United States
                   Environmental Protection
                   Agency
 Risk Reduction
 Engineering Laboratory
 Cincinnati, OH 45268
                   Research and Development
 EPA/600/S2-90/037 Sept. 1990
&EPA          Project  Summary
                    Effect of Recycling
                    Thermophilic Sludge on the
                    Activated  Sludge  Process
                   T.B.S. Prakasam, S. Soszynski, D.R. Zenz, C. Lue-Hing, L. Blyth, and G.
                   Sernel
                    A  full-scale  investigation was
                   undertaken at the Hanover Park Water
                   Reclamation Plant (WRP) operated by
                   the Metropolitan  Water Reclamation
                   District of Greater Chicago to study
                   whether the  net  sludge production
                   from the WRP can be reduced  by
                   implementing a scheme reported  by
                   Torpey et al. This scheme involved
                   the recycling of sludge  withdrawn
                   from a thermophilic digester into the
                   aeration tanks of  an activated sludge
                   system.
                    The Hanover Park WRP, which has a
                   design flow capacity of 45,400 m3 per
                   day (12 mgd), was split into a control
                   section and an experimental section.
                   The  control  and  experimental sec-
                   tions were operated as nitrifying ac-
                   tivated sludge systems, except that
                   thermophilic drawoff was recycled  in-
                   to the aeration tanks  of the experi-
                   mental  section  from a  digester
                   system,  which consisted  of meso-
                   philic and thermophilic  digesters
                   operated in series.
                    In  contrast to the results reported
                   by Torpey et al., a significant reduc-
                   tion in the net sludge production was
                   not observed as  a consequence  of
                   recycling thermophilic sludge into
                   the aeration  tanks  of the  Hanover
                   Park WRP.

                    This  Project  Summary was
                   developed by EPA's Risk  Reduction
                   Engineering  Laboratory, Cincinnati,
                   OH, to announce  key findings of the
                   research project that  is  fully
                   documented in a separate report  of
the same  title (see Project Report
ordering information at back).

Introduction
  In  1987,  the Metropolitan  Water
Reclamation District of Greater  Chicago
(herein called "the  District") disposed of
or used 251,000 tonnes (277,000 tons) of
dry sludge  solids produced at its seven
sewage treatment facilities. Costs of
treatment and disposal of sludge were a
major fraction of total operating costs.
Consequently, the  District  has  a
continuing  interest in  investigating
processing  approaches  that  reduce
volume and mass of sludge produced in
wastewater treatment.
  A novel approach for reducing sludge
production has been reported by Torpey
and coworkers(l) and  demonstrated at
the Rockaway Sewage Treatment Plant in
New York City. In  this process, sludge
from a mesophilic digester is processed
through a thermophilic digester and then
a portion of the thermophilic digester
drawoff is recycled through the  aeration
tanks of an activated sludge system. The
remainder of the thermophilic drawoff is
passed  through  a rethickening and
elutriation tank. The authors reported that
the volume  and quantity  of the sludge,
when  compared with that produced by
the activated sludge plant using meso-
philic digestion with no recycle, was
reduced by  about 60%. The reduction in
sludge  quantity  was attributed  to
additional destruction of solids achieved
in the thermophilic  digester and by the
oxidation of  some of the solids contained
in the thermophilic sludge recycled to the

