576 A DISCUSSION OF THE ADVANTAGES OF CO-THICKENING ...

A DISCUSSION OF THE ADVANTAGES OF CO-THICKENING OF
PRIMARY AND SECONDARY SLUDGES IN DISSOLVED AIR FLOTATION THICKENERS
Abstract
Richard Butler, King County Department of Metropolitan Services (Metro)
Richard E. Finger*, King County Department of Metropolitan Services
East Division Reclamation Plant
1200 Monster Rd. S.W.
Renton, WA 98055-2962
Phone (206) 684-2412
Jim Pitts, King County Department of Metropolitan Services
Barbara Strutynski, King County Department of Metropolitan Services
Dissolved air flotation thickeners (DAFTS) are commonly used to thicken waste activated sludge;
however, their use for co-thickening primary and waste activated sludges is not commonly practiced. This
paper describes the results of a full-scale DAFT co-thickening operation. Soluble BOD reduction across
the DAFTS is approximately 80% while soluble COD reduction is approximately 60%. This represents a
reduction in BOD loading to the activated sludge facilities of approximately 4%. In addition, substantial
amounts of fine grit is removed from the primary sludge during the flotation process. Thus, DAFT cothickening
of primary and secondary sludges can significantly reduce secondary organic loading while
concurrently reducing the amount of grit being discharged to the digesters. It also eliminates the need to
independently thicken primary sludge. These impacts lead to reductions in secondary operating costs as
well as reduced operating costs associated with digester cleaning and thickening of primary sludge.
Keywords
Dissolved Air Flotation Thickening, Soluble BOD Removal, Solids Thickening, Grit Removal
Introduction
The East Division Reclamation Plant (EDRP) of King County’s Department of Metropolitan Services
(Metro), which was formerly known as the Municipality of Metropolitan Seattle’s Renton Treatment
Plant, is a secondary activated sludge treatment plant, which was originally constructed in the early
1960’s. The original plant design, which was described by Uhte (1964), did not include solids
processing; all EDRP solids were pumped to Metro’s West Point facility for treatment. The EDRP was
expanded in 1972 to increase its liquid stream capacity from 1.05 m 3
/s (24 mgd) to 1.58 m 3
/s (36 mgd);
however, solid stream processing was again not included. A second expansion, completed between 1985
and 1988, doubled the secondary capacity to 3.15 m 3
/s (72 mgd) and constructed solids handling facilities
for the first time. The solid stream processing facilities included thickening, anaerobic digestion and belt
filter press dewatering.
576

