Electrochemical removal of sulfide on porous carbon-based flow-through electrodes.

Electrochemical oxidation of hydrogen sulfide and its separation from the waste stream in the form of sulfur was studied at low-cost carbon-based porous materials, activated carbon felt (ACF) and graphite felt (GF). Both materials were capable of selective HS- oxidation to elemental sulfur in low-conductivity solutions (i.e., <1 mS cm-1), as well as in raw sewage. The HS- removal rate was ten times faster at ACF compared with GF electrode due to the higher surface area and chemisorption of HS-. To address the electrode passivation with the electrodeposited sulfur, different electrochemical recovery strategies were tested. GF could be only partially regenerated (i.e., 30% efficiency) using cathodic polarization. Also, both anodic and cathodic polarization improved the sulfide removal in the subsequent working cycle due to the introduction of new redox-active oxygen containing functional groups. Sulfur deposited at the ACF electrode could not be recovered by any of the investigated strategies. Thus, sulfur was incorporated into the carbon matrix and strongly bonded with the carbon functional groups at both GF and ACF electrodes. Although carbon-based electrodes have been widely investigated for electrochemical sulfide removal, this study demonstrates that their application is limited by low regeneration efficiency of the electrodeposited sulfur.

Sewers as potential reservoirs of antibiotic resistance.

Wastewater transport along sewers favors the colonization of inner pipe surfaces by wastewater-derived microorganisms that grow forming biofilms. These biofilms are composed of rich and diverse microbial communities that are continuously exposed to antibiotic residues and antibiotic resistant bacteria (ARB) from urban wastewater. Sewer biofilms thus appear as an optimal habitat for the dispersal and accumulation of antibiotic resistance genes (ARGs). In this study, the concentration of antibiotics, integron (intI1) and antibiotic resistance genes (qnrS, sul1, sul2, blaTEM, blaKPC, ermB, tetM and tetW), and potential bacterial pathogens were analyzed in wastewater and biofilm samples collected at the inlet and outlet sections of a pressurized sewer pipe. The most abundant ARGs detected in both wastewater and biofilm samples were sul1 and sul2 with roughly 1 resistance gene for each 10 copies of 16s RNA gene. Significant differences in the relative abundance of gene intI1 and genes conferring resistance to fluoroquinolones (qnrS), sulfonamides (sul1 and sul2) and betalactams (blaTEM) were only measured between inlet and outlet biofilm samples. Composition of bacterial communities also showed spatial differences in biofilms and a higher prevalence of Operational Taxonomic Units (OTUs) with high sequence identity (>98%) to well-known human pathogens was observed in biofilms collected at the inlet pipe section. Our study highlights the role of sewer biofilms as source and sink of ARB and ARGs and supports the idea that community composition rather than antibiotic concentration is the main factor driving the diversity of the sewage resistome.

Control of sulfide and methane production in anaerobic sewer systems by means of Downstream Nitrite Dosage.

Bioproduction of hydrogen sulfide (H2S) and methane (CH4) under anaerobic conditions in sewer pipes causes detrimental effects on both sewer facilities and surrounding environment. Among the strategies used to mitigate the production of both compounds, the addition of nitrite (NO2(-)) has shown a greater long-term inhibitory effect compared with other oxidants such as nitrate or oxygen. The aim of this study was to determine the effectiveness of a new method, the Downstream Nitrite Dosage strategy (DNO2D), to control H2S and CH4 emissions in sewers. Treatment effectiveness was assessed on H2S and CH4 abatement on the effluent of a laboratory sewer pilot plant that mimics a full-scale anaerobic rising sewer. The experiment was divided in three different periods: system setup (period 1), nitrite addition (period 2) and system recovery (period 3). Different process and molecular methods were combined to investigate the impact of NO2(-) addition on H2S and CH4 production. Results showed that H2S load was reduced completely during nitrite addition when compared to period 1 due to H2S oxidation but increased immediately after nitrite addition stopped. The H2S overproduction during recovery period was associated with the bacterial reduction of different sulfur species (elemental sulfur/thiosulfate/sulfite) accumulated within the sewer biofilm matrix. Oxidation of CH4 was also detected during period 2 but, contrary to sulfide production, re-establishment of methanogenesis was not immediate after stopping nitrite dosing. The analysis of bulk and active microbial communities along experimental treatment showed compositional changes that agreed with the observed dynamics of chemical processes. Results of this study show that DNO2D strategy could significantly reduce H2S and CH4 emissions from sewers during the addition period but also suggest that microbial agents involved in such processes show a high resilience towards chemical stressors, thus favoring the re-establishment of H2S and CH4 production after stopping nitrite addition.

