-------
DIGESTER PROCESS CONTROL
INTRODUCTION
The first thought that comes to most opera-
tor's minds when the,,subject of control is
mentioned is laboratory testing. However,
there'.are a. variety of "tools" that should not
be overlooked in addition to what the;lab
results can provide. These include:
1. Eyes (to judge sludge thickness, superna-
tant quality, desirable color of digested
sludge, etc.)..
2. Ears (changes in thickness of raw sludge
can be detected by listening to a piston
pump "hammer" when sludge gets too
thin).
3. Nose (some industrial wastes, such as
phenolic, that can cause "digester problems
can be "smelle,d"Jn time to prepare for
handling them).
4. Hands (feeling texture of sludge can tip
the experienced operator to sand, grease
or uncomminuted components).
Nonstandard tests, are'also .used. These are
described on page 2-29,
A common question that the operator may
ask is; "What is normal for my digester?" This
has to be considered for the individual plant.
Some insight may be gained by answering the
following questions: : •'.-•-
1. Is the digester operation taking more
hours than it should? .(See the section on
Manpower Requirements, Part III.)
2. Is the digester causing problems in other
parts of the plant?
a. Supernatant in the primary clarifier? .
b. Supernatant in the aerators?
c. Foaming over the digester walls?
d. Excess BOD, SS or turbidity in the :
effluent?
3. Is the digester causing problems off-site?
a. Odors from trie digester?
b. Odors from the sludge beds?
4. Is the system costing too much money to
maintain? -• - •- •
a. Some estimate the average annual
maintenance . cost at 4% of • capital
• cost. •
b. Others use 2% of capital cost for the
first 10 years of use and 5% after this
time, as an estimate.
c. EPA cost estimates.show that digester
operation costs are about 10% of the
annual plant operation and mainten-
ance costs and drying bed /operation
runs about 5% of plant costs.
5. Is the system being upset by industrial
wastes?
6. Are operating procedures letting the di-
gester become upset? .
Some of these problems can be resolved by
looking at the way certain operations are
done and revising them to prevent possible
problems and improve digestion results.
A number of operation procedures are review-
ed in the following pages. An operation check
list is included at the close of the section.
2-17
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HOW TO SET UP A FEED SCHEDULE
First, the difference between feeding and
loading should be explained. Feeding con-
cerns only the raw sludge system while
loading considers both the feed and the
volume and contents of the digester.
Keeping excess water at a minimum and
feeding at regular intervals are important
features of a feeding schedule.
Control of Excess Water
Controlling the solids concentration going to
the digester may be done in several ways as
described in Operation Guide 1.
Total solids is the normal method for des-
cribing solids concentration. This test is des-
cribed on page 4-20 and Appendix E-9.
Three other methods can be substituted for
the total solids test by correlating them with
the total solids test results. These are: lab
centrifuge readings, motor amperage and the
Imhoff cone test. An example of how this is
done is given below using the lab centrifuge to
estimate solids.
Example: Take six samples ranging from thin
to thick sludge, approximately (1% to 8%
total solids) run both tests on each concen-
tration and plot results. Run centrifuge at
maximum speed for 15 minutes. Be sure test
is run at same speed for same time period
each time.
1. Record total solids and value of the other
indicator at 5 or 6 points between 1% and
8% total solids. :
2. Plot on a graph, drawing a line connecting
the points as shown on Figure 2-2.
3. Determine the lowest desired solids feed
level and set up system to stop pumping
when below that percent solids.
Feed Schedule Interval
Although the frequency of pumping may vary
from once a day to continuous, operators
should review this schedule to see if it can be
improved. The best feed schedule is contin-
uous at a low rate. The next best is frequent
pumping for short periods and once a day is
the worst. Several methods are discussed in
Operation Guide 1.
A caution about pumping to the digester:
do not allow the pump to be left on accident-
ally. Hydraulic washout is one of the major
causes of digester upset and all too often it is
traced back to an operator who left a raw
sludge pump control in the "on" or "hand"
position, overnight.
8
7
6
V)
Q .
13 5
84
ss
3
2
1
FIGURE 2-2
CORRELATION GRAPH
Scale % Solids
Draw Line
Plot Points
Shut Off Pump
Below This Level
I I I
5 10
ML. CENTRIFUGE TUBE
15
2-18
-------
OPERATION GUIDE 1 DIGESTER FEEDING
DESIRED GOAL
PLANT EQUIPMENT/
CONDITIONS
METHOD
A. Don't pump excess water
to the digester.
1. Sludge drawn to pit or
vault before pumping to
the digester.
2. Sludge drawn directly
from clarifier hopper or
gravity thickener with
positive displacement
pump.
1a. Watch sludge while being drawn. Shut -
off when too thin.
b. Sample and compare different sludge
concentrations by running lab centrifuge
tests or use Imhoff cone for quick
estimate.
2a. Check pump discharge gauge, higher
pressure generally indicates thicker
sludge.
b. Compare sound of pump with sludge
thickness. Excessive hammering of
piston pump indicates thin sludge.
c. Coordinate total solids with pump
amperage. Tie ammeter to pump
controls to shut it off if sludge is too
thin. See page 3-34, Gadgets.
d. Install solids concentration meter that
reads percent solids and use signal from
meter coordinated with time clock to
control feed solids concentration.
Pump at regular intervals
to prevent adding food too
fast for bacterial action
and prevent temperature
change.
1. Single stage digesters.
2. Two stage digesters.
1a. Pump at least several times per day by
hand and stop when solids drop too low.
b. Install time clock control if none
provided and set schedule for
10 a.m.—midnight. Let settle overnight
with no mixing and draw supernatant in
early morning.
2a. Spread pumping over 24 hours unless
freezing weather makes this unsafe when
plant is not manned. Control pumping so
that excessive water is not pumped
during low flow periods. See
Methods 2c and 2d in A above.
C. Review pumping sched-
ules and respond to
changing conditions.
1. Winter vs. summer.
2. I ndustrial wastes.
1a. Cold weather operation causes problems
with sludge bridging over in rectangular
clarifiers. Closer control must be
exercised to avoid reducing digester
temperatures.
b. Increase pumping time during storms
because of increase in solids. Decrease
afterwards because solids accumulate in
sewer lines and do not reach the plant.
2a. Most vegetable processing wastes
- increase the volume of sludge and .
decrease solids content. Adjust pumping
rates to match changing conditions.
2-19
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HOW TO CONTROL LOADING
General Loading Guidelines
In order to calculate loading, the operator
must have a record of the pounds of volatile
solids per day being fed to the digester and
must also know the volume of usable capacity
in tank. This calculation is given in Part IV,
The Basics, on page 4-20.
As noted in the introduction to this section,
two of the three major causes of digester
upset are hydraulic and organic overload. In
the first case excessive amounts of water flush
out the methane formers, leaving the tougher
acid formers to increase and cause volatile
acids to use up the buffering capacity.
In the case of organic overload, either the
amount of volatile solids increases due to an
excess of food or the digester capacity is re-
duced by scum and grit accumulations, mak-
ing the effective volume too small for the
amount of food being handled.
One approach to making a loading survey is
presented below.
Organic Loading Survey
To get a representative idea of an average
loading, take a series of grab samples on the
raw sludge feed three or four times through-
out the day and on several days of the week.
Use the procedure given on page 2-21 and the
calculations presented on page 4-21 and cal-
culate the actual available volume of digester
space and the volatile solids expressed in
pounds per day. Do the calculations and com-
pare the figures with those listed on page 2-4.
If the numbers are significantly more than
those listed in the manual as being average or
normal, it's time to remove the grit and sand
from the bottom of the digester and restore
the original available volume.
Some general guidelines that apply to loading
control are noted here and should be included
when the procedures for digester loading con-
trol are written for individual plants.
Operators generally have no control over the
characteristics or the total pounds of solids to
be fed to the digester. They do, however, have
control over the concentration and frequency
of feeding. These are two very important con-
trols, and they are also the ones which cause
the greatest number of sludge-handling
problems.
The operator must maintain the best possible
balance between the incoming raw sludge and
the sludge already in the digester. This is best
done by:
1. Establishing a feed schedule which is fre-
quent and in small amounts. A time clock
control on,the pump will allow this. How-
ever, the schedule should be set so that
excess water is not pumped at night.
2. Feeding the highest solids concentration
possible. Typical total solids ranges for
various sludges are:
primary raw sludge 5-8%
waste-activated 1 !/2-2%
trickling filter humus 1-3%
mixed primary/waste activated 3-5%
3. Obtaining good mixing throughout the
tank. A general rule of thumb is to recycle
a quantity equal to the volume of the
tank once a day. If the primary digester
has a volume of ,, 250,000 gallons
(950000 I) the mixer should be capable of
moving at least 175 gpm (11 I/sec.;).
4. Not overfeeding. One rule of thumb says
that the volatile .solids in the total,,daily
feed should not exceed 5% of the volatile
solids already in the digester.
5. Controlling digested sludge withdrawal to
keep buffering capacity high.
6. Maintaining an efficient grit removal
system.
2-20
-------
Methods for Determining Digester Capacity
1. Measure the amount of grit and inorganic
material in the bottom of the digester by
probing with a long stick or piece of pipe
and estimate the total cubic feet occupied
by this material. Another method of find-
ing the top of the grit layer is to take tem-
perature readings at lower digester depths.
The grit layer will .be several degrees
cooler.
2, If scum blankets have formed at the sur-
face, of the tank, they should be mea-
sured. One method of measuring this is to
use a stick with a hinged flap of. metal.
, When the. stick is passed down through
the scum layer and then lifted up, the flap
will open up underneath the scum blan-
ket. This device is discussed more fully on
page 3-31.
OPERATION GUIDE 2 DIGESTER LOADING
DESIRED GOAL
A. Prevent hydraulic overload.
PLANT EQUIPMENT/
CONDITION
1. Single stage digester,
manual sludge pumping.
2. All types digesters, auto-
matic pumping.
B. Prevent organic overload.
1. Single stage digesters.
2. Multiple tanks.
METHOD
1a. Pump thickest sludge possible, taking
care not to leave pump on accidently.
2'a. Control pump schedule so that pumping
rate equals sludge accumulation rate.
b. Impress personnel with the impor-
tance of not over-pumping or leaving
pump on accidently.
1a. Spread feeding over maximum portion
of the day.
b. Clean digester on regular schedule {every
2-3 years). See Part 111 on Digester
Cleaning.
c. Control industrial loading by ordinance
adoption and enforcement.
d. Monitor volatile solids loading and
VA/Alk ratio and be prepared to take
corrective action if necessary to restore
buffering.
e. Graph digester lab test data and watch
for trends.
2a. Consider the information in B-1a-1e
above.
b. Spread loading between several tanks if
one tends toward upset.
c. Recycle from the bottom of a secondary
digester or another well buffered
primary digester at a rate of 50% of raw
feed per day.
d. Adjust temperature-find most efficient
level for particular waste.
e. Increase mixing to maximum capacity.
2-21
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HOW TO CONTROL DIGESTER
TEMPERATURE
Specific temperature control methods will de-
pend on equipment used for heating the diges-
ter. Because it is important to hold constant
temperatures, the operator should be sure of
the following:
1. The temperature should be measured at a
point that represents the active part of the
digester.
2. The heating system should control the
temperature evenly so that it is not caus-
ing digester upset.
3. If cold weather makes temperature con-
trol erratic (changes of more than 2°F per
day), lower the operating temperature to
a level that can be kept more constant.
Some operation suggestions are given on
Operation Guide 3 and problems with temper-
ature control are discussed in Troubleshooting
Guides 5 and 6. Heating equipment operation
is discussed in Equipment Operation Guides 5
and 6.
HOW TO CONTROL MIXING
The goals of mixing control are to bring bac-
teria in contact with the food as it is added
and to keep scum and grit formations at a
minimum. Internal, external and recirculation
methods are discussed in Part IV, The Basics
Troubleshooting Guides 7, 8 and 9 give more
information on the subject.
OPERATION GUIDE 3 DIGESTER TEMPERATURE CONTROL
DESIRED GOAL
PLANT EQUIPMENT/
CONDITION
METHOD
A. Get accurate readings.
1.
No temperature gauges or
installed thermometer.
2.
Installed measuring device
giving questionable readings
1a. Allow recirculation pump to run for at
least 10 minutes, pulling from "active
zone." Pull sample and let bucket come
to sludge temperature, dump first
sample, draw another and measure
temperature using lab thermometer.
b. Lower sampler into digester, pick
samples at various levels according to
procedure in 1a and measure with lab
thermometer. (See page 3-29, Gadgets.)
2a. Use either method 1a or 1b above to
check temperature device, taking sample
as close as possible to where the device
measures temperature.
b. If device is in a recirculation pump line,
be sure pump is operating and pulling
representative sample.
B. Change operating temper-
ature up or down by at
least 5 deg. F. (2.8 deg. C.)
1. Changing weather condition
or waste characteristics.
1a. Adjust heat controls such that
temperature does not change more than
1 deg. F. (0.5 deg. C.) per day.
C. Desire to try thermo-
phylic range.
1. Heating equipment cap-
able of maintaining 130
deg, F. (54 deg. C.), mul-
tiple tanks available.
1a. Consult references in Appendix B.
b. Use only one tank at a time.
c. Change temperature at a rate of 1 deg. F.
(0.5 deg. C.) per day or 3 deg. F.
