11    SLUDGE QUALITY INDEX - EVALUATION OF SLUDGE
SETTLEABILITY
Richard Fox, Project Engineer
Angelbeck Environmental Engineers, Inc.
Toledo, Ohio 43606
Donald Angelbeck, Associate Professor
Department of Civil Engineering
University of Toledo
Toledo, Ohio 43606
INTRODUCTION
Major advances in process control of biological activated sludge operation have been made possible
through the continued development of solids flux analysis. Widespread availability of microcomputers and technical software have only recently opened the potential benefits of solids flux analysis
for daily operational control at municipal and industrial wastewater treatment facilities. Solids flux
analysis involves measuring sludge settleability which leads to a sludge settling constraint. This
information can be used to estimate operational levels of mixed liquor concentration within the
aeration tanks and underflow concentration of the final clarifier for different combinations of
influent and recycle flows.
The variable SQI, or Sludge Quality Index, has been devised to quantify sludge settleability as it is
measured and used in solids flux analysis. This paper will report the field data which supports the
theoretical development and definition of the Sludge Quality Index. Two traditional solids flux
models along with an empirical SVI model will be investigated to obtain solids flux curves and the
associated Sludge Quality Indexes.
The Sludge Quality Index, a single quantitative measure of sludge settleability, is needed for two
reasons. First, this quantity can be directly compared with other process variables to develop relationships that can be used to determine operational corrections to the activated sludge system. Secondly,
the measured SQI quantity along with SQI trending over the recent past can be used in algorithms to
calculate control adjustments of flow splits within a step-feed activated sludge process.
DEFINITION OF THE SLUDGE QUALITY INDEX
The Sludge Quality Index can be calculated from any batch solids flux curve. Initial settling curves
are obtained by conducting settleability tests (Settleometer Tests) on several concentrations of a waste
activated sludge. This data can be used to construct the solids flux curve either graphically or with
mathematical modeling. Traditional plots of batch solids flux settling capacity versus concentration
produce curves which vary in both shape and location depending upon overall sludge settling characteristics. Models used to construct these curves normally involve two variables, neither of which singly
or adequately represent changes in the overall sludge settleability. Furthermore, a general index for
sludge settleability should not be dependent on any one model for a solids flux curve.
The definition for the Sludge Quality Index is relatively simple once a solids flux settling curve has
been obtained. Place a recycle operating line, with a slope equal to a reference recycle flow rate
divided by the area of the clarifier, tangent to the solids flux curve. The value of the concentration
intercept of this line is defined as the SQI (Figure I). Physically, the SQI is the minimum underflow
sludge concentration that could be expected from operation using a reference recycle pumping rate or
the maximum recycle pumping rate. We have chosen a standard recycle flux of 700 gpd/ft2 to define
SQL If SQI data are compared within a specific plant, then the SQI defined by the maximum recycle
flux would be a more realistic variable which describes the minimum recycle sludge concentration at a
given time. However, if sludges between different plants are to be compared, the SQI defined by a
reference recycle flux must be used since each plant may have different recycle pumping capacities.
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