Aerobic waste sludge digester-thickener orbital system and method

A thickener well is provided in one end of a digester orbital ditch having a pair of channels and an elongated center partition spaced from a pair of ditch endwalls, the overall ditch forming a basin for digestion of wasted sludge. A curved endwall of the ditch provides a common curved top outer peripheral wall of the thickener well. The remainder of the well has a cylindrical configuration integral with the orbital ditch bottom and extending downwardly therefrom to a position below the horizontal plane of the ditch bottom. A sludge raking structure is positioned at the bottom of the well for periodically raking settled sludge into a sludge outlet at the bottom of the well. A surface aerator is provided between and spaced from the other ditch endwall and a juxtaposed end of the partition or other aeration apparatus mounted in a channel(s) to aerate and propel a liquid-solids sludge mixture wasted from a secondary treatment apparatus and fed into the orbital ditch. The mixture is circulated in the ditch and across and through a top portion of the well. The orbital ditch may be decanted by turning off the aerator, allowing a clear liquid top layer to form in the ditch and decanting the clear liquid from the tank by lowering a weir or by adding additional activated sludge to the ditch to displace clear liquid over a fixed weir.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a system of aerobically treating waste sludge. 
More particularly, the invention is directed to an orbital system and 
method for treating by digestion and thickening bacterial 
microorganism-containing sludge wasted from an industrial or municipal 
wastewater treatment process and apparatus. 
2. State of the Art 
In a wastewater process employing the activated sludge process waste water 
impurities including domestic wastes, sugars, proteins, carbohydrates and 
other nitrogen-containing materials are decomposable by microorganisms, as 
is well known in the art. As the impurities are decomposed a sludge of 
settled material and microorganisms is wasted from the process either on a 
continuous or non-continuous basis. The purpose of sludge wasting is to 
keep solids from building up in the system. Sludge from the process is 
normally transported by pumping to a digester for treatment prior to 
landfill or other disposal so as to reduce the volatile organic content of 
the sludge, reduce the sludge volume, reduce the pathogenic organisms 
present in the sludge, reduce its odor potential and improve sludge 
dewaterability, and for other reasons of lesser importance. Various prior 
art types of digesters and various digestion and stabilization processes 
have been proposed and used. 
The present invention is directed to an apparatus and method involving 
improvements in aerobic digestion. Current practice for aerobic digestion 
is to first transport the waste sludge generated in a secondary activated 
sludge system into a separate adjacent thickening tank of the gravity or 
other type and then transport the thickened sludge to a separate aerated 
digester for 10-30 days. Biological activity in the tank digests the 
sludge over that time. 
In addition, these digesters provide sludge holding capability for periods 
of time when the ultimate disposal method is not available (i.e., the 
sludge truck is broken or the landfill is closed, etc.) and they provide a 
means of thickening the sludge, called decanting. Decanting is simply 
allowing the sludge to settle by turning off the aeration for a period of 
time until a clear liquid layer forms on top of the sludge. This clear 
liquid is then removed from the digester tank which leaves behind a 
thicker sludge having less volume. Decanting can be a fill and draw 
operation where the water level in the tank varies or the clear liquid can 
be displaced by sludge being fed to the tank. There are also some floating 
weir decanters in use. In some cases the sludge is thickened in a separate 
thickener after digestion. The purpose of this is again to thicken the 
sludge and reduce its volume. 
Aerobic digestion is described more fully on pages 5-20 to 5-28 of U.S. 
Environmental Protection Agency (EPA) Publication 62511-74-006 entitled 
"Process Design Manual for Sludge Treatment and Disposal" dated October 
1974. As pointed out on page 5-23 aerobic digestion has disadvantages 
which may include high operating costs and poor dewatering characteristics 
on vacuum filters. A relatively long detention time of 10-30 days or more 
is necessary to break down the organic matter and control pathogenic 
organisms and is common practice. In small treatment systems (0.1 mgd) the 
digestion tank can also act as a sludge thickening unit. The current 
practice is to separately pump air or oxygen into the digester through 
diffuser pipes in the liquid-solids mass in the tank in an amount of from 
about 20-60 cfm/1000 cubic feet, dependent on the presence of primary 
and/or excess activated sludge, to keep the solids in suspension and 
maintain a dissolved oxygen (DO) between 1-2 mg/l (page 5-26). 