(1.7 deg. C.) in two days at maximum.
d. Be prepared for a month period of
transition.
e. Control using VA/Alk ratio and hold
below 0.1.
2-22
-------
The mixing schedule will vary depending on
the type of equipment and >tank configura-
tion. The important things to consider are:
1. Does the mixing system do a good job of
bringing food in contact with the
bacteria? , ,
2. Are scum blankets and grit accumulations
reducing the volume of the tank enough
to cause organic'overload?
3. Is the mixing (particularly gas-type) caus-
ing supernatant quality to upset the
plant? ...
Scum Blanket Control
gesters were installed with no mixers or in-
adequate, mixing devices. Under these condi-
tions, the scum blanket is a major problem.
'Keeping the scum blanket moist will normally
prevent the problem. This allows gas to pass
through and assist in preventing the blanket
from becoming too thick. A maximum depth
should be less than 24 inches.
Scum blankets in digesters usually have a
rolling movement if mechanical or natural di-
gester gas mixing is adequate. This movement
can be observed through the the thief hole. If
.movement is slight or not present, the opera-
tor should check mixer operation or probe
the scum blanket for thickness.
""Adequate mixing normally prevents scum
from forming a blanket. However, many di-
OPERATION GUIDE 4 HOW TO CONTROL MIXING
Several suggestions on mixing control are
given in Operation Guide 4.
DESIRED GOAL
PLANT EQUIPMENT/
CONDITION
METHOD
A. Keep scum and grit accum-
ulation at a minimum and
provide good contact be-
tween food and bacteria.
1. Single stage digester.
2. Two stage digesters.
1a. Run mixing equipment following each
sludge addition but shut off when it
affects supernatant quality. Time clock
control on both sludge pump and mixer
helps accomplish this.
b. If mixer fails, check possibility of using
raw sludge pump to provide some mixing"
when not pumping raw sludge.
c. Draw level down to minimum and
recirculate and mix simultaneously for
24-48 hours every six months if buildups
are a problem.
2a. Run mixer continuously in primary
digester unless secondary supernatant
quality goes above 5,000 mg/l total
solids. If above 5,000 mg/l, decrease
mixing time by manual or time clock
. control. Measure scum and grit accumu-
lation to find optimum mixing time.
B. Break up scum blanket
1. Mixing equipment not
operable, several digesters
available.
1a. Adjust number of tanks to allow loading
ratio of 0.3-0.4 Ib. VS/cu.ft./day
(4.8-6.4 kg/m3/day). Gas generation in
the tank will cause natural mixing.
Caution: The loading rate must be held in
this range continuously and the VA/Alk
ratio monitored to keep the tank in
control.
b. Introduce compressed digester gas into
the bottom sludge draw-off line and
allow bubbles to provide limited mixing.
2-23
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HOW TO CONTROL SUPERNATANT
QUALITY AND EFFECTS
One of the traps that some operators fall into
is believing that all process problems in other
parts of the plant are caused by outside
sources, when many times the trouble is from
digester supernatant returning to the head-
works or other points in the plant. Each plant
will have its own limits. However, problems
begin to develop in most plants if the superna-
tant total solids exceed 0.5 to 0.75% (5000 to
7500 mg/l).
General Guidelines for Supernatant Control
When drawing off supernatant in unmixed
digesters, the operator should select a draw-
off point which will give the best supernatant.
In single tanks with internal mixers, the oper-
ator should stop mixing for periods of 6-12
hours or plan for intermittent mixing to allow
settling before selecting and drawing off
supernatant. This will also require program-
ming sludge feed and sludge withdrawal.
Effects of Tank Types on Supernatant
Quality. Where two tank systems are operat-
ing, the active sludge mixture is transferred
from the primary digester to the secondary
where the sludge is detained without mixing.
The supernatant qualities obtained from the
secondary digester will depend on the deten-
tion time and the type of sludge feed.
Single stage tanks with moderate loading will
generally produce good supernatant if the
operator can find the right layer. Part of the
key to success is having several drawoff points
and selecting the best one. Several patented
supernatant selectors are installed in digesters
but even the best ones are subject to plugging
with hair, rags and other stringy debris. The
superior "selector" is the vigilant operator
who is willing to experiment until he finds the
optimum pattern for mixing, resting and
drawing the sought after "clear" supernatant.
The type of plant also affects supernatant
quality as shown by Table 11-1 below.
Table 11-1
TABLE OF EXPECTED RANGES
OF SUPERNATANT QUALITY
FOR DIFFERENT TYPE PLANTS
Suspended
solids
BOD5
COD
Primary
Plants
(mg/l)
200-1,000
500-3,000
1,000-5,000
Trickling
Filters*
(mg/l)
Activated
Sludge Plants*
(mg/l)
500- 5,000 5,000-15,000
500- 5,000 1,000-10,000
2,000-10,000 3,000-30,000
Ammonia
asNH3
Total
phosphorus
as P
300- 400
50- 200
400- 600
100- 300
500- 1,000
300- 1,000
* Includes primary sludge.
Some of the best results are obtained by
drawing the supernatant into an open lagoon
or drying bed and skimming the layer that
forms with wide circular decant pans. Greater
efficiency results when the, surf ace area to
depth ratio is large. If land is available and
problems exist, this solution should be
considered.
High rate gas mixing tends to homogenize the
sludge and contribute to poor quality super-
natant. One operator had success in improving
supernatant quality by adding about 3 mg/l of
water soluble anionic polymer to the gravity
thickener and reducing the total operating
hours of the gas mixer. This reduced the de-
tention time in the digester because thicker
sludge was pumped and the reduced mixing
time produced lower sol ids in the supernatant.
The product was Zimmite ZT-650.
Other considerations are summarized in
Operation Guide 5. Also see Troubleshooting
Guide 2.
2-24
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OPERATION GUIDE 5 SUPERNATANT CONTROL
DESIRED GOAL
PLANT EQUIPMENT/
CONDITION
METHOD
A. Liquid quality that will
riot affect the rest of
the plant.
1. Single tank fixed cover.
2. Single digester, floating
cover, single draw-off.
1a. Feed digester at as slow a rate as pos-
sible, do all mixing after supernatant has
quit displacing and allow tank to set
without mixing 8-12 hours before
feeding again.
b. Make up jars containing samples of
supernatant stabilized with formal-
dehyde, which can be used as a standard
by operators showing what is and is not
acceptable quality.
2a. Adjust tank level until best quality liquid
is found and operate within these limits.
b. Install swivel joint and 4-6 foot length of
pipe to draw-off line to allow selection
over wider range. See page 3-35,
Gadgets.
B. Prevent problems of over-
load to gravity thickener.
1. Poor quality supernatant
due to overloaded
digester.
1a. Add polymer to sludge going to
thickener to increase solids, decrease
quantity of supernatant and increase
digester detention time.
C. Prevent high demand
, supernatant going to
aerators.
Sidestream treatment.
1. Poor quality due to over-
loaded digester.
1a. If extra aerator is available, preaerate
before discharging to aerator. -• .. .
b. Aerobically digest supernatant to reduce
demand. Air demand will be high for first
few days but will taper off to 20-25 cfm/
- cu.ft. (20-25 m3/min./m3) tank capacity.
D. Eliminate all recycle
to process.
1. Poor quality supernatant
due to overload or poor
separation.in digester.
1a. Discharge to available drying bed or
• lagoon and spray irrigate decantate.
b. Haul or process digested sludge at a rate
that will prevent supernatant return.
c. Sell to firms or individuals requiring
liquids for composting processes.
HOW TO CONTROL SLUDGE
WITHDRAWAL
When sludge is drawn put of the digester,
either to beds or other sludge handling facil-
ities, there - are several important
considerations.
1. In small plants, particularly with single-
stage digesters, at least 12 hours-should
lapse between pumping raw sludge and
sludge withdrawal. Additionally, the con-
tents should be well mixed to prevent
• pulling out raw sludge that could create
odors as well- as contain pathogens.
-2: Care must be taken to prevent pulljng air.
into fixed cover digesters when sludge is
.withdrawn. Sludge from" multiple tank
• •- systems can be drawn at a rate that will
allow gas from another tank to be pulled
, back into the emptying tank. Single tank
...... operators,should pull sludge out slowly
• . enough that .air is not pulled in or kept at-
a minimum. Explosive conditions exist
when the methane concentration is below
20% on a volume basis.
2-25
-------
HOW TO USE LAB TESTS AND OTHER
INFORMATION FOR PROCESS CONTROL
Just as the driver of a car does several things
at once to keep control of the car, the oper-
ator looks at several indicators to keep the
digester from "upsetting." And, like the driv-
er who uses the steering action to keep the car
on the road, the operator can use lab tests,
such as the volatile acids and alkalinity, for
process control. Other tests are also needed to
give the full picture and these will be discuss-
ed in the following pages.
Methods for running lab tests are found in
Appendix E, and a discussion of what the
various parameters show is covered in Part IV,
The Basics, starting on page 4-1 1.
Important Indicators
There are certain indicators which measure
the progress of sludge digestion and warn
about impending upset. No one variable can
be used alone to predict problems; several
must be considered together. Control indica-
tors in order of importance are:
1 . Volatile acids to alkalinity ratio.
2. Gas production rates, both CH^ and CG^.
3. pH.
4. Volatile solids reduction (digester
efficiency).
None of the above used singly can indicate
the condition of the digestion process. For
.example, volatile acid readings may increase,
but which does it indicate:
1 , A decrease in percent methane (a rise in
percent
2. A decrease in alkalinity?
3. No change in percent methane production?
4. A decrease in pH?
2-26
5. A problem or no problem?
Large increases in volatile acids may take
place before pH is changed if the digester is
heavily buffered (has high alkalinity). Changes
in volatile acids mean more when considered
with alkalinity.
Obviously, the operator needs more informa-
tion before responding to the indicator.
The operator is cautioned against looking at
an absolute number. The rate of change is
much more significant. In summary, then,
trends of these indicators are the most useful
to predict the progress of digestion and as
signals of process upset. A discussion of
trends is in Part IV, The Basics, page 4-16,
and Appendix, G-2.
Importance of Samples in Process Control
Sampling is the first step in waste analysis. It
is absolutely necessary to take good samples
to get reliable usable tests. Good samples are
obtained by following a few simple rules:
1. The sample must be representative. For
example, when drawing samples from an
on-off pumping operation, allow pump to
run for several minutes to clear the line;
then make a composite sample during the
time the pump is running. This is done by
drawing three samples, at the beginning,
at mid-point and at the end of pumping
period. Equal volumes of sample should
be mixed together.
2. Always run pH and temperature tests im-
mediately (5-10 minutes) to avoid deteri-
oration. If samples are allowed to set too
long, CC>2 will be released, causing the pH
to rise. Always use the same length of
time from collection to determination for
each test run. It is important to standard-
ize when taking temperatures. Don't use
a warm bucket or thermometer one day
and a cold one the next day.
-------
3. Always refrigerate the sample if tests are
not run immediately. When storing a raw
sludge sample in a refrigerator, it's a good
idea to use a plastic wrap over the top of
'the jar with a rubber band on it to hold it
in place. This will allow any gases that
might collect in the sample to expand
without bursting the jar.
4. The container should, be cleaned thor-
oughly before and after use.
Sample Points for Control Information
There is no specific set or list of tests than can
fit all digester systems due to the variability
and complexity of systems.. However, in gen-
eral, the following points are usually sampled
for digester monitoring and control.
1. Raw sludge
2. Digester sludge (active zone)
3. Digested sludge
4. Supernatant
5. Digester gas
Raw Sludge. Tests performed on samples of
raw sludge tell an operator what .type of food
is being fed to a digester. The operator is
actually feeding a tank full of hungry organ-
isms their daily rations, much as a zookeeper
would distribute food to cages full of animals.
By knowing the condition (pH and tempera-
ture) and thecontent (total and volatilesolids),
the operator can predict to some degree how
the d.igester will react.
This sample is. normally taken at the raw
sludge pump or from a well-mixed portion of
a sludge pit or vault.
Digester Sludge. The second major sample
should be taken from a point in the digester
that represents the well-mixed active portion
of the primary, digester. This determines what
is happening inside the tank. This sample gives
the operator information on the alkalinity
and. volatile acids as well as on the solids that
will be used in other calculations (described
on page 4-20).
Samples may be taken from sample lines,
frorrj,w.overflow boxes where sludge passes
from' a primary to a secondary digester or
from a recirculation pump or line where a
corporation-cock is installed.
Digested Sludge. The contents of the bottom
sludge in the digester is another important
point which gives the operator information
on how the process is proceeding. This sample
may represent what is being transferred from
the bottom of a primary digester to a secon-
dary digester, or it may represent the bottom
sludge being withdrawn for disposal.
Quantities of sludge transferred from one
digester to another or from a digester to a
drying bed can usually be determined by:
1. Calculating the volume added to a drying
bed, sludge truck or dewatering unit and
recording .it as gallons per day or gallons
per month.
2. Calculating the diameter of a circular
digester, the number of gallons per inch
or per foot which measure the change in
liquid depth, and calculating and record-
ing the volume.
Supernatant. Grab samples of supernatant, if
they are fairly uniform and continuous, will
give a good idea of what is happening. How-
ever, some method must be used to decide
when to begin and stop transferring superna-
tant back to the process. Many times this is
done by visual observation. Some operators.
use an Imhoff cone with a cutoff point of 50
milliliters per liter after 30 minutes of settling
to tell them when to stop the flow of super-
natant and let the digester rest.
