18    IMAGE ANALYSIS OF BIOMASS IMMOBILIZED ON
MICRO-CARRIERS
PR. Senthilnathan, Graduate Student
Da-Hong Li, Graduate Student
J. J. Ganczarczyk, Professor
Department of Civil Engineering
University of Toronto
Toronto, Ontario
CANADA M5S 1A4
INTRODUCTION
The application of micro-carriers such as sand, activated carbon, and coke in modifying the
activated sludge systems has resulted in "fluidized bed systems"1 and "carrier activated sludge systems."2,3 The presence of these micro-carriers in the activated sludge reactors provide a significant
surface area (10,000 to 100,000 m2/m3 of carriers) for the formation of biofilm and thus aids in
retaining a high concentration of biomass in the reactors.
The biomass concentration in carrier activated sludge systems can vary from 5000 to 30,000 mg/L
as MLVSS depending on the number of carrier particles per unit volume of reactor, and their size,
shape, density, porosity, and other surface characteristics. The potential biomass holding capacity of
the carriers vary dramatically with respect to their size. The characteristics of substrates and of
microorganisms present in the activated sludge will also affect the biomass holdup.
To date, only a few attempts have been made to study the interactions between the microorganisms
and micro-carriers due to the difficulties involved in differentiating the attached biomass in the form
of biofilm from the non-attached suspended biomass.
The aim of this chapter is to develop an effective method for differentiating the attached biomass
from the non-attached biomass by means of image analysis, and to demonstrate the proposed method
for some selected activated sludge systems containing activated carbon as micro-carriers.
BACKGROUND
Currently, the interest in immobilizing biomass in activated sludge reactor is growing, and various
process modifications utilizing biomass carriers are available for commercial application.3 One of the
modifications, known as "Carrier Activated Sludge Process," utilizes a substantial amount of micro-
carriers (2000 mg/L to 15,000 mg/L) in the aeration basin. The micro-carriers provide an extremely
large surface area for passive immobilization of microorganisms. By means of these carriers it is
possible to achieve two-to fivefold increase in biomass concentration in the aeration tanks compared
with that in the conventional activated sludge reactors. The increased concentration of biomass in
these systems results in reduced volume of the reactor, increased stability, and improved
performance.
It is expected that the formation of biofilm on micro-carriers depends on the characteristics of the
selected micro-carriers, the substrates, and the microorganisms. However, for a given substrate the
amount of biofilm holdup on micro-carriers drastically vary with the size and surface characteristics
of micro-carriers. These carriers may be inert/neutral, adsorptive, or chemically reactive. Adsorptive
carriers enhance the performance of carrier activated sludge process by adsorbing substrates and
metabolic end products.
In order to study the effect of micro-carriers on the characteristics of activated sludge floes, the
biomass holdup by micro-carriers should be determined. Traditionally, direct microscopic observations were used to estimate the biofilm thickness on carriers and the quantity of attached biomass.
With this technique it is difficult or even impossible to determine accurately the perimeter and the area
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea. Michigan 48118.
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