Experimental and modeling investigations on the unexpected hydrogen sulfide rebound in a sewer receiving nitrate addition: Mechanism and solution.

Biogenic hydrogen sulfide is an odorous, toxic and corrosive gas released from sewage in sewers. To control sulfide generation and emission, nitrate is extensively applied in sewer systems for decades. However, the unexpected sulfide rebound after nitrate addition is being questioned in recent studies. Possible reasons for the sulfide rebounds have been studied, but the mechanism is still unclear, so the countermeasure is not yet proposed. In this study, a lab-scale sewer system was developed for investigating the unexpected sulfide rebounds via the traditional strategy of nitrate addition during 195-days of operation. It was observed that the sulfide pollution was even severe in a sewer receiving nitrate addition. The mechanism for the sulfide rebound can be differentiated into short-term and long-term effects based on the dominant contribution. The accumulation of intermediate elemental sulfur in biofilm resulted in a rapid sulfide rebound via the high-rate sulfur reduction after the depletion of nitrate in a short period. The presence of nitrate in sewer promoted the microorganism proliferation in biofilm, increased the biofilm thickness, re-shaped the microbial community and enhanced biological denitrification and sulfur production, which further weakened the effect of nitrate on sulfide control during the long-term operation. An optimized biofilm-initiated sewer process model demonstrated that neither the intermittent nitrate addition nor the continuous nitrate addition was a sustainable strategy for the sulfide control. To minimize the negative impact from sulfide rebounds, a (bi)monthly routine maintenance (e.g., hydraulic flushing with nitrate spike) to remove the proliferative microorganism in biofilm is necessary.

[The diagnostic utility of the Macao predictive values of impulse oscillometry for chronic obstructive pulmonary disease in patients over 45 years old].

OBJECTIVE: To investigate the clinical use of Macao predictive values of impulse oscillometry(IOS) for chronic obstructive pulmonary disease (COPD) in patients aged over 45 years. METHODS: We measured lung impedance with IOS and spirometry in healthy subjects (n=168) and patients with COPD (n=281) aging over 45 years. The spirometric parameters were compared with those of IOS calculated by Macao predictive equations with Lechtenboerger equations. RESULTS: Respiratory impedance (Zrs), respiratory resistance at 5 Hz (R5), R5-R20 in female COPD group were (0.72±0.28), (0.63±0.23)and(0.23±0.16) kPa·L(-1)·s(-1), respectively, and Fres was (22±7)Hz; while in male group, the value of each parameters was (0.56±0.21), (0.50±0.17) and(0.18±0.12) kPa·L(-1)·s(-1), Fres was(21±7)Hz, which were all greater than that of the healthy group(t value was 5.05, 4.30, 5.10, 6.05 and 8.27, 6.62, 12.68, 14.59, respectively; P value were all<0.01). Reactance at 5 Hz(X5) in the COPD group[(-0.30±0.21) kPa·L(-1)·s(-1) in female, (-0.26±0.16) kPa·L(-1)·s(-1) in male] was significantly lower than that in the healthy group[female group: X5=(-0.16±0.06) kPa·L(-1)·s(-1,) t value was -5.38; male group: X5=(-0.10±0.05) kPa·L(-1)·s(-1,) t value was -11.96, P value were all<0.01]. Zrs, R5, R5-R20 and Fres were negatively correlated with parameters (FEV1/FVC, FEV1%Pre) of spirometry, while X5 was positively correlated with them. Compared with the ROC areas under the curve(AUC), the AUC of Zrs(A/P2) (0.786 in female, 0.773 in male) was same as that of Zrs(A)(0.744 in female, 0.764 in male; χ(2) value was 4.96, 0.89, respectively, P value were all >0.05), the AUC of R5(A/P2)(0.754 in female, 0.741 in male) was larger than that of R5(A/P1) (both were 0.716; χ(2) value was 4.24, 6.38, respectively, P value were all <0.05). The AUC of X5(P2-A) was larger than that of X5(P1-A) in the male group, and it was same as in the female group. The first two AUC of IOS parameters were Fres and R5-R20. In the 2 groups, when Zrs (A/P2)% was larger than 130, R5(A/P2)% was larger than 130, X5(P2-A)was larger than 0.1, Fres was larger than 15 in male, 20 in female, their each Youden's index was 0.463, 0.398, 0.662 and 0.594, each accuracy was 84%, 71%, 81% and 82%, which were all greater than that of Lechtenboerger equations(66%, 63%, 80% and 50%). CONCLUSION: There are good correlations between spirometry and respiratory impendance measured by IOS in the diagnosis of COPD. The Macao predictive equations have higher sensitivity and specificity for diagnosing COPD.