Coordinated management of combined sewer overflows by means of environmental decision support systems.

During heavy rainfall, the capacity of sewer systems and wastewater treatment plants may be surcharged producing uncontrolled wastewater discharges and a depletion of the environmental quality. Therefore there is a need of advanced management tools to tackle with these complex problems. In this paper an environmental decision support system (EDSS), based on the integration of mathematical modeling and knowledge-based systems, has been developed for the coordinated management of urban wastewater systems (UWS) to control and minimize uncontrolled wastewater spills. Effectiveness of the EDSS has been tested in a specially designed virtual UWS, including two sewers systems, two WWTP and one river subjected to typical Mediterranean rain conditions. Results show that sewer systems, retention tanks and wastewater treatment plants improve their performance under wet weather conditions and that EDSS can be very effective tools to improve the management and prevent the system from possible uncontrolled wastewater discharges.

Life cycle assessment of urban wastewater systems: Quantifying the relative contribution of sewer systems.

This study aims to propose a holistic, life cycle assessment (LCA) of urban wastewater systems (UWS) based on a comprehensive inventory including detailed construction and operation of sewer systems and wastewater treatment plants (WWTPs). For the first time, the inventory of sewers infrastructure construction includes piping materials and aggregates, manholes, connections, civil works and road rehabilitation. The operation stage comprises energy consumption in pumping stations together with air emissions of methane and hydrogen sulphide, and water emissions from sewer leaks. Using a real case study, this LCA aims to quantify the contributions of sewer systems to the total environmental impacts of the UWS. The results show that the construction of sewer infrastructures has an environmental impact (on half of the 18 studied impact categories) larger than both the construction and operation of the WWTP. This study highlights the importance of including the construction and operation of sewer systems in the environmental assessment of centralised versus decentralised options for UWS.

Impact of in-sewer transformation on 43 pharmaceuticals in a pressurized sewer under anaerobic conditions.

The occurrence of 43 pharmaceuticals and 2 metabolites of ibuprofen was evaluated at the inlet and the outlet of a pressure sewer pipe in order to asses if in-sewer processes affect the pharmaceutical concentrations during their pass through the pipe. The target compounds were detected at concentrations ranging from low ng/L to a few µg/L, which are in the range commonly found in municipal wastewater of the studied area. The changes in concentrations between two sampling points were negligible for most compounds, i.e. from -10 to 10%. A higher decrease in concentrations (25-60 %) during the pass through the pipe was observed for diltiazem, citalopram, clarithromycin, bezafibrate and amlodipine. Negative removal was calculated for sulfamethoxazole (-66 ± 15%) and irbesartan (-58 ± 25%), which may be due to the conversion of conjugates back to their parent compounds in the sewer. The results show that microbial transformation of pharmaceuticals begins in sewer, albeit to different extents for different compounds. Therefore, the in-sewer transformation of pharmaceuticals should be assessed especially when their concentrations are used to estimate and refine the estimation of their per capita consumption in a catchment of interest in the sewage epidemiology approach.

Implications of Downstream Nitrate Dosage in anaerobic sewers to control sulfide and methane emissions.