Carbon Dioxide. This is a most easily mea-
sured component of the digester gas. Because
the sum of the CC>2 and CH4 is approxi-
mately 100%, the amount of CH^ can be
roughly estimated by measuring the CC^.
Well operating tanks range between 25-35%
CC>2. The percent of CC>2 can be an early
2-27
-------
indicator of approaching problems if the
trend is upward.
The percent CC>2 will increase shortly after
feeding, if sludge is fed two or three times per
day. Information should be obtained during
different times of the day to find normal
values for the plant.
Several CC>2 analyzers are on the market, such
as those manufactured by Hays or Orsat. The
C02 content of the gas coupled with the
quantity of gas produced shows the immedi-
ate response to how the food is being utilized.
If the CC>2 content stays consistently high, it
can be a trend toward excess acid production
and trouble.
Samples may be taken several places. If gas is
piped into the lab and used for Bunsen burn-
ers, this can be a very adequate sampling point.
It is important to purge the line before collect-.
ing a sample. This is done by lighting the burn-
er and letting it burn for a minute or so, turn-
ing it off to collect the sample. If samples can-
not be run in the lab, the sampling device can
be located at a sample point on the digester
gas line.
Suggested Tests and Frequency. The follow-
ing table lists the possible tests and suggested.
frequencies for a plant with approximately 1
to 2 mgd and two or more digesters.
This table is a suggestion only and would have
to be adapted to the type of sludge being re-
ceived at a plant, the severity of overloading
and a number of other factors, but gives some
idea of how often information could and
should be obtained. Two columns are shown,
the first showing the optimum, the second
showing the minimum test frequencies.
TABLE 11-2
SUGGESTED SAMPLE TESTS AND FREQUENCY
1-2 MGD PLANT, TWO DIGESTERS
Raw Sludge
Recirculation Sludge
Bottom Primary
Bottom Secondary
Supernatant
Gas
Scum
Grit
Depth Series
TEMP.
D
D
D
M/2
W
Y/4
(D)
(D)
(W)
(M)
(M)
(Y/2
TS
D
D
W
W
W
Y/4
Y/4
Y/4
(4/W)
(4/W)
(M)
(M)
(W)
(Y/2)
(Y/2)
(Y/2)
VS
D
D
W
W
W
Y/4
Y/4
Y/4
4/W)
4/W)
(M)
(M)
(W)
(Y/2)
(Y/2)
(Y/2)
CO?
D
pH
D
D
D
W
D
Y/4
(W)
(D)
(W)
(M)
(W)
(Y/2)
ALK
D
2W
Y/4
(W)
(W)
(Y/2)
VA
D
2W
Y/4
W)
W)
(Y/2)
QUANTITY
D
Da
Db
D
W
M
M
(D)
(Da)
(Db)
(D)
(Y/4)
(Y/4)
(Y/4)
() ~ Minimum frequency
C = Continuous
D = Daily
W = Weekly
M = Monthly
Y = Yearly
M/2 = Twice a month
Y/2 = Twice a year
4/W = Four times a week
a = Amount transferred to secondary, if applicable
b = Or as often as drawn to disposal point
2-28
-------
Non-Standard Tests
There are some non-standard tests whicrrare
not given in the books which will provide ad-
ditional useful information.
Visual Gas Test. A yellow flame with blue at
the base is normal at the waste gas burner;
When too much blue is present and the flame
will not stay lit, this may indicate too much
CC>2. An orange flame with smoke may be pre-
sent when the digester has a high sulfur contact.
Test for Grit. Estimates on the amount of grit
in the sludge may be obtained by allowing
tap water to run into an open beaker of sludge
at a slow rate to wash most-of the light solids
out, leaving the grit in the bottom of the beak-
er. If the amount of water run into the beaker
is the same each time, -then the operator can
get some visual feeling for the amount of grit
in raw sludge, the sludge being drawn out, and
the amount in the recirculated sludge. It is dif-
ficult to assign "numbers to these amounts, but
visually the operator can tell if the amount is
increasing or decreasing. Using this informa-
tion along with actual sounding of the digester
can give him a feel for the probable grit build-
up in the digester.
Sniff Test. Another bit of information can be
gained by the non-standard "sniff" test. Sim-
ply smelling the sludge samples can tell the
operator whether it's septic, sour, well-digest-.
ed or, in the case of raw. sludge samples,
whether there are chemicals such as oils, sol-
vents, or other types of materials that might be
harmful to the digester. Experience is the best
teacher for drawing conclusions from this type '
of a test, but it should not be ignored by the
operator. Examples of digester supernatant
sniff indicators are rotten egg odors which
may indicate organic overload and a rancid
butter smell which may be present when heavy
metals toxicity exists.
Digester Profiles
In addition to the above tests, samples should-
be taken inside the digester. This can be done
by lowering sample collectors into the tank at
least twice yearly. One procedure is to set aside
half a day, or a day if necessary', to take sam-
ples at five-foot intervals from top to bottom
of all digesters and set up total solids, volatile
solids, pH, temperature, and alkalinity on the
entire series. By'plotting the results after
they are obtained, it is possible to have a pretty
good idea of how much grit is on the tank
bottom, whether there are pockets of undi-
gested material, or whether the temperature is
not uniform all the way through. These sam-
ples can be taken, using a homemade samp-
ling device, one of which is described on
page 3-29. The important thing is to collect
a sample that represents the particular level
that the sample is takep'from. '
It is also a good idea to take samples from sev-
eral different locations and depths. Samples
can be taken-from prepared sampling holes
known as "thief holes." If the'tank has a float-
ing cover, it is possible to lower a sampling
device alongside the floating cover'into the
tank. It will probably be necessary to break
away the scum layer, and although this is not
the best location, it will give some information
if no other sampling points are available. As
a last resort a manhole cover can be taken off;
however, safety precautions should be strict-
ly observed. A floating cover should be down
on the corbels before the.manhole cover is
removed. :
At the same time the digester profile is being
done, both the amount of scum and the a-
mount of grit should be recorded. In order to
find the amount of grit that has accumulated,
several points in the digester should be sounded
using a, long stick or a piece of pipe to deter-
mine-where the top of the grit layer is. Then
force the stick down through it until the floor
is reached and record the difference in the two
measurements. If,the plans on the digester are
available, the grit layer can be estimated by us-
ing the top of the wall as a reference point and
measuring down to the top of the.grit layer,
noting the difference between these measure-
ments.
2-29
-------
OPERATIONS CHECK LIST
The following list is prepared to help you make up your own check
list and may include items not within your process. Use only those
which apply to your plant. Suggested
Frequency
A. Feed Sludge
1. Record volume pumped for a 24-hour period. Daily
2. Run total solids test and compare with amount pumped in to be sure Weekly
too much water is not being fed. (1-3 times)
3. Check pump operation for packing gland leaks, proper adjustment of Daily
cooling water, unusual "noises, undue bearing heat, and suction and
discharge pressures.
4. Monitor pump time clock operation for proper control and check Daily
running time with sludge consistency.
B. Recirculated Sludge
1. Record temperature of recirculated sludge. Daily ;
2. Collect sample of recirculated sludge and run tests. Weekly
3. Check boiler temperatures, burner flame, and exhaust fan for proper (1-3 times)
operation. Daily
4. Check hot water circulating temperatures. Daily
5. Check and record heat exchanger inlet and outlet temperatures. Daily
6. Check for leaks in sludge lines. Weekly
7. Check pump operation — packing gland leaks, proper adjustment of Daily
cooling water, unusual noises, undue bearing temperatures and suc-
tion and discharge pressures.
C. Digesters
1. Gas manometers for proper digester gas pressure. Daily
2. Drain condensate traps — more often if needed. 4/daily
3. Drain sediment traps. Daily
4. Waste gas burner for proper flame. Daily
5. Record gas pressures. Daily
6. Record floating cover position, check cover guides and check for gas Daily
leaks.
7. Record digester and natural gas meter readings. Dail'y >
8. Check and record fuel oil. Daily
9. On gas mixers check flow of gas to each feed point. Daily
10. Check internal moving mixers for proper operation. Daily
11. Pressure relief and vacuum breaker valves — Verify operation with Daily/Weekly
manometer and check for leaking gas.
12. Check supernatant tubes for proper operation, collect sample, and Daily
hose down supernatant box.
13. Check level and condition of water seal on digester cover. Daily
14. Check flow meters for correct flows, leaks and vibration. Daily
15. Check feed sludge, density meter for correct density, leaks and Daily
vibration.
16. Check scum blanket through sight glass. Daily
17. Check gas storage tank for gas leaks and odor. Record readings on Daily
pressure gauges and drain condensate traps.
2-30
-------
18. Check all gas line piping for leaks. Test with soapy solution if a leak
is suspected.
19. Check gas pressure regulators and verify with manometer reading.
2Q. Check flame trap arresters by noting the pressure drop across unit or
that equipment downstream is working.
21. Check scum bjanket for dryness and depth.
22. Clean and fill manometers. . •
23. Remove, clean and check all safety devices for proper operation.
24. Flush and refill digester dome seals.
25. Sound digester by sampling from bottom up at 5-foot intervals. •
26. Remove digester from service and clean and repair unit.
D. Sludge Withdrawal •'...':
1. Check volatile content of bottom sludge, if below 50% and nuisance
odors not present it should be ready to remove.
2. Frequency of removal will vary with method of dewatering and/or
•disposal. Some plants that haul wet sludge to land sites or dewater
..,. on filters pull out daily. Plants that have drying beds in wet climates
may draw out only in summer and fall months. .
3. Collect several samples and composite for calculation of digester
efficiency.
E. Compressors .
1. Check for proper operation of unit by looking at the oil level, drive
belts and discharge pressure.
F. Piston Type Sludge Pumps
1. Check for proper operation of pump and motor by looking at the
automatic oiler. Make sure that the eccentric is dripping at a regular
rate, packing is adjusted properly, drive belt tension is OK. Note
the vacuum and discharge pressures and record revolution counter
reading and reset. .
2. Collect sample of sludge when operating.
G. Fire Fighting Equipment
1. Be sure all units are in place and that unit is still within its inspection
date.
H. First Aid Kit
1. Be sure they are in place and that all items match the inventory sheet.
Suggested
Frequency
Weekly
Monthly
Monthly
Monthly
6 Months
6 Months
6 Months
6 Months
3-8 Years
Weekly
Variable
When drawing
Daily
4 Hours
Daily
Monthly
Monthly
2-31
-------
PREVENTIVE MAINTENANCE CHECK LIST*
1. Exercise the Variable Speed Drive (Raw Sludge Pump)
2. Exercise the Variable Speed Drive (Digested Sludge Pump)
3. Inspect Pump (Centrifugal, Hot Water Recirculation)
4. Inspect Floating Cover for evenness and gas leaks
5. Inspect First Aid Kit
6. Inspect Pump (Centrifugal, Sludge Recirculation)
7. Inspect and clean Motor (Raw Sludge Pump Drive)
8. Inspect and lubricate Raw Sludge Pump (Piston Type, Belt Driven)
9. Inspect Piping and Exercise Valves
10. Inspect and clean Motor (Sludge Recirculation Pump)
11. Inspect and clean Motor (Gas Recirculation Compressor)
12. Inspect and clean Motor (Digested Sludge Pump Drive)
13. Inspect and clean Motor (Gas Storage Compressor)
14. Verify accuracy of Raw Sludge Flow Meter (Magnetic)
15. Inspect and lubricate Digested Sludge Pump
16. Check accuracy of Raw Sludge Density Meter (Nuclear)
17. Lubricate Coupling (Hot Water Recirculation)
18. Lubricate Coupling (Sludge Recirculation)
19. Lubricate Coupling (Gas Recirculation)
20. Clean and fill Gas Manometers
21. Inspect Compressor (Gas Recirculation)
22. Disassemble and clean Gas Water Traps (Condensate)
23. Disassemble and clean Flame Arresters (Gas Piping)
24. Check for support and leaks on Digester Internal Piping (Gas Mixing)
25. Inspect and clean Gas Storage Compressor
26. Clean Heat Exchanger
27. Disassemble and clean Gas Pressure & Vacuum Relief Valves (Digester Cover)
28. Check Fire Fighting Equipment
Weekly
Weekly
Weekly
Weekly
Weekly
Weekly
Monthly
Monthly
Monthly
Monthly
Monthly
Monthly
Monthly
Monthly
Monthly
Monthly
Quarterly
Quarterly
Quarterly
Semi-annually
Semi-annually
Semi-annually
Semi-annually
Semi-annually
Semi-annually
Semi-annually
Semi-annually
Semi-annually
Review the equipment manufacturers' recommended preventive maintenance procedures
and schedules. They should be followed. Also refer to your plant's O & M manual for more
detail on preventive maintenance for the plant.
2-32
-------
CHEMICALS USED IN DIGESTER
CONTROL
Chemical usage in the digester falls into two
categories: pH adjustment and metal toxicity
control. This section covers" pH adjustment.
The subject of toxicity/is discussed .in a separ-
ate section on toxic material in Part III,
page 3-21.