It has been common for some years to use a so-called orbital oxidation 
system or oxidation ditch to biologically treat wastewater. Such systems 
are exemplified by the U.S. patent to Klein 3,510,110 where sidewalls and 
a center partition form a race track-like essentially closed circuit with 
a surface aerator positioned adjacent to one end of the partition to both 
aerate the mixed liquor in the tank and to provide the motive force for 
circulation of the mixed liquor around the circuit. Horizontal brush-type 
aerators, jet-type aerators or rotating disc-type aerators have also been 
employed in activated sludge sewage orbital purification processes. As 
seen in Stensel et al., (4,303,516) clarifiers, (termed intraclarifiers) 
which have a sludge thickening function have been placed into a channel of 
an orbital ditch and function to decant off clarified liquid from a 
portion of the overall channel mixed liquor flow introduced into the 
clarifier and to allow settled sludge to be returned to the channel from 
the intraclarifier. 
Further, it has been known to use an oxidation ditch as an aerobic digester 
per se. Reference is also made to McKinney, U.S. Pat. No. 3,462,360 which 
shows a loop-type secondary aerobic treatment apparatus in which liquid 
waste is treated and a separate settling basin provided defining a 
quiescent liquid zone for conducting tertiary treatment. Settled algae may 
be returned from the settling basin to the loop or withdrawn from the 
facility. 
SUMMARY OF THE INVENTION 
The present invention utilizes an orbital basin combined with thickener 
well in a common vessel. Such digester-thickener apparatus provides a very 
large surface area which promotes settling and aids in decanting the 
vessel, provides a sludge thickener mechanism which results in a highly 
dense discharge sludge for disposal, provides for minimal mixing energy to 
maintain the settling sludge in the vessel in suspension during the 
digestion phase of the method, and minimizes capital and operational costs 
due to the combination of two unit processes in a single vessel. 
Further, the continuous flow of liquid across and through the thickener 
keeps the sludge from becoming septic, since the bulk of the liquid is 
constantly being exposed to aeration as it orbitally circulates. This 
helps control odors and floating sludge. The sludge which falls out of 
suspension into the thickener area is the densest, most digested material, 
while the lighter, more volatile material will tend to recirculate in the 
active channel portions of the digester. Any sludge or scum which tends to 
float in the thickener area is entrained in the flow of the ditch and 
reaerated. This contrasts with an external thickener where the scum and 
floating material build up and become a problem. 
In the system of the invention the flow of the sludge around the ditch 
tends to elutriate or wash the sludge in the thickener which has been 
shown to promote better thickening. Additionally, the flow of the 
liquid-solids in the ditch will prevent freezing which is a problem in 
external thickeners in cold climates since the thickener normally has a 
quiescent top surface. Still further, the aerobic liquid-solids 
circulating over the thickener top will prevent odors from becoming a 
problem. In many separate prior art thickeners covers and expensive odor 
control systems are necessary. 
The above advantages result by providing a method utilizing an orbital 
vessel with an appropriate surface aerator at one end of the partition 
wall or other aeration/propelling mechanism in one or more of the flow 
channels. The channels generally define an aerobic digestion section or 
zone of the ditch. At the other end of the partition and with an adjacent 
end wall of the vessel a thickener section or zone is defined with a 
depressed cylindrical portion integral with the ditch bottom. The aeration 
device also functions as a mixer. A rake assembly shaft driven from above 
the vessel is provided at the bottom of the thickener section in the 
cylindrical thickener portion. Sludge is removed from the center of the 
bottom of the cylindrical thickener portion. A decant weir(s) is provided 
extending along the outside periphery of a channel(s) and a sludge inlet 
provided downstream from the aerator/mixer normally in a first channel.