Nitrate (NO3⁻) is commonly dosed in sewer systems to reduce sulfide (H2S) and methane (CH4) produced in anaerobic rising main pipes. However, anoxic conditions along the whole rising pipes are difficult and costly to maintain since nitrate is added at the upstream sections of the sewer. In this study we tested the effects of the Downstream Nitrate Dosage strategy (DND) in anaerobic pipes in a specially designed laboratory-scale systems that mimics a real rising main. Effectiveness of the strategy was assessed on H2S and CH4 abatement on the effluent of the lab sewer system. A combination of process (Normal Functioning monitoring and batch tests) and molecular (by 454-pyrosequencing) methods were used to investigate the impacts and microbial activities related to the nitrate addition. Results showed a complete abatement of H2S generated, with a fraction transformed to elemental sulfur (S°). Methane discharged was reduced to 50% while nitrate was added, due to the CH4 oxidation in the anoxic conditions established at the end of the pipe. Both sulfidogenic and methanogenic activities resumed upon cessation of NO3⁻ dosage. An increase of microorganisms of the genera Simplicispira, Comamonas, Azonexus and Thauera was detected during nitrate addition. Regarding anoxic methane oxidation, only one Operational Taxonomic Unit (OTU) was identified, which is likely related with this metabolism. Obtained results are relevant for the optimal management of nitrate dosage strategies in sewer systems.

Assessment of pH shock as a method for controlling sulfide and methane formation in pressure main sewer systems.

Caustic dosing to raise pH above 10.0 for short periods (hours) is often used by water utilities for controlling sulfide formation in sewers. However the effectiveness of this strategy is rarely reported and the impact of pH level and exposure time on the effectiveness is largely unknown. The effectiveness of this strategy under various pH levels (10.5-12.5) and exposure time (0.5-6.0 h) in controlling sulfide and methane production was evaluated in laboratory scale anaerobic sewer reactors and then in a real sewer system. Laboratory studies showed that the sulfide production rate of the laboratory sewer biofilm was reduced by 70-90% upon the completion of the pH shock, while the methane production rate decreased by 95-100%. It took approximately one week for the sulfate-reducing activity to recover to normal levels. In comparison, the methanogenic activities recovered to only about 10% in 4 weeks. The slow recovery is explained by the substantially loss of cell viability upon pH shocks, which recovered slowly after the shocks. Laboratory studies further revealed that a pH level of 10.5 for 1-2 h represent cost-effective conditions for the pH shock treatment. However, field trials showed a higher pH (11.5) and larger dosing times are needed due to the pH decreases along the sewer line and at the two ends of the caustic-receiving wastewater slugs due to dilution. To have effective sulfide and methane control, it is important to ensure effective conditions (pH > 10.5 and duration >1-2 h) for the entire sewer line.

Laboratory assessment of bioproducts for sulphide and methane control in sewer systems.

The effectiveness of three bioproducts (also known as biomaterials) for liquid-phase biological treatment (LPBT) of sewer biofilms to control detrimental build-up of sulphide (H(2)S) and methane (CH(4)) in sewers was tested in a laboratory system mimicking a rising/force main sewer pipe. Bioproduct A claims to disrupt cell-to-cell communication of sewer anaerobic biofilms while Bioproducts B and C claim to enhance sulphidotrophic (sulphide-oxidising) capacity of the sewer biofilm, to avoid sulphide accumulation. The results demonstrated that all three bioproducts tested had no or negligible impact on sulphide or methane control, as opposed to traditional sulphide-controlling chemicals widely used by the wastewater industry such as oxygen, nitrate, iron salts and magnesium hydroxide. Those had previously been demonstrated to be effective using the same laboratory system with the same testing protocol. The implications of the findings are discussed. It is concluded that field application/trials of these three bioproducts are not warranted. It is recommended that other bioproducts should be subject to similar rigorous tests prior to being taken up by the water industry for field trials/application.

SCORe-CT: a new method for testing effectiveness of sulfide-control chemicals used in sewer systems.

