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The sudden increase in indicator bacteria, including fecal
coliforms (FCs) and E. coli, was evaluated at several full-scale
facilities, in addition to the increase measured during cake
storage. The results showed that the sudden increase was a
statistically verifiable occurrence at some facilities, but not
all, as was the additional increases measured during cake storage.
The sudden increase and growth were much more prevalent in
processes that utilized centrifuge dewatering compared to belt
filter press dewatering. The sudden increase appears to be a result
of the reactivation of indicator bacteria that become reversibly
non-culturable (RNC) during digestion. Although other hypotheses,
such as contamination and presence of inhibitors, cannot be ruled
out in all cases. Only one plant that was sampled with high solids
centrifugation did not show reactivation and/or regrowth and this
plant was different from others in that it utilized thermophilic
reactors in series. The results showed a good correlation between
the digestion temperature and the reactivation potential and amount
of reactivation measured after dewatering. As temperature of
digestion increased, the amount of reactivation increased (for
plants with reactivation). Similarly, this was generally true, on
average for the extent of regrowth. The digestion SRT and VS
reduction did not correlate well with reactivation or regrowth.
Polymer demand varies considerably for different digestion
processes as well as the same digestion processes at different
locations and the reasons for these differences are not known. The
objectives of this research were to develop a mechanistic
understanding for these differences. The differences were
hypothesized to depend on both the amount of charge in a sludge
sample and the characteristics of the polymer. The first phase of
the research was aimed at establishing the component of the sludge
that creates the polymer demand and the second phase was aimed at
understanding the interactions of polymers with the components of
sludge. A number of samples were collected and analyzed for factors
affecting polymer demand. The samples incluced MLSS, RAS,
conventional aerobically and anaerobically digested, thermophilic
anaerobically digested, temperature phased anaerobically digested,
and an auto-thermal aerobically digested sample. Results from the
first phase showed a good linear correlation between the optimum
polymer dose (OPD) and biocolloid concentration as measured by
soluble protein and polysaccharide concentration. In other words,
these small particles (less than 4.2 mm created a significant
portion of the polymer demand, especially for digested sludges.
Conditioning samples with different polymers (varying charge
density and configuration, linear, branched and hydrophobic) showed
that the key characteristic in determining OPD is the charge
density of the polymer. The shear associated with several
full-scale dewatering devices was measured and quantified using the
unitless Gt term. In addition, several field trials were performed
to show the impact of Fe addition on polymer demand as well as
methods to reduce the shear associated with high solids
centrifuges.
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