CONTROL OF pH
Several chemicals are available which can be
used as caustic agents in digesters to raise or
control pH. Each has advantages and disad-
vantages. The choice of which one to use
largely depends upon availability, cost, stor-
age and handling preference. In all cases of
caustic addition, care must be taken to pro-
vide mixing. Mixing is essential to be sure
that the caustic solution will be distributed
throughout the tank' contents and prevent
localizing the caustic. This section discusses
the use of various chemicals in digester
operations.
Lime
Lime is one of the most common caustic
agents due to .its availability, relatively low
cost and ease of handling. Lime is usually
used in starting a digester because it speeds up
gas production and lowers volatile acids_con-
centration. One limitation to the use of lime
is its inability to maintain the pH at higher
levels than about 6.,8. When lime is added to a
digester it combines with CC^, removing C02
from the liquid. This combining reaction
forms calcium bicarbonate when the digester
is below 6.7 or 6.8 and the bicarbonate
alkalinity is between 500 and _ 1,000 mg/l
(NOTE: This is not total alkalinity). Calcium
bicarbonate becomes a buffering agent,
neutralizing the acids in the digester and
allowing the digester to return to normal.
Too much lime causes insoluble calcium
carbonate to form. Like grit, calcium carbon-
ate settles out, takes up space, and may be
very difficult to remove. A further disad-
vantage is that it may create a vacuum in the
digester because C02 is removed, causing a
decrease in gas pressure inside the.digester.
This occurs when excess lime is added.
If the operator were to continue adding lime
after 'the digester pH has reached between 6.5
and 6.8 and the C02 were to continue drop-
ping, a dangerous situation might result. The
C02 content might drop until it reached
about 10% and the pH would start to increase
to about 8.0.
As the C02 percentage drops, the pressure
lowers and a vacuum results. With biological
activity continuing, the percent of C02 in-
creases,'rising again to the 10% level, at which
point the pH drops to below 7.0. Additions of
lime beyond that necessary to neutralize the
acids or indicated by pH are wasteful. They
may result in lowered gas pressure, a vacuum
inside the tank, and a collapsed cover.
However, it is reported by Perry L. McCarty
that some excess calcium carbonate has .some
benefits: it prevents calcium toxicity and
when the pH drops to about 6.5, the insoluble
calcium dissolves forming additional bicar-
bonate alkalinity.
Lime is available in two forms:
1. As unslaked lime, or calcium oxide (CaO),
2-33
-------
often called quicklime. It is hygro-
scopic, which means that it takes up
water or moisture quite readily. The
maior disadvantage is that quicklime
must be "slaked" (water must be added
in a controlled way) before it can be
used. This requires special equipment.
CAUTION: ALWAYS ADD QUICKLIME TO THE
WATER TO PREVENT AN EXPLOSION WHICH
MAY SPLATTER THE OPERATOR WITH LIME
AND CAUSE SKIN BURNS. QUICKLIME MUST
BE STORED IN A DRY PLACE.
2. Hydrated lime (calcium hydroxide
Ca(OH)2) is the preferred form since
it is already slaked and ready to use.
Lime is always mixed with water to
form a slurry using about 100 pounds
(45.4 kg) of lime to 50 gallons
(189 1) of water before being fed to
the digester. Most operators add the
lime slurry into a sludge or scum pit
at the side of the primary clarifier
and pump it along with the sludge.
LIME DOSAGES. Two quick methods can
be used as rough approximations for
the amount of lime additi'ons.
1. Apply a dosage of 1 pound of lime
for every 1,000 gallons (37850 1) in
the digester. This is risky. Too much
or too little may be added. A better
way is given below.
2. The empirical method
PROCEDURE:
a. Obtain a sample of representa-
tive sludge (about 5 gallons)
from the digester and record
the exact amount.
b. Carefully add calculated
amounts of lime to sample
until pH reaches about 6.7
or 6.8, then stop. Record
total amount of lime used.
c. Calculate the amount of lime
needed to treat the digester
using the results of the above
step.
The following example shows the
calculation:
Assume 0.1 pounds of lime was re-
quired to treat the 5-gallon sample.
If the digester volume is 100,000
gallons (378500 I) then,
100,000 gallons
5 gallons
=20,000 times the sample volume
and 20,000 times as much lime would
be needed to treat the digester.
Therefore 0.1 times 20,000 equals
2,000 pounds (907 kg) of lime re-
quired. To get a better estimate,
the experiment could be done three
times and the results averaged.
Another method is to add enough lime
to neutralize the volatile.acids.
Use the following procedure to find
pounds of lime needed:
Calculate the amount of volatile
acids in mg/1 times 8.34 times
volume of digester in million gallons
equals pounds of volatile acids. The
pounds of lime needed equals .62 times
the pounds of volatile acids. :
For example: Assume volatile acids to
be 1800 mg/1 and digester volume to be
150,000 gallons (0.15 million gallons),
then, 1,800 times 8.34 times .15
million gallons equals 2,252 pounds of
volatile acids. The amount of lime
needed is 2252 x .62 equals 1396 Ibs.
of lime.
NOTE: The amount of alkalinity already
in the digester in this case would be
in excess and is considered a cushion.
The following procedures are recom-
mended for lime addition:
1. Begin adding lime if the pH drops
below 6.6.
2. Check the vacuum relief device on
the digester to be sure it is working
(the addition of lime can cause a
vacuum inside the tank). Stop lime
addition if vacuum relief begins opera-
ting and wait 24 hrs.before starting
lime again.
2—34
-------
3. Add the lime slurry only while mixing
and/or recirculating the? digester and
continue for at least an hour or more after
the last addition. Cheek the pit frequently.
4. Stdp adding lime when pH reaches 6.8.
Anhydrous Ammonia
Anhydrous ammonia is a gas and is available
in pressurized cylinders. It may be used for
pH adjustment under controlled conditions.
However, lime, or other caustics, are recom-
mended for the'smaller plants for safety
reasons.
Several precautions are noted below for those
using anhydrous ammonia: ;
1. There is the possibility of ammonia tqx-
icity if the neutral pH is overshot. The
toxic level depends on other buffering
agents in the digester but the concentra-
tion should not exceed 1,400 mg/l as N in
any case.
2. The gas cylinders should be handled using
all the precautions normally employed
with gases under pressure; i.e., do not
drop or strike with sharp objects, keep a-
way from excessive heat and use approved
regulating valves.
Several feeding procedures are noted below:
1. Make up tight ammonia connections from
cylinder to aluminum pipe. Insert the pipe
through a thief hole in the top of the di-
gester. The pipe should go to a depth of
• 10-15 feet (2.5-3.2 m) in the digester. A
1/8" reducing elbow can be attached to
the lower end of the pipe so that the pipe
can be rotated in a full circle to distribute
the gas addition.
2. Make up a connection to a recirculation
line which allows gas feed into the sludge
while it is being recircula'ted. The connec-
tion may be made using a corporation-
cock and necessary fittings to mate with
the feed system.- Precautions'to be ob-
s,eryed include:
a. Use materials in connection and feed
piping that are not affected by am-.
monia. DO NOT use copper or brass
fittings.
b. Feed ammonia only when sludge is
circulating and downstream valves are
open. .-',.--
The digester pH should be carefully watched
when using ammonia. The greatest danger lies
in ammonium toxicity (see Toxic Materials,
Part 111, page 3-21). ...
The following example shows how to find the
pounds of ammonia needed:
1. Determine desired amount of excess alka-
linity. Suggested amoupt equals 500 mg/l.
2. Determine alkalinity needed to neutralize
volatile acids. When alkalinity is expressed
as mg/l CaC03, the amount needed to
neutralize volatile acids is:
ALK = 0.833 x VA
Example: If the alkalinity equals 2,000
and VA equals 3,000, then the amount of
additional buffering alkalinity needed
would be:
2,000 - 0.833 (3,000) = -500 mg/l
(NOTE: the minus sign 'shows that this
amount is needed in addition to excess.)
3, Determine amount of ammonia needed by
the following formula:
Ibs. of 100%NH3
= 2.78 x vol. of dig. in gal.
x needed alkalinity in mg/l CaCO3
(excess + buffering) +- 1,000,000.
a. Assume:
Digester volume 250,000 gal.
Alkalinity 2,000 mg/l
Volatile acids 3,000 mg/l
Excess alkalinity
desired , 500 mg/l
2-35
-------
b. Find amount of alkalinity needed to
neutralize the VA. (See Step 2 above.)
c. Find total alkalinity needed.
500 + 500 = 1,000 mg/l as CaC03
4, Find amount of ammonia needed:
Ibs. of 100%NH3
= 2.78 times dig. vol. in gal.
times mg/l alk. needed per 1,000,000 gal.
= 2.78 times 250,000 times 1,000 per
1,000,000 gal.'
= 2.78 times .25 times 1,000
= 695 Ibs. (315kg)
5. Commercial anhydrous ammonia is about
80% ammonia. Correct for this amount
by:
100%
x 695 = 869 Ibs (537 kg) 80%
80%
ammonia
6. Find feed rate: ., .',..,
The feed rate should be about 0.85
lb./1,000. .gal. (0.1Q2 kg/1000 I) digester
volume per hr. at a pressure of 50 psi
(345 KN/m2).
Feed rate
= 0.85
per hour
1 ,000
= 212lbs./hr. (96 kg/hr.)
Other Chemicals Used for pH Adjustment
A table entitled "Chemicals Used in Control
of Digesters," Table ll-4on page 2-38, lists
other chemicals used for pH control in
addition to chemicals used for other purposes.
In order to use the pH control chemicals, it is
helpful to know how to figure the. amount
needed based on the alkalinity expressed as
CaCO3.
Two factors must be considered, the equiv-
alent weight of the chemical and the percent
available. The following example shows how
to make the calculation using the information
given:
"Digester volume 250,000 gal.
Volatile acids (VA) 3,000 mg/l
Chemical bicarbonate of soda
Equiv. wt. from Table 84
% available from supplier 99%
CaCOs equiv. wt. 50
Alkalinity needed/lb. VA 0.833
1. Find pounds of volatile acids in the
digester:
3,000 x 8.34 x 0.25 = 6,255 Ib. (2837 kg) VA
2-Find pounds of alkalinity as CaCOo needed:
6,255 x 0.833 = 5,210 Ibs (2363 kg) CaC03
3. Find pounds 100% bicarbonate
(NaHC03) needed:
5,210 Ibs alkalinity as CaCC>3
''"• '' - '"" times'eqbiv- wt NaH'CQ3
: ' equiv. wt. CaCOs
equals Ibs. 100% NaHC03
84
5,210 times
,50
equals 8,753 Ibs. (3970 ks) 100% NaHCOs
4. The amount available is 99'% not 100%,
therefore:
Q 7<^+- 100%
8,753timesavai|ab|e0/0 =
=8,841 Libs. ( 4010kg) of 99'-% NaHC03
100
99
As a practical matter, the total amount
needed would not be added at one time but
rather spread out over three to four days in
equaj increments. Volatile acids, alkalinity
and pH should be monitored in the active
zone of the digester and records kept on the
progress toward recovery.
2-36
-------
Another way to estimate chemical dosage is
given in conjunction with Table 11-3 where
the percent concentration of acid in the
digester is used to find the appropriate
amount of neutralizing chemical.
Using the same information as in the previous
example and assuming liquid caustic soda
(NaOH) is to be used, the steps would be as
follows:
1. The pounds of acid were:
3,000 x 8.34 x 0.25 = 6,255 Ibs. (2838 kg) VA
2. Pounds per 100 gal Ions of digester volume:
Ib. VA _ 6,255 Ib. VA
vol. dig',/100 250,000/100 gal/
2.5 Ib. ,0.3 kg.
100 gal..1001
3. Read across from the column "pounds of
acid per 100 gal." that reads 2.5 to the
column "NaOH liquid caustic soda"
which reads 3.32 Ibs.
4. Find total number of pounds needed:
3.32 Ibs. NaOH
100'gal.
-x dig. cap. in gal.
x 250,000 = 8,300 Ibs. (3766 kg)
100
NaOH
Approximate amounts of other chemicals can
be determined by the same method using the
information in Table 11-3.
Note:
1 kg = 2.205 Ib.
11 =0.264 gal.
Actual
Ibs. of
Acid, per
100 gals.
.834
1.67
2.50
3.34
4.17
5.00
5.84
6.67
7.51
8.34
•16.71
25.10
33.51
41.96
50.42
84.5
171.3
TABLE 11-3
QUANTITIES OF VARIOUS ALKALIES
TO
NHs
Anydrous
Ammonia
Ibs.
.236
.472
.708 -
.944
: 1.18
1 .42
1.65
1.89
2.12
2.36
4.73
7.11
9.49
11 88
14.27
23.92
48.5
NEUTRALIZE
NH4OH
Aqua
Ammonia
gals.
.197
.216
.322
.429
.536
.645
.750
.859
.963
." 1.07
2.15
3.23
4.31
5.40
6.49
10.87
22.05
REQUIRED
VOLATILE ACIDS
Na2CO3
Anhydrous
Soda Ash
Ibs.
.736
1.47
2.21
2.94
3.68
4.43
5.14
•-' 5.89
6.61
7.36
14,74
22.16
29.58
36.84
... 44.48
74.56
1.51.17
NaOH
Liquid
Caustic
Soda, Ibs.
1.11
2.22
3.32
4.44
5.54
6.68
.- 7.76
8.88
9.96
11,10
22.24
33.42
44.60
55.84
67.06
112.42
227.96
NaOH
Flake
Caustic
Soda, Ibs.
