Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Table of Contents
1. Background And Theory Of Aerobic Digestion ............................................................................................ 2
2. Aerobic Digestion Performance Study At Plum Creek Waste Water Authority .................................... 9
3. Aerobic sludge digestion At Paris W.W.T.P., Illinois ................................................................................ 21
4. What Do You Mean, We Can’t Meet 503 With Aerobic Digestion? ...................................................... 27
5. High Solids Aerobic Digestion At Los Lunas, New Mexico ....................................................................... 44
6. The P.A.D. Process And Design Of Aerobic Digester System Incorporating Recent Experience ....... 60
BACKGROUND AND THEORY OF
AEROBIC DIGESTION
GLEN T. DAIGGER, Ph.D.,P.E. DEE
Senior Vice President
CH2M HILL
The initial set of slides deals with the theoretical aspects of aerobic digestion. We are not going to
focus on the traditional kinetics. I think everybody has seen the first order of decay of volatile solids
in aerobic digestion systems in textbooks. What we’re going to talk about is how we can use some
ofthe things we’ve been learning elsewhere, but take and apply them to improve the performance
of aerobic digestion systems. For example, the use of nitrification and denitrification to improve the
performance of aerobic digester.
Advantages and disadvantages of conventional aerobic digesters: 1. Simple to implement 2. Modest capital requirements 3. Simple to operate 4. High energy costs 5. Cold weather efficiency 6. Nitrification/pH effects 7. Poor pathogen reduction
Biochemical conversions in aerobic digesters include:
Destruction of Biomass: C5H7O2N + 5O2 = 4CO2+H2O+NH4HCO3
NITRIFICATION OF RELEASED NH3‐N: NH4
++2O2+ = NO3‐+2H+ +H2O
With complete nitrification C5H7O2N + 7O2 = 5CO2+3H2O+HNO3
Biochemical conversions in aerobic digesters include (continued):
With partial nitrification 2C5H7O2N + 12O2 = 10CO2+5H2O+NH4++ NO3
‐
Stabilization Using NO3‐N: C5H7O2N + 4NO3
‐ + H2O = NH4++5HCO3‐+ 2N2
With complete Nitrification/Denitrification C5H7O2N + 5.75O2 = 5CO2+2N2+4H2O
So, Advantages of aerobic/anoxic operation include:
A reduction in process oxygen Requirements by 17% from 7 to 5.75 mole O2/mole biomass
No alkalinity depletion, as denitrification produces alkalinity needed for nitrification
Nitrogen removal
Destruction of biodegradable organic matter releases heat
3.6Kcal released/g VSS oxidizes, just like for composting
Used to Heat Digester Contents
Off‐sets Digester Heat Losses
Can cause Summertime Temperature excursions beyond the mesophilic range
Physical constraints on maximum digester solids concentration include:
Mixing: Reactor contents turn over; the ability to maintain suspended solids in suspension
Oxygen transfer: Digestion solids release non‐biodegradable organics which are surface active and change bubble size and shape
Sludge thickening offers severals benefits to aerobic digester:
Increased SRT
Temperature elevation: I. 21˚C for 2% feed sludge with 30% destruction II. Same principle as ATAD,Except maintain mesophilic temperatures
Cold weather efficiency‐ the adverse effects of cold temperatures on the performance of the
process.
The third item: nitrification and pH effects‐ as you release ammonia and it’s nitrified, you can
get dramatic and drastic drops in pH.
The fourth item, as driven by the 503 regulations, is the challenge of getting acceptable
pathogen reduction in aerobic digestion system.
We’re going to talk through all four of these disadvantages, and what we can do about
them. We can’t necessarily make them go away, but there are some things that we can do address
these factors in the design and operation of aerobic digesters. In particular, some of the recent
innovations including aerobic and anoxic operation for pH control by denitrifying to recover
alkalinity, and also by denitrifying to reduce our energy requirements. Temperature control by pre‐
thickening ‐‐ then staged operation to both improve efficiency and pathogen destruction. This will
set the stage for the case histories that you’re going to see.
Bio‐chemical conversions:
We’re going to do a little bit of chemistry and bio‐chemistry. C5H7O2N is the classic formula for
biomass in an activated sludge system. It’s also the formula for you and me. This would be the
chemical composition for you and me. The first item, the destruction of biomass in an aerobic
digester, uses oxygen to oxidize organic material to carbon dioxide and water. The first equation
shows the release of ammonia. The important thing here is that it’s NH3 which is released, which
then combines with some of the CO2 that’s produced to form ammonium bicarbonate. So digestion,
aerobic or anaerobic, creates alkalinity because it releases ammonia, which then reacts with carbon
dioxide to form ammonium bicarbonate.
In an aerobic digester you see the second reaction, nitrification of the released ammonia to
create nitrate and 2 moles of acidity. That’s shown as the two moles of hydrogen (H+). So the
destruction of biomass produces 1 mole of alkalinity as ammonium bicarbonate (from the first
equation), then nitrification destroys 2 moles of alkalinity. Complete oxidation and nitrification
takes 7 moles of oxygen for just oxidation of biomass.
In an aerobic digestion process, depending on the amount of alkalinity that you have in the
feed sludge, and see dramatic drops in pH down into the 5’s, which begins to adversely affect the
digestion process.
What we often see in fact is partial nitrification and also a portion of the nitrogen being left
as ammonia. The system will nitrify until the pH drops enough that it begins to inhibit the nitrifying
bacteria, as illustrated by the equation called partial nitrification. This would be the case where
there is only an inconsequential amount of alkalinity in the sludge that’s fed into the digester. You’ll
end up with a mixture of both ammonia and nitrate being produced. And that’s what we see in
many, many digesters, some ammonia and some nitrate.
If we use the oxygen in the nitrate as an oxygen source, just like we do in liquid stream
process, we can nitrify and denitrify. There’s no reason we can’t do that in an aerobic digester. You
see this in the middle equation, taking biomass plus nitrate, releasing ammonia, and producing
nitrogen gas. So, nitrate is used to oxidize the biomass.
You will also see that,
Alkalinity produces biocarbonate
Digestion produces alkalinity
Denitrification produces alkalinity
Nitrification consumes alkalinity
The equation on the bottom shows a balanced stoichiometric equation. If we nitrify and denitrify all
of the ammonia that’s released, we have balance.
Biomass + O2 CO2 + N2(g)+ H2O
So if we can balance nitrification and denitrification, we can operate and maintain a neural pH. That
is a very interesting finding based on the theory of these systems.
This provides the opportunity to
1. Reduce our process oxygen requirements because we’re making use of the nitrate. (a 17%
reduction)
Avoid alkalinity depletion because we’re using the alkalinity produced in denitrification to offset the
remaining alkalinity that’s required for nitrification. Nitrogen removal:
It may or may not be consequence. Nut first two mean operating savings, and those are
important things to look at. But the first two mean operating savings, and those are important things
to look at.
Destruction of biodegradable organic matter also releases heat. It releases 3.6Kcal per gram
of volatile solids oxidized, just like a compost pile. So, just like in a composting system, heat is
released when you burn organic matter.
Issue: Can we capture and use it in the process?
It certainly can be used to heat the digester contents and offset digester heat losses. It can also
cause us problems in the summer times, particularly if we have temperature excursions above about
35 to 40 degrees Centigrade. By destroying organic matter that’s been grown under mesophilic
conditions, and those bacteria grow well between about 10 degrees up to about 35 degrees
centigrade. Above 35 degrees centigrade, we begin cooking them, and they release cell contents
that cause foaming.
Sludge thickening offers several benefits.
Increase the SRT
Potential to capture the heat and control temperature.
For example:
If you had 2% feed sludge and you are destroying 30% of it, you would have the potential if
you conserved all that heat to increase the digester contents’ temperature by 21 degrees. That’s
pretty nice in the winter, but it can cause problems in the summer as described previously.
If we have dual operating modes, though, we can operate to meet both of these
requirements. In the winter: When we have high heat losses, we maximize the use of the released
heat to heat water. In the conventional digestion process, the reason you don’t see temperature
increase is that you put thin sludge in at 1‐ ½ %, and as it digests, you supernatant out of the
digester. As the heat is released, it is used to heat a lot of water so the temperature elevation is
small.
In the winter In the summer We would want to go through some sort
of thickener to take the water out so that there’s a lower mass of water going into the digester.
As heat is released, less mass of water has to be heated and we get higher temperatures.
To avoid excessive temperatures that give us operating problems, we could put water into the digester and take it back out. You put the thin sludge into the digester, then thicken the digester contents.
The heat released goes to heat a larger mass of water.
Therefore the temperature elevation is less, and we can avoid the operating problems caused by temperatures in the 40‐degree centigrade range.
Obviously these are two extremes. You can control the digester temperature by how much pre‐
thickening and how much out‐of‐digester thickening you do. So, with the right facilities, you can
control this yourself.
Physical constrains
Mixing Oxygen transfer
Keep solids in suspension
It is limited because as you digest sludge, you release some non‐degradable organic matter into solution that can change the surface tension characteristics of the fluid and interfere with the ability to transfer oxygen.
In some of the case histories we’re going to see some instances where that has occurred. As you put
sludge in the digester, and as it gets thicker and thicker, you can reach the point where you can’t mix
it. You can also reach the point where you just can’t transfer the oxygen.
Staged operation?
We can look at that on both a continuous flow and on an intermittent basis. In a continuous flow
and on basis it is accomplished by operating digesters in series. On an intermittent basis, it’s
operating one digester, then the other digester.
Plug flow conditions give us better efficiency so we can get higher volatile solids reduction
efficiency or a lower specific oxygen uptake rate out of a system.
