Conference - Michigan Water Environment Association

Are you spending
quality time with your clarifier
Part 2 – Analysis of final clarifier performance
Sam Jeyanayagam, Ph.D., P.E., DEE
Senior Associate, Malcolm Pirnie Inc.
Phone: (614) 430-2611 e-Mail: Sjeyanayagam@pirnie.com
INTRODUCTION
This article outlines tools available to designers and operators
to analyze and predict clarifier behavior based on site-specific
data. Part 1 of this article, which appeared in the last issue of the
Matters, addressed factors impacting final clarifier performance.
Figure 1 illustrates a typical activated sludge system, which
consists of the bioreactor where the biological reactions occur
and the final clarifier, where the biomass is separated from the
wastewater. A relatively small portion of the settled sludge is
wasted from the system to maintain the desired biomass inventory
while the rest is returned to the bioreactor as return activated
sludge (RAS) to seed the incoming wastewater. Consequently, the
performance of the solids separation process is closely linked to the
performance of the biological process and vice versa. The failure
to understand this interdependency has led to poor clarifier design
and operation.
The capacity of a clarifier is related to the rate at which the
incoming solids can be separated and conveyed to the sludge
collection mechanism at the bottom. This rate of solids conveyance
is primarily impacted by the following factors:
Sludge (MLSS) settleability
Operational parameters
• Influent flow rate
• RAS flow rate
• MLSS concentration
Hydraulic characteristics of the clarifier
State point analysis (SPA), is a practical tool available to
designers and operators to examine the behavior of the final
clarifier. It incorporates the impacts of all of the above factors
except the hydraulic characteristics of the clarifier.
STATE POINT ANALYSIS
Solids Flux Theory
The State Point Analysis (SPA), first proposed by Keinath, is an
extension of the solids flux theory, which describes the movement
of solids through a clarifier. In final clarifiers, Type III settling
is the predominant solids removal mechanism (see Part 1 of this
article). Type III settling is characterized by flocculated particles
settling as a zone or blanket. As they settle, the particles maintain
their position relative to each other. The zone settling velocity
(ZSV) is a function of solids concentration (X) and is commonly
expressed by the Vesilind equation:
Figure 1: The Activated Sludge System
GRAVITY SETTLING IN FINAL CLARIFIERS
The activated sludge floc is composed mostly of bacteria, which
constitutes 70 – 80 percent water. In addition, the relatively small
flocs (less than 0.2 mm diameter) formed in the aeration basin
entrap considerable amount of bound water. Hence, the density
difference between the aeration basin floc and water is so small
that solids separation in the clarifier is difficult. Hence, a properly
designed clarifier must encourage flocculation, which results in
larger and heavier particles (less than 0.2 to 2 mm diameter), and
create quiescent conditions for the particles to settle.
ZSV = Vo e-kX (1)
Where Vo and k are settling constants obtained from a series
of settling tests. A good settling sludge is characterized by high
Vo and low k. The solids flux (G), lb/sf/d, represents the mass of
solids transported per unit area of the clarifier per unit time. It
is obtained by multiplying the zone settling velocity by the solids
concentration.
G = (ZSV) X (2)
By combining equations (1) and (2) we obtain:
G = (X) (Voe-kX) (3)
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MWEA MATTERS: FALL 2005