a cost and a hazard. An issue limiting the practical implementation of this approach is the occurrence of membrane scaling due to the almost universal presence of Ca(2+) and Mg(2+) in wastewater. It results in a rapid increase in the cell voltage, thereby increasing the energy consumption of the system. Here, we propose and experimentally demonstrate an innovative solution for this problem involving the inclusion of a middle compartment between the anode and cathode compartments. Caustic was efficiently produced from wastewater over a period of 12 weeks and had an average Coulombic efficiency (CE) of 84.1 ± 1.1% at practically relevant caustic strengths (∼3 wt %). Neither membrane scaling nor an increase in the cell voltage was observed throughout the experiments. In addition, dissolved oxygen was produced in the anode, resulting in continuously oxygenated wastewater leaving the three-compartment cell. This membrane-scaling control strategy represents a major step forward toward practical implementation of on-site simultaneous electrochemical caustic and oxygen generation for sulfide control in sewers and also has the potential to be applied to other (bio)electrochemical systems receiving wastewater as source for product recovery.

Feasibility of sulfide control in sewers by reuse of iron rich drinking water treatment sludge

Dosage of iron salt is the most commonly used method for sulfide control in sewer networks but incurs high chemical costs. In this study, we experimentally investigate the feasibility of using iron rich drinking water treatment sludge for sulfide control in sewers. A lab-scale rising main sewer biofilm reactor was used. The sulfide concentration in the effluent decreased from 15.5 to 19.8 mgS/L (without dosing) to below 0.7–2.3 mgS/L at a sludge dosing rate achieving an iron to total dissolved inorganic sulfur molar ratio (Fe:S) of 1:1, with further removal of sulfide possible by prolonging the reaction time. In fact, batch tests revealed an Fe consumption to sulfide removal ratio of 0.5 ± 0.02 (mole:mole), suggesting the possible occurrence of other reactions involving the removal of sulfide. Modelling revealed that the reaction between iron in sludge and sulfide has reaction orders of 0.65 ± 0.01 and 0.77 ± 0.02 with respect to the Fe and sulfide concentrations, respectively. The addition of sludge slightly increased the total chemical oxidation demand (tCOD) concentration (by approximately 12%) as expected, but decreased the soluble chemical oxidation demand (sCOD) concentration and methane formation by 7% and 20%, respectively. Some phosphate removal (13%) was also observed at the sludge dosing rate of 1:1 (Fe:S), which is beneficial to nutrient removal from the wastewater. Overall, this study suggests that dosing iron-rich drinking water sludge to sewers could be an effective strategy for sulfide removal in sewer systems, which would also reduce the sludge disposal costs for drinking water treatment works. However, its potential side-effects on sewer sedimentation and on the wastewater treatment plant effluent remain to be investigated.

Real-Time Multistep Prediction of Sewer Flow for Online Chemical Dosing Control

Chemical dosing is the most common strategy for sulfide control in sewers. Recent research has shown that online control of chemical dosing can significantly reduce dosing costs, while achieving better control performance. One of the bottlenecks of online control is the prediction of sewage retention time in sewers, governed by future sewage flows. This study developed a methodology for real-time future flow prediction in sewers based on autoregressive moving average (ARMA) models and multistep iterative prediction. This methodology was validated with flow data collected from two pumping stations with different flow characteristics and different wet-well storage capacities. The results showed that the proposed methodology was capable of predicting future flow rates with good accuracy under different weather conditions. Online control of chemical dosing with real-time sewer flow prediction was tested through a simulation study. Results showed that future flow prediction improved sulfide control and significantly reduced chemical dosage.

Impact of oxygen injection on CH4 and N2O emissions from rising main sewers

Oxygen injection is a commonly used mitigation strategy for sulfide control in sewers. Methane, a potent greenhouse gas, is also produced in sewers. Oxygen injection may reduce methane generation/emission, but could potentially lead to N2O production due to the development of a nitrifying microbial community. The impact of oxygen dosing for sulfide control in sewers on CH4 and N2O production was assessed in this study in laboratory sewer reactors. Results showed that oxygen injection is able to reduce CH4 formation in sewers, although full control of CH4 was not achieved, likely due to partial oxygen penetration into sewer biofilm. The experimental results also revealed a nitrogen loss of around 5 mN/L. However, no significant N2O accumulation was detected.

In-situ caustic generation from sewage: The impact of caustic strength and sewage composition

Periodic caustic dosage is a commonly used method by the water industry to elevate pH levels and deactivate sewer biofilms responsible for hydrogen sulfide generation. Caustic (NaOH) can be generated in-situ from sewage using a divided electrochemical cell, which avoids the need for transport, handling and storage of concentrated caustic solutions. In this study, we investigated the impact of caustic strength in the cathode compartment and the impact of sodium concentration in sewage on the Coulombic efficiency (CE) for caustic generation. The CE was found to be independent of the caustic strength produced in the range of up to ∼3 wt%. Results showed that a caustic solution of ∼3 wt% could be produced directly from sewage at a CE of up to 75 ± 0.5%. The sodium concentration in sewage had a significant impact on the CE for caustic generation as well as on the energy requirements of the system, with a higher sodium concentration leading to a higher CE and lower energy consumption. The proton, calcium, magnesium and ammonium concentrations in sewage affected the CE for caustic generation, especially at low sodium concentrations. Economical assessment based on the experimental results indicated that sulfide control in sewers using electrochemically-generated caustic from sewage is an economically attractive strategy.

Dosing free nitrous acid for sulfide control in sewers: Results of field trials in Australia

Intermittent dosing of free nitrous acid (FNA), with or without the simultaneous dosing of hydrogen peroxide, is a new strategy developed recently for the control of sulfide production in sewers. Six-month field trials have been carried out in a rising main sewer in Australia (150 mm in diameter and 1080 m in length) to evaluate the performance of the strategy that was previously demonstrated in laboratory studies. In each trial, FNA was dosed at a pumping station for a period of 8 or 24 h, some with simultaneous hydrogen peroxide dosing. The sulfide control effectiveness was monitored by measuring, on-line, the dissolved sulfide concentration at a downstream location of the pipeline (828 m from the pumping station) and the gaseous H2S concentration at the discharge manhole. Effective sulfide control was achieved in all nine consecutive trials, with sulfide production reduced by more than 80% in 10 days following each dose. Later trials achieved better control efficiency than the first few trials possibly due to the disrupting effects of FNA on sewer biofilms. This suggests that an initial strong dose (more chemical consumption) followed by maintenance dosing (less chemical consumption) could be a very cost-effective way to achieve consistent control efficiency. It was also found that heavy rainfall slowed the recovery of sulfide production after dosing, likely due to the dilution effects and reduced retention time. Overall, intermittent dose of FNA or FNA in combination with H2O2 was successfully demonstrated to be a cost-effective method for sulfide control in rising main sewers.