Page 793 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

748 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

%VSS (destroyed)
0.80 g–0.24 g
=———–————
0.80 g 0.56 gm
=0.80% =0.70
VSS o 0.80 gm Digestion

VSS=55% 0.24 gm

Ash 0.20 gm Ash 0.20 gm
(a) (b)
FIGURE 23.15 Illustration of method of calculation of percent reduction in volatile suspended solids. (a) Sludge to digester and (b) sludge
from digester.

Example 23.4 Calculation of VSS Destroyed oped. The approaches were empirical and experience was
assimilated in converging on guidelines and practices that
Given worked; theory had not taken hold.
Figure 23.15a shows that VSS o 0.80 for a sludge entering a From about 1950 to the present, theory has helped to com-
digester. For the sludge leaving the digester, let VSS 0.55 in plement practice. The idea of a tank as a ‘‘reactor’’ started to
Figure 23.15b. Both were measured quantities at the plant.
emerge in the 1950s. A ‘‘golden age’’ of research occurred,
Required arguably, through the 1950s and into the 1970s, providing a
Calculate the fraction of VSS(destroyed) by the anaerobic knowledge base of principles. A defining break with the past
process.
was the 1970 paper of Lawrence and McCarty (1970) that
Solution delineated these principles with guidelines for design and
a. Ash
operation. The 1972 Metcalf and Eddy book codified what
The ash mass remains constant and is 0.20 g.
had been learned in practice coupled with guidance from the-
b. VSS mass leaving digester
ory. Other books, e.g., by Orhan and Artan (1994), Grady et al.
For the sludge leaving the digester,
(1999), Rittman and McCarty (2001), added scientific ration-
f(VS) ¼ VSS=(ash þ VSS) ale, essentially summarizing the state of the art gleaned from
0.55 ¼ VSS=(0.20 g þ VSS) the last fifty years. In 1987 the IWA model provided a means to
VSS ¼ 0.24 g actually model the activated-sludge process and provided a
flexible framework for modeling the anaerobic process.
c. VSS destroyed
Demands for a comprehensive control of pollutants during
VSS(destroyed) ¼ VSS o VSS the 1970s led to sophisticated activated-sludge and bio-filter
¼ 0.80 g 0.24 g practices to remove nitrogen and phosphorous as well as
¼ 0.56 g
organic carbon. The anaerobic process remains less glamor-
f(VSS destroyed) ¼ 0.56 g=0.80 g ous but has been understood since the 1960s as a complex
0.70 biochemical process which has been assimilated in design and
operation. Operation is understood at a different level also
Discussion than was the case through the 1960s. Plant operators run a
Without a ‘‘tip’’ a person new to the field is likely not to
consider the ash. Also, a sketch helps to keep the calcula- complex chemical plant with monitoring and surveillance
tions straight. Sampling to calculate the f(VS destroyed) is governed by restrictive regulations; the training and profes-
part of daily monitoring. sionalism has been commensurate.

23.5 SUMMARY
23.5.2 PARAMETERS
A perspective on the state of the art and its evolution is
The bases for most rational parameters are reaction stoichiom-
outlined. In addition activated-sludge parameters are pro-
etry, kinetics, and reactor mass balance. From these relations a
vided, both as a summary and as a convenience.
number of parameters may be derived. Because they are often
scattered indifferent sections ofa bookas related to their respect-
23.5.1 STATE OF THE ART
ive presentations, they are summarizedinTable 23.10 asa matter
Biological treatment, as a technology, is still evolving; like of convenience for retrieval. Tchobanoglous and Burton=
most unit processes it has not reached closure. In the first Metcalf and Eddy (1991) was the reference used for most of
50 years of the twentieth century technologies such as septic the equations. In some cases the nomenclature was altered.
tanks, Imhoff tanks for anaerobic treatment, two-stage Also, this text used fewer defined coefficients than found in the
digesters, activated sludge, trickling filters, etc. were devel- literature, in order to keep a more fundamental orientation.