Reduction of VOC Emissions in High Purity Oxygen Activated Sludge Wastewater Treatment Process: Toxchem Based Fate & Emissions Modeling Case Study

Reduction of VOC Emissions in High Purity Oxygen Activated Sludge Wastewater Treatment Process: ToxChem Based Fate & Emissions Modeling Case Study

^aMalcolm Fabiyi, ^bRajeev Goel, ^bSpencer Snowling, ^aRichard Novak

^aPraxair, Inc. 7000 High Grove Boulevard, IL

^bHydromantis, Inc. 1 James Street South, Hamilton, ON

 

Introduction

In the United States, 188 organic compounds have been designated as Hazardous Air Pollutants (HAPs), and facilities which generate or handle these air toxics are subjected to permitting, monitoring and reporting requirements (Woodward & Curran, 2006). Although extensive emissions controls efforts have been implemented at many industrial facilities, a significant amount of Volatile Organic Compounds (VOCs) can still appear in the wastewater, where the VOCs can be stripped during conveyance or biological treatment.  

Despite the increasing adoption of high purity oxygen (HPO) systems for wastewater treatment, due in part to reduced VOC emissions relative to conventional aeration systems, commonly available fate & emission models do not have default aeration modules that enable their ready application for modeling VOC emissions associated with HPO systems (Levine et al, 2010; Lubkowitz-Bailey et al, 2002; Rodieck et al, 2001).  High efficiency oxygenation systems have been demonstrated to provide over 90% reduction in VOC emissions relative to comparative diffused aeration system (see Figure 1; Rodieck et al, 2001; Levine et al, 2010).

Objectives

This paper develops a methodology for establishing the modifications for default diffuser and mechanical surface aeration models in ToxChem™ that would enable VOC emission characteristics associated with HPO systems to be modeled. The paper will discuss the development and testing of a custom module for evaluating VOC emissions in ToxChem™, and will provide a protocol for adapting fate & transport models for use with HPO systems. 

Methodology

VOC emission models were developed in ToxChem™, using published field data on comparative VOC emissions from HPO and conventional diffused air processes applied for treating API Pharmaceutical Wastewater (Lubkowitz-Bailey et al, 2002; Rodieck et al, 2001) and Methyl Ethyl Ketone spiked loads to a municipal wastewater plant (NYSERDA, 2000).

The default diffused aeration modules in ToxChem™ were applied in modeling the air based tests.  Modifications to the default air and mechanical surface aeration modules were made to enable the simulation of the HPO emissions data. Critical model parameters that were modified included comparative gas flow rates, oxygen transfer efficiencies, O₂ mass fraction in the gas, ratio of the gas and liquid film mass transfer coefficients (K_g/K_l), aeration power, standard oxygen transfer rate (kg O₂/kWh), and mass transfer correction factor (see Tables 1& 2).

Results & Observations

Our results indicate that the VOC behavior of the HPO system is captured effectively by modeling the mass transfer characteristics as that of a fine bubble diffuser system, with adjustments for gas flow, O₂ purity levels and oxygen transfer efficiency of the HPO device (see Table 3, also NYSERDA, 2000; Levine et al, 2010; Rodieck et al, 2001). Attempts to utilize the default mechanical surface aeration module indicated that the mass transfer characteristics of surface aeration systems cannot be readily adapted to HPO systems, even though they are both mechanical mixing systems. The ability of diffused aeration models to capture the dynamics of VOC emission from mechanical HPO oxygenation systems suggests that the dominant mechanism for VOC emissions in HPO systems is likely to be due to VOC attachment to entrained bubbles (Chern and Yu, 1995), rather than losses due to surface turbulence or from exchange between the air and volatiles in airborne water droplets (Roberts & Dandliker, 1983).

These results have a good mechanistic basis. Given that air contains about 21% oxygen, SOTE values for diffused aeration systems in the range of 20%-30% imply that only about 4%-6% of the volumetric air flow supplied to a basin using a diffused aeration system is actually dissolved. The undissolved 94%-96% of the entrained air flow can serve as attachment sites for VOCs, which are volatilized with the off-gas stream (Chern and Yu, 1995). In contrast, HPO systems like Praxair's In-Situ Oxygenation system use pure oxygen (up to 100% O₂) and have SOTE values of +90% (PMS, 2003), implying that the relative gas flows to meet the same O₂ demand in Diffused Aeration vs. HPO systems can be ≥ 25. A linear correlation is observed between VOC stripping and the volumes of gas flow supplied to a wastewater treatment process (Figure 2).  Therefore, the VOC emissions from HPO aeration are only a fraction of those observed in diffused aeration systems.

References

1.      Jia-Ming Chern and Cheng-Fu Yu. Volatile Organic Compound Emission Rates from Diffused Aeration Systems. 1. Mass Transfer Modeling. Ind. Eng. Chem. Res. 1995, 34, 2634-2643.

2.      Levine et al, 2010. Pure Oxygen Activated Sludge Unit Effectively Controls Volatile Organic Compound Emissions from a Mixed Petrochemical Wastewater, WEFTEC 2010

3.      Lubkowitz-Bailey et al, 2002. Start-up of a High Purity Oxygen Sequencing Batch Reactor System for Treatment of an Active Pharmaceutical Ingredient Wastewater. WEFTEC 2002

4.      NYSERDA Report, 2000. In-Situ Oxygenation (I-SO™) for Volatile Compound Emission Control.

5.      Praxair Report of Clean Water Tests at Philadelphia Mixing Solutions (PMS), 2003.

6.      Roberts, P. V., & Dandliker, P. G. Mass Transfer of Volatile Organic Contaminants from Aqueous Solution to the Atmosphere during Surface Aeration. Environmental Sci. Tech. Vol 17, No. 8, 1983.

7.      Rodieck et al, 2001. Alternative Treatment Strategy for High Strength Pharmaceutical Wastewater. WEFTEC 2001.

8.      Woodward & Curran, 2006. Industrial Waste Treatment Handbook

Figure 1. In-Situ Oxygenation (I-SO™) system

Figure 2. Effect of gas flowrates on Toluene emissions in API pharmaceutical wastewater showing a linear relationship between VOC emissions and gas flowrates in the range of flows analyzed (Data on which model is based is sourced from Rodieck et al, 2001 and is utilized in the ToxChem 4.0 model used in this study)

Table 1: Results from field tests (source: Rodieck et al, 2001).  Detailed influent characteristics were provided in the paper for HPO Baseline Test 2 and Diffused Aeration (DA) Baseline Test. Comparison of volumetric Toluene emissions from HPO Baseline test 2 vs. Diffused Air Baseline test show a 95% reduction in VOC emissions for HPO vs. DA system (i.e., 10.5% Toluene emissions with DA system vs. 0.5% emissions with HPO system).

Table 2. Model input variables utilized in Rodieck et al, 2001. These values were applied to the ToxChem 4.0 model used in this paper.

 

  

Table 3: Summary of Modeling and Empirical Values for VOC reduction using I-SO™ HPO Systems (a) Gas flowrates used for Pharmaceutical Waste - 0.61 cfm HPO system, 35 cfm Diffused Aeration (DA) system (refer to Table 2 and Figure 1 to see effect of air flow rates on emissions). (b) Gas flowrates used for Municipal (MEK) wastewater - 27 cfm HPO system, 2400 cfm Diffused Aeration (DA) system