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aeration tanks of an  activated  sludge
system.
  Torpey's findings have been supported
by  further work  by New  York City's
Department of Environmental Protection.
Carrio and coworkers (2) reported in 1985
that  recirculation of digested sludge to
the activated sludge aerators  reduced
total sludge  production  at  the  Newton
Creek, Bowery Bay,  26th Ward,  and
Oakwood  Beach wastewater treatment
plants.
  In  view  of the potential advantage of
substantially  reducing sludge production,
the District undertook a demonstration at
its Hanover Park WRP of Torpey's con-
cepts; that  is, in  a  plant using  the
activated  sludge  process, digest  the
sludge first by mesophilic and  then by
thermophilic  digestion, and then divide
the digested  sludge into two  portions,
one for discharge  and one for recycle to
the  activated  sludge  aeration.  The
Hanover Park WRP  was  selected for the
study because it was easily divided  into
two  nearly  identical  modules, which
would provide  a control section for com-
parison with the experimental section.
  The following were the main objectives
of the study  conducted  at the  Hanover.
Park WRP:
  1.To demonstrate  on a  full-scale
    whether  retrofitting  an  existing
    activated  sludge  system with  a
    mesophilic-thermophilic (M-T)
    digestion scheme  and recycling the
    drawoff  from  the  thermophilic
    digester into the aeration tanks of an
    activated  sludge  system would
    reduce the volume  and quantity of
    volatile  solids produced when
    compared with the  volume  and
    quantity  of volatile solids produced
    from a conventional activated sludge
    system  using mesophilic digestion
    and no recycling of digested sludge.
  2. To evaluate  the  quality of  the
    effluents obtained  under the various
    phases of the M-T digestion scheme
    and compare  these  with the results
    obtained  under the  conventional
    mode of  operating  an activated
    sludge system.
  3.To determine the  dewatering
    characteristics of  the  sludges
    resulting  from the recycling  of
    thermophilic  digester  drawoff  and
    compare  them v/ith those of  the
    drawoff derived from a conventional
    mesophilic digester.
  4. To evaluate the odor characteristics
    of the sludge derived from  the  M-T
    digester sequence  and  compare
    them  with those of the mesophilic
    digester drawoff.
  5. To compare the removal of heavy
    metals  observed in a conventional
    activated I sludge  system  with  the
    removal observed  using the recycle
    of the slucJge from the M-T digestion
    scheme.  I
Procedures
  The Hanover Park WRP with a nominal
dry weather capacity of 45,400 m3  per
day  (12  MGJD),  employs  primary
treatment,  secondary treatment by  the
activated sludge process, followed by
sand  bed  filtration.  The  sludge  is
anaerobically digested,  pumped  to
lagoon storage, and  ultimately used on
an adjacent farm. A schematic diagram of
the plant is shown in Figure 1.
  Wastewater | enters the treatment plant
through  coarse bar screens, and flows to
primary  settling  tanks.  There are two
settling  tanks'for each of four aeration
batteries (A,B,C, and  D).  Two aeration
tanks comprise a battery. Primary influent
flows are 3,780 m3 per day (1.0 MGD) for
each aerator  pf Battery A, 7.57 m3  per
day (2.0 MGD) for each aerator of Battery
B, and 5.68 m3  per day (1.5 MGD) for
each aerator of Batteries C and D. There
is one clarifier for each  of the  eight
aerators. Excess activated sludge  is
wasted to the primary tanks.  After  the
secondary effluent from the clarifiers is
chlorinated  with  sodium hypochlorite it
then flows to rapid sand filters.  Sludge is
digested  in  [four  circular digesters
equipped  with  floating  covers.  The
digesters  normally  are  maintained
between 29 and 35°C and  are contin-
uously mixed. The digesters are 12.2 m
(40 ft) in diameter with a variable depth of
between 4.4 m (14.5 ft) and  5.9 m (19.5
ft). Digested sludge is pumped to storage
basins and  is [applied to the sludge farm
in the summer.
  During the  [experimental  program  the
plant  was divided  into  two similar
modules. Batteries A and  B, the control
section,   were   operated  in  the
conventional activated sludge mode with
mesophilic  digestion  of the  mixed
primary and waste activated sludge
withdrawn  frorn the primary tanks. The
sludge was digested at  35°C  in Digesters
1  and  2  operated  in  series.  Both
digesters wer^ heated  and continuously
mixed.       |
  The experimental section consisted of
Batteries C and D and Digesters 3 and 4.
Digester 3  received the mixed sludge
from the primary tanks and was operated
at 35°C. The sludge from Digester 3 was
fed in series  jb Digester 4 where it was
digested at 53°C.
  The  incoming wastewater  flow  was
divided between control and experimental
sections  to give  similar  hydraulic
retention  times  in the activated  sludge
aerators. After appropriate time periods to
ensure steady state,  increasing portions
of the thermophilic  sludge  from  the
experimental section were recirculated to
the aerators to study the effects  on the
activated  sludge process and  sludge
production. There were five  operating
phases: phase 3—no re-circulation, phase
3a-10%  recirculation,  phase  3b--30%
recirculation,  phase  3c~40%  recircula-
tion, and phase 3d--50% recirculation.
  Analyses were conducted by standard
methods.  An appropriate quality  assur-
ance program was carried out to  ensure
accuracy of all analytical measurements.

