CALCULATION OF HYDROGEN SULFIDE (H?S) CONCENTRATION AND EVAPORATION RATE IN THE WET WELL OF THE SEWAGE PUMPING STATION

Hydrogen Sulfide (H?S) Concentration and Evaporation Rate Calculation in Sewage for the Golapbag Sewage Pumping Station

Hydrogen sulfide (H?S) is a toxic and odorous gas commonly found in sewer systems due to the anaerobic decomposition of organic matter. Proper estimation of H?S concentration and evaporation rate is crucial for designing ventilation, odor control, and safety measures for a sewage pumping station.

1 Hydrogen Sulfide (H?S) Concentration Calculation

Various factors, including temperature, sulfate concentration, organic matter content, and detention time, influence H?S concentration in sewage. The process follows international standards such as ASTM D 5708, AWWA Manual MOP-60, and ASCE standards for sewer design.

1.1 Required Data for Calculation

To estimate H?S concentration, the following data are needed:

1. Sulfate (SO?2?) concentration in wastewater (mg/L)

2. pH of sewage (typically ranges from 6 to 8)

3. Temperature of sewage (°C)

4. Dissolved oxygen (DO) concentration in wastewater (mg/L)

5. Biochemical oxygen demand (BOD?) (mg/L)

6. Sewage velocity in wet well (m/s)

7. Sewage detention time in the wet well (hours)

8. Alkalinity and presence of iron compounds (affecting H?S precipitation)

1.2 Test Data Required

? Wastewater Sampling: Laboratory tests should be conducted to determine sulfate, pH, temperature, DO, and BOD.

? Gas Measurement: Field gas monitors (e.g., Jerome H?S Analyzer, OdaLog Logger) can measure actual H?S levels.

1.3 Calculation Steps

Step 1: Sulfate Reduction and H?S Production

The production of H?S follows the anaerobic reduction of sulfate :

The amount of H?S produced can be estimated using empirical relationships:

Where:

? H?S produced = Total mass of H?S produced (mg/L)

? BOD5= biochemical oxygen demand over 5 days (mg/L)

? Sulfate Concentration = Measured sulfate content in sewage (mg/L)

? Sewage Detention Time = Time sewage remains in the wet well (hours)

Step 2: H?S Gas Equilibrium in Liquid

The dissolved hydrogen sulfide in wastewater follows Henry’s Law:

Where:

? Cg= Concentration of H?S in the air (ppm)

? H= Henry’s Law constant for H?S (varies with temperature)

? C1 = Dissolved H?S in sewage (mg/L)

For typical wastewater conditions, an empirical equation is:

Where:

? Sewage Flow Rate (m3/s)

? Ventilation Rate (m3/s)

Step 3: H?S Volatilization and Release Rate

The mass transfer coefficient determines the rate at which H?S is released from the liquid to the gas phase.

Where:

? KL = Mass transfer coefficient (m/s)

? A =Surface area of sewage exposed to air (m2)

? CL= Dissolved H2S in sewage (mg/L)

? CG= H?S concentration in air (mg/L)

2 Evaporation Rate Calculation

The evaporation rate of volatile substances (H?S, ammonia, etc.) depends on environmental conditions, including temperature, wind speed, and surface area.

2.1 Required Data for Calculation

1. Temperature of sewage (°C)

2. Wind speed over the wet well (m/s)

3. Humidity and air pressure

4. Surface area of exposed sewage (m2)

5. Air exchange rate in the wet well (m3/h)

6. H2S concentration in air (mg/L)

2.2 Calculation Steps

Step 1: Mass Transfer Rate of H?S Evaporation

Evaporation follows the empirical relation:

Where:

? E= Evaporation rate of H?S (g/m2·s)

? k = Mass transfer coefficient (depends on wind speed, turbulence)

? A= Surface area of exposed sewage (m2)

? CL =Dissolved H2S concentration in sewage (mg/L)

? CG= H2S concentration in air (mg/L)

? H= Henry’s Law constant

Step 2: Adjusting for Environmental Factors

? High temperature increases evaporation rate.