Pathogen reduction is disinfection process, and what we want it is plug flow. We can
accomplish that either by staging or by batch operation. So these are the concepts that we’re
looking at, and some of the tools that we have to improve the operation of aerobic digester. That’s
the framework that we’ll be working from.
AEROBIC DIGESTION
PERFORMANCE STUDY AT
PLUM CREEK WASTEWATER
AUTHORITY CASTLE ROCK,
COLORADO
Presented By Tim Grotheer, Plant
Superintendent
Led By Jeff Mahagan, Operations
Supervisor
Who we are & what we do:
Serve the town of castle rock and castle pines area
3.55 MGD capacity/1.7 MGD current flow
Activated sludge/nitrification/denitrification
Phosphorous removal
Reuse to treated effluent
Projected biosolids for 1998: 380‐400 dry tons
About the digester
Converted an abandoned secondary plant to a plug flow digester
Modifications completed and started up in November of 1996
Expandable plant:
Ultimate expansion is somewhere around 12MGD. Right now it’s at 3‐½MGD as of last December,
and current flow is about half that. We have a conventional activated sludge plant. We do
nitrification and we also do denitrification through the on/off aeration process.
We remove phosphorous to protect a reservoir downstream of us and we reuse treated
effluent, a lot of our effluent on a couple of golf courses, one of them being a championship PGA
course you may have seen, the International Tournament is held there each year.. And projected
biosolids for 1998 is around 400 dry tons.
Our Mission:
To treat the wastewater and reuse and recycle as much as we can.
Digester performance study:
Originally a 0.6 MGD Enviroquip treatment plant. It was actually two treatment plants and it could
be operated by whole number of different ways, because it’s actually two plants and it could be
operated by a whole number of different ways, because it’s a very flexible little facility. And we had
been using it to store biosolids, and we thought that we could take out the clarifier equipment that
was in there and install some of the aeration equipment from our old plant, create a digester out of
it and save some money in pouring concrete tankage.
We have a gravity belt thickener and we use it to thicken to about 5% solids. The digester
was designed to get Class B biosolids out of it, and so far it’s done fine, and also to meet vector
attraction requirements so that we could do surface application in those areas that we have the
ability to do that.
It was the original plan of operation. (Figure below)
We had initial problems from the beginning of very low volatile solids reduction.
Design Criteria
Meet pathogen destruction criteria at full capacity <2,000,000 colony forming units Or 60 days detention time at 15˚C
Meet vector attraction reduction 38% volatile solids reduction
In the beginning The staff decided to begin the study using the original recommended operating procedures, thickening to 5 % solids before digestion. The following would be the focus of the study
WAS‐TSS & TVSS
GBT‐TSS & TVSS
Each basin’s TSS, TVSS, pH, dissolved O2, temperature, and volatile solids reduction.
Initial digester performance problems
Extremely low VSR (<16%)
Numerous odor complaints
Numerous odor complaints
Fearing a lynch mob, we decreased the solids concentration at the end of March. This was an attempt to increase the D.O. concentration and reduce odors
Our hypothesis Poor digestion performance is due to:
Lack of oxygen transfer at 5% solids
Lack of proper mixing (Both are caused by a combination of shallow basins and high solids concentrations)
Odors began to subside and the VSR began to rise A new problem
High ammonia concentrations caused low applications rates and a serious lack of available biosolids application sites.
Hypothesis:
Our hypothesis was that we are not getting the oxygen transfer that we needed due to the
higher solids because we were not getting enough mixing. And one of the contributing factors is
shallow basins. Two of the basins that we have in the system were previously clarifiers. And, also,
another thing that we did was operate our gravity belt thickening process on a batch basis and we
would heavily load the digester a couple of times a week rather that spread that load over a period
of time.
Special study:
The data we wanted to collect was total solids and volatile solids on the waste activated
sludge after the gravity belt thickener and in each of the basins, along with pH, dissolved oxygen,
temperature, and the volatiles solids reduction.
However, we continued to have odor complaints and we decided that we just had to do something
to mitigate that problem. And so we decreased the solids so centration going into digester,
essentially started taking the waste activated sludge directly into the digester instead of the
thickened waste activated sludge and then started thickening after digester. We went from
approximately 4.8% solids to about 3.8% solids fairly quickly.
And this is what happened, a dramatic change in performance. At the end of March is about
when we began to decrease the solids, you can see where the curve really drops off. See the
dramatic increase in volatile solids reduction took place in April. But at the same time as we were
dropping the solids concentration, we were also seeing a rise in temperature as warmer weather hit
the Rocky Mountain region. So there are two variables there that we threw into the study at the
same time, which makes it a little difficult.
This was the original plan of operation. The estimated detention time was going to be about
60 days, and the flow would come in from our plant to the WAS holding tank. We expanded the
original plan to include this tank to give ourselves more detention time between operations of the
gravity belt thickener. So the thickened biosolids would come into this tank and then flow, plug flow
through the rest of the tankage.
We had initial problems from the beginning of very low volatile solids reductions.
Phase #2 Objectives
Reduce sludge detention time from 52 days at 4% solids to 40 days at 2.5% solids, but still maintain 38% volatile solids reduction.
Reduce ammonia concentrations from over 4% to less than 1%.
Phase #2 Producing excellent results
39% VSR in only 19 days,16 days under aeration and 3 days in anoxic zone
Ammonia concentrations dropped from >4% to <.5%
What’s next
Continue phase #2 throughout the winter to see what effect temperature has on the digestion process at 2.5%
Operations of phase #3 will be designed after evaluating the data of phase #1 & phase #2
Design criteria Design volume 86,000 C.F
Design aeration solids 3 Percent
Design max. aeration solids 5 Percent
Design population equivalent 25,800 P.E
Design volatile solids reduction 38+ SCFM
Air requirements (both process units 8.0 psi discharge pressure)
2000 SCFM
Min. mixing (per MFG.) 2,510 SCFM
Design 2,580 SCFM
IEPA criteria (30 SCFM/1,000 CF) 2,582 SCFM
MAX.(per mfg) 3,450 SCFM
MAX. air available (with largest blower out) 5,200 SCFM
Design SOTR (Clean water transfer rate) 230 #O2/HR
Sludge transfer pump 20‐150 GPM @ 38 PSI
Blowers
Two‐50 H.P.,NOM 8.0 PSIG 850 SCFM
Two‐100 H.P.,NOM 8.0 PSIG 1,750 SCFM
Dedicated 460 V/3/60 HZ service & meter 1,200 AMPS
Problem:
High ammonia concentration. We were not nitrifying in the digester, so now we introduced
the waste activated sludge. Then we turn these two smaller basins into anoxic zones. And
then turn the air off. And then once a day somebody will go n turned the air on, just to
prevent too much deposition of solids. And it helped us to get back in shape.
pH issue:
In the first tank especially, the basin no. 2, there is a significant change in pH. By
reducing the total solids, we saw a great reduction in ammonia over 4% to less than 1%.
New storage capacity:
It’s around 1.8 million gallons, and this is after digestion and thickening. So we will get
additional volatile solids reduction in this tankage.
Phase 2:
We were digesting about 2‐½ %, solids, we’re getting the 38% volatile solids reduction
required for vector attraction requirements in about 19 days; 16 days under aeration
and 3 days in anoxic zones and still we’re getting volatile solids reduction even in those
anoxic zones, which is interesting as well. And they are helping to replace alkalinity.
Solids concentration dropped.
The percent or the reduction itself dropped. And the reason for that is because we
added yet another one of our aeration tanks on line and increasing our aeration
capacity by about 25%, so we were actually operating as an extended air plant doing
most or a good deal of our volatile solids reduction in the treatment plant itself before it
ever got to the digester.
We’re achieving the volatile solids reduction in less than 20 days, and we’re really not
getting much more reduction in the last three tanks here.
AEROBIC SLUDGE DIGESTION
AT
PARIS W.W.T.P., ILLINOIS
Presented by
Mr. Richard Yates, P.E.
FRANCIS AND ASSOC.
Introduction:
Due to mechanical failure of existing worn‐out anaerobic digestion equipment, the City of
Paris was faced for several years with the task and cost of disposing of raw primary and waste
activated sludge (WAS) to a landfill located more than twenty (20) miles away. A grant was obtained
by the Engineer to develop an alternate, and after evaluation of several alternatives, it was decided
to convert the existing tankage to aerobic sludge digestion, hopefully as a first step in a process
train meeting Federal EPA a 503 regulations.
Basis of design:
The basis of design shown in Fig.3.1contained herein. Although Paris, IL. Has a people
population of 9500, its’ P.E. equivalent is over 25,000, due to several industries, including two
involved in food processing. These wastes are loaded with starches, organics, and fat/oil/grease
(F/O/G) components.
The treatment process, which is approaching an overloaded state, is a conventional activated sludge
until built in 1968, and has primary clarification, fine bubble activated sludge, and conventional
clarification units. In general, the design of the plant and nature of the waste stream contribute to
digester design conditions outside those normally encountered in “conventional” municipal design.
Nevertheless, after considerable analysis, it was decided to select a highly mixed process based on
the Enviroquip equipment package, and to utilize provisions in the Agency’s design criteria allowing
higher loading rates and in‐basin solids concentrations in excess of two percent (2%). The means to
achieve this design variance was to utilize the top belt of a sludge press for dewatering and return
to the digester basins, and to also decant supernatant from both basins for the same purpose.