Results
  The  entire  experimental  program
including  modification to  the  plant eind
cleaning of digester took almost 4  yr. The
program  was divided into three  main
phases. In Phase 1, the plant was  divided
into the  control  and  experimental
sections,  the thermophilic  digester was
started up, and  steady  state operation
was achieved. In Phase 2, background
data on performance of  the control and
experimental sections were  obtained.
Phase  3, the experimental phase of the
program, was subdivided into Phases 3a
to 3d.  In  these  phases, recirculation of
thermophilic sludge withdrawn  from
Digester  4  and recycled to  aeration
Batteries C and D was increased in steps:
zero in Phase 2 to 10%,  30%, 40%, and
50% of its output.

Thermophilic Digester Operation
  The thermophilic digester (Digester 4~
the second digester in the experimental
section) was operated successfully at
53°C.  Figure  2 shows  the period  of
startup and initial operation.  Instability
occurred  at about  55°C.  Although
operation  may  have been  possible at
55°C, temperature was reduced to  53°C
to reduce system sensitivity. Except for
occasional volatile acid excursions (see
Figure  2),  operation  at 53°C  was
uneventful. During the  data  collection
period  that followed, volatile acids  leaving
Digester 4 ranged from 89 to  233 mg/L.
Volatile acids from the second mesophilic
digester in the  control section  (Digester
2) ranged  from 18 to 86 mg/L.
  The processing of approximately half of
the sludge output of the plant through the
thermophilic digester as a final digestion
step created problems at the  sludge
farm.  Numerous  complaints  about
unpleasant odors were received  from

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                                                                                                                                         I
                     NWIS-20A
                        f
                          42"RCP
      to
   ox. pond
   to
ox. pond
                                                                                                   to lagoons
                                                                                                   or farm
Notation:
NWIS 20, 20A: Northwest interceptor sewers; W.W.: wet well; S. BOX: splitter box; P.S.: primary sludge; WAS: waste activated
sludge; R.S.: returned sludge; ML: mixed liquor; P.E: primary effluent; S.£: secondary effluent; OX. POND: oxidation pond
Figure 1.  -  Schematic of Hanover Park Water Reclamation Plant.

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  Figure 2.  Volatile acids and temperature profile of digester 4.
persons  living  near the  sludge farm
during the experimental  program  as
contrasted to practically no  complaints
when only mesophilically digested sludge
was  being discharged. The odor  of
thermophilically  digested sludge evi-
dently was either more offensive or more
identifiable  than  the   odor   of
mesophilically digested sludge.


Fate of Volatile Solids
  About 2% to 5% more  volatile solids
were destroyed  in  the  experimental
section  digesters  than in the  control
section's  mesophilic  digesters when
thermophilic sludge drawoff was recycled
(up to 40% of the volume withdrawn) into
the aeration tanks of the experimental
section. When the  thermophilic  sludge
drawoff recycle rate to the  aeration tanks
was  increased to  about  50%  of  the
volume withdrawn, the volatile  solids
destruction in  the  experimental  section
digesters was found to be lower by about
12.5% than that observed in  the control
section  digesters.   At  this high
recirculation,  the experimental  section
digesters  were overloaded.  Total resi-
dence times in control  and experimental
section  digesters were  respectively 21.5
and  8.8 days at 50% recycle. A volatile
suspended  solids (VSS) balance  around
the aerator-clarjifier revealed  a net VSS
production  for  both  control  and
experimental  systems.  Scatter was too
great to allow [a quantitative  conclusion
about the influence of  recirculation of
digested solids Ion VSS production.
  When masses of VSS  in  the  waste
activated sludge (WAS) streams  from
control Batteryj B  were compared with
those from  experimental Batteries C and
D, recirculatiori  of up  to 40%  of the
meso-thermophilically  digested  sludge
did not increase the mass of WAS per
unit volume of wastewater treated.
  The difference in the total digested
sludge  ultimately  disposed from the
control and experimental sections is not
statistically  significant. Table 1 shows the
magnitude of the differences for Phase 2
and  Phases 3 a-c.  Recirculation of  up to
30% of the  thermophilic sludge produced
a  negligible  [effect on  total  sludge
disposed per u|nit volume of  wastewater.
At 40%  recirculation,  the experimental
section produced 13% less sludge, which
was  not  statistically significant. At 50%
recirculation (npt shown in Table 1), the
experimental  unit produced more sludge
than  the contrbl. This  result  is atypical
because the experimental digesters were
overloaded  at this high recycle.
Effluent Quality and Plant
Performance

