Final Report: VOC Emissions from Sewers Process Drains and Drop Structures

EPA Grant Number: R823335
Title: VOC Emissions from Sewers Process Drains and Drop Structures
Investigators: Corsi, Richard L.
Institution: The University of Texas at Austin
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1995 through September 1, 1998
Project Amount: $271,896
RFA: Exploratory Research - Engineering (1995) Recipients Lists
Research Category: Air Quality and Air Toxics , Engineering and Environmental Chemistry , Land and Waste Management

Objective:

As a result of the Clean Air Act of 1990, several industries have come under increased regulatory scrutiny as sources of hazardous air pollutant (HAP) emissions to the ambient atmosphere. For example, one source of fugitive emissions that has recently received significant attention is on-site industrial sewers, including process drains, conveyance channel, and drop structures associated with junction boxes and wet wells. Publicly-owned treatment works (POTWs) have also received attention as sources of HAP emissions, with a focus on emissions from treatment systems due to a general lack of information related to emissions from public sewers.

An understanding of, and an ability to accurately estimate, the extent and nature of gas-liquid mass transfer in municipal and on-site industrial sewers is important for identifying the need for source-specific National Emission Standards for Hazardous Air Pollutants (NESHAPs). Such information would provide the USEPA with scientific criteria for development of NESHAPs, and would also provide municipalities, industry, and regulators with the ability to routinely and accurately estimate HAP emissions from sewers. Finally, such information could ultimately provide municipalities and industry with tools for determining the feasibility of low-cost methods for reducing HAP emissions, e.g., through minor process modifications.

As stated in the original proposal for this project, the primary goal of the research was to develop a significantly improved understanding of VOC emissions from municipal and industrial sewers. Specific objectives were stated as:

Process Drains:

  1. Determination of the important mechanisms that cause VOC emissions from industrial process drains, and the sensitivity of those mechanisms to environmental factors and drain design and operating conditions.
  2. Estimation of VOC stripping efficiencies for drains under a wide range of VOC physicochemical properties, environmental conditions, and drain design/operating conditions.
  3. Development of a state-of-the-art model that will allow estimation of VOC stripping efficiencies at process drains.
Drop Structures:
  1. Determination of VOC stripping efficiencies over a wide range of VOC physicochemical properties and system operating conditions.
  2. Determination of the importance of air entrainment and gas-phase resistance to mass transfer below drop structures.
  3. Development of a protocol for estimating VOC stripping efficiency at drop structures based on routine oxygen absorption measurements or estimates.
All three objectives associated with industrial process drains and the first objective associated with drop structures were achieved during the course of this study. Although an explicit determination of the importance of air entrainment below drop structures was not completed, a unique experimental system was constructed as part of this study and will be used to study the importance of air entrainment in the future. The importance of gas-phase mass transfer below drop structures has been addressed and will continue to be studied in the future. Based on a review of the published literature it was determined that any attempts to relate oxygen and VOC transfer are limited theoretically, and were thus abandoned as a research objective. However, as part of this study empirical relationships were developed to allow estimates of mass transfer coefficients given drop structure operating conditions, in particular the rate of potential energy dissipation at drop structures. When these mass transfer coefficients are coupled with two-phase mass balances (on liquid and gas phases), VOC emissions can be estimated directly based on knowledge of VOC physicochemical properties.

Summary/Accomplishments (Outputs/Outcomes):

This study addressed the needs described above through the use of several experimental systems intended to simulate industrial process drains and municipal or industrial drop structures. A total of 91 experiments were completed (25 drop structure experiments and 66 process drain experiments) to study VOC emissions over a wide range of system operating conditions, environmental conditions, and chemical properties. In each case, a cocktail of volatile tracers with varying chemical properties was introduced into the experimental system. Specific tracers that were used over the course of this study included acetone, ethyl acetate, methyl ethyl ketone, toluene, ethylbenzene, and cyclohexane. The use of liquid and gas phase sampling allowed for mass closure analyses and the determination of tracer-specific stripping efficiencies. The latter allowed for the determination of fundamental mass transfer parameters, including gas and liquid-phase mass transfer coefficients and, in the case of process drains, air entrainment rates. These parameters were determined over a wide range of system operating conditions.

The results of this study were used to develop semi-empirical mathematical relationships that link chemical properties, system operating conditions, and environmental conditions to chemical stripping efficiencies. Those relationships were incorporated into a computer model (naUTilus) that integrates the results of this project with several other studies that were completed at The University of Texas at Austin, and funded by the American Petroleum Institute, the U.S. Chemical Manufacturer's Association, and the Gulf Coast Hazardous Substance Research Center. The naUTilus model has since been used to simulate VOC emissions from entire industrial sewer systems, and large portions of municipal wastewater collection systems.

Conclusions:

The results of this study indicate that significant mass transfer can occur from wastewater to adjacent air at both industrial process drains and industrial or municipal drop structures. However, mass transfer processes are complex and affected by process operating conditions, environmental conditions, and chemical properties. It is concluded that a large fraction of VOCs may be emitted from industrial and municipal sewers before wastewater reaches the end of the sewer system. This is an important conclusion given that the past emphasis of research related to VOC emissions from wastewater has clearly been in treatment systems. This conclusion is particularly true for hydrophobic VOCs such as cyclohexane. Hydrophilic compounds, e.g., methyl ethyl ketone, are nearly always characterized by a rapid approach to chemical equilibrium, and subsequent mass transfer limitations that lead to very low stripping efficiencies, even from entire sewer systems. Emissions of VOCs with mid-range Henry's law constants, e.g., benzene, are highly dependent on system operating conditions and can vary greatly from system-to-system. However, it appears that large fractions of these compounds can be removed from wastewater prior to entering municipal wastewater treatment plants.

