Keshab Sharma

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.

Sulfur transformation in rising main sewers receiving nitrate dosage

The anoxic and anaerobic sulfur transformation pathways in a laboratory-scale sewer receiving nitrate were investigated. Four reactors in series were employed to imitate a rising main sewer. The nitrate-dosing strategy was effective in controlling sulfide, as confirmed by the long-term sulfide measurements. Anoxic sulfide oxidation occurred in two sequential steps, namely the oxidation of sulfide to elemental sulfur (S(0)) and the oxidation of S(0) to sulfate (SO(4)(2-)). The second oxidation step, which primarily occurred when the first step was completed, had a rate that is approximately 15% of the first step. When nitrate was depleted, sulfate and elemental sulfur were reduced simultaneously to sulfide. Sulfate reduction had a substantially higher rate (5 times) than S(0) reduction. The relatively slower S(0) oxidation and reduction rates implied that S(0) was an important intermediate during anoxic and anaerobic sulfur transformation. Electron microscopic studies indicated the presence of elemental sulfur, which was at a significant level of 9.9 and 16.7 mg-S/g-biomass in nitrate-free and nitrate-exposed sewer biofilms, respectively. A conceptual sulfur transformation model was established to characterize predominant sulfur transformations in rising main sewers receiving nitrate dosage. The findings are pertinent for optimizing nitrate dosing to control sulfide in rising main sewers.

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.

Dynamics and dynamic modelling of H2S production in sewer systems

Accurate and reliable predictions of sulfide production in a sewer system greatly benefit formulation of appropriate strategies for optimal sewer management. Sewer systems, rising main systems in particular, are highly dynamic in terms of both flow and wastewater composition. In order to get an insight in sulfide production as a response to the dynamic changes in sewer conditions, several measurement campaigns were conducted in two rising mains in Gold Coast, Australia. The levels of various sulfur species and volatile fatty acids (VFAs) were monitored through hourly sampling for periods ranging from 8 to 29 h. The results of these field studies showed large temporal as well as spatial variations in sulfide generation. A dynamic sewer model taking into account the hydraulics and the biochemical transformation processes was formulated and calibrated and validated using the data collected during the four measurement campaigns at the two sites. The model was demonstrated to reasonably well describe the temporal and spatial variations in sulfide, sulfate and VFA concentrations. Application of the model was illustrated with a case study aimed to optimize oxygen injection to one of the two mains studied, which is being used as a means to control sulfide production on this site. The model predicted that, moving the current oxygen injection point to a location close to the end of the sewer line could achieve the same degree of sulfide control with only 50% of the current oxygen use. This study highlighted that the location at which oxygen is injected plays a major role in sulfide control and a dynamic model could be used to make a proper choice of the location.

Development of a model for assessing methane formation in rising main sewers

Significant methane formation in sewers has been reported recently, which may contribute significantly to the overall greenhouse gas emission from wastewater systems. The understanding of the biological conversions occurring in sewers, particularly the competition between methanogenic and sulfate-reducing populations for electron donors, is an essential step for minimising methane emissions from sewers. This work proposes an extension to the current state-of-the-art models characterising biological and physicochemical processes in sewers. This extended model includes the competitive interactions of sulfate-reducing bacteria and methanogenic archaea in sewers for various substrates available. The most relevant parameters of the model were calibrated with lab-scale experimental data. The calibrated model described field data reasonably well. The model was then used to investigate the effect of several key sewer design and operational parameters on methane formation. The simulation results showed that methane production was highly correlated with the hydraulic residence time (HRT) and pipe area to volume (A/V) ratio showing higher methane concentrations at a long HRT or a larger A/V ratio.

Impact of nitrate addition on biofilm properties and activities in rising main sewers

Anaerobic sewer biofilm is a composite of many different microbial populations, including sulfate reducing bacteria (SRB), methanogens and heterotrophic bacteria. Nitrate addition to sewers in an attempt to control hydrogen sulfide concentrations affects the behaviour of these populations, which in turn impacts on wastewater characteristics. Experiments were carried out on a laboratory reactor system simulating a rising main to determine the impact of nitrate addition on the microbial activities of anaerobic sewer biofilm. Nitrate was added to the start of the rising main during sewage pump cycles at a concentration of 30 mg-N L(-1) for over 5 months. While it reduced sulfide levels at the outlet of the system by 66%, nitrate was not toxic or inhibitory to SRB activity and did not affect the dominant SRB populations in the biofilm. Long-term nitrate addition in fact stimulated additional SRB activity in downstream biofilm. Nitrate addition also stimulated the activity of nitrate reducing, sulfide oxidizing bacteria that appeared to be primarily responsible for the prevention of sulfide build up in the wastewater in the presence of nitrate. A short adaptation period of three to four nitrate exposure events (approximately 10 h) was required to stimulate biological sulfide oxidation, beyond which no sulfide accumulation was observed under anoxic conditions. Nitrate addition effectively controlled methane concentrations in the wastewater. The nitrate uptake rate of the biofilm increased with repeated exposure to nitrate, which in turn increased the consumption of biodegradable COD in the wastewater. These results provide a comprehensive understanding of the impact of nitrate addition on wastewater composition and sewer biofilm microbial activities, which will facilitate optimization of nitrate dosing for effective sulfide control in rising main sewers.

Reducing sewer corrosion through integrated urban water management

Sourcing corrosive sewer sulfides Sewer systems are corroding at an alarming rate, costing governments billions of dollars to replace. Differences among water treatment systems make it difficult to track down the source of corrosive sulfide responsible for this damage. Pikaar et al. performed an extensive industry survey and sampling campaign across Australia (see the Perspective by Rauch and Kleidorfer). Aluminum sulfate added as a coagulant during drinking water treatment was the primary culprit in corroding sewer systems. Modifying this common treatment strategy to include sulfate-free coagulants could dramatically reduce sewer corrosion across the globe. Science, this issue p. 812; see also p. 734 Decreasing sulfate added during drinking water treatment can prevent corrosion of sewers caused by wastewater. [Also see Perspective by Rauch and Kleidorfer] Sewer systems are among the most critical infrastructure assets for modern urban societies and provide essential human health protection. Sulfide-induced concrete sewer corrosion costs billions of dollars annually and has been identified as a main cause of global sewer deterioration. We performed a 2-year sampling campaign in South East Queensland (Australia), an extensive industry survey across Australia, and a comprehensive model-based scenario analysis of the various sources of sulfide. Aluminum sulfate addition during drinking water production contributes substantially to the sulfate load in sewage and indirectly serves as the primary source of sulfide. This unintended consequence of urban water management structures could be avoided by switching to sulfate-free coagulants, with no or only marginal additional expenses compared with the large potential savings in sewer corrosion costs.

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.

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.

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.