As a part of the predesign work for the second expansion, pilot testing of dissolved air flotation
thickening (DAFT) and gravity thickening was conducted. Before solids handling facilities were
constructed at the EDRP, no attempts were made to thicken primary sludges in order to minimize
residence time in the forcemain used to transport solids to West Point. As a result, the plant staff were
interested in evaluating options which would allow the continued rapid withdrawal of primary sludge
from the primary units. DAFT pilot testing therefore included an evaluation of co-thickening of primary
and secondary sludges as well as thickening of secondary sludge alone. The results of that pilot testing
were described by Anwar, Finger, Merrill and Uhte (1983).
The results of the pilot testing suggested several advantages to co-thickening primary and secondary
solids using DAFTS. First, the need for separate thickeners for primary and secondary sludges would be
eliminated, thus reducing the number of operating thickeners and associated capital and operating costs.
Second, DAFT co-thickened sludge produced a higher solids concentration than could be achieved using
gravity thickeners for primary solids and DAFTS for secondary solids. Third, solids loading for cothickening
was twice the normal design loading used for the thickening of secondary solids alone. Thus
no increase in DAFT size was required over that needed for secondary thickening. Fourth, data
suggested the potential for a reduced BOD concentration in the recycle stream, presumably due to the
removal of soluble BOD in the DAFT. Fifth, the data suggested that there was the potential for
coincidental degritting of the primary sludge.
Pilot testing results indicated that DAFT co-thickening would result in the lowest overall operating
costs and the lowest total present worth, even when the potential savings from improved degritting and
BOD removal were ignored. As a result, it was decided to proceed with a design which included cothickening
of primary and secondary sludges using DAFTS even though it was a new operational
concept. The solid stream construction was completed in late 1987. Since solid stream operation was
new to the plant staff, the first several years were spent learning the system operation. In addition, a
number of operational, equipment and control strategy modifications were implemented to improve the
thickening operation. These modifications and initial DAFT performance have been described by Finger,
Grenet, Pitts and Uhte (1992). Since this work was reported, performance has continued to improve
with float solids in the range of 6% or greater being achieved while polymer usage has dropped to the
range of $2.98 per metric ton ($ 2.71 per ton) of solids treated. While efforts to optimize the DAFT
performance are continuing, additional work has been directed towards verifying the results of the pilot
testing with respect to BOD and grit removal. The results of this testing are discussed below.
The design and operation of the DAFTS at the East Division Reclamation Plant has been thoroughly
discussed by Finger, Grenet, Pitts and Uhte (1992); however, a summary is provided below for reference.
Feed to the DAFTS, which is referred to as mixed sludge (MS), is a mixture of the primary and
secondary waste activated sludges, filtrate from the belt filter presses and thickened bottom sludge return.
The thickened bottom sludge return is the centrate from solids which had settled in the DAFTS and
which had been centrifuged to separate the organic fraction from the inorganic fraction. The inorganic
fraction is discharged along with the grit to a collection area from which it was hauled to landfill for
disposal. MS is ground using in-line grinders and discharged to a distribution box which feeds four 17.8
meter (55 foot) diameter circular DAFTS by gravity. Equal flow distribution is achieved using overflow
weirs.

Each DAFT is provided with two float collection troughs located 180 degrees apart with float being
continuously removed by 16 hinged flights which are supported by an eight armed carousel. Float level is
automatically maintained by a level control algorithm which controls the rate at which the clarified liquid,
which is called thickener overflow (TO), is removed. Both the level setpoint and the collector speed can
be varied to help optimize unit operation. Each DAFT has two dedicated air dissolution systems, either
or both of which can be used to maintain the desired air to solids ratio. Polymer is fed to each DAFT,
with the rate controlled automatically based on the turbidity of the TO. TO turbidity is measured using a
Hach surface scatter turbidimenter. Settled solids, which are called thickened bottom sludge (TBS), are
transported to a central hopper by a scraper, which is coupled to the float collectors. The TBS is
processed as described above. The capability to degrit the TBS was incorporated into the design as a
result of the pilot testing, which showed a significant amount of grit in the solids which accumulated on
the bottom of the test unit.
Methodology
Procedures to evaluate BOD and COD removal are relatively easily documented as compared to grit
removal, which depends as much on anecdotal as on quantitative data. The MS and TO samples were
composited on a flow-paced basis. Mixed sludge feed to the DAFTS was sampled using an automatic
solids sampler as described by Finger (1992). The MS sampling rate was proportional to the DAFT feed
rate as measured by two 0.3-meter magnetic flow meters. TO, or clarified subnate, was collected using a
dipper-type sampler which collected a fixed volume sample per volume of flow discharged.
MS and TO were analyzed for total BOD, total COD, soluble BOD and soluble COD. BOD analyses
were performed in accordance with Standard Methods. COD analyses were performed using a modified
version of that described in Standard Methods with digestion time reduced to 20 minutes. The lower
digestion time was based on analytical data which indicated this time was adequate to ensure complete
oxidation of the species present. Soluble portions were prepared by centrifugation and then filtration
through 0.045 m μ membrane filters.
Other analyses were performed on the solid stream processes as a routine part of the plant process
monitoring and control program. Methodologies contained in Standard Methods were utilized in
conducting these analyses. Once the analytical testing was completed, routine monitoring data was
entered into a computerized laboratory data base which was developed by Metro staff. In addition to
storing raw data, this system performed process calculations and provided data retrieval and report
generation capabilities.
Manual probing of the digester, which has been practiced since the digesters were placed in service,
was used to indicate grit and solids carryover from the DAFTS. The probe consisted of a series of rod
sections which could be screwed together to reach the bottom of the digester, which was about 10.7
meters (35 feet) below the cover. The first rod section had a small disk on the end to produce a
resistance as it encounters more dense layers. The rod was inserted into the digesters through access
ports on the floating cover. After the initial resistance of the scum layer, the rod was lowered until a
significant increase in resistance was encountered. The rod was then lowered until the bottom was
reached. The distance between the digester bottom and the depth where resistance increased was defined
as the thickness of the digester bottom sludge accumulation.