Specific conclusions are listed below and are divided as those pertaining to process drains and those pertaining to drop structures.

Process Drains:

  1. Stripping efficiencies for industrial process drains are highly dependent on physicochemical properties, temperature, and hydrodynamic conditions. Fractional stripping efficiencies ranged from 0 to 45% depending on experimental conditions.
  2. Liquid-phase mass transfer coefficients in the process sewer channel located immediately below a drain are reasonably well correlated with process flow rate for both sealed and unsealed drains.
  3. For well-ventilated systems, stripping efficiencies below a water trap are approximately equal to those with no water seal in place.
  4. For drains that contain a water seal (trap), air entrainment into the seal causes gas-liquid mass transfer and "above trap" emissions that increase with increasing process flowrate for both disintegrated and intact films.
  5. The ratio of air entrainment rate to liquid flow rate is greater for disintegrated films and leads to higher stripping efficiencies for such conditions, i.e., relative to intact (solid) films.
  6. The degree of equilibrium between entrained air bubbles and surrounding water is a strong function of liquid flow rate and Henry's law constant. The degree of chemical equilibrium exponentially approaches unity at decreasing Hc.
  7. For the range of conditions tested, emissions from water seals appear to be significantly less than those from well ventilated channels below a drain. However, preliminary findings indicate only about a 50% reduction in emissions of toluene after the installation of a trap. These results suggest that the use of water seals to reduce emissions from industrial process drains may not be as effective as previously assumed.
  8. It is possible to employ design modifications to process drains in order to reduce VOC emissions. These modifications include nozzle extension with tangential entry to the underlying sewer channel, and the use of flexible shrouds between a nozzle and drain hub to allow for reduced ventilation and suppression of mass transfer.
Drop Structures
  1. As with industrial process drains, chemical stripping efficiency at drop structures varies over a wide range and is sensitive to both drop structure operating conditions and chemical properties. For this study, VOC stripping efficiencies for a single drop structure varied from a low of 0.14% (acetone) to a high of 36% (cyclohexane).
  2. Chemical stripping efficiency cannot be directly predicted from VOC physicochemical properties and drop structure operating parameters without using mass transfer kinetics.
  3. The product of the overall mass transfer coefficient and interfacial area (KLA) increases with increasing wastewater flow rate. However, the effects of this increase are counter-acted by a decrease in the hydraulic residence time below a drop structure at higher wastewater flow rates.
  4. The ratio of gas- to liquid-phase mass transfer coefficients varies widely and is highly dependent on system operating conditions. As such, the relative importance of gas- and liquid-phase mass transfer resistance, particularly for compounds such as benzene that have mid-range Henry's law constants, is also system-specific and may vary significantly for the same system given different operating conditions.
  5. For compounds with low Henry's law constants, e.g., acetone and methyl ethyl ketone, gas-phase resistance accounts for the majority of overall resistance to interfacial mass transfer. Liquid-phase resistance to mass transfer becomes more important as Henry's law constant increases. For compounds with large Henry's law constants, i.e., cyclohexane, gas-phase resistance to mass transfer tends to be negligible. In all cases where gas-phase resistance is not negligible, it accounts for a larger fraction of overall resistance to interfacial mass transfer when the drop structure is operated under conditions of low tailwater depth than under conditions of high tailwater depth.
  6. Liquid-phase mass transfer coefficients are strongly correlated to the rate of potential energy dissipation at drop structures, especially at high tailwater depths.


Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 14 publications 6 publications in selected types All 6 journal articles
Type Citation Project Document Sources
Journal Article Olson D, Rajagopalan S, Corsi RL. Ventilation of industrial process drains:Mechanisms and effects on VOC emissions. Journal of Environmental Engineering, ASCE 1997;123(9):939-947. R823335 (1998)
R823335 (Final)
  • Abstract: ASCE - Abstract
    Exit
  • Journal Article Olson DA, Varma S, Corsi RL. A new approach for estimating volatile organic compound emissions from sewers: methodology and associated errors. Water Environment Research 1998;70(3):276-282. R823335 (1998)
    R823335 (Final)
  • Abstract: Ingenta-Abstract
    Exit
  • Other: University of Bologna - Abstract
    Exit
  • Journal Article Olson DA, Stubbe JK, Corsi RL. A mechanistic model for estimating VOC emissions from industrial process drains part I: the underlying channel. Environmental Progress 2000;19(1):1-10. R823335 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Other: University of Bologna - Abstract
    Exit
  • Journal Article Olson DA, Stubbe JK, Corsi RL. A mechanistic model for estimating VOC emissions from industrial process drains part II: the water seal. Environmental Progress 2000;19(1):11-17. R823335 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Other: University of Bologna - Abstract
    Exit
  • Supplemental Keywords:

    Scientific Discipline, Air, Water, POLLUTANTS/TOXICS, air toxics, Wastewater, Environmental Chemistry, Chemicals, Environmental Engineering, drop structures, hazardous air pollutants, mass transfer mechanisms, Clean Air Act, mechanistic behavior of HAP emissions, industrial sewers, process drains

    Progress and Final Reports:

    Original Abstract
  • 1996
  • 1997