The Determination of Bacterial Kinetics in A Powdered
Activated Carbon Reactor
BRIAN P. FLYNN, Environmental Engineer
Environmental Control Division
E.I. Du Pont de Nemours & Co.
Deepwater, New Jersey 08023
INTRODUCTION
One of the most recent approaches to operating an activated sludge process is on the
basis of the sludge age concept (1). However, this approach was developed for biological
systems and has not been applied to a biological system with powdered carbon addition (Du
Pont PACT process). In order to design and operate a 30MM gpd waste treatment plant
based on the PACT process, a procedure for analyzing data to produce the required sludge
age was needed. The procedure developed is outlined along with laboratory-scale and pilot
plant data. Dependence of BOD removal on temperature and sludge age is illustrated. The
effects of carbon and biomass on COD, TOC, and color removal are separated.
Du Pont Chambers Works is a diverse organic chemicals plant producing many types
of dyes, organic intermediates, fluorocarbons, and lead alkyl compounds. The acidic
wastewater is colored and contains heavy metals, with biodegradable and nonbiodegradable organics. It is difficult to treat by the conventional activated sludge process
with any high degree of reliability. It has been discovered at this location by other
investigators(2) that the most reliable and economical treatment process is the addition of
powdered activated carbon to an activated sludge system. Figure 1 illustrates the process.
Figure 1 — Carbon   addition   to   activated sludge.
Primary clarified effluent is fed to a complete mixed activated sludge reactor with fresh
(or regenerated) powdered activated carbon. The carbon is used to adsorb high molecular
weight organics and toxic compounds while the bacteria in the process degrade the lower
molecular weight materials. Carbon and bacteria form a fine, dark sludge which settles well
in the final clarifier where overflow effluent of tertiary quality is obtained. The system is
capable of being run on the conventional sludge age concept by wasting sludge either from
the aerator or clarifier underflow. The wasted sludge can be regenerated to recover reusable
powdered activated carbon.
The sludge age approach has been formally derived in terms of Monod biological
kinetics. A brief synopsis based on Lawrence's paper (1) with modifications for carbon
addition follows.
The sludge age is defined as the total activated microbial mass in the treatment system
(Xy) divided by the total active microbial mass wasted daily (AX/At)T. This wasting is both
deliberate and inadvertent.
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