A new method for testing the effectiveness of chemical products for sulfide control in sewers is reported. The method, called SCORe-CT (Sewer Corrosion and Odour Research - Chemical Testing), consists of two specially designed laboratory-scale systems that mimic sulfide production in real rising main sewers, and a multi-phase and multi-facet testing protocol. The monitoring tools/methods include both routine chemical analysis of various sulfurous and carbonaceous compounds in liquid and their on-line monitoring using advanced sensors. Molecular methods and microelectrodes can also be employed to examine the microbial structure and activity of sewer biofilms. The SCORe-CT method is not proposed to replace field trials but to screen chemicals prior to their often costly trials/applications in field conditions. For effective chemicals the method helps to reveal the mechanisms involved, and assists with the design of optimal dosage strategies, which would significantly reduce application costs. In this paper, the method is explained in detail and demonstrated with several case studies.

Chemical dosing for sulfide control in Australia: An industry survey.

Controlling sulfide (H(2)S) production and emission in sewer systems is critical due to the corrosion and malodour problems that sulfide causes. Chemical dosing is one of the most commonly used measures to mitigate these problems. Many chemicals have been reported to be effective for sulfide control, but the extent of success varies between chemicals and is also dependent on how they are applied. This industry survey aims to summarise the current practice in Australia with the view to assist the water industry to further improve their practices and to identify new research questions. Results showed that dosing is mainly undertaken in pressure mains. Magnesium hydroxide, sodium hydroxide and nitrate are the most commonly used chemicals for sewers with low flows. In comparison, iron salts are preferentially used for sulfide control in large systems. The use of oxygen injection has declined dramatically in the past few years. Chemical dosing is mainly conducted at wet wells and pumping stations, except for oxygen, which is injected into the pipe. The dosing rates are normally linked to the control mechanisms of the chemicals and the dosing locations, with constant or profiled dosing rates usually applied. Finally, key opportunities for improvement are the use of mathematical models for the selection of chemicals and dosing locations, on-line dynamic control of the dosing rates and the development of more cost-effective chemicals for sulfide control.

Optimization of intermittent, simultaneous dosage of nitrite and hydrochloric acid to control sulfide and methane productions in sewers.

Free nitrous acid (FNA) was previously demonstrated to be biocidal to anaerobic sewer biofilms. The intermittent dosing of FNA as a measure for controlling sulfide and methane productions in sewers is investigated. The impact of three key operational parameters namely the dosing concentration, dosing duration and dosing interval on the suppression and subsequent recovery of sulfide and methane production was examined experimentally using lab-scale sewer reactors. FNA as low as 0.26 mg-N/L was able to suppress sulfide production after an exposure of 12h. In comparison, 0.09 mg-N/L of FNA with 6-h exposure was adequate to restrain methanogenesis effectively. The recovery of sulfide production was well described by an exponential recovery equation. Model-based analysis revealed that 12-h dosage at an FNA concentration of 0.26 mg-N/L every 5 days can reduce the average sulfide production by >80%. Economic analysis showed that intermittent FNA dosage is potentially a cost-effective strategy for sulfide and methane control in sewers.

The strong biocidal effect of free nitrous acid on anaerobic sewer biofilms.

Several recent studies showed that nitrite dosage to wastewater results in long-lasting reduction of the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms. In this study, we revealed that the quick reduction in these activities is due to the biocidal effect of free nitrous acid (FNA), the protonated form of nitrite, on biofilm microorganisms. The microbial viability was assessed after sewer biofilms being exposed to wastewater containing nitrite at concentrations of 0-120 mg-N/L under pH levels of 5-7 for 6-24 h. The viable fraction of microorganisms was found to decrease substantially from approximately 80% prior to the treatment to 5-15% after 6-24 h treatment at FNA levels above 0.2 mg-N/L. The level of the biocidal effect has a much stronger correlation with the FNA concentration, which is well described by an exponential function, than with the nitrite concentration or with the pH level, suggesting that FNA is the actual biocidal agent. An increase of the treatment from 6 to 12 and 24 h resulted in only slight decreases in microbial viability. Physical disrupted biofilm was more susceptible to FNA in comparison with intact biofilms, indicating that the biocidal effect of FNA on biofilms was somewhat reduced by mass transfer limitations. The inability to achieve 2-log killing even in the case of disrupted biofilms suggests that some microorganisms may be more resistant to FNA than others. The recovery of biofilm activities in anaerobic reactors after being exposed to FNA at 0.18 and 0.36 mg-N/L, respectively, resembled the regrowth of residual sulfate-reducing bacteria and methanogens, further confirming the biocidal effects of FNA on microorganisms in biofilms.