Results
Soluble COD and BOD removal across the DAFTS were evaluated over two periods: February 1 to
June 23, 1994, and September 20 to October 24, 1993. These periods represent distinctly different
seasonal periods which should provide some insight into whether organic uptake in the DAFTS is
dependent on seasonal conditions. Only soluble COD analyses were performed before April 17 (first half
of Period One). Both soluble COD and soluble BOD testing were conducted for the remainder of Period
One and all of Period Two. Analyses were seven days per week except during the second part of Period
One when analyses were limited to weekdays. The results of soluble COD testing are summarized in
Table I, while Table II summarizes the soluble COD loading to the DAFTS for the two periods.
Figure 1 presents data for both test periods including MS and TO soluble COD concentrations and
soluble COD removal efficiencies.

.
800
700
600
500
400
300
200
100
0
The results of the soluble BOD testing are summarized in Table III, which includes concentration data
for the two test periods. Table IV summarizes the soluble BOD loading data for the same periods.

solids buildup in the digesters as measured indirectly by probing and directly during digester cleanings,
the comparative volatile content of the DAFT thickened sludge (digester feed) and the DAFT bottom
sludge (TBS), and the primary sludge and TBS quality during extreme storm events.
Manual probing of the digesters has been conducted on a monthly basis since the solids handling
facilities came on-line in 1988. The intent of the probings was to measure solids accumulation in order to
schedule digester cleanings without seriously impacting plant operations. Over the past six years, the
probings have consistently indicated no appreciable solids accumulation in the digester.
Although digester probings had proven “negative”, one digester was drained and cleaned in 1992 to
test the digester cleaning system (which had not been tested since it was installed in 1988) and to verify
the results of the digester probing. A significant accumulation of plastics and rags was found on the
digester bottom. However, only a minor amount of grit was found amongst the accumulated solids. As a
result of the significant amount of plastics and rags found during the first digester cleaning, digester
cleanings have continued on a yearly basis. Two more digesters have subsequently been drained and
cleaned (as a part of the annual digester cleaning routine) and found relatively free of accumulated grit,
though plastics and rags accumulation are a recurring problem. Steps are in progress to address the issue
of rags and plastics through an upgrade to the plant raw sewage screening system.
The comparative volatile content of the THS and TBS is an another indication of the DAFTS
effectiveness at separating grit. Grit settles in the DAFTS and is incorporated in the TBS. TBS is
subsequently recycled through degritting classifiers prior to it’s return to the DAFTS as part of the MS.
The average mass of TBS produced during the test periods was 18,695 kg/d (41,130 lb/d), and the
volatile content of the THS and TBS averaged 80% and 75%, respectively. Assuming the difference in
volatile content represents the amount of grit removed during the co-thickening process, daily grit
removal in the DAFTS averaged approximately 935 kg (2057 lb). This represents about 9% of the total
grit and screenings hauled from the plant on an average day. The grit being remove in the TBS has a
generally fine sandy consistency based on a visual examination of the channel used to convey TBS from
the sampling point to the plant drain. Flushing water is required to move the TBS to the drain and the
resultant agitation results in the separation of the grit from the solids and subsequent settling in the drain
channel.
The effectiveness of the DAFTS to separate grit from primary sludge was demonstrated during an
extreme storm event in November 1990. The EDRP collection system is primarily a separated system
resulting in moderate storm flows. However, significant inflow and infiltration, a result of high water
tables during Pacific Northwest winters, can produce significant peak flows. During a November 1990
storm, which was rated a 100 year event, plant flow peaked at the plant’s maximum hydraulic capacity of
10.5 m 3
/s (240 mgd). These extreme flows significantly reduced the effectiveness of the plant’s aerated
grit removal system. The resulting carryover of grit to the primary system caused a significant drop in the
volatile content of the primary sludge; however, the THS from the DAFTS was not significantly affected.
At the same time, there was a significant increase in the quantity of TBS produced in the DAFTS. It took
several days to return to normal, during which time grit production remained elevated; however, digester
operation was not affected because of the effectiveness of the DAFTS at removing the grit. During the
day of the storm and the following day, the mass of TBS increased by over 27,262 kg (60,000 lbs) while