.555
1.11
1.66
2.22
2.77
3.34
3.88
4.44
4.98
5.55
11.12
16.71
22.30
27.92
33.53
56.21
113.98
2-37
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-------
PART 3
POTPOURRI
MANPOWER REQUIREMENTS FOR ANAEROBIC DIGESTER OPERATION
AND MAINTENANCE
SAFETY
DIGESTER START-UP, INTERRUPTION AND CLEANING
TOXIC MATERIALS
CASE HISTORIES
GADGETS
-------
MANPOWER REQUIREMENTS FOR
ANAEROBIC DIGESTER OPERATION
AND MAINTENANCE
INTRODUCTION
Hiring of personnel, based solely on their
abilities for operation of anaerobic digesters,
is seldom done except in very large plants.
Nevertheless, a plant that has digesters must
also employ people with the necessary skill to
maintain good operation.
This section covers several aspects of staffing
for operation and maintenance of digesters.
The three areas of discussion include pro-
cedures for estimating time requirements,
job descriptions and some aspects of training
associated with digesters.
ESTIMATING TIME REQUIREMENTS
Three prime references compiled for EPA are
available for estimating manpower require-
ments for treatment plants. These include
sections that deal with anaerobic digestion
specifically and give man-hour estimates on
an annual basis.
The three publications are:
1. Estimating Staffing for Municipal Waste-
water Treatment Facilities, EPA Report,
Contract No. 68-01-0328 (March 1973).
2. Estimating Costs and Manpower Require-
ments for Conventional Wastewater Treat-
ment Facilities, EPA Report, Contract
No. 14-12-462 (October 1971).
3. Estimating Laboratory Needs for Munici-
pal Wastewater Treatment Facilities. EPA
Report, Contract No. 68-01-0328
(June 1973).
The first two publications use graphs to assist
in arriving at annual man-hours required for
various unit processes including digestion. The
first is most useful for plants from 0.5-25 mgd,
the second for 2-100 mgd.
A method is given in the first reference for
making estimates based on specific plant
conditions. The second publication breaks
manpower requirements into the areas of raw
sludge pumping, sludge holding tanks,
anaerobic digesters and sludge beds.
In considering the number of man-hours
required for a specific segment of the plant,
remember that personnel perform'dual func-
tions. The operator may pick up samples from
the final clarifier and check the condition of
the supernatant while making a general inspec-
tion of the plant. In doing this, it is difficult
to separate the number of hours per day spent
specifically on digestion. However, the esti-
mates will give some guidance on manpower
needs, particularly to operators facing plant
expansion which includes expanded digestion
facilities.
Generally, the activities to consider when
estimating time spent specifically on digester
operation and maintenance include:
1. Sample collection and analyses.
2. Equipment maintenance on units directly
associated with tank structures and
mechanical devices used in the digestion
process.
3. Operation activities associated with moni-
toring and/or changing raw sludge pump-
3-2
-------
ing, supernatant withdrawal, sludge recjr-
culation, etc. The Equipment and Process
Operation .Guides in Part" 2 of this manual
will help identify all the functions opera-
tors must perform.
Using information from the above three
sources to estimate time requirements for
operation and maintenance functions, each
operator should be able to arrive at informa-
tion specific to his own treatment plant.
JOB DESCRIPTIONS
Many of the functions performed by opera- -
tion personnel in other areas of the plant are
duplicated.in digester operation. In addition
to equipment surveillance and routine process
adjustments, an understanding of the biologi-
cal process is helpful to give a reason for the
changes that may be required.
Detailed job descriptions for specific occupa-
tions in treatment plants are listed in the EPA
publication, Estimating Costs and Manpower
Requirements for Conventional Wastewater
Treatment Facilities, pages 149-196 (cited in
the previous section).
Three major categories summarize the tasks
performed by personnel necessary for.most
plants. These are Operations Tasks, Mainten-
ance Tasks and Laboratory Tasks. A summary
from the EPA publication. Estimating Staff ing
for Municipal Wastewater Treatment Facilities,
pages E-1, E-2 and E-4, with specific adapta-
tions, presents a good overview of digester
personnel responsibilities. In, small plants,
part of, or all three functions may be done by '
one person while larger plants will, divide
them between several persons.
Operations Tasks
Included in these tasks are various activities
that are commonly identified with the
mechanics of plant operation. The following
are examples:
1.-Operation of process equipment, valves,
sludge pumps, mixers and boilers.
2. Cleaning of equipment, bar screens, and
other items necessary for proper unit
process function.
3. Taking sludge samples as required.
4. Operation of electrical controls
(timers, etc.).
5. Monitoring of gauges, meters and control
panels.
6. Monitoring of sludge and supernatant
quality.
Maintenance Tasks
Maintenance has been divided into two types:
preventive and corrective maintenance. These
can be defined as "what you do to keep
equipment from breaking (preventive), and
what you do to fix broken equipment (correc-
tive)." Some of the activities you might per-
form in both types of maintenance are the
following: . . .
1. Lubricate equipment and check for equip-
ment malfunctions.
2. Replace packing in sludge pumps, sludge
mixers, and gas and sludge valves. .
3. Service and replace bearings in motors and
other equipment.
4. Install and start up new equipment.
5. Clean out pipes (sludge lines).
6. Do some minor plumbing.
7. Do some welding and cutting.
8. Calibrate and repair meters, gauges, and
manometers (although this is sometimes
done by an electrician or by outside
contract).
3-3
-------
9. Set up and maintain a regular program of
lubrication and replacement of critical
parts (bearings).
10. Inspect and service mechanical and electri-
cal control systems (timers, level control-
lers, etc.)
11. Service and repair gas safety and control
devices.
12. Service, inspect and repair sludge heating
and mixing equipment.
Laboratory Tasks
In small plants, these tasks may be handled by
those spending time at either supervisory or
operations tasks. Thus, the supervisor might
also be the laboratory technician. Most of the
tests associated with digester control do not
require a high degree of skill, but do require
the ability to obtain repeatable results. Large
plants that run metals and nutrient tests will
require technically trained personnel. Some of
the activities involved in laboratory work are
the following:
1. Collecting samples (sewage-and receiving
water).
2. Performing laboratory analyses—both
simple and complex.
3. Assembling and reporting data obtained
from tests.
4. Evaluating data in terms of plant process
performance.
5. Preparing common chemical reagents and
bacteriological media.
6. Recommending process changes based on
laboratory data.
7. Reporting to regulatory agencies on the
operation of the plant.
TRAINING DIGESTER OPERATION
PERSONNEL
Preparing new personnel and upgrading exper-
ienced operators should be ongoing programs
in any plant. The resources available to the
facility will vary with geographical location
and size of community, but each individual
responsible for training his personnel should
review the following list to see which sugges-
tions are useful for his situation.
Publications
1. Operation and Maintenance Manual for
the Plant.
2. This Manual.
3. Operation of Wastewater Treatment
Plants, Chapter 8, California State Univer-
sity, Sacramento (6000 J Street, Sacra-
mento, California 95819) (1970).
4. Anaerobic Sludge Digestion—MOP 16,
Water Pollution Control Federation,
Washington, D.C. (1967).
5. Operation of Wastewater Treatment
Plants-MOP 11, Chapter 7, Water Pollur
tion Control Federation, Washington,
D.C. (1970).
6. McCarty, P.L., "Anaerobic Waste Treat-
ment Fundamentals—Parts I, II, III and
IV," Public Works, September: p. 107;
October: p. 123; November: p. 91;
December: p. 95 (1964).
7. Specific magazine articles are listed in
Appendix B.
Correspondence Courses
1. The manual listed as item 3 above may also
be used as the text for a correspondence
course. Information may be obtained
from the address noted.
3-4
-------
2. Information on a correspondence course :
prepared for operators in the State ,of
South Carolina may be obtained from:
Dr. John H. Austin
Environmental Systems Engineering
Clemson University
Clemson, South Carolina 29631
3. Information on correspondence courses
from the University of Michigan can be
obtained by writing:
University of Michigan
Department of Civil Engineering
Ann Arbor, Michigan 48104
4. Catalogs are available in state and local
areas from universities, colleges and
community colleges that give instructions
and details on correspondence courses.
Other Training Opportunities
1. State sponsored short schools—contact
state water pollution control regulatory
agency.
2. EPA—each regional office has a Manpower
Development and Training Officer with
information on training opportunities on
a regional level.
3. Local community college—many com-
munity colleges have credit or noncredit
courses that are offered either at night or
in the day time. Contact the registrar's
office for details.
3-5
-------
SAFETY
BASIC CAUTIONS
Sludge handling areas and equipment are
potentially among the most dangerous in a
wastewater treatment plant.
Plant operators should be thoroughly familiar
with the problem areas, the safety devices
that should be used, the precautions to take
and some general rules for working safely.
Pump rooms can accumulate combustible
gases, deplete oxygen in the air and be the site
of mechanical problems. Pump rooms should
be adequately ventilated and provided with
low-level oxygen alarms. Pumps should have
isolation valves on the suction and discharge
side for isolating the unit. Piping, connections
and equipment should be checked on a
frequent basis for leaks.
Dried sludge and powdered chemicals present
dust problems. Operators should wear goggles
and face-type breathing filters when working
with these compounds.
Methane gas is explosive when in contact with
air. Avoid mixing air with methane in the
range of from 20:1 to 5:1. Maintain a positive
pressure in all gas lines to prevent leakage of
air into the pipeline. Methane gas is also
produced from digested or partially digested
sludge, found in holding tanks. Therefore,
wherever gas may be present, there should be
no smoking, sparks or any open flame. Gas
detectors must always be used before entering
any empty digester.
Electrical installations, including light
switches, temporary devices or fixtures must
be of the explosionproof type.
Mechanical equipment should always have
machine guards in place. Operators must be
trained in their proper use and follow all
applicable safety rules.
MAINTENANCE SAFETY
The following rules apply at all times when-
ever working on equipment:
1. Lock out and tag main switch to prevent
accidental starting.
2. When working on pumps, be sure suction
and discharge valves are fully closed and
tagged. Be sure pump is vented and
drained.
3. Isolate fuel lines as applicable.
DANGER AREAS
Digester
When you must enter the digester, observe
the following basic rules for your protection.
1. Provide adequate ventilation to remove
gases and to supply oxygen. Be sure
exhaust fan is on.
2. Never enter the digester alone. Always
have someone to help in the event of
trouble.
3. Use safety harness equipped with safety
line.
4. Check for gases with explosimeter.
5. Be extremely careful about footing.
3-6
-------
6. Use bucket and rope to lower tools and
: equipment.
Laboratory Safety
The handling of wastewater and numerous
chemicals creates a potential hazard to the
health and safety of individuals in the lab.
Danger originates when lab workers fail to use
caution in handling these materials, fail to
read labels or fail to follow directions as to
use and procedure. There always exists the
possibility of inadvertent .or accidental spills
which will require immediate, specific and
correct-.action to minimize a potential hazard.
Inhalation of vapors must be avoided since
many chemicals or compounds are dangerous
•jnnhis respect. Most hazards caused in the lab
result from inattention, carelessness and poor
housekeeping. Some specific rules are listed
below:
1. Use chemicals with due respect. Know
their properties and how to use them.
2. Be sure each bottle or container is labeled
for contents, date, warnings, etc.
3. Read and follow directions carefully.
4. Arrange and store chemicals according to
•• poison, flammability, explosiveness, etc.,
.-•' and in proper areas. ,
':' 5. Use existing ventilation.
6. Wear proper clothing; i.e., rubber gloves,
aprons, safety glasses, etc.
7. Know the antidote - for poisonous
chemicals and keep these posted in lab.
8. When collecting samples, use appropriate
sample collecting devices.
9. Use the eye wash in the lab to flush
harmful chemicals accidently splashed on
the face and the emergency shower to
flush chemicals off other parts of the
body.
General Plant Safety
All personnel are to assume the responsibility
of keeping walking areas safe and free of
tools, debris, spills, grease, etc., checking to
see that guards are in place on operating
equipment, chain rails are in place and all
areas properly lighted. • •••••
Electrical Safety
1. Lock out and tag main switch of electrical
equipment before working on it.
2. Do not remove tag without first checking
with person who initialed the tag.
3. Notify plant superintendent in the event a
motor circuit breaker trips out.
4. Only trained plant personnel are to open
motor control center panels to perform
authorized work.
5. Report and log any unusual motor temp-
erature, noise, vibration, etc.
The safety material presented in this manual
is an incomplete summary of. general safety
procedures. All plant operators should review
their practices from time to time. One of the
best manuals on plant safety for operators is
Safety in Wastewater Works MOP No. 1, 1975
Edition published by the Water Pollution
Control Federation. .
The following charts summarize details associ-
ated with devices and their function in
digester safety.
3-7
-------
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Showers
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SAFETY RULES AND REGULATIONS
FOR THE PREVENTION OF ACCIDENTS
1. Protect your head! Wear a hard hat at all
times. Except in the office, lab or break
areas.
2. Prevent falling! Keep all areas clear and
clean.
o Pick up all loose objects, tools, trash,
ladders, hose, etc.
o Clean up all oil or grease spills
immediately.
3. Prevent body infections and disease!
o Do wash hands.
o Do wear gloves when working on or
with sewage equipment or collecting
samples.
o Do shower and change clothing
before going home.