  The overall performance of the control
and experimental sections of the  Hanover
Park WRP was comparable during all the
phases  of the study.  The  recycling of
thermophilic sludge drawoff, even at the
rate of about  50%   of  the  volume
withdrawn  from  the digester into  the
aeration tanks  of the  experimental
section, did not have an adverse effect on
the secondary effluent quality.
  The  overall BOD removals of  the
control  and  experimental sections were
comparable  and  were  in the range of
91.7%  to  97.2%  and  94.2%  to 97.7%,
respectively, during the various phases of
the study. The corresponding  secondary
effluent BOD values for  these  sections
were in the range of 3 to 9 and 3 to  6
mg/L,  respectively.  Recycling of up to
50%  thermophilic  sludge  had  no
noticeable effect on BOD removal.
  The  overall suspended solids  removal
ranged  from 83.9%  to 94.2%  for  the
control section and from 89.4% to 94.7%
for the experimental  section.  The
secondary effluent suspended solids
concentration in these sections were
comparable and  were  in the ranges  of 6
to 15 mg/L (control) and 5 to  10 mg/L

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           Table 1.
Average Volatile Solids Production Sent to Farm During the Various Experimental Phases
Phase
(Recirculation)
2(0%)
3a(10%)
3b(30%)
3c(40%)
Sludge Production (Ib/mg)*
Control (Q
372
340
274
288
Experimental
(E)
364
334
265
250
Difference
C-E
8
6
9
38
(C-E)(100)
C
2.2%
1.8%
3.3%
13.1%
  * Ib/mg XO.J2 =
                                     ^ or mglL
(experimental).  Again,  recycling  of
digested sludge had no noticeable effect
on percent removal.
  The  percent removal  of NH4-N was
within the range of 91.0 to 98.6 percent in
the control and  experimental  sections.
That the  NH4-N concentration  of the
secondary  effluent  was  less  than 1.2
mg/L in both of these sections indicated
that the recycling of thermophilic sludge
did not  have  an  adverse  effect  on
nitrification.
  The  recycling  of  thermophilic  sludge
drawoff into the aeration tanks  of the
experimental section did  not  have  a
discernible  influence on  the  sludge
volume index (SVI) values of the mixed
liquors. However, there  was  too much
scatter in  SVI  values  for  any  firm
conclusions on the effect of recycling.
  A comparison  of  the  mixed  liquor
dissolved oxygen  concentration and
oxygen uptake rate  data  for the control
and experimental section aeration tanks
revealed no significant trends. Scatter in
the results  prevented drawing any firm
conclusions on the effect  of recycling on
these variables.
  The  recycling  of  thermophilic  sludge
into  the  aeration  tanks  of the
experimental section  did  not  seem  to
alter the microbiological  profile  of the
mixed  liquor when compared with that of
the mixed liquor of the  control  section
aeration tanks.
  The  soluble  concentration of  Zn,  Cd,
Cu, Cr,  Ni,  Pb,  and   Hg remained
approximately the same  in the  digester
feeds  and draws; this indicated that  no
significant solubilization of these metals
occurred  during  mesophilic  or thermo-
philic  digestion.  However, the soluble
concentration of iron in the digester feed
of the  control and experimental sections
                  decreased significantly  as it underwent
                  digestion  in  the primary  digester.  No
                  further  resolubilization of this metal
                  occurred in the secondary digesters.

