Hydrogen Sulfide, Corrosion and BOD5 Modeling in InfoSWMM and InfoSWMM SA

The control of odorous gases and the corrosion of sewers are the two most important problems in operating wastewater collection systems. Evaluation of existing or potential odor or corrosion problems, and identification of where such problems will occur is, therefore, highly essential. In sanitary sewer systems, odors are produced as a result of biological decomposition of organic matter, particularly those containing sulfur and nitrogen, under anaerobic conditions prevailing in the slime layer of gravity pipes, force mains, and wet wells. Hydrogen sulfide and ammonia are the only malodorous inorganic gases produced from the decomposition. Other odor producing substances include organic vapors such as idoles, skatoles, mercaptans and nitrogen-bearing organics. However, Hydrogen Sulfide (H2S) is the most commonly known and prevalent odorous gas associated with domestic wastewater collection and treatment systems. InfoSWMM H2OMap SWMM InfoSWMM SA gives wastewater engineers a powerful Operations and Maintenance ( O&M) tool to readily model and analyze entire sewer collection systems for sulfide generation and corrosion potential under varying conditions anticipated throughout the life of their systems.

Hydrogen sulfide has a characteristic rotten egg odor, is extremely toxic, is corrosive to metals, and is a precursor to the formation of sulfuric acid (which corrodes concrete, lead-based paints, metals, and other materials). The conditions leading to the formation of Hydrogen Sulfide generally favor the production of other odorous organic compounds. Therefore, investigation of the conditions favoring the Hydrogen Sulfide formation not only helps to quantify the potential for odor generation from other compounds, but also it aids in identifying potential corrosion problems in the collection system.

The occurrence of Hydrogen Sulfide in wastewater collection systems, other than that added from industrial sources and infiltrated groundwater, is primarily the result of the reduction of sulfate ion (), one of the most universal anions occurring in natural waters, under anaerobic conditions, as shown by the following reaction.

The molecular Hydrogen Sulfide, formed from sulfate reduction, dissolves in the waste water and dissociates in accordance with reversible ionization reactions, expressed as:

The partitioning of the hydrogen sulfide into these components (i.e., (H2S) aqueous, HS- ion, and S= ion) depends primarily on the temperature and the pH of the wastewater, although ionic strength, as represented by dissolved solids or electrical conductivity, also affects the partitioning. The HS- ion and S= ion produce no odors. Some of the aqueous hydrogen sulfide will escape into the sewer atmosphere causing the odor problem. The concentration of hydrogen sulfide gas in the atmosphere will vary with the concentration of (hydrogen sulfide) aqueous according to Henry’s law. The rate of escape of hydrogen sulfide gas is a function of the difference between the saturation or equilibrium concentration determined by Henry’s law and the actual concentration of hydrogen sulfide in the sewer atmosphere. The EPA provides a figure that shows hydrogen sulfide in the sewer atmosphere in equilibrium with the given concentrations of aqueous hydrogen sulfide concentration in the wastewater at the respective temperatures, for a pressure of one atmosphere.

Prediction of the rate of sulfide buildup and corrosion potential is an essential element in the design of new sewer systems as well as in the evaluation of existing systems. The rate of sulfide buildup depends on a number of environmental conditions, including concentration of organic material and nutrients, sulfate concentration, dissolved oxygen (DO), pH, temperature, stream velocity, surface area, and detention (residence) time. Accounting for all these environmental conditions, InfoSWMM predicts sulfide buildup in sewer collection systems for gravity sewers, force mains, and wet wells using the Pomeroy-Parkhurst equations. InfoSWMM enables wastewater utilities to pinpoint odor and corrosion problems, develop effective monitoring programs, alert plant operators and sewer maintenance workers to potential danger and the need to observe safety practices, evaluate and implement effective control system such as aeration, chlorination, and mechanical cleaning. As described above, hydrogen sulfide is an acutely toxic material. It can cause serious health hazards even at very low concentrations. The physiological effects (i.e., toxicity spectrum) of hydrogen sulfide are summarized in another figure provided by EPA.

The control of odorous gases and the corrosion of sewers are the two most important problems in operating wastewater collection systems. Evaluation of existing or potential odor or corrosion problems, and identification of where such problems will occur is, therefore, highly essential. In sanitary sewer systems, odors are produced as a result of biological decomposition of organic matter, particularly those containing sulfur and nitrogen, under anaerobic conditions prevailing in the slime layer of gravity pipes, force mains, and wet wells. Hydrogen sulfide (H2S) and ammonia are the only malodorous inorganic gases produced from the decomposition. Other odor producing substances include organic vapors such as idoles, skatoles, mercaptans and nitrogen-bearing organics. However, H2S is the most commonly known and prevalent odorous gas associated with domestic wastewater collection and treatment systems. H2S DetectorTM extension for InfoSewer/Pro suite gives wastewater engineers a powerful Operations and Maintenance (O&M) tool to readily model and analyze entire sewer collection systems for sulfide generation and corrosion potential under varying conditions anticipated throughout the life of their systems.