the volatile content dropped to approximately 60%. The system thus prevented approximately 10,910
kg (24,000 lbs) of grit from reaching the digesters during that storm event alone.
Discussion
A significant reduction in soluble BOD and COD occurred across the DAFTS. Soluble BOD removal
averaged almost 80% or 1387 kg/d (3051 lb/d) during the test period. This removal represents a 3 .6%
reduction in secondary BOD loading, which averaged 38,182 kg/d. The reduction is probably a result of
the biologically active solids from the activated sludge system and the favorable aerobic environment of
the DAFTS. Interestingly, DAFT effluent soluble BOD remained fairly constant though influent soluble
COD varied considerably, which accounted for the change in percent soluble BOD removal.
While BOD removal in the DAFTS represents a significant impact, operating conditions at EDRP tend
to minimize soluble BOD production. First, wastewater temperatures in the Pacific Northwest are lower
than those in much of the country throughout the year, which helps to limit the development of anaerobic
conditions in the collection system and within the plant. Thus, soluble BOD concentrations of the plant
influent and feed to the thickening system can be expected to be lower than in other parts of the country.
Second, primary sludge is removed from the EDRP clarifiers without attempting to achieve thickening.
This tends to minimize the production of soluble BOD associated with anaerobic conditions. In fact,
work done at Metro’s EDRP and West Point facilities found that attempts to thicken sludge in the
primary clarifiers led to a significant increase in primary effluent soluble BOD. Third, gravity thickeners,
if utilized at EDRP, would be expected to promote even more solubilization, resulting in significantly
higher recycle soluble BOD than is encountered at the EDRP. Thus, the soluble BOD removals and
reductions in secondary organic loadings observed at EDRP represent a lower baseline. Potentially much
higher removals and reductions would occur if gravity thickening of primary sludge was practiced.
The reduction in BOD loading to the secondary presents several advantages. First, there would be a
direct reduction in the amount of air required in the secondary process. Using current operating data at
the EDRP, this would translate to an annual savings in electrical cost for aeration of $20,250 based on a
electrical rate of $0.037 / kwh. Savings in other areas of the country where electric rates are significantly
higher would be correspondingly greater. As discussed previously, elimination or avoidance of processes
or procedures which promote BOD solubilization would lead to an even greater savings. Since the
treatment of soluble BOD in the secondary process would result in the production of biological solids, a
reduced solids loading to the secondary clarifiers and reduced pumping of waste sludge, both of which
have either direct or indirect cost savings to the facility, would result from the removal of soluble BOD
during the DAFT thickening process.
In addition to the removal of soluble BOD, DAFT co-thickening results in an improved capture of grit
and thus a reduced rate of grit accumulation in the digesters. The mass of grit being removed under
normal conditions is fairly low at the EDRP, due in part to the separated nature of the collection system.
Operation at the West Point treatment plant, which is on a combined system and which utilizes primary
tanks to thicken the primary sludge, has shown a significant accumulation of grit in the digesters which
necessitates frequent digester cleaning. Personnel from both plants have commented on the significant
difference in the grit accumulation in the digesters of the two facilities. While the average mass of grit
removed was low, the data on storm conditions suggests that there is a significant potential for reducing