Design criteria Design volume 86,000 C.F
Design aeration solids 3 Percent
Design max. aeration solids 5 Percent
Design population equivalent 25,800 P.E
Design volatile solids reduction 38+ SCFM
Air requirements (both process units 8.0 psi discharge pressure)
2000 SCFM
Min. mixing (per MFG.) 2,510 SCFM
Design 2,580 SCFM
IEPA criteria (30 SCFM/1,000 CF) 2,582 SCFM
MAX.(per mfg) 3,450 SCFM
MAX. air available (with largest blower out) 5,200 SCFM
Design SOTR (Clean water transfer rate) 230 #O2/HR
Sludge transfer pump 20‐150 GPM @ 38 PSI
Blowers
Two‐50 H.P.,NOM 8.0 PSIG 850 SCFM
Two‐100 H.P.,NOM 8.0 PSIG 1,750 SCFM
Dedicated 460 V/3/60 HZ service & meter 1,200 AMPS
Paris ,IL‐ Process Flow Schematic
I.D.number Description Volume Concentration
① Thickened WAS + primary sludge to belt thickening
1,000 to 1,500 GPD 19,000 GPD
0.5 TO 1.0% 1.0 TO 6.0%
② Belt thickened combination 1˚ & WAS to EAST DIGESTER
8,000 GPD 8.0%
③ Flow‐through EAST DIGESTER to WEST DIGESTER
8,000 GPD 2.4%
④ Pump from WEST DIGESTER to EAST DIGESTER
8,000 GPD 1.9%
⑤ Sludge from press 3,500 lbs/day 20‐25%
⑥ Pump from either tank to top belt for thickening tank contents
‐‐‐ ‐‐‐
Design details
As is often the case in old anaerobic digester design, the two (2) 45‐ft. dia. x 25’. SWD tanks
are separated by a building meant to accommodate piping, pumps, valves, and heating equipment
for the anaerobic process. When this was all stripped out, there was plenty of space in the basement
for the piping revisions and variable speed Moyno pumps. Installing two (2) 100‐ HP and two (2) 50‐
HP centrifugal compressors on the upper floor presented some more challenging geometry.
Extensive use of hot‐dip galvanized prewelded steel components from the equipment
manufacture, such as blower piping manifolds and even a stairway eliminated many of the problems
associated with plumbing large diameter, multi‐fitting piping runs.
All new sludge piping was executed in PVC with ductile fittings, and after an attempt at
restoration, all plug valves were replaced with resilient‐seat gate valves (RSGV’s).
Since the recommended depth of air discharge for the coarse air injector units in the
digester was 16’ per manufacturer’s recommendation after surveying basin geometry it becomes
apparent that the air backpressure for the digester was close to the same as measured for the
aerobic treatment process of the main plant. Therefore, a tie‐in line was bid as an alternate and
subsequently built. At present the blowers, although sized to IEPA standards, which are basically Ten
State Standards, are adequate to aerate both the digestion process and the plant using 200‐HP of
the 300 available. This has allowed the operator to cease using old P‐D units located in the basement
of the Main Operation Buildings.
Performance
Although the unit was recently in the “start‐up” mode, enough data and sludge has been
produced to assure the Operator and Engineer that the unit is capable of producing a well‐stabilized
and non‐putrescent sludge. During startup, some dips in pH were noted, but were easily corrected
by lime addition and by turning off the air for a while to allow denitrification to occur, thereby
putting back some alkalinity removed by the nitrification process. It is felt that the decision to rely
upon the sludge press dewatering method, with wasting of the pressate , may have inadvertently
resulted in the alkalinity problem. It is also suspected that the nature of waste, with high volatiles
present, may be a cause of intermittent pH problems. In either case, the problem has lessened at full
load and as familiarity with the process is gained. Although an inordinate increase in sludge quantity
and a decrease in dewater ability were feared during design, due to Paris’ reliance on a single old
sludge press, neither of course the decant from the process and the pressate are much more
manageable than those previously experienced in the anaerobic days.
VSS reduction exceeding 50% (Van Kleeck Method) are commonplace. It is felt that these
consistently higher VSS reduction rates are due to the higher influent ratios, which are 80% and
above for the raw primary sludge. The 40% range for digested sludge from the secondary digester is
felt to represent the effective practical lower limit for the Paris sludge stream, and is indeed a good
dewatering, low‐odor sludge.
Temperature runs 10‐15 degrees F above ambient, which is due to basin geometry, the insulting
effect of a foam layer, the heat from the air compressors, and heat internally generated by the
process itself.
Although not shown in the data, solids are currently maintained in the 2.4% range for the primary
digester and 1.9% for the secondary digester. The latest calculated SRT was 34 days, which is
comfortably within or above ranges suggested in engineering research literature, if not within the
40‐60 day range suggested by the 503 regulation. It is felt that higher temperatures and most
importantly, through mixing contribute to the performance of these units.
Summary and General Observations:
A. Above all else, mixing seems to be the key to performance.
B. The aerobic digester, when loaded in the middle ranges, is inherently self‐stabilizing and
operator‐friendly.
C. Higher VSS loads can occasionally result in D.O. levels below 2.0 mg/L without process upset.
Should additional oxygen be needed, it can easily be added by a few fine‐bubble diffusers. It
should be noted that this comment applies more to the high‐strength primary sludge
situation in Paris than to most municipalities.
D. It appears that attainment of consistent “class B” sludge can occur with lower SRT’s that
those suggested in the 503 Regulation.
PARISDIGESTER‐OPERATINGDATA
Weekof‐‐‐‐,1997
WESTUNIT EASTUNIT VSSREDUCTION FECALCOLIFORMTempRange
pHRange
TempRange
pHRange
RAW Fin. %Red.* High Low GeoMean**
6/30 ‐ 7.0‐7.1 86‐88 7.0‐7.2
7/7 ‐ 7.0‐7.2 86‐90 6.9‐7.2
7/14 ‐ 6.9‐7.1 92‐94 6.8‐7.2
7/21 ‐ 6.9‐7.1 92‐94 6.9‐7.1
7/28 ‐ 6.9‐7.2 91‐96 6.7‐7.0
8/4 85‐90 6.9‐7.0 88‐91 6.8‐7.0 0.82 0.51 77 590 1000 800
8/11 85‐87 6.7‐6.9 89‐93 6.9‐7.2 0.84 0.52 80
8/18 86‐87 6.7‐7.1 88‐94 6.9‐7.2 0.66 0.52 44
8/25 86‐89 6.8‐7.1 89‐95 6.9‐7.1
9/1 89‐91 6.8‐7.2 89‐96 6.9‐7.0
9/8 88‐90 6.9‐7.1 86‐90 6.9‐7.0
9/15 88‐89 7.0‐7.3 6.7‐7.1
9/22 86‐88 6.9‐7.3 90‐93 6.9‐7.1
9/29 83‐86 6.7‐7.0 86‐92 6.8‐7.1 1200 1100 1200
10/6 83‐84 6.7‐6.9 87 6.9‐7.0
10/13 78‐82 6.7‐6.9 78‐82 6.9‐7.0 0.62 0.48 50
WHAT DO YOU MEAN, WE
CAN’T MEET 503 WITH
AEROBIC DIGESTION?
Presented By James P. Scisson, Jr.
Operations Specialist
Jones & Henry Engineers, Ltd.
And
Fred Craig
Superintendent
Clyde WWTP
Ohio
Introduction
There are those who believe that aerobic digestion is not an appropriate sludge stabilization
process in areas where the winter temperature is often near or below 0˚C.Reasearch of basic
principles and practical, full scale pilot testing at the Clyde, Ohio WWTP prove otherwise.
The Clyde WWTP has a design capacity of 1.9mgd and a peak capacity of 4.8 mgd. The current daily
average flow is 1.4mgd. The plant is secondary treatment plant with screening and grit removal
preceding a Burns‐McDonnel Treatment System (BMTS),which temperature is often near or below,
which is an oxidation ditch with diffusers for aeration, propeller mixers for MLSS suspension, and an
intra‐channel clarifier. Effluent disinfection is by chlorination.
The Clyde WWTP produces approximately 2200lbs/day of WAS, and 1770 lbs/day VSS. The solids
handling train consists of gravity thickening of waste mixed liquor(WAS),aerobic digestion, sludge
concentration by gravity belt thickening(GBT), and sludge storage prior to land application. The two
aerobic digesters are 28’ dia. and 21’ SWD. and have a capacity 97,000 gallons. The digesters and
storage tanks are aerated with point‐type diffusers {(figures 2A, 2B(digesters 1 & 2) figures 3A and
3B (sludge storage 1/digester 3 and sludge storage 2/digester 3)}. Air is supplied by four, 400 SCFM
positive‐displacement blowers; two for the aerobic digesters and two for the holding tanks.
The digesters were not performing well. The biosolids were not in compliance with Class B pathogen
reduction criteria, and had to be subsurface injected to meet the vector attraction reduction
requirements (VARR). In addition, there were continual odor complaints from neighbors. Jones &
Henry Engineers was commissioned to determine how to best solve this problem. The study had two
phases, the first from july 1996 to October 1996 and the second from October 1996 to May 1997.
Part one phase one study
Initial observations
Site visits were made before beginning analyses. The following observations were made during the
visits:
1. The digesters were narrow and deep. The diffusers produced a vigorous aeration pattern.
The diffusers produced a vigorous aeration pattern. The diffuser system manufacturer
claimed a dirty‐water oxygen transfer efficiency of 14%.
