(580f) 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