DETAILED DESCRIPTION 
The combined orbital digester and thickener system 10 is seen in FIGS. 1 
and 2. The system includes a basin 16 formed by vertical or slightly 
inclined parallel sidewalls 11 and 12 and normally curved upstanding 
endwalls 14 and 15 integral therewith. A central elongated longitudinal 
partition 19, as is known in the art, separates the basin 16 into parallel 
channels 17 and 18 forming with the curved endwalls an essentially closed 
flow loop as indicated by the arrows. Aeration and a flow propelling force 
is provided by a surface aerator 20 supported by a walk platform 21 
extending across endwall 14. The surface aerator is generally aligned with 
the partition longitudinal axis and spaced from the partition end 6. 
Rotation of the aerator mixes and aerates the waste activated sludge in 
the basin and propels that solids-liquid mixture around the partition into 
channel 17. Wasted activated sludge is fed into the basin normally at or 
near the aerator to provide rapid mixing with the solids-liquid mixture 
already flowing around the orbital pathway and entering the aerator from 
channel 18. The surface aerator is of the type having a series of radial 
blades 23 shaft-driven by a motor 22 which blades beat air from above the 
solids-liquid surface level 24 of the basin into the water-containing 
activated sludge and which keep the solids-liquid mixture in motion 
through the channels. In a typical application the velocity of the 
solids-liquid mixture is from about 0.7 ft. per second to about 1.3 ft. 
per second. Such velocity is high enough to continue the flow without 
sludge settling out in the channels but low enough to have a sufficient 
lowering of that velocity in the larger transverse cross-sectional area 
adjacent and between the endwall 15 and partition end 7 forming the 
thickening zone in the basin. 
The thickening zone is bounded by semicircular endwall 15 and a thickener 
well 25 constructed integral with endwall 15 so that endwall 15 is common 
to the overall basin and to the upper outer periphery of the thickener 
well 25. The remainder of well 25 takes the form of a half right circular 
cylinder 27 integrally extending downwardly from channel bottom 29 inward 
of partition end 7 and faired into a lower portion of semicircular endwall 
15. Partition end 7 extends toward the rake vertical shaft a sufficient 
distance into the thickener zone so as to (1) minimize short circuiting of 
the flowing liquid-solids mixture so that most passes through the 
thickening zone and (2) still permitting a reduction in flow velocity in 
the thickening zone so as to assist in sludge settling in that zone. The 
well 25 is closed at its bottom in this preferred embodiment by an inverse 
conical surface 35 having a sludge removal outlet 36 at its center. A rake 
mechanism 28 includes rake arms 26 which extend radially along the conical 
surface 35 to transport solids settling on the rake and conical surface 35 
to the sludge outlet 36. Digested sludge may be withdrawn by a pump (not 
shown) through line 32. 
Wasted sludge is fed to the basin through valve 30 into an aeration zone 
surrounding aerator 20 and propelled around the basin as shown by the 
arrows (FIG. 1). The horizontal arrows in FIG. 2 illustrate the 
solids-liquid flow path through channel 17, across and through a top 
portion of well 25 and then back through channel 18. Vertical arrows 
represent settling of the sludge into the cylindrical bottom portion of 
the thickener deep well. The full cylindrical lower section of the well 
above the rakes and below ditch bottom 29 is a relative quiescent zone 
little impacted by the circulation flow in the channels and around the 
basin ends. 
FIGS. 3 and 4 show a second embodiment of an orbital digester-thickener 40 
in which a pair of channels 47 and 48, curved endwalls and a partition 41 
spaced therefrom form an orbital flow path in a basin. A pair of 
horizontal rotary brush aerators 49 are positioned so that the radial ends 
of the brushes extend into the liquid-solids surface level 59 in channels 
47 and 48 to both aerate and orbitally propel the liquid-solids waste. As 
in FIGS. 1 and 2, a deep thickener well is formed integral with the one 
endwall 45 with a cylindrical section 57 extending integrally from the 
basin bottom 60 to a position lower than the basin bottom 60. Radial rake 
arms 42 with attached angular rake blades 43 are shaft-driven by a motor 
55. In this alternative embodiment, a flat circular floor 61 is provided 
in the thickener zone with radial rake arms and angular blades scraping 
settled sludge radially outwardly to a sludge pocket 62 along the outer 
peripheral wall 45 for removal. Waste activated sludge is fed into the 
basin through line 50 and digested sludge withdrawn for removal through 
line 58 extending from the bottom sludge pocket in the well. Clear liquid 
may be decanted over vertically adjustable or fixed weir 46 and into a 
trough 44 for disposal. A suitable weir jack mechanism 51 as known in the 
art may be utilized for adjusting weir 46. 