2. The sludge in the digesters was very dark, and emitted a punagent, amine‐like odor. The
digester solids content was 3.5‐4% total solids (TS)
3. The digested sludge in the storage tanks had been thickened to a concentration of 8‐10% TS.
The sludge was thickened to this concentration to reduce the digested sludge volume,
maximize the mass of sludge production that could be stored on‐site, and minimize sludge
hauling costs. The sludge is thixotropic at this concentration, and acts as a solid until
sufficient energy is imparted to it. The diffused air system could not mix or aerate solids at
this concentration. Sludge around each diffuser was fluidized; the remainder was an
unaerated quivering mass, the storage tanks foul, septic, fecal odors
Data analysis
Mass balance
A mass balance was prepared for the solids handling process. The mass balance is shown in
figure 1 and also in table 1.
Table 1
Solids handling mass balance
WAS THICKENED SLUDGE TO DIGESTER
DIGESTED SLUDGE
THICKENED SLUDGE
SLUDGE TO
LAND
FLOW .115 mgd 7100 gpd 6600gpd 2200gpd 3043 gpd
TS% 2382mg/L 3.87 3.48 9.85 8.34
VS% 77 65 60.5 59 56
Lbs/day/TS 2256 2287 1917 1808 2061
Lbs/day/VS 1773 1483 1160 1074 1159
Aerobic digester performance
Critical performance indicators for the aerobic digestion process are shown in table 2.
Table 2
DIGESTER PERFORMANCE INDICATORS
1995‐96
Dig. Feed pH
Dig. Feed SOUR*, mg,O2,/hr/g m VSS
Digester HRT**/SRT*** days
Digester D.O
Digester pH
Digester SOUR
VS REDUCTION %
6.63 7.4 26/32 .48 7.3 3.0 20.8
* SOUR is specific oxygen uptake rate
** HRT is hydraulic retention time
*** SRT is solids retention time
The data shows that:
1. The digester SRT, is, on average, too short to allow for 503 compliance by the time and
temperature method. The digester met the SRT/temperature criteria for pathogen reduction
only in July‐September 1994 and June‐august 1995. Bio‐solids are hauled from the plant
several months each year.
2. The average D.O. is too low to maintain aerobic conditions. As shown in figure 4, the D.O is
often below 0.2 mg/L
3. The SOUR is above the EPS criteria for vector attraction requirements.
4. The volatile solids reduction is poor indicating that sludge is not stabilized
5. The digested sludge pH is higher that the feed sludge pH. Under normal conditions, aerobic
digestion of WAS will convert organic nitrogen to nitrate. Nitrification and cell lysis will
create acid, lowering the Ph. Under anaerobic conditions, organic nitrogen is degraded to
ammonia, which will raise the pH and alkalinity.
Oxygen supply vs oxygen demand
The SOUR data was used to chart the sludge oxygen demand against digester temperature and the
theoretical; oxygen supply of 53 lbs/digester, and 106 lbs/hr total oxygen supply
1. The air supply cannot satisfy the oxygen demand
2. The cycle or SOUR and temperature are perfectly opposed. Microbial activity is temperature
dependent. In the winter microbial activity is greatly reduced, and solids are not stabilized.
The aeration equipment supplier was contacted. The supplier indicated that the aeration system was
designed for an SRT OF 40 DAYS, AS REQUESTED BY THE City’s former consulting engineer. Since the
actual SRT is 23‐32 days, the aeration system was not designed with sufficient capacity to stabilize
the solids.
Pilot testing and monitoring
Plant designs that work on paper may fail in the field. Conversely, design that are proven unfeasible
on paper work perfectly well in the real world. During this study, some full scale pilot tests were
done to gain information about how best to modify plant operations and/or redesign the plant to
improve the digestion process. These tests were:
1. Batch operation of a digester to determine how many days of aeration with no addition
sludge addition are required to stabilize the sludge, and if any inhibitory substances might be
present.
2. Use the storage to tanks as digesters determine the real‐world SRT required to stabilize the
solids.
3. Additional monitoring of the solids handling process to determine the fate of fecal coliform
bacteria through the process, and testing for other indicators of sludge stability such as
alkalinity and ammonia concentration.
Batch operation
To determine how much time was required to stabilize the sludge, Digester #2 was operated
as a batch reactor. The empty tank was half‐filled with sludge from digester #1, filled with thickened
WAS and isolated from the sludge feed. The pH, SOUR and fecal coliform data are shown in Figures 8
and 9. The batch operation shows:
1. The sludge can be digested. There are no inhibitory substances to hinder digestion.
2. Continuous, incremental decreases in the volatile solids content and the SOUR, indicating
that the sludge is being digested at a fairly constant rate.
3. A continual decrease in the fecal coliform count.
4. A marked decrease in pH and increase in DO after 15 days of operation. The changes in pH and
DO are good indicators of stability. The pH decrease is caused by nitrification of organic
nitrogen released by digestion, which produces acid. The sharp drop in pH indicates all the
alkalinity has been destroyed. The abrupt rise in DO indicates that the oxygen demand has
been satisfied, and there is now a surplus of air.
Additional Detention Time (Series Operation)
To determine the benefits of additional digester volume, sludge storage tank #3 was
converted to an aerobic digester (aerobic digester #3). Aerobic digester #3 was operated in
series with digesters #1 and 2. Half of the digesting sludge was transferred, without further
thickening, from digesters #1 and 2 to digester #3. The remainder was thickened as normal
and discharged to storage tank #2. Digesters 41 and 2 were operated in parallel, the normal
operation. The results are shown in Table 3.
The additional detention time reduced the fecal coliform count by more than 90%, and
reduced the SOUR to well below the level of 1.5 mg o2/hr/gm TS required for compliance
with 40 CFR part 503 VARR. The drop in pH and rise in DO indicate that the additional
detention time is sufficient to stabilized to solids.
Additional Monitoring
Fecal coliform density and alkalinity were measured in the WAS, digesters #1&2, and digester #3 to
track pathogen reduction across the entire process, and confirm anaerobic conditions in the aerobic
digesters. The results are shown in Table 4. The data shows that 1) the fecal coliform density is
declining across the process and pathogens are not regrowing in the solids handling process, 2)
digesters 1 and 2 are sub stoichiometric for oxygen. Some organic nitrogen is being degraded to
ammonia and 3) digester 3 is nitrifying arid depleting all the alkalinity.
FINDINGS, PART ONE
The study found that the two existing aerobic digesters are undersized and do not supply sufficient
oxygen to stabilize the sludge. Using the sludge storage tank as a digester allowed the Clyde WWTP
to comply with the class B pathogen requirement of 40 CFR Part 503. An HRT/SRT of 45 days
produces a well‐digested sludge that complies with Class B pathogen reduction and VAR
requirements.
The study also found the sludge storage volume is relatively small. To maximize the volume of
solids that could be stored in the holding tanks, and to reduce hauling costs, the digested solids
were thickened to 8‐10% The aeration system was not designed to aerate these solids. The solids
putrefied in the tanks, and generated odors. Additional sludge storage is needed so storage tanks 3
and 4 can be used as digesters.
RECOMMENDATIONS, PART ONE
Based on the findings from the study, it was recommended that one storage tank be used as an
aerobic digester. Digested sludge could then be transferred to the other storage tank and mixed with
thickened sludge from the GBT. The digester and holding tank fecal coliform densities and alkalinity
levels in the should be monitored weekly.
To determine the best long‐term solution to the problem, four treatment alternatives were
examined. These were:
Convert both storage tanks to aerobic digesters and construct additional above‐
ground storage to provide storage for 150 days production of digested sludge
thickened to 8% TS and mixed with a pumped mixing system.
Construct an auto thermal aerobic digester (ATAD) and additional sludge storage.
Construct a liquid lime stabilization system and provide additional sludge storage.
Cover and heat the aerobic digesters. Provide additional sludge storage.
Alternative A was the easiest and least costly long‐term solution (figure 10)
Table 3
ADDITIONAL DIGESTER VOLUME TEST RESULTS
pH DO, mg/L
SOUR mg O2,/hr/g m VSS
Fecal coliform count,#/100mL
pH DO,mg/L SOUR mg O2,/hr/g m VSS
Fecal coliform count,#/100mL
7.4 .21 3.0 2,048,557 5.7 3.26 0.64 141,004
Table 4
MEAN COLIFORM AND ALKAILNITY CONCENTRATIONS.
WAS DIGESTER 1 & 2 DIGESTER 3
FECAL COLIFORM DENSITY, #/gm TS
7,500,000 2,048,000 141,004
ALKALINITY, mg/L 71 586 10
PHASE TWO
CONTINUED MONITORING
The city continued to operate the storage tanks as digesters. The solids handling system was
monitored, and another study was commissioned. The study period ran from October 1996
through May 1997. The study was commissioned, in part, to:
1. Evaluate the new operating mode during winter conditions, when pathogen and solids
destruction are reduced.
2. Monitor odor complaints.
3. Determine the digester requirements when the plant flow increases to 1.9 mgd.
4. Monitor the effect of the new operating mode on plant operations.
Operational Review
TWAS was pumped from the gravity thickener to digesters I and 2, operating in parallel. Digesting
sludge was transferred from digesters 1 and 2 to digester 3. Digester 3 transferred to digester 4 by
gravity. Approximately one‐third of the sludge from digesters 1 and 2 was thickened with the GBT
before being transferred to digester 3. The digesters 3 and 4 solids content varied between 3.5
and 5.5 % TS. Digesters 3 and 4 were partially emptied each month and the digested biosolids land
applied.
Odor Complaints
There has been only one odor complaint since the new operating mode began.
Pathogen Reduction
Storage tanks #2 and 3 had been used as aerobic digesters since October 1996. The tanks were
operated in series, with sludge transferred from digesters #1 and 2 into digester #3, and digester #3
into digester #4. The additional detention time in digester 3 is about 20 days. The fecal coliform
counts for digesters #1,2,3 and 4 are shown in Table 7 and figure 11.