A laboratory assessment of the impact of brewery wastewater discharge on sulfide and methane production in a sewer.

The impact of brewery wastewater discharge on sulfide and methane production in a sewer was assessed. Experiments were carried out on laboratory scale sewer reactors consisting of both an experimental and a control reactor. The control reactor was intermittently fed with real fresh sewage while the experimental reactor was fed with a mixture of brewery and domestic wastewater at two different proportions (10 and 25% v/v). 10% v/v discharge of brewery wastewater increased the H2S and CH4 production rates in the sewer reactor by 40% and 30%, respectively. When the brewery wastewater fraction was increased to 25% v/v, the H2S production rate of the experimental reactor decreased to the level of the control reactor. In contrast, the CH4 production rate maintained at a level that was 30% higher than that in the control reactor. These results indicate that the discharge of brewery wastewater into sewers can give negative impacts in relation to odour and corrosion management of the systems and will increase the greenhouse gas emissions from sewers. The study also reveals that the impact of trade waste on the biological reactions in sewers is complex, and requires careful experimental assessment in each case.

Effects of nitrite concentration and exposure time on sulfide and methane production in sewer systems.

Nitrite dosing is a promising technology to prevent sulfide and methane formation in sewers, due to the known inhibitory/toxic effect of nitrite on sulfate-reducing bacteria (SRB) and methanogenic Archaea (MA). The dependency of nitrite-induced inhibition on sulfide and methane producing activities of anaerobic sewer biofilms on nitrite levels and exposure time is investigated using a range of nitrite concentrations (40, 80, 120 mg-N/L) and exposure time up to 24 days. The recovery of these activities after the 24-day nitrite dosage was also monitored for more than two months. The inhibition level was found to be dependent on both nitrite concentration and exposure time, with stronger inhibition observed at higher nitrite concentrations and/or longer exposure time. However, the time required for achieving 50% recovery of both sulfate-reducing and methanogenic activities after the cessation of nitrite dosage only marginally depended on nitrite concentration. Model-based analysis of the recovery data showed that the recovery was likely due to the regrowth of SRB and methanogens. The lab studies and mathematical analysis supported the development of an intermittent dosing strategy, which was tested in a 1-km long rising main sewer. The field trial confirmed that intermittent dosing of nitrite can effectively reduce/prevent the formation of both sulfide and methane.

Iron salts dosage for sulfide control in sewers induces chemical phosphorus removal during wastewater treatment.

Chemical phosphorus (P) removal during aerobic wastewater treatment induced by iron salt addition in sewer systems for sulfide control is investigated. Aerobic batch tests with activated sludge fed with wastewater containing iron sulfide precipitates showed that iron sulfide was rapidly reoxidised in aerobic conditions, resulting in phosphate precipitation. The amount of P removed was proportional to the amount of iron salts added, and for the sludge used, ratios of 0.44 and 0.37 mgP/mgFe were obtained for ferric and ferrous dosages, respectively. The hydraulic retention time (HRT) of iron sulfide in sewers was found to have a crucial impact on the settling of iron sulfide precipitates during primary settling, with a shorter HRT resulting in a higher concentration of iron sulfide in the primary effluent and thus enabling higher P removal. A mathematical model was developed to describe iron sulfide oxidation in aerated activated sludge and the subsequent iron phosphate precipitation. The model was used to optimise FeCl(3) dosing in a real wastewater collection and treatment system. Simulation studies revealed that, by moving FeCl(3) dosing from the WWTP, which is the current practice, to a sewer location upstream of the plant, both sulfide control and phosphate removal could be achieved with the current ferric salt consumption. This work highlights the importance of integrated management of sewer networks and wastewater treatment plants.