? A higher air exchange rate reduces H?S buildup.

? High wind speed over sewage surfaces enhances mass transfer.

Step 3: Estimating the Total Emission Rate

For a wet well with volume V and ventilation rate Q, the total H?S emission can be estimated using:

Where:

? Etotal = Total H?S release (g/s)

? Q = Air exchange rate (m3/h)

3 Hydrogen Sulfide (H?S) Concentration Modeling in SewerGEMS

Also, we can calculate the Hydrogen Sulfide (H?S) Concentration by the modelling of SewerGEMS. Bentley SewerGEMS manual for the Hydrogen-Sulfide (H2S) Modeling [Link]

4 References:

The calculations of hydrogen sulfide (H?S) concentration and evaporation rate in sewage systems, following international standards and best practices:

Hydrogen Sulfide (H?S) Concentration Calculation References

1. Assessment of hydrogen sulfide emission in a wastewater pumping station-Mohamed Hamoda, Kuwait University (KU)· Department of Civil Engineering Ph.D. Env. Eng., University of Toronto, Canada [Link]
2. Metcalf & Eddy, Inc. (2014). "Wastewater Engineering: Treatment and Resource Recovery" (5th Edition) – McGraw-Hill Education. [Link] This book provides detailed equations and methodologies for estimating gas emissions, including H2S, from wastewater.
3. DESIGN OF MUNICIPAL WASTEWATER TREATMENT PLANTS-WEF Manual of Practice No. 8 ASCE Manuals and Reports on Engineering Practice No. 76 Fifth Edition. [Link]
4. WEF Design of Water Resource Recovery Facilities, MOP 8, 6th Edition [Link] . Covers the estimation of sulfide generation in wastewater collection systems.
5. AWWA MOP-60: Control of Odors and Emissions from Wastewater Treatment Plants – American Water Works Association (AWWA). [Link]. Provides methods for calculating H?S gas emissions based on environmental conditions.
6. ASTM D 5708 – Standard Test Methods for Determining Sulfides in Wastewater – American Society for Testing and Materials (ASTM). ?[Link]. Specifies laboratory testing methods for sulfide concentrations in wastewater.
7. Pomeroy, R.D. and Bowlus, F.D. (1946). "Progress Report on Corrosion Studies" – Sewage Works Journal, Vol. 18, No. 4. [Link]. Introduced the Pomeroy Equation, which estimates sulfide generation in sewer systems.
8. Odour prevention strategies in wastewater treatment plants [Link]
9. USEPA (United States Environmental Protection Agency) – Odor Control in Wastewater Treatment Plants (EPA/625/R-92/014). [Link]. Provides guidelines for estimating and mitigating hydrogen sulfide emissions.

2. H?S Evaporation Rate Calculation References

1. Henry’s Law Constants for Various Substances in Water – Sander, R. (2015). "Compilation of Henry's Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry" – Atmospheric Chemistry and Physics.[Link]. Used for determining the equilibrium concentration of H?S in air and water.
2. ASCE (American Society of Civil Engineers) Manual and Reports on Engineering Practice No. 89: Gravity Sanitary Sewer Design and Construction. [Link]. Details empirical equations for mass transfer of gases from wastewater.
3. Industrial Ventilation: A Manual of Recommended Practice (ACGIH, 2013) – American Conference of Governmental Industrial Hygienists (ACGIH). [Link]. Provides standard methodologies for estimating gas evaporation and ventilation design.
4. Hvitved-Jacobsen, T. (2001). "Sewer Processes: Microbial and Chemical Process Engineering of Sewer Networks" – CRC Press. [Link]. Covers sulfide production and transfer mechanisms in sewers.
5. Estimation of hydrogen sulfide emission rates at several wastewater treatment plants through experimental concentration measurements and dispersion modelling [Link]. Provides methods for calculating H?S emissions and its fate in sewer networks.