4. Do use common sense when moving or
lifting heavy objects.
o Use proper equipment.
o Lift with your legs—not your back.
5. Do not RUN to answer the telephone!
6. Use handrails on stairways.
7. NEVER work on equipment without:
o Locking it out at push button or
circuit breaker.
o Tagging main circuit breaker.
8. Know where safety equipment is and
how to USE it!
9. Know locations of all fire extinguishers
and how to use them!
10. All injuries, even scratches or skin
abrasions, MUST be reported and first
aid given!
11. BE ALERT to safety conditions around
the plant!
If something is out of place or not work-
ing, fix it! Examples: light bulbs burned
out, safety chains not in place, padlocked
equipment not locked.
3-10
-------
DIGESTER STARTUP, INTERRUPTION
AND CLEANING
INTRODUCTION
Digester operation may be a difficult enough
procedure on a day-to-day basis; however, the
procedure is further complicated by the need
to start new digesters, shut down and clean an
existing unit or interrupt the operation of the
tank for mechanical or process reasons. This
chapter will discuss some of the methods used
by operators in each of the above situations.
STARTING A NEW DIGESTER
The goal for starting new digesters is to begin
reducing the volatile matter as soon as
possible and produce burnable gas under
stable operating conditions. A stable con-
dition usually means the proper volatile acids-
alkalinity ratio and near-neutral pH without
continued addition of chemicals. Several
factors enter into choosing the best startup
method. These include:
1. Availability of seed sludge (active solids
from a well operating digester).
2. Ability to control feed rate.
3. Type of feed.
4. Availability of other digesters and condi-
tion of their contents.
Several methods will be discussed briefly, as
follows:
o Single Digester—No Seed Available
o Multiple Digesters—No Seed Available
o Single Digester—Seed Available
o Multiple Digesters—Seed Available
o High Rate Digesters—No Seed
Single Digester—No Seed Available
1. Fill digester with raw sludge and sewage
by pumping continuously to level that
will cause a seal to be formed around.the
floating cover. In a fixed cover tank, fill
up to the supernatant overflow. This will
be defined as the operating level.
2. When the.tank.is full, begin heating the
contents to bring the temperature to
approximately 95 degrees Fahrenheit (35
degrees centigrade) as rapidly as possible.
3. Begin mixing and/or recirculating at
maximum, rate when operating level is
reached. .
4. Begin feeding raw sludge at a uniform
rate. Gradual addition over 24 hours is
preferable. Maximum feed rate would be
an even feed o/er an 8-hour period.
5. Records should be kept and results put in
a .graph form "for the following
information:
a. Quantity of Raw Sludge Fed
b. ,Raw Sludge, Total and Volatile Solids
c. Total and Volatile Solids of Digester,
Contents '
d. Volatile Acids, Alkalinity and pH of
Contents
e. Temperature, Gas Production,. C02
Content of Gas
The section on Data Review and Graphing
in Appendix G discusses the use of lab
results and graphing of data.
3-11
-------
6. At low feeding rates, it may be possible to
bring the tank into normal operation
without adding anything for pH control.
If VA/Alk ratio rises to 0.8 or more and
pH is below 6.5, addition of some buffer-
ing agent such as lime or soda ash should
be considered. The amounts and condi-
tions for use are discussed in the section
entitled Chemical Usage in Digester
Control, page 2-33.
7. Fairly stable conditions should be reached
in 30 to 40 days if loadings do not exceed
0.06 Ib./day/cu. ft. (0.96 kg/day/m3).
The addition of chemicals for pH control is a
hotly debated subject among operation per-
sonnel. However, chemicals can be the means
of preventing digester failure if used properly.
If the operator is in a location where chemicals
are not available or has definite reservations
about their use, there are alternatives
available. Feed as much sludge as the digester
will handle without going below pH 6.5
and/or above VA/Alk ratio of 0.8 and:
1. Haul the balance to another treatment
plant in a tank truck or septic tank pump
truck, or,
2. If aeration capacity is available,
aerobically digest the extra sludge. The
stabilized sludge may be added later, or, if
volatile content is reduced to approxi-
mately 60 percent, it may be disposed of
directly on drying beds or applied to
other land disposal sites, if available.
Multiple Digesters—No Seed Available
Follow the procedure cited in Single
Digester—No Seed Available, except both
tanks should be filled with sewage. Raw
sludge may be fed to the primary, letting the
supernatant transfer to the secondary. The
secondary may be used as a means of keeping
the loading to the primary low [less than 0.06
Ib./cu.ft./day (0.96 kg/m3/day)] as follows:
1. Ten to twenty percent of the raw sludge
3-12
may be directed to the secondary for
several weeks or more if necessary. If,
after two to three weeks, the bottom
sludge from the secondary has higher
alkalinity and pH than the primary, this
may be recycled back to the primary to
act as a buffer.
2. As the primary approaches stable condi-
tion, more bottom secondary sludge may
be -recycled back to the primary to
increase the buffering capacity and
increase gas production.
Single Digester— Seed Available
If sufficient buffered seed sludge is available
from a nearby treatment plant, this can
reduce the start-up time to two weeks or less.
The amount of seed required is about 20
times the anticipated volatile solids in the raw
sludge. Example: If it is estimated that the
raw sludge will be about 1,000 gallons per day
(3785 I per day) at 4 percent solids and 80
percent volatile, then the amount of volatile.
solids in the feed would be:
0.04 x 1,000x0.8x8.34 = 270lbs (122 kg)
Thus, 270 times 20 equals 5,400 pounds
(2450 kg) of volatile solids would be needed
as seed sludge.
If the seed sludge averages 5 percent solids
arid 50 percent volatile, the amount to be;
hauled would be:
°approx. 25.900 gal.
0.05
x 0.5
The procedure would be as follows:
1. Haul seed sludge and transfer into digester
directly, if possible.
2. Fill tank with sewage and follow steps 2
through 6 of Single Digester—No Seed
Available.
-------
3. No chemicals should be necessary if raw
sludge is fed evenly. However, if records
show buffering is needed, some sludge
might be hauled to the plant where the seed
was obtained while more seed is hauled to
replace sludge drawn out.
Multiple Digesters—Seed Available
In a new plant with multiple tanks, starting
up with seed available from another .plant is
essentially the same as under Single Digester-
Seed Available. The same recycle capability as
discussed under Multiple Digesters-No Seed
Available should be applied. . , .-.
With the availability of more than one tank, it
is possible to distribute loading between tanks
to control the seasoning process of the
primary tanks. Also, stable conditions may be
established more rapidly, and future startups
can be accomplished using seed from an
existing active digester. • •--. ;
High-Rate Digesters-No Seed
The term "high-rate" generally refers to the
rate of loading and/or detention,time of the
tanks. Generally, detention times of 10 to 15
days are considered in the high-rate range.
Startup can be accomplished in 30 to 60,days
using the method detailed under Single
Digester-No Seed Available, if mixing is.
continuous and pH is adjusted and rnamtained
in the range of 6.8 to 7.2. Feed rates should
be maintained that would allow a hydraulic
detention time of not less than 20 days until
normal levels of gas production are reached.
Some general rules that apply in all cases of
startup are noted below:
1. When the desired temperature is reached,
it should be held within plus or minus one
degree Fahrenheit continuously.
2. Maximum mixing should be used to keep
the surface material in contact with the
bacteria and prevent raw sludge pockets
from collecting in the tank bottom.
3..Foaming may result if too much feed is
added and mixing is not adequate. If it is
not possible to mix continuously, mix
during and after feeding.
4. Gas production should start to increase
within a few days of startup and rise
rapidly as volatile solids decrease. Gas
production normally will reach 'some
- maximum point, then decrease to a stable
:, level. --.-••
5. Best results in startup will be obtained if
volatile acids, alkalinity, pH, loading and
gas production, are monitored daily in
large plants, twice weekly in small 'plants
and .results plotted on a graph. Trends can
be noted and corrections made before
problems develop. .
INTERRUPTION OF NORMAL PROCESS
WITHOUT DRAINING
There are times when digester operation must
be interrupted for varying periods of time
when it is neither possible nor desirable to
empty the tank. When this is necessary,
certain precautions heed to be taken and
procedures • followed when putting the
digester back in operation. Several situations
are described:
o Mechanical Repairs
o Temperature Loss
o Hydraulic Washout
o Organic Overload
o Toxic Loading
Mechanical Repairs
Pumping equipment failure may interrupt the
process for several hours or up to a period of
several days. Mixing and heating should be
continued even though feed to the digester
has been stopped. Sludge may be stored in the
clarifier for 24 to 48 hours in warm weather
or longer in cold weather.
3-13
-------
When normal feeding is resumed, care must be
taken not to slug the digester. Restarting will
be assisted by maximum mixing. Frequent
monitoring of volatile acids will indicate di-
gester reaction to restarting. High feeding
rates should be spread out over as long a time
period as possible.
Repairs to equipment may require opening
access holes in the digester dome. In that
case, precautions against explosion and poor
breathing conditions must be taken. When the
digester is producing gas, pressures above
atmospheric normally will prevent air from
entering the tank. However, the first gas
removed from the tank following the resump-
tion of operation should be vented to the
atmosphere for two to three hours before
ignition.
Temperature Loss
When normal heating is interrupted and the
tank contents cool down, gas production will
normally drop off. The following items
should be considered in this situation.
1. Feed rate should be kept as low as
possible.
2. Only the thickest sludge should be
pumped. (Clarifier scum, if it is normally
fed to the digester, might be removed and
buried until operation has resumed.)
3. Mixing should be confined to the lower
portion of the tank to prevent heat loss
through the dome.
4. When normal operation has resumed, and
temperature loss is not more than 10 or
15 degrees, bring heat up at about one to
two degrees per day and maintain mixing
at the maximum rate. '
5. Monitor volatile acids and be prepared for
high gas production and possible foaming
as a result of available food digesting at
faster reaction rates. Correct pH by recir-
.culating from the bottom of another
3-14
digester or with chemicals if necessary.
Hydraulic Washout
This occurs when abnormal amounts of thin
sludge are received due to industrial waste, or
accidental overpumping. Digester contents are
replaced with water. The buffering capacity
of the contents is lowered and temperature
may be reduced.
The digester may react similarly to start-up
conditions and the same procedures may be
followed to bring it back to normal.
1. Recirculate sludge from an active, well
buffered digester, if possible, to bring
buffering capacity back to normal.
2. Restore temperature to normal at 1-2
degrees per day.
3. Be prepared to correct pH if necessary.
4. Monitor a.nd record results of the indi-
cator tests. (These are discussed in Part
IV, page 4-r27 and Part III, page 3-28.)
5. Keep feed rate as low as possible.
Organic Overload
Organic overload may result from solids that
settle in the sewer system and are carried to
the plant during rain storms. Additionally,
some industrial wastes cause increased organic
loads. The procedures followed are the same
as for hydraulic problems but more care must
be taken to prevent foaming. Mixing from the
top to the bottom will minimize this problem.
Recycling from the active zone or from the
bottom of the secondary digester may speed
recovery and prevent further problems.
Toxic Loading
Various materials may be toxic to the digester
and these are discussed in detail in Part IV,
page 4-20 and Appendix G. Two methods of
preventing problems or restoring normal oper-
ation more rapidly are considered:
-------
1. The preferred method is to isolate the
toxic material in the primary clarifierand
either neutralize it or haul it out of the
plant for disposal.
2. The best procedure to follow in case toxic
material is discharged to the digester before
discovery is to stop the addition of raw
sludge and recycle sludge from another
digester back to the affected tank. If no
sludge is available for recycle, pump in
heated sewage to dilute and displace the
tank contents. Dilution with hot sewage
; pumped through the'heat exchanger will
maintain the 'digester temperature while
reducing the concentration of the toxic
'-.- substance.
CLEANING OF DIGESTERS
Operators from various plants were question-
ed on the frequency of tank cleaning. Answers
ranged from every other year to "I've been
here for nigh on to 20 years and never cleaned
the thing:" Most of those who set up pro-
cedures for regular cleaning find that, opera-
tion is more efficient and mechanical
problems are reduced. The most frequent
cleaning interval of those contacted was
approximately three years.
When to clean the tank depends on a number
of things which include:
1. Grease accumulation and efficiency of
grease removal.
2. Grit accumulation and grit removal
efficiency.
3. Types of waste treated.
4. Efficiency of mixing. ;
5. Structure of the tank.
6. Condition of the internal equipment.
7. Alternative ways of treating sludge when.
the tank is out of service.
Part IV, page 4-22, describes the method for
determining the amount of nondigesting ma-
terial' in the tank "that is reducing the available
space for digestion. Information is also given
beginning on page 2-8 for determining the
equipment conditions.
When it is decided that cleaning will be
done, other factors must be considered.
The following questions should be answered
in preparation for the cleaning operation:
1. What will be done with' raw sludge while
the tank is out of service? ..•:.-
2, Will the job be done using plant person-
nel, outside contractors, or both?
3. What equipment is: available for accom-
plishing the job?
4. Where will digested sludge be disposed of? .
Some possible answers to these questions will
be considered in the remainder of this section.
The answer to the first depends largely on the
availability of more than one tank. For plants
using single tanks, this can pose some difficult
problems which will be considered below:
o Single Digester Plants
o •• Multiple Tank Plants
Single Digester Plants
The. major problem with cleaning single
digester plants is What to do with raw sludge
during the cleaning operation. Several alter-
natives are possible:
1, Concentrate as much as possible and haul
by tank truck to ;a nearby treatment plant.
