Modeling Hydrogen Sulfide Emission Rates in Gravity Sewage Collection Systems

Abstract

The factors affecting sulfide buildup in gravity sewers are complex, consisting of biological and physical processes, both in the aqueous and the gas phases of the sewer. The rate of each of these processes varies (among other parameters) according to flow characteristics, temperature, and pH. Under fast and turbulent flow conditions, the stripping of hydrogen sulfide into the gas phase may become the dominant process. The paper presents a semiempirical approach to the problem of quantifying hydrogen sulfide emission rates in sewers. Kinetics of hydrogen sulfide emission as a function of hydraulic parameters was measured in the laboratory using methods adopted from flocculation theory. A flocculation unit was used to impart a selected velocity gradient ͑G͒ into the water, and sulfide concentration was measured with time. The process was repeated for a number of G values. Regression analysis was then used to fit the rate of hydrogen sulfide emission equation against G. An equation was developed linking G to H L (head loss) in sewers assuming plug flow conditions. The hydraulic model and the kinetic model were linked (via G) to give the desired rate equation for hydrogen sulfide emission along a sewer line. The model was used to predict H 2 S emission from a uniform flow sewer and the effect of parameters such as pH, sewer slope and degree of fullness was studied. As expected, results show that low pH, high slope, and low degree of fullness enhance emission rates. Reasonable agreement was attained when the model output was compared with measured results from a field test sewer in Virginia, South Africa, under conditions where sulfide stripping was the rate-dominant process.

A Different Approach for Predicting Reaeration Rates in Gravity Sewers and Completely Mixed Tanks

“…The dependency of dO 2 /dt on G 2 under completely mixed conditions conforms well with previous results obtained from similar experiments, where the rate of H 2 S (aq) stripping was quantified ( Lahav et al, 2004 ). From a physical standpoint, the link between oxygen transfer rate and G 2 is very reasonable, as G 's mathematical definition is the square root of the power imparted for mixing, divided by the volume of the water sample (see ).…”

Why Aren't H<SUB>2</SUB>S Concentrations Logical? (What We Are Missing In Daily H<SUB>2</SUB>S Patterns)

“…Therefore, increased detention times due to diurnal flow rates do not necessarily result in greater H 2 S emission rates. The opposite was found by Lahav (2004) and by a simple computational analysis in this paper. The lower and slower flow results in an increased wastewater surface area versus the wastewater volume that results in greater reaeration and sulfide oxidation in the wastewater.…”

“…2 Where S 2 = predicted sulfide concentration at time t 2 (mg/L); S 1 = sulfide concentration at time t 1 (mg/L); S lim = limiting sulfide concentration (mg/L) = (M/m); = (M'/m) EBOD (su) -3/8 (P/b) EBOD = effective biochemical oxygen demand (BOD), BOD = BOD X 1.07 (T-20) (mg/L) T = wastewater temperature ( ) M' = effective sulfide flux coefficient in gravity sewers (m/h); m = empirical coefficient for sulfide loss; s = slope (m/m); u = stream velocity (m/s); t = flow time in a given sewer reach with constantly slope, diameter, and flow (h); d m = mean hydraulic depth, equal to area of flow divided by surface width (m); P = wetted perimeter (m); and b = width of wastewater stream at surface (m). Lahav (2004Lahav ( , p. 1383) explains the equation consists of two terms: the first predicts the rate of sulfide generation in the sewer and the second represents the rate of sulfide elimination from the liquid phase, i.e., the combined effect of biological sulfide oxidation, sulfide stripping, and indirectly, the effect of natural ventilation on the concentration of sulfide in the gas phase of the sewer. Figure 4 …”

“…Lahav (2004Lahav ( , p. 1383 states the weakest link in available models is related to H 2 S emission as a function of the hydraulic conditions. Lahav also states that the K L value depends on both temperature and mixing conditions in the water, and further, in the sewer environment, the mixing conditions will depend on hydraulic flow characteristics, which can be depicted by the head loss along the line of flow.…”

Response Surface Analysis and Statistical Modeling of Sulfide Generation from Municipal Wastewater

“…The H 2 S production reaction does not take place if dissolved oxygen (DO) or another more thermodynamically favored electron acceptor, e.g., nitrate, is present in wastewater [8]. Many research works have been recently conducted an attempt to better understand and model hydrogen sulfide formation in sewer networks [6,[9][10][11][12], which mainly focuses on the H 2 S (g) emission rate modeling under steady state condition. Despite the significance of the H 2 S (aq) generation rate only a handful of publications are found in the literature on this matter.…”