As before, digester #3 reduced fecal coliform densities by about 90%. Additional digestion provided
a modest decrease in fecal coliform density. Figure 12 shows the monthly fecal coliform densities
for October through March. Mean coliform counts were well below the 503 ceiling level, peaking in
December at 550,000, and declining to less than 50,000 in March. Digester temperatures during the
winter months (December‐March) were as low as 3.2° C.
FIGURE 11
8
14
13
10
save COLIFORM DONT a. NV, GOLFO.. COG, I
Vector attraction reduction
As before, the volatile solids reduction as measured from the gravity thickener discharge to land
application was not in compliance with 40 CFR Part 503. The SOUR from digesters 1 and 2 were in
compliance part of the time. The solids in digesters 3 and 4 were probably in compliance, but were
not measured. Table 8 shows the SOUR and volatile solids reduction for October 1996 through March
1997.
Digester 3 and 4 Aeration and Mixing Capacity
Although there had been only one odor complaint since October 1996, there was evidence that
digesters 3 and 4 could not maintain aerobic conditions. Digester DO, ammonia concentrations,
alkalinity and pH all show periods of anaerobic conditions. Visual inspection often showed the
digested sludge to be black (septic) instead of brown (aerobic). Data from October show that digester
3 ammonia levels rose from <1 mg/1 to 156 mg/L when the solids concentration reached 4% TS. Data
from February showed ammonia levels of 164 and 180 mg/L in digesters 3 and 4. Alkalinity levels at
times exceeded 2000 mg/i. Ammonia levels in digesters 1 and 2 ranged from 180 ‐ 360 mg/L.
Data analysis showed that septic conditions were related to the sludge solids content. Septic
conditions appeared each time the sludge solids content approached 4% TS. Aerobic conditions
would return when the solids content fell to 3.5% TS. The most likely causes of septic conditions are:
Increased absolute oxygen demand caused by the increased solids content. For example, if
the digested sludge in digesters 3 and 4 had an SOUR of 1.5 mg 02/hr/gm TS and a sludge
solids content of 4%, the oxygen demand would be 89 lbs/hr. The diffuser manufacturer
claims a dirty water oxygen transfer efficiency of 9% With the maximum air flow of 800 scfm,
the diffusers can transfer 68 lbs. of oxygen per hour, which is less than the oxygen demand.
The changed nature of the air/solids interface at higher solids concentrations. At higher
solids concentrations the sludge is viscous and no longer acts like water. This changes the
physical properties of the fluid and may hinder oxygen transfer between the air bubbles and
the sludge.
Observation of digesters 3 and 4 show that digester mixing is limited when the solids content is at or
above 4% TS. At one point during the winter the sludge froze at the surface as it was aerated. The
diffuser equipment manufacturer was notified of the findings. After consultation between the
engineer and the vendor, it was decided that an additional drop leg of 8 diffusers, installed
perpendicular to the current diffusers, should be installed, along with an additional 400 scfm blower.
Digester I and 2 Aeration Capacity
Digester 1 and 2 continued to have low DO, emit amine odors, and showed elevated
ammonia and alkalinity levels. Though the air supply was theoretically sufficient to maintain
aerobic conditions, the digesters were not truly aerobic. Once again, the high solids content
may have hindered oxygen transfer. The other potential cause of the septic conditions was
the series operation. Digesters 1 and 2 had half of the total digester volume and about half
of the total oxygen supply, but considerably more than half of the oxygen demand.
If the digester volume had been constructed as one tank, and the digested sludge not
rethickened with a GBT, the existing air supply would have been sufficient to maintain aerobic
conditions. Making use of existing structures divided the oxygen demand unequally between
the tanks, creating an oxygen deficit in digesters 1 and 2. Rethickening the digested sludge to
store more solids increased the oxygen demand and caused the oxygen deficit in digesters 3
and 4.
As with digesters 3 and 4, the vendor was consulted. After some discussion, it was decided
that the air supply to digesters 1 and 2 should be increased to 600 scfm each. The existing
blower motor drive sheaves could be replaced, and the blower capacity increased to 540
scfm. It was decided that this was "close enough".
Diffuser Fouling
The Clyde WWTP has no primary treatment, and has coarse (1") screens in preliminary
treatment. The rags and other debris that is not removed form mop strings that foul the
diffusers. All digesters have to be taken out of service at least once a year to remove mop
strings that foul the diffusers. Taking digesters out of service overloads the remaining
digesters and causes septic conditions.
FINDINGS, PART TWO
The aerobically digested sludge has been in compliance with the Class B pathogen reduction
requirement since October, 1996. Pathogen reduction was not affected significantly by
winter operation. There has only been one odor complaint since October, 1996.
The aeration capacity is, at best, barely adequate to maintain aerobic conditions. The oxygen
supply deficiency has two causes; 1) serial operation that places most of the oxygen demand
in digesters I and 2, while the digesters have only 50% of the air supply, and 2) the need to
thicken digested sludge in digesters 3 and 4 to reduce volume, which causes the oxygen
requirement to exceed the oxygen supply.
The diffusers are continually fouled with mop strings, reducing their efficiency and requiring
considerable labor for their removal.
RECOMMENDATIONS, PART TWO
The new operating mode produces Class B sludge year‐round. The Clyde WWTP should continue to
use the new operating mode. The solids content in digesters 3 and 4 should be maintained between
3.5 and 4 % TS. to minimize sludge volume and avoid septic conditions
The aeration capacity in digesters 3 and 4 should be increased by installing an additional 8 diffusers
in each digester, increasing the aeration capacity by 36% An additional 400 scfm blower should be
installed to supply the additional diffusers. The air supply to digesters 1 and 2 should be increased
from 400 scfm to 540 scfm.
A fine (3/8” or ¼”) screen should be installed in preliminary treatment to reduce diffuser fouling.
WHAT THE OPERATORS DID AFTER THE CONSULTANTS LEFT
During the summer of 1997, the Clyde WWTP staff monitored the digester performance and made
alterations to the aeration equipment and digester operation. In May 1997, digesters I and 2 were
drained and cleaned. At this time the diffuser assemblies were modified These modifications were
done to reduce diffuser fouling and improve oxygen transfer.
In June the operation of digesters 1 and 2 were modified. Digesters 1 and 2 are normally operated
in parallel, with the digester feed changed from one digester to the other every few days. The
digester feed was changed from parallel operation to series operation. The TWAS feed was always
sent to digester 1. Sludge from digester 1 transferred to digester 2 by gravity overflow. At this time
there was an immediate reduction of the SOUR in digester 2 (figure 13). Long‐term operation in
this mode also reduced anaerobic conditions in digesters 3 and 4. Digester 3 and 4 ammonia levels
were reduced from 169 mg/I to less than 1 mg/1, and the ammonia levels in digesters I and 2
decreased from 250 mg/I to 40 mg/l(figure 14). The alkalinity in digesters 3 and 4 was also reduced
from 2300 mg/L to 1250 mg/L. This decrease in anaerobic byproducts indicates that the oxygen is
being used more efficiently.
The level of anaerobic byproducts is also reflected in the pH. The pH in all digesters fell from a
level of 7.6‐7.8 to near neutral (figure 15). Further study reveals that a drop in the digester pH is a
seasonal occurrence (figure 16). Additional monitoring is needed to determine if the 3‐stage
series operation continues to reduce ammonia and alkalinity levels.
TABLE 7
DIGESTER COLIFORM LEVELS
DIGESTER 1 AND 2
DIGESTER 3 DIGESTER 4 503 REQUIREMENTS
MAXIMUM
FECAL COLIFORM COUNT,#gr TS
11,787,470
572,900
479,495
N/A
GEO MEAN
FECAL COLIFORM COUNT,#gr TS
1,700,000
280,000
230,000
2,000,000
Table 8
VS reduction and sour data,
Digester and sour data,
Digesters 1 and 2
OCTOBER NOVEMBER DECEMBER JANUARY FEBRUARY MARCH
SOUR, mg O2,/hr gm TS
1.6 2.2 2.5 3.2 4.1 3.5
VS reductoion %
13.53 24.55 21.87 14.73 5.56 8.02
Conclusion
Thickened WAS can be aerobically digested and comply with the pathogen reduction
requirements of 40 CFR part 503 at modest SRTs and low sludge temperatures. The aeration
requirements of TWAS must be taken into account during phase of any project. Textbook values for
aeration requirements may not be sufficient. When possible, field SOURs of the TWAS and the
expected final SOUR should be used to size the aeration equipment. The TWAS concentration in an
aerobic digester should not exceed about 4% TS. At solids concentrations above 4% it may become
difficult to maintain aerobic conditions due to concentrating the oxygen‐demanding biomass into
such a small volume. In addition, aerating to store digested sludge becomes thixotropic. If it is
desirable above 4% TS, the sludge should not be aerated, but mechanically mixed.
HIGH SOLIDS AEROBIC DIGESTION
AT
LOS LUNAS, NEW MEXICO
Presented By
Henry J. Hervol, P.E.
With Data Provided By
Molzen‐Corbin & Associates
Of Albuquerque, New Mexico
And The City Of Los Lumas, New Mexico
Background
The original wastewater treatment facility at Los Lunas, New Mexico was built in 1982 and went on‐
line in 1983. It has won an EPA award as “plant of the year” in its respective for EPA region VI.
The original plant design was for a capacity of 0.7 MGD with a peak flow capability of 2.1 MGD. The
process chosen by the engineer in the original design was the extended aeration configuration for
the activated sludge process, utilizing a 22 foot deep tank approach.