                  Dewaterability
                    Capillary suction time (CST) measure-
                  ments  at various -polymer  dosages
                  indicated that mesophilic sludge required
                  a  lower  polymer dosage than  did  the
                  thermophilic sludge (10  vs. 22.5 kg/dry
                  tonne) to achieve the minimum CST that
                  was possible. The  thermophilic  sludge,
                  however, exhibited a higher floe strength
                  than did the mesophilic sludge.
                    Pilot scale centrifuge studies confirmed
                  that the  thermophilic  sludge required a
                  higher polymer dosage  than  did  the
                  mesophilic sludge. At optimum polymer
                  dosages, these studies also indicated that
                  the mesophilic sludge approached 100%
                  solids capture whereas  the thermophilic
                  sludge approached  a maximum  of 96%
                  solids capture. The lower solids  capture
                  with  thermophilic  sludge probably
                  resulted from the higher concentration of
                  fine particles  in it than in the mesophilic
                  sludge.
                    The  percent cake solids achieved in
                  pilot-scale centrifuge studies appeared to
                  be  similar at  any given  polymer dosage
                  for the mesophilic and  thermophilic
                  sludges.


                  Discussion and Conclusions
                    The   recirculation   of   meso-
                  thermophilically  digested sludge to  the
                  activated sludge aerator caused a minor
                  reduction  in sludge mass discharged at
                  Chicago's Hanover  Park WRP that was
                  not statistically significant. There were no
                  adverse  effects on quality  of  treated
                  wastewater.
  Although the Torpey  Process did  not
degrade  the quality  of  the treated
wastewater,  it  did  not  produce  the
desired reduction of mass  of solids that
must be discharged to  the environment.
This result contrasts sharply with  results
obtained at New York City plants. One
possible reason  for the disagreement is
that the  New York  City plants  use high
rate activated  sludge  processes. The
waste biological sludge from these plants
could very well be  substantially reduced
in  mass by  continued  recirculation
through the anaerobic/aerobic processes
in Torpey's scheme.
  In Torpey's work,  sludge was elutriated
before disposal. This process is not used
in Chicago. It also could be responsible
for  loss of suspended solid material that
would not occur at the Hanover plant.
  Recirculation of sludge at rates above
40% overloaded  the  experimental
section's digesters at the Hanover WRP.
Although treated wastewater quality was
not  degraded,  further increases  in
recirculation rate  would  inevitably cause
this  effect. An  existing  facility should
have excess  digestion  capacity before
the Torpey Process  can be adopted.
  The  objectionable  odor  of  the
thermophilically digested sludge caused
so  many complaints that use of this
process as the terminal  digestion step is
ill-advised if sludge is to be utilized on
farmland with close neighbors.

Recommendations
  Based on the lack of  effect on sludge
mass and the  increase  in digestion
capacity required, the Torpey process is
not  recommended  for   Chicago's
conventional rate activated sludge plants.
Nor is  thermophilic  digestion as the
terminal  sludge  digestion  process

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recommended if the sludge is to be used
at a site with nearby neighbors.
  The  full  report was  submitted  in
fulfillment of Cooperative Agreement No.
CR-811925 by the Metropolitan Water
Reclamation District of Greater Chicago,
Chicago, IL, under the sponsorship of the
U.S. Environmental  Protection Agency.
         Literature Cited
           1. Torpey, W. N., J. Andrews, and J. V.
           Basilico, J. Wat. Poll. Contr. Fed., 56,
           62 (1984)," Effect of Multiple Digestion
           on Sludge."  !
2.  Carrio, L. A., A. R. Lopez, et al., J.
Wat. Poll. Contr. Fed., 57,  116  (1985),
"Sludge  Reduction  by  In-Plant
Modification:   New  York  City's
Experiences."
   T.B.S. Pra!
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