Simultaneous online measurement of sulfide and nitrate in sewers for nitrate dosage optimisation.

A new method for the simultaneous online measurement of sulfide and nitrate in wastewater is developed. A UV-VIS spectrometer was used. The sensor was calibrated by means of simultaneous online and offline measurements of sulfide and nitrate in batch tests carried out on a laboratory-scale sewer system. The developed calibration algorithm was successfully validated for both sulfide and nitrate measurement, with confidence limits of 2.7 mg S/L for total dissolved sulfide, and 7.5 mg N/L for nitrate. The online measurement of sulfide and nitrate enabled detailed evaluation of seven nitrate dosing strategies in the laboratory-scale sewer system, providing strong support to process optimisation. The dosage optimisation revealed that nitrate should be added at a location close to the point of sulfide control rather than at the beginning of a rising main, at a rate proportional to the expected hydraulic retention time (HRT) of the wastewater in the sewer section between the point of nitrate addition and the point where sulfide control is desired.

Effects of long-term pH elevation on the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms.

The dosage of alkali is often applied by the wastewater industry to reduce the transfer of hydrogen sulfide from wastewater to the sewer atmosphere. In this paper the activities of Sulfate Reducing Bacteria (SRB) and Methanogenic Archaea (MA) under elevated pH conditions (8.6 and 9.0) were evaluated in a laboratory scale anaerobic sewer reactor. Compared to those in a control reactor without pH control (pH 7.6+/-0.1), the SRB activity was reduced by 30% and 50%, respectively, at pH 8.6 and pH 9.0. When normal pH was resumed, it took approximately 1 month for the SRB activity to fully recover. Methanogenic activities developed in the control reactor in 3 months after the reactor start-up, while no significant methanogenic activities were detected in the experimental reactor until normal pH was resumed. The results suggest that elevated pH at 8.6-9.0 suppressed the growth of methanogens. These experimental results clearly showed that, in addition to its well-known effect of reducing H(2)S transfer from the liquid to the gas phase, pH elevation considerably reduces sulfide and methane production by anaerobic sewer biofilms. These findings are significant for the optimal use of alkali addition to sewers for the control of H(2)S and CH(4) emissions. A model-based study showed that, by adding the alkali at the beginning rather than towards the end of a rising main, substantial savings in chemicals can be achieved while achieving the same level of sulfide emission control, and complete methane emission control.

Evaluation of oxygen injection as a means of controlling sulfide production in a sewer system.

Oxygen injection is often used to control biogenic production of hydrogen sulfide in sewers. Experiments were carried out on a laboratory system mimicking a rising main to investigate the impact of oxygen injection on anaerobic sewer biofilm activities. Oxygen injection (15-25mg O(2)/L per pump event) to the inlet of the system decreased the overall sulfide discharge levels by 65%. Oxygen was an effective chemical and biological oxidant of sulfide but did not cause a cessation in sulfide production, which continued in the deeper layers of the biofilm irrespective of the oxygen concentration in the bulk. Sulfide accumulation resumed instantaneously on depletion of the oxygen. Oxygen did not exhibit any toxic effect on sulfate reducing bacteria (SRB) in the biofilm. It further stimulated SRB growth and increased SRB activity in downstream biofilms due to increased availability of sulfate at these locations as the result of oxic conditions upstream. The oxygen uptake rate of the system increased with repeated exposure to oxygen, with concomitant consumption of organic carbon in the wastewater. These results suggest that optimization of oxygen injection is necessary for maximum effectiveness in controlling sulfide concentrations in sewers.