The city’s engineer, molzen‐corbin & associates of Albuquerque, new mexico, added an aerobic
digester with a detention time of 60 days ± in 19929 that is the subject of this discourse.
Existing (typical) influent design values for the current plant are as follows:
BOD5 = 270 mg/L
TSS = 220 mg/L
TkN = 23.5 to 43.4 mg/L
The process utilized in treating the influent wastewater to plant consists of:
Mechanical bar screens
Aerated grit basins
Activated sludge (conventional, but with long detention times at present)
Final sedimentation
Post aeration
UV disinfection
Aerobic digestion (preceded by pre‐thickening)
Currently, solids are thickened in a gravity belt thickener (provided by Enviroquip , Inc.) and then
aerobically digested prior to surface injection disposal. The GBT was designed to handle all of the
sludge from the existing facility running only two (2) days per week, so there is plenty of excess
capacity available in the unit (for future unit).
It is expected that by the year 2020, the average influent flow to this facility will increase to 2.2 MGD
with a peak rate at 4.64 MGD.
During the 1992 expansion (addition of aerobic digesters and GBT), four (4) 10,000 gallon ± aerobic
digester basins were added to bring the plant into compliance with 40 CFR 257 Federal Sludge
Regulations. Since the start‐up of the 1992 addition, the new Sludge Regulations (40 CFR 503) have
come into effect. The significance of the regulation is that both pathogen and vector attraction
reduction can be achieved without Aerobic Digestion. Although Aerobic Digestion is not required, it
can be utilized for increased plant operations flexibility and also to limit the total nitrogen load
applied to the land for final sludge disposal, if opened load applied to the land for final sludge
disposal, if operated in a nitrification/denitrification mode.
Aerobic digester design (1991 standards):
The engineer that accomplished the design at Los Lunas, Molzen–Corbin & Associates of
Albuquerque, New Mexico, designed a unit to have a minimum detention time of 60 days and a
minimum operating temperature of 15˚C (59˚F). Molzen–Corbin did a detailed heat balance on the
unit. Remember that Los Lunas is about thirty (30) miles south of Albuquerque, New Mexico, and
typically sees winter temperatures down in the 20’s, for short periods of time.
The plan was to build four (4) basins, each about 25 feet by 25 feet by 22 feet SWD = 13,750 CF =
102,850 gallons. Three (3) basins would be full and provide the required 60 days detention time,
while the fourth basin would be allowed to rise and fall and be used for loading the trucks to haul
the digested sludge to the land disposal site. The fourth basin would have an average level of 1 feet
SWD ±.
The design temperatures used in analyzing the proposed aerobic digester were;
Outside air (minimum long term mean) = 33 ˚F
Sludge (WAS) minimum =55 ˚F
Aeration Air = 150 ˚F
Digester Contents (15˚C) = 59 ˚F
The aerobic digester heat loss calculations were done for a 1.0 MGD design flow and an expected
sludge wastage rate of 1500lbs solids (dry basis) per MGD, with an expected thickened sludge solids
from the GBT at 5.5 %, which would give 27,300lbs of wet sludge applied per day.
Enviroquip, Inc. of Austin, Texas, provided mixing data (curves) for their Shear Tube Aeration system
that showed the design digester airflow rate would be about 20 SCFM/th CF of tank contents at 5.5
% solids concentration. This would mean that about 30,000lbs of air would be applied to each of the
segments daily, for optimum mixing.
The engineer expected a biological heat generation as follow:
3.5 Kcal per gram COD oxidized and 3.97 BTU/ Kcal, or a total of 6300 BTU/lb of COD destroyed. He approximated 1lb of VSS to 1lb COD, for the purpose of the heat balance computations.
Allowing that 70% of the influent solids are volatile, that would give 1000lbs VSS/day. Destroying
35% of the VSS in the aerobic digester would give 350lbs/day destroyed. If a third of this is done in
one (1) basin (worst condition), that would be about 117lbs/day. The BTU/hour generated in each
basin was expected to be about 29,100 BTU/hour.
A schematic diagram showing the heat losses and heat gains in the covered aerobic digester at Los
Lunas is shown in fig.2 attached hereto. This data is also tabulated in table 1 attached hereto.
Note: All of the values are for one (1) basin (25 feet squared). The heat balance for the actual
installation is shown in figure 3.
Conclusion
1. Digester must be covered to achieve a positive heat balance and thus maintain the required
59˚F.
2. Both the walls and cover do not have to be insulted; however, one(1) or the other must be
insulted.
3. At low (initial) flows, a positive heat balance would be achieved since the quantity of sludge
wasted and hence the biological activity would be reduced (from the design point). At a flow
of 0.5 MGD, everything would be reduced by 50%, but we should still have a positive heat
balance of at least 1300 BTU (15,900 – 29,100 + 14,500). A wastewater flow of 0.5 MGD,
however, is 40% of the digester design flow of 1.3 MGD. If two (2) of the three (3) active
compartments were used in this case, a detention time of about 100‐days { (60‐ days) (1.3
MGD/ 3 BASINS) / (0.5 MGD / 2 BASINS) } would be achieved which is more than the 60 days
required. This should increase the biological heat gain in each basins being used to 21,700
BTU/hours (14,500 x ⅔) which would result in a heat balance of at least 8500 BTU/hour
(15,900 – 29,100 + 21,700).
DESCRIPTION WITHOUT WALL insulation without cover
WITHOUT WALL insulation with uninsulated cover
WITHOUT WALL insulation with insulated cover
With wall insulation with insulated cover
With wall insulation with insulated cover
With wall insulation with insulated cover
Heat losses / requirements
Walls above soil
17,000 17,000 17,000 2,000 2,000 2,000
Walls below soil
4,700 4,700 4,700 1,200 1,200 1,200
Floor 1,600 1,600 1,600 1,600 1,600 1,600
Roof 17,900 1,300 17,900 1,300
Surface convection
105,000 105,000
Surface evaporation
5,400 5,400
Aeration air* (see below)
Heating sludge in
1,500 1,500 1,500 1,500 1,500 1,500
Total losses 135,200 42,700 26,100 116,700 24,200 7,600
Heating gains
Aeration air (net)
12,900 12,900 12,900 12,900 12,900 12,900
Biological activity
29,100 29,100 29,100 29,100 29,100 29,100
Total gains 42,000 42,000 42,000 42,000 42,000 42,000
Heat balance ‐93,200 ‐700 +15,900 ‐74,700 +17,800 +34,400
Note: all quantities are BTU’s/hr
Note, however that with only two (2) basins in use, there would be heat loss through common wall
with the unused basin. If the walls were not insulated and this wall were considered like an outside
wall, the loss would be about 11,900 BTU/hr (17,000 + 4700 / 2 walls]. Which would result in a
negative heat balance of 3400 BTU/hr for the insulated cover/uninsulated wall case (8500 = 11,900),
and a negative heat balance of 1500 BTU/hr (34,400 – 15,000 – 34,00).
A layout of the aerobic digester system designed by Molzen‐Corbin & associates for the Los Lunas
STP is shown in figure.
Operation Data:
The aerobic digesters at Los Lunas have been doing some remarkable things. A sludge analysis
performed on 07/18/1997 showed the following results.
Nitrate = 1,790 mg/L
Ammonia nitrogen = 300 mg/L
Total Kjeldahl Nitrogen = 850 mg/L
Total dissolved solids = 19,000 mg/L
Total solids = 6.3 %
On August 6, 1997, the plant operator calculated his volatile solids reduction at 54.6%. Note the
sludge had been in the digester for 136 days.
In august 6, 1997, temperature were recorded in four (4) cells as follow:
NE 35.2 ˚C 95 ˚F
NW 34.1 ˚C 94 ˚F
SW 34.9 ˚C 95 ˚F
SE Empty
The city of Los Lunas has done project performance logs on the unit since April 12, 1995, which are
tabulated monthly.
According to 40 CFR 503 regulations, in order to classify the sewage sludge as class B with respect to
pathogen reduction, temperatures in an aerobic digester must either be maintained at or above
15˚C for a mean cell residence time (MCRT) of 60 days, or 20 ˚C for 40‐days MCRT. The Los Lunas
Wastewater Treatment Plant has achieved these temperature requirements as show in figure.
In the final project performance report dated May 1996, the engineer stipulated that the average
temperature at 20.5 ˚C and 33.1 ˚C, respectively.
THE FINAL DIGESTION INCLUDED INSULATED COVERS, INSULATED AIR LINES, AND UNINSULATED WALLS.
The average MCRT in the digester was 45.2 days (refer to table 2), and the pathogen reduction
requirements were met for the fourth quarter by the temperature and MCRT requirements.
Vector attraction reduction requirements for Class B sludge state that volatile suspended solids (VSS)
must be at least 38% from the initial VSS going into the aerobic digester. Figure 5 shows that the
percent the percent reduction in VSS for the fourth quarter of operation met this requirement. In
that quarter, volatile solids reductions as high as 77.95% were recorded.
The mixing and aeration equipment furnished have been shown to adequately mix and aerated the
thickened WAS in the digester. The type of system furnished by the manufacturer breaks the floc on
each pass through the aeration system.
Five (5) each 24‐inch tubes are applied in each cell. At the design air application rate on 6% sludge,
based on enviroquip’s test data, each tube would pump at a rate of 9.74CFS (4369 GPM). At that
pumping rate, the entire contents of each quadrant go through the diffusion system every
102,850/4369=23.5 minutes.
There is enough flexibility in the air delivery system, to increase the airflow rate to 30 SCFM/th CF,
which decreases the turnover time to 19 minutes.