Hydrogen sulfide has a characteristic rotten egg odor, is extremely toxic, is corrosive to metals, and is a precursor to the formation of sulfuric acid (which corrodes concrete, lead-based paints, metals, and other materials). The conditions leading to formation of H2S generally favor the production of other odorous organic compounds. Therefore, investigation of the conditions favoring H2S formation not only helps to quantify the potential for odor generation from other compounds, but also it aids in identifying potential corrosion problems in the collection system.

The occurrence of H2S in wastewater collection systems, other than that added from industrial sources and infiltrated groundwater, is primarily the result of the reduction of sulfate ion ( ), one of the most universal anions occurring in natural waters, under anaerobic conditions, as shown by the following reaction.

The molecular H2S, formed from sulfate reduction, dissolves in the waste water and dissociates in accordance with reversible ionization reactions, expressed as:

The partitioning of the hydrogen sulfide into these components (i.e., (H2S) aqueous, HS- ion, and S= ion) depends primarily on the temperature and the pH of the wastewater, although ionic strength, as represented by dissolved solids or electrical conductivity, also affects the partitioning. The HS- ion and S= ion produce no odors. Some of the aqueous H2S will escape into the sewer atmosphere causing the odor problem. The concentration of H2S gas in the atmosphere will vary with the concentration of (H2S) aqueous according to Henry’s law. The rate of escape of H2S gas is a function of the difference between the saturation or equilibrium concentration determined by Henry’s law and the actual concentration of H2S in the sewer atmosphere. The EPA provides the figure below that shows H2S in the sewer atmosphere in equilibrium with the given concentrations of aqueous H2S concentration in the wastewater at the respective temperatures, for a pressure of one atmosphere.

Prediction of the rate of sulfide buildup and corrosion potential is an essential element in the design of new sewer systems as well as in the evaluation of existing systems. The rate of sulfide buildup depends on a number of environmental conditions, including, concentration of organic material and nutrients, sulfate concentration, dissolved oxygen (DO), pH, temperature, stream velocity, surface area, and detention (residence) time. Accounting for all these environmental conditions, H2S Detector predicts sulfide buildup in sewer collection systems for gravity sewers, force mains, and wet wells using the Pomeroy-Parkhurst equations. H2S Detector enables wastewater utilities to pinpoint odor and corrosion problems, develop effective monitoring programs, alert plant operators and sewer maintenance workers to potential danger and the need to observe safety practices, and evaluate and implement effective control system such as aeration, chlorination, and mechanical cleaning. As described above, H2S is an acutely toxic material. It can cause serious health hazards even at very low concentrations. The physiological effects (i.e., toxicity spectrum) of H2S are summarized in the following figure provided by EPA.

HYDROGEN SULFIDE PARAMETERS

These parameters are available when the H2S option is selected. Hydrogen Sulfide results are generated in addition to any other pollutants entering the system. Note that BOD must be defined as a pollutant and enter the system to simulate Hydrogen Sulfide generation and propagation.

• Pollutant - Enter the pollutant ID used to predict Hydrogen Sulfide buildup and transport. This pollutant (BOD) must enter the drainage system via runoff, groundwater, or node inflow for any hydrogen sulfide buildup or corrosion prediction to occur.
• Reaction Rate Coefficient - A first order coefficient (per day) which varies with the type of waste. The default value is 0.23/day.
• Temperature - average daily temperature for the region (in oC).
• pH - The pH of the wastewater. The normal pH range of municipal wastewater ranges from 6.0 to 8.0.
• Sulfide Flux Coefficient - The effective sulfide flux coefficient for sulfide generation by the slime layer in gravity sewers (meter/hour). For conservative analysis (i.e., observed sulfide buildup generally less than predicted), the suggested values of this parameter is 0.00032.
• Sulfide Loss Coefficient - A dimensionless coefficient to account for sulfide losses by oxidation and escape to atmosphere. For conservative analysis (i.e., observed sulfide buildup generally less than predicted), the suggested values of this parameter is 0.64. For moderately conservative analysis a value of 0.96 is suggested.
• Ionization Coefficient - A logarithmic ionization constant for hydrogen sulfide (unit less), a function of temperature and specific electrical conductance of the waste water. Its value generally varies from 6.67 (at a temperature of 40oC and specific electrical conductance of 50,000 micromhos/cm) to 7.74 (at a temperature of 10oC and specific electrical conductance of 0 micromhos/cm).
• Soluble Sulfide (%) - The percent of total sulfides that occur in the soluble (dissolved) form for the wastewater, most frequently known to vary from 70 to 90 percent.
• H2S Aqueous = 1.0625*(1.0/(1.0 + -Ionization Constant/-pH)) * Soluble Sulphide % soluble sulphide and ionisation constant are user inputs, rather than calculated values. “...at which point does the concentration of sulphate become a limiting factor?”
• Limiting Sulfide = Sulfide Flux Coefficient / Sulfide Loss Coefficient * Effective BOD * Slope Velocity Term * Flow Perimeter / Top Width