grit loading to the digesters as a result of coincidental grit removal during the DAFT co-thickening. The
cost savings associated with reduced digester cleaning frequency and reduced abrasion on downstream
equipment will depend on the magnitude of the problem at each utility. As a consequence, no attempt
has been made to assign a dollar value to this benefit.
Conclusions
Co-thickening of primary and waste activated sludge using dissolved air flotation offers several
opportunities for savings over the separate thickening of the two sludges. Based on pilot testing and full
scale operation, DAFT loading rates can be increased over those conventionally used for thickening of
wasted activated sludge alone so that the total size of the thickeners does not have to be increased
significantly. In addition, the thickened sludge from the co-thickening DAFT operation has a higher
solids concentration than can be achieved using separate gravity thickening for primary solids and DAFT
thickening for secondary solids.
Elimination of the separate primary thickening process has direct cost savings associated with reduced
siteing needs, reduced capital costs and reduced operation and maintenance costs. The elimination of
gravity thickening also has a positive effect in that it avoids the potential for BOD solubilization during
the thickening process with a subsequent increase in secondary BOD loading. Coupled with this is the
enhanced removal of soluble BOD which occurs as a direct result of the mixing of primary and secondary
solids in the DAFT under aerobic conditions. This effect can be enhanced by routing all solid streams
processes having high soluble BOD levels, such as dewatering filtrate or centrate, through the DAFTS.
In addition to helping to eliminate BOD loading on the secondary, the solids removal which occurs
reduces the secondary solids loading associated with the solid area recycle streams. All of these factors
translate into reduced operating costs and improved process reliability.
Coincidental degritting of the primary solids provides another benefit to DAFT co-thickening. The
reduced grit load on the digesters can reduce the accumulation of grit and thus the frequency with which
the digesters must be cleaned. This has advantages from a operational standpoint as well as providing
greater process stability by preventing the need to operate with reduced numbers of digesters and/or with
digesters having reduced active capacity due to grit accumulation. The removal of grit during the
thickening process also serves to limit abrasion on downstream equipment.
While DAFT co-thickening may not be the answer for all treatment plants, there are compelling
reasons related to ease of operation and reduced operating costs why this option should be considered
when new plant deign or upgrade of existing plants is being undertaken. In addition, plants presently
having both DAFTS and gravity thickeners could benefit from the conversion to a DAFT co-thickening
operation. As discussed above, there are a number of operational advantages including the potential for
significant cost savings.
Acknowledgments
The work reported in this paper was performed as a part of the routine operation of the King County
Department of Metropolitan Services East Division Reclamation Plant and could not have been
accomplished without the fill support of the plant’s Operations, Maintenance and Process Control Staff

The authors would especially like to thank the Process Laboratory Specialists, who’s analytical work was
instrumental in documenting the results of this work. A special acknowledgment is extended to Warren
Uhte, the principal designer of the original Renton Plant and the three subsequent enlargements, who
deserves credit for many of the innovations which have helped make the East Division Reclamation Plant
the outstanding facility it is.
References
Anwar, P., Finger, R., Merrill, S. and Uhte, W. (1983) A New Look at Liquid, Solids Separation
Predesign and Design for the Renton Treatment Plant. presented 50th Annual Pacific Northwest Pollution
Control Association Conference.
Finger, R. (1 992) Development and Testing of an On-Line Automatic Sampler for High Solids Samples.
presented Water Environment Federation 65th Annual Conference
Finger, R., Grenet, D., Pitts, J., and Uhte, W. (1992) Metro Seattle Renton Treatment Plant Sludge
Thickening Update.
Standard Methods for the Examination of Water and Wastewater (1989) 17th Ed., Am. Public Health
Assoc., Washington, D.C.
Uhte, W. R. (1964) “Metropolitan Seattle’s Renton Treatment Plant.” Journal Water Pollution Control
Federation, 36, 475 (1964).