The aeration system furnished has proven itself to mix and aerate sledges at 6 to 7 % (±) in the Los
Lunas facility. The system furnished has shown the following advantages:
1. Oxygen is controlled by varying the air supply rate
2. Compressed air ads heat to the system to improve efficiencies.
Dirty water and sludge mixing effects
One of the points that should be made is that the total function of the aeration equipment must be
viewed in any discussion of oxygen transfer into a tank full of dirty water (wastewater) or sludges. A
perfectly efficient, properly designed transfer system must do more than simply dissolve air in
water. It has to provide an environment which is optimum for bacteria to carry on their intended
processes. As such, the aeration device is called on to perform a much broader class of service. It is
also the essential mixer that carries on all of the basic mixing functions within the aeration tank or
aerobic digester.
Mixing is required for several reasons.
A. A velocity needs to be introduced into the tank which is sufficient to hold the solids in
suspensions.
B. Sufficient shearing forces must be provided such that the floc in the wastewater or sludge
does not become overly dense.
C. The mixing also needs to be gentle enough that the floc is not really dispersed and the
bacterial colonies are maintained and allowed to function.
D. The mixing must also transport the bacteria and food into close proximation so that the
desired reactions can occurs.
The method of mixing will determine the degree to which the process approaches a plug flow or
completely mixed system. It also influences the energy required for oxygen transfer. An example of
this is the case where the raw waste or sludge exhibits low alpha, say 0.5, but the treated effluent
alpha is 1.00. If the system operates in a plug flow mode, then the energy required for oxygen
transfer can be about 30%more than for a completely mixed system. The dispersion of the organics
and nitrogen throughout the aeration tank (or aerobic digester) is principally affected by the mixing
patterns induced by the aeration equipment. Geometry is very important to the design of any
aeration system, especially Enviroquip’s shear tube aerators.
One of the principal reasons for the success that Los Lunas has had with their aeration system is the
fact that they have had such good mixing and been able to get essentially complete mixing within
the basins to quickly disperse high organic loads and nitrogen, and get the bacteria within the basin
to work on these loads as quickly as possible. The high pumping rate affects oxygen uptake rate
(disperses it). The diffuser supplied constantly renews interfacial areas across which diffusion occurs
within the shear tubes. Good mixing can lengthen the duration of contact time between the bubble
and the liquid, and thus increase the oxygen stripping efficiency. Another benefit of the shear tube
system is that the rapid blending of toxic or other feed upsets will dilute their impact on the
biological process.
In summary, it appears that the Los Lunas diffusion system offers (when properly applied) include:
A. High oxygen stripping efficiency. This gives the relatively high oxygen transfer rates in
pounds per horsepower hour, in dirty water or sludges.
B. High pumping capacity and ideal flow patterns for the suspension of solids, at optimum
installation geometry.
C. Full access to all orifices entirely above and outside of the wastewater sludge system under
aeration.
D. No “dead spots” in the event of mechanical equipment failure.
E. Full capability to change orificing at any time in order to taper or otherwise adjust oxygen
introduction to the basin(s).
F. Enviroquip’s engineering know‐how to adjust orificing and diffuser bubble break‐out levels to
maximize the turndown efficiency of any system.
G. As close to zero maintenance as you can get for any type of oxygen transfer and mixing
equipment.
H. Ability to turn the system “on and off” with minimal maintenance requirements.
Shear tube aerators provide the added benefit of a high “g” value at the point of application of the
oxygen into the system. In a shear tube aerator, the air is pumped into a tube – usually into a 24 inch
tube through a 2 inch diffuser with the point of injection roughly 14 feet below the liquid surface.
Application of the air induces “pumping” through the tubes at velocities of 3 to 5 FPS (dependent on
sludge thickness and viscosity). Typical basin turn‐over rates are 5 to 30 minutes (dependent on air
requirements and solids concentration). Within the tubes, the sludge solids (flocs) particles are
sheared at the point of injection of the air (oxygen source) which allows for the introduction of the
oxygen in close proximity to the bacteria that will utilize it in the breakdown of the digester solids
within the tankage. On exiting the top of the shear tube, the solids are “re‐flocced”.
Also, in a system undergoing concurrent nitrification‐denitrification within a sludge (floc) particle,
the shearing action will allow for nitrified material to be released to provide for an additional oxygen
source within the basins. The surface turbulence in the area immediately above each tube may also
allow for stripping of carbon dioxide and nitrogen from the system. All of these effects should be
beneficial to thr operator of high solids aerobic digesters.
Another advantage is that the rapid turnover time allows for easy pick‐up of any settled solids within
the “flux” in the aerobic digester and re‐entrainment of solids (in the tank) within that flux.
TABLE2.Third&FourthQuarterAerobicDigesterTemperatures
Temperature°C
Date NE Cell NW Cell SW Cell SE Cell
11/05/95 30.8 31.6 30.8 26.0
11/06/95 31.0 31.3 31.1 26.1
11/10/95 29.7 29.3 29.7 25.6
11/12/95 29.7 29.7 29.5 25.6
11/14/95 30.4 30.5 30.0 25.7
11/17/95 30.5 30.4 30.0 25.7
11/20/95 30.3 30.7 31.0 25.9
11/27/95 29.9 30.1 30.4 ‐
12/03/95 30.7 31.6 31.1 25.0
12/04/95 38.8 31.7 31.1 25.4
12/18/95 30.2 34.8 35.8 30.9
12/30/95 30.4 35.3 35.3 30.4
01/06/96 31.1 33.8 33.7 24.6
01/07/96 31.4 32.0 33.8 25.0
01/17/96 27.9 31.8 33.6 24.9
01/26/96 25.2 32.1 34.1 ‐
02/01/96 25.2 31.0 29.9 ‐
02/03/96 27.5 30.0 28.8 24.5
02/12/96 26.3 30.3 29.8 28.0
02/19/96 28.0 30.5 30.3 ‐
02/24/96 29.4 29.5 31.8 ‐
03/02/96 26.3 29.0 29.8 28.3
03/05/96 26.2 29.2 29.2 25.8
03/10/96 25.6 28.6 28.1 24.4
03/11/96 26.1 29.1 28.8 25.2
03/18/96 27.0 30.4 31.6 ‐
03/23/96 24.8 30.2 32.8 27.9
03/25/96 27.6 31.6 33.1 26.0
03/31/96 24.3 20.5 30.8 32.8
04/06/96 24.0 32.3 32.9 31.0
04/14/96 25.2 30.2 32.2 25.3
04/15/96 26.0 32.1 32.6 27.0
Source: City of Los Lunas W.W.T.P. Facility
Quarterly & Final Project Performance Report
Prepared by Molzen‐Corbin & Associates
Albuquerque, New Mexico
AdditionaldataprovidedbyCityofLosLunas
Date NECell NWCell SWCell SECell05/97 25.6 32.7 32.4 ‐
05/97 27.3 30.8 29.2 ‐
05/97 27.1 31.0 29.4 ‐
06/97 27.1 29.2 28.6 27.6
06/97 29.0 31.1 30.5 28.8
06/97 29.4 31.2 30.6 29.0
07/97 29.8 31.6 31.0 29.3
07/97 30.1 32.0 31.8 30.1
07/97 33.3 32.5 32.2 31.0
08/97 35.2 34.1 34.9 ‐
08/97 35.4 34.2 35.0 ‐
TABLE 3. VSS Reduction
Third & Fourth Quarters of Operation
Dates %VSSReduction SludgeRetention
11/17/95 59.32 293 days
12/19/95 64.53 296
01/21/96 71.92 211
02/15/96 64.29 50.76
03/13/96 46.06 50.50
04/24/96 77.95 43.22
Source: City of Los Lunas W.W.T.P. Facility
Quarterly & Final Project Performance Report
Prepared by Molzen‐Corbin & Associates
Albuquerque, New Mexico
TABLE 4. Aerobic Digester
Dissolved Oxygen Levels
DateDissolvedOxygen,mg/L
NECell %Solids NWCell SWCell SECell
11/06/95 2.68 3.01 4.67 5.00
11/17/95 2.50 (5.4) 3.00 4.91 5.12
11/27/95 2.70 3.21 5.11 5.00
12/04/95 2.67 (4.6) * 3.19 4.74 4.91
12/18/95 0.21 (11.0) 1.36 3.17 4.40
01/26/96 0.27 (4.8)' 2.00 3.65 ‐
02/01/96 0.41 2.17 3.85 ‐
02/12/96 0.18 (5.4) 2.61 3.59 4.10
02/19/96 0.18 (6.63) 3.11 3.23 ‐
03/11/96 0.28 2.44 2.54 3.13
03/25/96 0.28 1.91 1.33 2.50
04/15/96 5.54 (8.41) 4.66 4.64 4.34
*Note: The initial thickened sludge pump furnished was found to not be able to
handle 6%(±) solids, so a different type of unit was furnished by the
Contractor. For that reason, the Belt Filter Press had to initially be run at a
reduced solids concentration in the cake.
Source: City of Los Lunas W.W.T.P. Facility
Quarterly & Final Project Performance Report
Prepared by Molzen‐Corbin & Associates
Albuquerque, New Mexico
THE P.A.D. PROCESS
AND
DESIGN OF AEROBIC DIGESTER
SYSTEM
INCORPORATING RECENT
EXPERIENCE
Presented By
Elena Bailey
Aeration Product Manager
Enviroquip, Inc.