Activated waste sludge as used herein includes both primary sewage from a 
primary clarifier whose effluent is fed into the aeration basin of a 
secondary treatment activated sludge system as well as waste activated 
sludge from a secondary clarifier. Effluent from secondary treatment is 
normally sent to a secondary clarifier which may be a separate vessel 
sited away from the aeration basin or an intraclarifier in the aeration 
basin. Waste activated sludge from the secondary clarifier may in part be 
returned to the secondary aeration vessel or sent to a sludge digester for 
digestion. Either or both of these sludge sources may comprise the feed to 
the orbital digester-thickener of this invention. 
Normal operation of the above described apparatus permits performance of a 
method of digesting sludge wasted from a secondary activated sludge 
treatment apparatus or process. The method comprises providing an orbital 
ditch having a pair of extended channels essentially forming a digestion 
zone; forming a cylindrical deep well integral with one end of the orbital 
ditch and the ditch bottom to form essentially a circular thickening zone; 
feeding water-containing activated sludge into the ditch at a position 
displaced from the deep well; aerating, mixing and propelling the 
activated sludge through a channel, across and through an upper part of 
the thickening zone and into the other channel; thickening the sludge by 
allowing settling of dense portions of the sludge into the deep well; and 
withdrawing settled sludge from the bottom of the well. To aid in removal 
of settled sludge the sludge may be raked into a center or side outlet at 
the well bottom. The method also may include a step of decanting clear 
liquid from the ditch preferably at a position downstream of the 
thickening zone. This step preferably may be done by periodically stopping 
the surface aeration and propelling means and closing valve 30 and sludge 
outlet 36 so that a clear liquid layer forms at the top of the basin and a 
portion of that layer is decanted either by lowering the outlet weir or by 
opening valve 30 and feeding additional wasted sludge to the basin so as 
to displace clear liquid over a fixed weir. Thickened digested sludge may 
be withdrawn from the bottom of the well at any time during the 
settling/feed/decant cycle or during normal operation of the aerator. 
______________________________________ 
Design Example 
______________________________________ 
MGD Activated Sludge Plant 
3000 lbs/d of waste activated sludge 
40,000 gpd of waste activated sludge 
Digester Design Criteria 
Temperature, 20.degree. C. 
Sludge Age, 40 days 
Solids concentration, 2.5% TS 
Volatile solids destruction, 40% 
Volume, 430,000 gal 
DO in aeration zone, 2 mg/l 
Digester Size 
Length 155' 
Width 43' 
Channel width 20' 
Channel depth 10' 
Thickener diameter 41' 
Thickener side wall depth 
12' 
(outer periphery) 
Digester Operation 
Feed schedule 40,000 gal/day 
Decant rate 10-12 in/day 
Aerator size 40 HP 
Minimum aerator horse power 
25 
Maximum aerator horse power 
40 
Thickened digested sludge rate 
2150 lb/day 
Thickened digested sludge 
3.5-4.0% TS 
55-60% VS 
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Digester/Thickener Operation (Typical) 
Sludge Wasting: 
Pump 40,000 gal of waste activated sludge to digester each day on the 
average. Prior to initiating the feed sequence, turn the aerator off for 
2-6 hours to allow a clear liquid supernatant to form at a depth of at 
least 18-24". Lower the digester weir by 10-12" and decant the clear 
supernatant. Raise the weir and begin the feed pumping sequence. 
At 2-5 day intervals as practical, withdraw the entire contents of the 
sludge thickener i.e. the bottom cylindrical volume, for disposal by 
actuating the rake mechanism so that settled sludge is moved into the 
center or side pocket outlet for pumping to a truck or other transport 
means to fill disposal. Such procudure can be done in lieu of supernating 
as noted above. 
The above description of embodiments of this invention is intend to be 
illustrative and not limiting. Other embodiments of this invention will be 
obvious to those skilled in the art in view of the above disclosure.