If tank trucks are not available, septic tank
pumpers might be used. Costs for the ser-
vices-would be approximately $1.00/100
gal. (based on 1975 cost data), however,
some metropolitan areas may run as high
as $3-5/100 gaL
2. In activated sludge plants, an available
3-15
-------
aeration tank might be converted to a
temporary aerobic digester. The sludge
can be fed back into the digester over a
period of time when operation resumes.
3. A primary clarifier might be converted to
a temporary digester if provisions are
made for covering with heavy-ply vinyl
material and odor control measures are
taken.
4. If sufficient land is available and regula-
tory agency approvals are obtained, a
temporary anaerobic lagoon might be set
up. This would entail constructing a
watertight lagoon or converting an exist-
ing tank into a holding pond, filling par-
tially with water and pumping in the
sludges that accumulate. Heating might
be done with a portable steam cleaner.
5. Lime or other caustic may be used as a
stabilization procedure during transfer or
as an aid for odor control.
Multiple Tank Plants
Plants with more than one tank are able to
distribute feed to other primary tanks or, if
there is only one primary and one secondary,
the secondary can be used to receive raw
sludge.
When one or more tanks are taken out of
service, the remaining units will receive higher
loads. Closer control will be necessary and ade-
quate mixing must prevail. Where the second-
ary is used to receive raw sludge, recirculation
and/or mixing must be practiced.
In-House or Contract?
Another important consideration is determin-
ing who will, do the work. Several nationwide
firms are actively soliciting work for cleaning
digesters and guarantee completion within a
specific time period. For a small town with
limited facilities or plants that require mini-
mum down time,this is an attractive possibility.
Advantages of using plant personnel include
^heir familiarity with piping^and their capabil-
ity to work the operation into the regular
schedule. If a plant normally operates a tank
truck for sludge disposal, it may be used in the
cleaning operation.
To make the decision between using in-plant
forces or contracting the job,out, the fol-
lowing should be considered:
1. The amount of time the tank can be down.
2. Type of equipment available on-site.
3. Costs of rental equipment.
4. Number of man-hours available from the
plant or municipality staff.
5. Disposal site and how sludge will be
transported.
An example of performing a job by plant per-
sonnel and contracting out is given on
page 3-27.
BASIC EQUIPMENT NEEDED FOR
CLEANING
The simplest equipment for digester cleaning
is an open valve to a drying bed and a wash-
down hose. The most extensive might include
a crane to lift the cover off the digester and a
clamshell to scoop the thick sludge out to be
hauled away in dump trucks. Most plants will
fall between these extremes and the following
list shows the types of equipment which may
be used in the emptying and cleaning proce-
dure. It may serve as a check list for the oper-
ator when preparing for the job of emptying
and cleaning the tank. Not all the items would
be used in every job but these are presented
to give the person responsible a quick method
for reviewing those which apply to his parti-
cular plant.
1. Sludge line valves. Must be free to operate,
not obstructed, and accessible.
3-16
-------
2. Sludge lines (permanent). Should be free
of obstructions, may require rodding
and/or flushing either before,'during or
. after the operation. '
3. Sludge lines (temporary). May be alumi-
num, plastic, steel or heavy duty hose.
Should'have tight joints and quick coup-
lings, if possible. Minimum size 4" (100
mm); 6" (150 mm) and larger are preferred.
(Might be rented from an equipment rental
firm, a farm irrigation pipe company, or
other sources. Fire departments sometimes
have hose available that is being phased
out of water service:)
4. Access to inside of digester. Manholes or
hatch covers should be available to allow
washdown water to be added and men to
enter the tank for final cleanup.
5. Explosionproof ventilation .fans. Used to
exhaust harmful gases and to supply air
for breathing when personnel are inside
the tank (might be obtained from a fire
department).
6. Explosion meter. Used to monitor atmos-
phere inside the digester. Must be used.to
verify safety of entrance into tank (might
be obtained from, a fire department).
7. Ladder with safety apparatus. Manhole
rungs built into tank walls may be deteri-
orated and should.be avoided. A sturdy
ladder with protection from slipping and
tipping should be used.
8. Self-contained breathing apparatus. Used
any tirne work is done before a safe atmos-
phere exists and whenever entrance into
the tank is necessary. . .
9. Safety harness. Used when entrance is nec-
essary while wearing safety self-contained
breathing apparatus and entering a tank
partially filled with sludge.
10. Explosionproof lights. Used inside the
tank at any time.
11. Source of water. Should be air-gapped and
capable of supplying a pressure in excess
of 60 psi '{400 kN/m2) (up to 100 psi
(700 kN/m2) is preferred).
12. Washdown water,hose. Should be 1-inch
(25 mm) or larger.
13. Fire hose type nozzle'with shutoff. Should
be 1-inch (25 mm) or larger. ••••.-.••
14. Auxiliary washdown'water pump. May be
/used when other suitable sources are not
available. The pump can take suction'/from
the final effluent; precautions must then
/ be ta.ken to warn workmen that it is non-
potable water (Fire departments may have
•" surplus hose'available.)
15. Sludge pumps'(fixed). The. operator should
•review pump and piping arrangements for
emptying the- -digester. Recirculation
" pumps may be'used for part of the drain-
ing procedure.' Generally, these pumps have
:..;• a higher volume output than positive dis-
placement pumps used-for heavy solids
handling.
16: Sludge pumps (portable). Portable pumps
' may be placed in several configurations
and may'be positive displacement or cen-
trifugal .with special adapted impellers.
Pump motor ratings should be checked
for compatibility with available power
; sources (single-phase, three-phase, etc.).
- '-They may be temporarily wire'd in.to
circuits used by: equipment that will be
•"•••' -out of service during the cleaning opera-
tion, such as mixers, recirculation pumps,
etc. (Pumps may be on-site, borrowed
from nearby plants, or rented.; In some
cases where several tanks are involved, it
'may pay to purchase a portable dewater-
:- 'ing pump.) The following' are several
possible configurations for pumps:1--;"
'a. -Mounte'd in-lih'e, to pull from existing
- suction line and discharge through
., ' - fixed discharge.
3-17
-------
b. Mounted in-line, to pull from existing
suction and discharge through a
portable line.
c. Mounted at some location to take
suction from portable suction line and
pump into disposal site or portable
discharge line. Motors or engines must
be explosionproof if located near
openings in the digester cover. (Gas
driven pumps must be located so that
fumes are not drawn into the tanks
through ventilation systems when
people are inside the tank.)
d. Open impeller pumps, with special
cutter blades and explosionproof
motors may be lowered into the tank
and flexible discharge hose attached
to temporary or permanent discharge
lines. These may be submersible or
nonsubmersible types.
Normally, the pump is lowered
through the manhole using a tripod or
other safe and controlled method of
keeping the pump at the desired level.
e. Positive displacement or centrifugal
pumps may be lowered into the tanks
and placed on a temporary platform
or tank floor to provide minimum
suction. Discharge may be either,
through fixed or portable lines.
17. Turret nozzle. Firefighting equipment
supply houses have special turret nozzle
apparatuses that may be adapted for use
in digesters. These may be programmed to
direct high pressure water to localized
portions of the tank automatically. These
are particularly useful to plants with more
than two digesters and where cleaning is
done at two- to three-year intervals.
18. Tripod or hoist. These may be mounted
on the tank roof above the manhole to
facilitate removal of equipment, and rais-
ing or lowering the dewatering pump (it
might be rented or obtained from a ma-
chine shop or other municipal depart-
ment).
19. Tank truck. If this is not a part of on-site
equipment, rental companies in larger
cities may be able to .provide this equip-
ment. Charges are based on distance to
disposal site and type of material hauled.
Contracts might be made with septic
tank pumping firms as well. Nearby treat-
ment plants may also have equipment
available. Care must be taken to provide
means of handling grit, scum and other
debris that might not normally be in
sludge hauled from operating tanks under
day-to-day conditions.
20. Crane. Mechanized hoist vehicles of vari-
ous sizes might be used in the cleaning
operation. In extreme cases of heavy grit
and/or scum accumulations, it may be
necessary to remove floating covers and
use a clamshell to remove digester con-
tents. Cranes for this operation are
normally rented. Care must be taken in
removing floating covers to lift them by
specific and structurally defined points in
order to prevent their collapse. Refer to
the tank fabrication or contractor's draw-
ings and contact the supplier to make sure
that the correct lift points are used.
The preceding list is summarized in Table 111-1
with columns designating equipment that
might be used if sludge is handled by:
Column Conditions
A Gravity Flow to an On-site Disposal Area.
B Pump to an On-site Disposal Area.
C Pump to Tank Truck for Off-site Disposal.
D Solidified Solids, Heavy Grease and Grit
Accumulation.
The operator can use the blank column to
check those items that apply to his plant and
may also find additional items that are
specific to his plant.
3-18
-------
Table 111-1
DIGESTER CLEANING CHECK LIST
.-."',-; . "-$^'' ' _ •
A B C D Other
1. Sludge line valves
2. Sludge line (permanent)
3. Sludge line (temporary)
4. Digester access
5. Explosionproof vent fan .
6. Explosion meter
7. Safe ladder
8. Self-contained breathing apparatus
9. Safety harness
10. Explosionproof lights
11. Water source
12. Wash down hose .
13. Nozzle with shutoff
14. Wash water pump
15. Fixed sludge pump
16. Portable sludge pump
17. Turret nozzle
18. Tripod or hoist
19. Tank truck
20. Crane "
X
X
X
X
X
X
X
X
X
X
X
X
X
0
o
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
0
0
0
o
o
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-X
X
X
X
X
X
X
X = definitely needed
O = possibly needed
A Gravity Flow to an On-site Disposal Area.
B Pump to an On-site Disposal Area.
C Pump to Tank Truck for Off-site Disposal.
D Solidified Solids, Heavy Grease and Grit
Accumulation.
SAFETY PRECAUTIONS
Precautions must be observed to prevent the
following:
1. Falls (use of safe ladders; harness, etc.).
2. Infection (basic hygiene and protection of
open cuts).
3. Injuries during use of equipment (staying
clear of moving equipment, staying out
from under objects overhead, and wearing
a safety helmet).
4. Asphyxiation or suffocation (testing at-
mosphere for oxygen content and use of
breathing apparatus).
5, Explosions (testing for explosive condi-
tions, use of spark-free equipment and
explosionproof motors).
In,conjunction with item 5, the operator must
keep in mind the fact that as sludge is pulled
out of the tank that air will be pulled in, either
through open hatches or through the vacuum
relief. The most explosive condition exists
when the methane-air mixture is such that
methane is between 5-20%. This is the reason
the atmosphere in the tank should be moni-
tored and any equipment that could cause
sparking must not be used.
When the level of a fixed-cover tank is lower-
ed as when sludge is pulled out for disposal,
the gas from another tank should be allowed
3-19
-------
to equalize in the tank. If air is admitted, the
gas system should be isolated and vented to
the atmosphere to prevent pulling air into the
entire multiple tank system.
There must be at least two men on top of a
digester for every man inside the tank to re-
move the worker in case of emergency.
Each job will have special problems which re-
quire thinking ahead about safety of individ-
uals and equipment used on the job. Neces-
sary safety considerations must also be in-
cluded with the list of materials, equipment
and procedures which are developed for the
job.
GENERAL INFORMATION
Following are some general observations from
experience of operators in cleaning tanks
which may be of use to personnel working on
their first unit.
I.The first consideration is to prepare
plans for the disposal of digester contents
as well as wash water.
2. The approximate volume of water needed
to move the solids to the disposal point
(either on-site or to a tank truck) is from
two to four times the volume of the di-
gester. As an example, a digester contain-
ing 200,000 gallons may require an addi-
tional 400,000 to 800,000 gallons of wash
water to move the contents to the dispos-
al site.
3. Commercial haulers generally charge
$1.00/100 gallons for hauls for 20 miles
or less for disposing of liquid sludge.
This will vary from locality to locality.
4. Contractors. for cleaning digesters give
free estimates and costs may range up-
ward from $2.00/100 gallons for cleaning
the tank and disposing of the sludge on
site. Higher costs are necessary if the
sludge must be disposed of off site.
5. Little information is available on the use
of catch-basin cleaning equipment, but
this might be used by enterprising opera-
tors for digester cleaning.
3-20
-------
TOXIC MATERIALS
INTRODUCTION
Toxic materials entering a digester are those
elements or components which cause the
bacteria to slow down or which kill them.
Most plants in the United States today have
potential toxicity problems, even those plants
serving domestic wastes only. Sources of these
problems come from accidental spills of
petroleum products such as fuel oil, auto-
motive greases and oils, etc. Other plants
serving communities connected to various
industrial facilities will have potential toxic
problems unique to the industrial area.
Typical toxic materials include:
o Heavy metals discharged by metal plating
firms, jewelry manufacturers, tanneries,
aircraft manufacturers, etc.
o Sulfides from metal manufacturers, mara-
schino cherry producers, salt water
intrusions, coal mines, and others.
o Phenols and plastic resins from petroleum
wastes, furniture manufacturing plants,
paint manufacturers or users, and coal tar
and gas producers.
o Ammonia from overloading digesters with
proteinous wastes.
o Cyanide wastes • from metallurgical
processes.
o Chemicals from chemical manufacturers.
o Insecticides and fungicides.