Hydrogen sulfide control in sewer systems: A critical review of recent progress.

In sewer systems where anaerobic conditions are present, sulfate-reducing bacteria reduce sulfate to hydrogen sulfide (H2S), leading to sewer corrosion and odor emission. Various sulfide/corrosion control strategies have been proposed, demonstrated, and optimized in the past decades. These included (1) chemical addition to sewage to reduce sulfide formation, to remove dissolved sulfide after its formation, or to reduce H2S emission from sewage to sewer air, (2) ventilation to reduce the H2S and humidity levels in sewer air, and (3) amendments of pipe materials/surfaces to retard corrosion. This work aims to comprehensively review both the commonly used sulfide control measures and the emerging technologies, and to shed light on their underlying mechanisms. The optimal use of the above-stated strategies is also analyzed and discussed in depth. The key knowledge gaps and major challenges associated with these control strategies are identified and strategies dealing with these gaps and challenges are recommended. Finally, we emphasize a holistic approach to sulfide control by managing sewer networks as an integral part of an urban water system.

A pilot-scale sulfur-based sulfidogenic system for the treatment of Cu-laden electroplating wastewater using real domestic sewage as electron donor.

Elemental sulfur (S0) reduction process has been demonstrated as an attractive and cost-efficient approach for metal-laden wastewater treatment in lab-scale studies. However, the system performance and stability have not been evaluated in pilot- or large-scale wastewater treatment. Especially, the sulfide production rate and microbial community structure may significantly vary from lab-scale system to pilot- or large-scale systems using real domestic sewage as carbon source, which brings questions to this novel technology. In this study, therefore, a pilot-scale sulfur-based sulfidogenic treatment system was newly developed and applied for the treatment of Cu-laden electroplating wastewaters using domestic sewage as carbon source. During the 175-d operation, >99.9% of Cu2+ (i.e., 5580 and 1187 mg Cu/L for two types of electroplating wastewaters) was efficiently removed by the biogenic hydrogen sulfide that produced through S0 reduction. Relatively high level of sulfide production (200 mg S/L) can be achieved by utilizing organics in raw domestic sewage, which was easily affected by the organic content and pH value of the domestic sewage. The long-term feeding of domestic sewage significantly re-shaped the microbial community in sulfur-reducing bioreactors. Compared to the reported lab-scale bioreactors, higher microbial community diversity was found in our pilot-scale bioreactors. The presence of hydrolytic, fermentative and sulfur-reducing bacteria was the critical factor for system stability. Accordingly, a two-step ecological interaction among fermentative and sulfur-reducing bacteria was newly proposed for sulfide production: biodegradable particulate organic carbon (BPOC) was firstly degraded to dissolved organic carbon (DOC) by the hydrolytic and fermentative bacteria. Then, sulfur-reducing bacteria utilized the total DOC (both DOC degraded from BPOC and the original DOC present in domestic sewage) as electron donor and reduced the S0 to sulfide. Afterwards, the sulfide precipitated Cu2+ in the post sedimentation tank. Compared with other reported technologies, the sulfur-based treatment system remarkable reduced the total chemical cost by 87.5‒99.6% for the same level of Cu2+ removal. Therefore, this pilot-scale study demonstrated that S0 reduction process can be a sustainable technology to generate sulfide for the co-treatment of Cu-laden electroplating wastewater and domestic sewage, achieving higher Cu2+removal and higher cost-effectiveness than the conventional technologies.

Effects of food waste addition on biofilm formation and sulfide production in a gravity sewer.