P.A.D PRE‐THICKENED AEROBIC DIGESTER PREOCESS
Objectives Achieve 2.5‐3.5% solids without mechanical dewatering. No polymer consumption required for thickening. Requires very little if any supervision
DIFFERENCES BETWEEN THE P.A.D And the CONVENTIONAL SYSTEM:
1. FLOW STREAM 2. OPERATION 3. WASTING PROCEDURE 4. DEWATERING POLYMER COSTS
Conventional system: Flow stream: The WAS is introduced into the Aerobic digester Pre‐mix Thickener Wasting occurs from the bottom of thickener
The P.A.D process and design of aerobic digestion system incorporating
recent experience
All the previous speakers have presented cases where the sludge was pre‐thickened using some type
of mechanical device. Even though all of them used a belt thickener it could have been any type of
mechanical dewatering system such as a centrifuge, or a rotating drum thickener. No matter what
the means of thickening the sludges going into the digesters normally anywhere from 3 to 10%
solids. Even though this is the direction our industry in taking, if you have a customer that feels more
comfortable working with sludges in the range of 2‐3% solids then P.A.D may be a better route to
take.
The P.A.D. process stands for PRE‐THICKENEE AEROBIC DIGESTION. The ORIGINAL P.A.D process was
developed a few years back before we even knew what Class B sludge meant. What we have found
out talking to Dr.Diagger in the last year or so, is that even though this process was developed with
very different objectives, because of the way it is set‐up it can also provide the necessary steps to
meet Class B sludge.
It was pioneered by Pete Czerwinski who is here in the audience today, and it was originally
designed to meet the following objectives: increase solids concentration in the range of 2.5‐3.5%
using a gravity thickener without any polymer and the expenses associated with polymers. The
concept was developed for owners who are either understaffed or have very limited resources and
run plants that have supervision for only 3 to 5 hours a day. The system is also fully automatic using
airlift pumps on timers that can perform each stage of the process.
The system is very different from the standard conventional system that you are probably familiar
with. The floe stream is different, the operation is different, the wasting procedure is different as
well as the cost associated with dewatering of the final product.
P.A.D. Flow Stream
A. Incoming flow: the WAS is introduced into the PRE‐MOX‐THICKENER‐AEROBIC‐DIGESTER 1 OR 2
B. Flow in the system : The flow is recycled from the Aerobic Digester to the Pre‐mix and then to the Thickener and back to the Aerobic Digester.
C. The water level: the water level in the digester remains constant. The water level in the pre‐mix and thickener is constant but is approximately 6” lower than the in loop aerobic digester basins.
3. P.A.D. – wasting procedure: When the sludge is ready to be sent to the sludge handling facility, the sludge is drawn from the isolated aerobic digester rather than from the isolated aerobic digester rather than from the bottom of the thickener , because the isolated digester has achieved pathogen kill and has the same concentration of sludge as the thickener.
4 P.A.D. – Dewatering polymer costs: Because the sludge is completely MIXED and HOMOGENOUS, the polymer dosage for the belt press or drying is constant which results in considerable polymer cost savings.
Overall result of P.A.D. system
I. Meets pathogen destruction driteria <2,000,000 colony forming units II. Meets vector attraction recustion
i) ≥ 38% volatile solids reduction or
ii) SOUR‐ standard oxygen uptake rate ≤1.5 mg o2/hr/grTSS
Conventional system: In this system the sludge is introduced to the aerobic digester basin. When sludge is fully digested it is then transferred to the pre‐mix if available and finally to the thickener for settling. The supernatant is transfer back to the head of the plant and the thickened sludge is wasted from the bottom of the thickener is taken and is taken either to the drying beds, belt press or centrifuge for dewatering or hauled to be liquid land applied.
The P.A.D. process: It consists of 4 basins: 2 digester, a pre‐mix and a thickener. The process is divided into two phases:
Phase 1
Digester no 1when digester no. 1 is part of the loop, it operates as a VOLATIZER basin. Result: Reduction of a majority of the volatile solids in this phase, as well as majority of the
pathogens. Thickener: The thickener serves as both a thickener and as a DENITRIFIER. There is no need to cycle the air on and off. Results: Thicken, denitrify, and recover up to 50% of the alkalinity lost in the system.
All the scum and WAS from the bottom of the clarifier enters the pre‐mix basin. The sludge is
aerated at a very high airflow for a very short time and then it flows into the thickener by gravity.
The thickened sludge from the bottom of the thickener is airlifted to Digester #1. The sludge is
aerated and then overflows back to pre‐mix.
Example:
Let’s assume that the WAS is around 0.5% solids when it enters the pre‐mix basin. After it settles in a
thickener for a while it may thicken up to 0.8%. When it is transferred to digester #1 it is still 0.8%,
then it is aerated for a while and overflows to the pre‐mix then the next time it goes through the
thickener the concentration will increase to 1%, 1.5% and so forth and so when it thickens to around
3% the digester is taken out of the loop , therefore phase 1 for digester #1stops.
At this point digester #1 will be taken out of the loop, all the incoming flow will be diverted
to aerobic digester #2.
Hydraulically, the water level in the pre‐mix and thickener is about the same, and the water level in
the Aerobic Digester in loop is about 6” higher that the other two structures. The water level in the
isolated digester may be all the way to the bottom if sludge is wasted out of the system, a gallon of
water has to go out in order to maintain hydraulic balance in the basin in loop. They way we achieve
that is by taking water from the thickener in the form of supernatant back to the head of the plant.
Doing so we achieve not only a hydraulically balanced system but also we enhance the thickening of
the sludge in the system without the need to turn the air off in the aerobic digester in loop.
Inphase1
Aerobicdigester#1
Thepre‐mixandthickenerareinloop
Inphase2
Aerobicdigester#1isisolated.
Whendigester#1isinloop,
Aerobicdigester#2isisolatedandviceversa.
Benefits:
It’s important to provide basins in series to prevent contamination of the waste and achieve
pathogen kill. This system is better than a series system because it provides full and complete
isolation. No new sludge can enter the system during Phase 2 so the maximum pathogen kill can
occur.
In addition, if you waste out isolated digester, the sludge is completely mixed and
homogeneous and polymer dosage for the belt press or drying beds is constant.
The digester in loop operated as a VOLATIZER while the sludge thickens.
The majority of the volatile solids reduction occurs in this phase.
Phase 1 aerobic digester #1 – “in loop”
1. WAS and scum from clarifier 2. Pre‐mix to thickener 3. Thickened sludge to digester #1 4. Digested sludge recycled to pre‐mix
Aerobic digester #2 – “isolated” 5. Sludge to dewatering
Phase 2 aerobic digester #1 – isolation 5. Sludge to dewatering. Aerobic digester #2 – “in loop”
6. WAS and scum from clarifier 7. Pre‐mix to thickener 8. Thickened sludge to digester #2 9. Digested sludge recycled to pre‐mix
Phase 2 Digester no 1 When digester no. 1 isolated, all raw waste is diverted to digester no 2. There is no further contamination of the waste in the Digester no 1 Air can be cycled on and off to achieve additional denitrifictaion if required. The basin is used as “PATHOGEN REDUCTION BASIN” Finally, all the 2.5‐3.5% sludge is pumped to dewatering, and the basin is ready for phase 1.
Controlled aerobic digestion
Sludge from clarifier to volatizer or thickener
Thickened sludge to volatizer
Volatizer to pathogen reduction #1
“POLISHED” sludge to dewatering.
CONTROLLED AEROBIC DIGESTER SYSTEM:
A. A minimum of 3 cells or tanks in series. B. All three tanks provided with SWD ≥20 ft. C. WAS or primary sludge can be pre‐thickened with a belt thickener or gravity
thickener prior to entering the first digester. D. Provide means to by‐pass thickening mechanism prior to entering the digester
complex. E. Provide means for thickening of partial flow from first tank prior to entering the
second tank F. First tank will serve as a VOLATIZER basin. G. Second tank will serve as the PATHOGEN REDUCTION BASIN #1. H. Third basin will serve as PATHOGEN REDUCTION BASIN #2.
Equipment required to provide a controlled environment:
A. First tank will be provided with a flat aluminum insulated Air beam® aeration cover to maintain 20‐20°C all year. Use shear tubes designed to maximize shearing if waste with high viscosity or high solids concentration is introduced to the system.
B. Provided means to provide Cycling or air (on and off) in all 3 basins. Control process by measuring changes in Nitrates, or pH.
C. The aeration system in the 3rd basin should be capable of providing adequate mixing at variable water levels.
D. The blowers should be capable of handling water level variations.
E. Capability of shift oxygen between cells and having to meet varying oxygen uptake
demands is essential.
Conclusion of the workshop
Provide a minimum of 3 cells in series or isolated tanks to provide the maximum pathogen kill
The deeper basins performed the best.
The sludge can either be pre‐thickened or the system should be such that provisions can be made to
bypass the thickener in case the temperature in the basin exceeds 35°C or in case the aeration
equipment can’t completely mix higher solids due to high viscosity.
Isolate the functions. Use the first basin as a Volatizer. The second and third basin can be used to
reduce the pathogens. The third basin should also be designed to provide adequate mixing at
variable water levels. Both the aeration equipment and blowers need to be designed to handle
variable water levels in the third tank.
Provide a cover on all the basins if possible if possible or at a minimum cover at least the first basin if
lower temperatures are expected in the basin.
Control ammonia‐nitrogen, nitrite‐nitrogen, nitrate‐nitrogen concentrations and pH and determine
the length of time that the air needs to be on and off.
Above all provide the most flexible design possible, both by shifting the oxygen between basins to
meet the varying oxygen demand either due to higher volatile solids feed going into the system or
due to higher solids concentration in the feed or due to additional air required to meet the SOUR
requirements.