44    OXYGEN AND VOC MASS TRANSFER
AT CLARIFIER WEIRS
Michal Labocha, Graduate Student
Richard G. Zytner, Assistant Professor
School of Engineering, University of Guelph
Guelph, Ontario, Canada
Richard L. Corsi, Assistant Professor
Department of Civil Engineering, University of Texas at Austin
Austin, Texas 78712
INTRODUCTION
Emissions of volatile organic compounds (VOCs) resulting from the operation of wastewater
treatment plant (WTP) facilities have received considerable attention in the past few years. Both the
surface volatilization from open tanks and the stripping occurring at various hydraulic structures
contribute to the overall release of VOCs to the atmosphere. Aeration tanks are considered the biggest
single source of VOC emissions and have already received considerable attention of researchers.
However, there are numerous other potential sources of significant VOC emissions such as sewage
inlets at wet wells of pumping stations, archimedes screw type pumps, aerated grid chambers, com-
minutors, flumes, clarifier weirs, trickling filters, and rotating biological contactors. Many of these
potential sources of air contamination are currently being investigated but some of them have not yet
received due attention.
The identification of mass transfer mechanisms and key operating parameters that influence the
mass transfer of VOCs is extremely important, both for the assessment of relative danger to the public
and for a conscious undertaking of preventative measures to reduce air pollution resulting from WTP
operations. Included in this is the development of accurate models to predict VOC emissions from
these sources. This paper deals specifically with mass transfer occurring at clarifier weirs.
The main objective with respect to clarifier weirs has been to asses the relative importance of weir
operating parameters such as drop height, weir loading, and tailwater depth, as well as the influence
of contaminant physico-chemical properties on the rate of stripping. Laboratory pilot scale studies
were conducted using clean water. Dissolved oxygen measurements were made since oxygen is an
excellent surrogate for mass transfer studies. Limited measurements were also made with clean water
spiked with a ten compound chemical cocktail. The following sections present and discuss the results,
and suggest recommendations on operational changes.
BACKGROUND
A common approach to the modeling of VOC mass transfer has been the use of oxygen as a
surrogate gas. It is based on the assumption that the stripping rate of any volatile compound can be
related to the simultaneously occurring oxygen uptake. The oxygen uptake is usually expressed as a
deficit ratio r„ defined as:1
(1)
where:         r„ = oxygen deficit ratio, dimensionless
C^ = dissolved oxygen concentration upstream of weir [mg/L]
Cdo = dissolved oxygen concentration downstream of weir [mg/L]
C„ = saturation concentration of oxygen at given temperature [mg/L]
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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