The conversion of food waste (FW) into the sewage system is regarded as a promising method of relieving the burden of solid waste management. However, knowledge about its effects on sewer processes is limited, particularly in terms of biofilm formation and sulfide production. In this study, a gravity sewer system was set up to investigate the effects of the addition of FW on biofilm formation, the sulfate-reducing bacteria (SRB) population, and the sulfide production potential. The sewer biofilm characteristics changed with long-term FW addition, and a greater thickness (by 32%), an increased dry density (by 13%), and more extracellular polymeric substance (by 141%) were observed. The thicker and denser biofilm limited oxygen diffusion, enlarged the anaerobic area in the sewer biofilm, promoted an increase in the SRB population, and enhanced the sulfide production potential in the gravity sewer. Substantial differences in the H2S profiles in the biofilm samples with and without the addition of FW were observed via microelectrode analysis. A model-based investigation of sewer biofilm formation with and without the addition of FW was conducted with a dynamic sewer biofilm model to gain further insights into sewer biofilm processes. The results suggest that the addition of FW can promote sulfide production and SRB growth in a sewer biofilm, which can be significantly affected by the ratio of FW to sewage. It is worth further investigations of the impacts of FW addition on the potential sulfide production in pressure sewers.

Systematic evaluation of a dynamic sewer process model for prediction of odor formation and mitigation in large-scale pressurized sewers in Hong Kong.

To evaluate and mitigate odor formation and emission in sewers, several sewer models have been developed. Although these models can predict the immediate effects of chemical dosing on odor emission control, the long-term effects due to the variation of biofilm dynamics were generally underestimated. Therefore, in this study, we developed a dynamic model to simulate sewer processes initiated by sewer. The dynamic sewer process model was calibrated and validated with experimental data collected from two pressurized mains in actual operation in Hong Kong (TCS and MH17). The results show that the dynamic model can satisfactorily predict the dynamic concentrations of sulfide and ammonium (with measured and simulated values differing by less than 6%). The model was employed to systematically assess the long-term effects of three commonly used control strategies, i.e. addition of nitrate salts, addition of biocides, and hydraulic flushing, on sulfide formation and to predict sewer biofilm compositions. The modeling results reveal that the effect of odor mitigation measures on sulfide control varied with time due to the re-establishment of sulfate-reducing bacteria community in sewer biofilm. The long-term effect of nitrate addition would be diminishing because of the growth of heterotrophic denitrifies in sewer biofilms (increased from 7% to 21% after 55 days of nitrate addition) to consumed more nitrate. After dosing biocide or hydraulic flushing in sewers, sulfide production would rebound in the following several days due to the regrowth of sewer biofilms, indicating that the optimization of odor mitigation strategies is necessary. This study highlights that the biofilm dynamics shall be involved in the simulation of odor formation and emission, to evaluate and optimize the long-term effects of mitigation measures.

Experimental and modelling evaluations of sulfide formation in a mega-sized deep tunnel sewer system and implications for sewer management.

Mega-sized deep tunnel sewer systems are indispensable infrastructures to convey the sewage and/or stormwater to the centralized sewage treatment works in large cities with dense populations and limited land. The rapid urbanization in China and other countries is boosting the construction of the deep tunnel sewer systems. However, the formation of sulfide, which induces serious odor nuisance and sewer corrosion, has not been investigated in such sewer systems. Taking a real Sewage Conveyance System (SCS) with 23.3 km-long and 70-160 m-deep interconnected tunnels in Hong Kong as a representative example, this study conducted experimental and modelling investigations to evaluate the sulfide formation in the mega-sized deep tunnel sewer systems. The field investigation revealed that the daily sulfide production rate in the SCS was up to 1410 kg S/d, suggesting the substantial sulfide production during the long-distance and long-time sewage conveyance. Using a validated Biofilm-Initiated Sewer Process Model (BISM), the sulfide formation in the SCS under the influences of various factors, which are relevant to the situations in China and other countries, were simulated. The simulation results showed that 89% of the total sulfide production in the SCS was generated in the two tunnels with long hydraulic retention times (HRT) and large flowrates. The specific sulfide formation rates exhibited a linear relationship with HRT (R2 = 0.61), but the linear relationship with the sewer diameter was weak. The sulfide production rate increased with increasing temperature (12 °C-32 °C) by 3.5 times, and it only decreased by 50% when the sulfate concentration decreased from 309 to 17 mg S/L, indicating that serious sulfide pollution could still happen in the sewers with a low concertation of sulfate in sewage. Increasing the organic levels in sewage would also promote the sulfide production in sewers. The flowrate would not influence the sulfide production rate significantly, but a storm event could remarkably reduce the sulfide production in rainy days. The findings unveil the potential serious sulfide problems in the mega-sized deep tunnel sewer systems, which are being increasingly constructed in China and other countries. To mitigate the odor and corrosion problems in the deep tunnel sewer systems, the sulfide control strategies should be considered during the sewer design and management.