GENERAL PLANS AND PROCEDURES
Plants exposed to potential toxic waste dis-
charges will need to develop plans and proce-
dures to identify all potential sources, to pre-
vent these wastes from entering a digester
in toxic concentrations, and. to implement
emergency response programs. The plan
should include sufficient laboratory equip-
ment and staffing to perform monitoring and
identification procedures. The laboratory staff
requirements will vary from plant to plant de-
pending on frequency, variability and com-
plexity of toxic waste discharges. Staffing re-
quirements will also depend on how effective
the industrial waste discharge ordinances.are.
Depending on need, the necessary lab work
may be performed by:
1. A lab group headed by a degreed chemist.
2. A lab technician.
3. Outside sources such as: ,
Community colleges
• High school science departments
Industrial chemists
Most problems result from the too common
practice of dumping concentrated solutions of
these toxic materials. Toxic materials entering
a digester are those elements or compounds
which will produce an inhibitory effect lead-
ing to a,bacterial kill. The best operating plan
for any plant is to prevent these materials
from entering a digester. The penalty of a
digester failure caused by toxicity is a severe
one—emptying the digester, disposing of its
contents and starting all over again.
3-21
-------
The most frequent cause for toxic conditions
is a slug dose of some toxic material. The only
real and effective cure is prevention. Keep
toxic materials out of the system. A good in-
dustrial waste ordinance which is enforced
will help here.
PREVENTION
If toxic concentrations of toxic materials are
likely to occur, the plant operator should take
the following steps:
1. Set up an industrial inventory, cataloging
all connected industries with their types
and volumes of wastes. This should
include normal concentrations and po-
tential for accidental spills, cleaning or
similar discharges.
2. Establish and enforce industrial waste
ordinances specifically prohibiting certain
untreated toxic discharges.
3. Establish an early warning system for the
industry to notify the treatment plant
when accidental discharges occur. This
means a good public relations program.
It's hard to do and hard to get coopera-
tion, but it is essential! Show plant
owners or managers what the wastes are
doing to the treatment processes.
4. Establish a training program to teach
operators how to look for the presence of
toxic materials in the plant's influent.
5. Establish a laboratory sampling and
testing program to monitor industrial
waste discharges.
6. Use holding tanks to contain the toxic
wastes. In many plants, this may be
difficult to do. However, take a close look
at all of the possibilities. You may find a
way to isolate the suspected toxic
material in the primary clarifier; this is a
particularly good solution if you have
several primary clarifiers.
7. Preplan actions to respond to a toxic
waste. For example:
a. Isolate and hold the waste (see item 6
above).
b. Dilute the waste below the toxic
level by:
o Using seed sludge from a second-
ary digester. NOTE: this action
tends to spread the toxic material
to both tanks and both tanks may
be affected.
o Using dilution water.
c. Forming an insoluble precipitate.
d. Using another compound which will
react with the toxic element to form
less harmful compounds. These are
called antagonistic compounds
because they neutralize the toxic
effect.
8. If the plant lacks the necessary lab equip-
ment or personnel to investigate and
identify toxic materials, use any available
resources to do the tests. Other resources
include:
a. High school science instructors and
laboratories
b. Community college instructors and
laboratories
c. Commercial laboratories
9. Improve digester operation—the tolerance
level of digesters to toxic materials is
increased when digesters are in a healthy
condition. This means a pH of 6.8 to 7.2
and plenty of buffering alkalinity.
The rest of this section discusses different
kinds of toxic substances and some of the
things that can be done to control their
effects on the process.
3-22
-------
Alkali and Alkaline Earth Salt Toxicity
Ammonia Toxicity
Inhibitory concentrations of sodium, potas-
sium, calcium and magnesium normally come
from industrial wastes, but sometimes the
operator causes them by overcorrecting a pH
problem. The following table lists the effects
of different concentrations of these salts on
digesting sludge.
Table 111-2
STIMULATORY AND INHIBITORY
CONCENTRATIONS OF ALKALI
AND ALKALINE-EARTH CATIONS
Cation
Concentrations in mg/l
Moderately Strongly
Stimulatory Inhibitory Inhibitory
Sodium 100-200
Potassium 200-400
Calcium 100-200
Magnesium 75-150
3500-5500
2500-4500
2500-4500
1000-1500
8,000
12,000
8,000
3,000
Stimulatory concentrations are desirable
because they improve the process and are
often used for control. Moderately inhibitory
concentrations can be tolerated except when
introduced in slug doses. Process recovery
may take up to a week. Strongly inhibitory
concentrations cannot be tolerated. Usually
the digester must be emptied and restarted
because the process cannot recover.
If one of these elements is present in toxic
concentrations, it can be controlled by adding
one of the others as an antagonistic element.
In an article appearing in Public Works,
November 1964, Perry L. McCarty reports,
"For example, if 7,000 mg/l of sodium were
present it is possible to add 300 mg/l of
potassium which will reduce the toxic effect
by 80 percent. Then, if 150 mg/l of calcium
were added, the toxic effect would be elimi-
nated. Antagonists are best added as chloride
salts. If these .are not available or are too
costly, the best method would be dilution."
Concentrations of ammonia between 1,500
and 3,000 mg/l can be inhibitory if the
digester pH is greater than 7.4. Under these
conditions, however, the volatile acid pro-
duction will increase, decreasing the pH.
This may relieve the inhibitory effect. The
volatile acid concentration will remain at a
high level unless the pH is reduced by adding
hydrochloric acid (HCI). The HCI should be
added until the pH is about.7.0. Add the HCI
slowly to keep the pH from going below 7.0.
At concentrations above 3,000 mg/l, ammonia
becomes toxic enough to cause digester fail-
ure regardless of pH. The best remedy is to
dilute the digester by withdrawing supernatant
or digested sludges and adding settled sewage.
Sulfide Toxicity
Su If ides in a: digester come from trie'normal
amounts contained in domestic wastewater,
from metallurgical industries as sulfate salts,
and from anaerobic protein decomposition.
They are in two forms: insoluble heavy metal
sulfides that precipitate from the solution and
are not harmful, and soluble sulfides. Like
oxygen, there is a demand for soluble sulfur,
which is necessary for bacterial growth. The
bacteria can tolerate between 50-100 mg/l of
soluble sulfide with little effect. They can
handle up to 200 mg/l of soluble sulfides in a
continuous operation. Concentrations above
200 mg/l are toxic and must be treated.
Treatment consists of:
1. Using iron salts to precipitate sulfides.
2. Dilution of the waste.
3. Identifying, separating, and containing the
toxic waste streams.
Heavy Metal Toxicity
Copper, nickel and zinc salts are soluble and
are toxic in low concentrations. The tolerable
concentration level of these soluble salts
3-23
-------
depends on the concentration of su If ides in
the waste stream. The best method to treat
these metals is to add sulfur in the form of
sulfuric acid or sulfides. When using these, the
operator must be careful not to create a toxic
sulfide level by adding too much. Ferric
sulfate, commonly called "Ferrisul" may be
added to counteract this and also to build up
a sulfide reservoir.
J. W. Masselli and others, in an article appear-
ing in the August 1967 Water Pollution
Control Federation Journal, report the use of
sulfuric acid for treating metal toxicity.
Sulfuric acid may be purchased in several
grades. The cheapest grade is recommended
for this use and is designated as Technical
Grade 70-90 percent. This acid is generally
available from commercial chemical supply
firms in 55-gallon (208 I) drums.
Those plants desiring to use sulfuric acid
should contact the supply firm and seek their
advice on application. Some of the cautions
and procedures are given below.
CAUTION: Sulfuric acid should be handled
with care using procedures marked on the
containers. These include:
1. Wearing rubber gloves and goggles.
2. Not getting the solution on clothing or
skin.
3. Not pouring water into the acid. A re-
action much like pouring water into a
hot pan of bacon grease will result.
The following procedure is outlined for
application of the sulfuric acid:
1.lf pH is below 7.2, add 11 pounds
(24.3 kg) of sodium hydroxide (caustic
soda) for each gallon of acid used.
2. Add sulfuric acid in daily doses of
1 gallon/10,000 gallons (11/100001) in
the digester. This adds a concentration of
176 mg/l of acid (173 mg/l sulfate) which
will be reduced to 58 mg/l of sulfide.
Continue daily doses for three to ten
days.
CAUTION: Do not add water to sulfuric
acid!
3. Make sulfide analysis on the digester gas
before each daily dose. When the first
trace (0.5 mg/l) appears, stop the daily
acid additions.
4. This treatment should be effective for up
to three months.
5. If continuous treatment is desired, use
Ferrisul at daily doses of 1 lb./1,000
gallons (120 g/1000 I) in the digester.
Two other methods may be used if a suitable
sulfur compound is not available.
1. Raise the pH contents of the digester to
8.0 by adding soda ash or sodium bicar-
bonate. Sodium hydroxide may be used,
but it may cause the carbon dioxide to go
into solution. Watch the C02 gas
production.
2. Transfer secondary digester sludge to the
primary to dilute the incoming sludge and
thus reduce the heavy metal
concentration.
More information on the subject of toxicity
will be found on Troubleshooting Guide 14 in
Part I.
3-24
-------
CASE HISTORIES
LOADING
Controlling Waste Activated Sludge Load to a
Digester -....-„ - *•• .,
A low solids, ,c,pncentralion,,Jn the digester
feed .caused..detention "time!: problems at a
5 nigd activated sludge'plant treating wastes
from" an industry producing' corn'chips. This
was a result of mixing waste -activated with
the raw'sludged;The::probl'em'was solved by
converting one-:of'the :tw<>- primary !clarifiers
to a thickener. All of the,waste activated
sludge was "then--diverted to the hew thick-
ener.'The thickened waste activated sludge is
then separately Digested' in-one primary and
one secondary'digester while raw sludge is
treated in another- -p&ir of digesters.'By
prethickening, the waste activated sludge^was
concentrated to approximately 3.3 percent
solids and' with' separate vdjges'tion the
digestion time was increased allowing -both
systems to function efficiently.. ;
Use of Soda Ash to Control Organic
Overloading
Vegetable processing plants seasonally cause
over 100 percent increase in the amount of
sludge handled at one plant. The operators
daily monitor the volatile acids and alkalinity
ratio for digester control during the pro-
cessing season. When the ratio climbs above
0.25, soda ash is added to bring it back into
control. As one example, when the ratio
reached 0,25, 500 pounds of soda ash were
added and then, seven days later when the
ratio again approached 0.25, 1,500 pounds
were added. Following these two additions,
the ratio dropped back down to jess than .1
and gas production increased to its, previous
level,. . . , .
Hydraulic Overload Control by Using Polymer
A 10 mgd primary plant was hydraulically
overloaded and detention times were less than
design. * , •
A program was implemented to decrease the
volume of sludge being fed to the digester.
This .was accomplished by adding polymer to
the thickener at about 0-2 milligrams per liter
dosage to reduce the volume of sludge being
pumped. The polymer used was Zimmite
No. 651.
Grit Removal in a Single Stage Digester
In a plant that.was handling twice,its design
load, a single stage digester finally failed to
operate due to a thick., scum blanket and
accumulation of grit. This, plant operator
corrected the problem by opening all possible
openings, such as manhole covers and sample
vents, and allowing the digester to sit idle
with no recirculation. The scum blanket
formed a cover thick enough to prevent odors
in the area. • ' .
In order to move excess grit from the bottom,
an air compressor, with a long pipe was
obtained -and air was fed into the .bottom of
the digester while sludge was being drawn off
to the beds. If tried in other locations, this
procedure might be safer using steam.
Breaking up a Scum Blanket with a Pump
How can a scum blanket be broken up
without emptying the digester? A plant in the
'Northwest which had an eight-to-ten-foot
scum: blanket in an existing digester, solved
the problem by inserting a. large-capacity
3-25
-------
chopper type pump (Vaughan Scum Gun)
through a digester manhole. Several pre-
cautions were necessary in this operation.
1. Safety precautions were exercised to pre-
vent explosive situations during the
installation.
2. Very rapid breakup of the scum caused a
load on the digester because food, which
had been tied up in the scum, was released
into solution very rapidly. It was neces-
sary to monitor volatile acids and alka-
linity frequently, similar to any heavy
organic loading.
3. Floating covers must be balanced to
counter loads caused by placement of the
pump. This is particularly important if the
pump is placed off center.
MIXING
Use Motor Amperage Readings to Indicate
Impeller Wear
A plant in Washington noted progressively
worse mixing results in a digester with a draft
tube. This unit had a reversible propeller
mounted on it. When the unit was pulled for
inspection, the propeller which was originally
20 inches in diameter, had been worn to a
10-inch diameter. Amperage readings were
compared and it was noted that the amperage
had been getting progressively lower because a
smaller volume of sludge was being moved.
Regular monitoring of the motor's amperage
would have warned the operator about this
problem.
LINE PLUGGING
How to Unplug a Supernatant Line
Continuous plugging of supernatant lines by
scum can be a serious problem, particularly in
a fixed^ cover digester. In one plant, a one-inch
pipe was passed through a rubber plug. The
plug was fit tightly into the supernatant line
and high-pressure water discharged through
the pipeline into the digester, dislodging scum.
Freezing a Sludge Line to Install a Valve
During the remodeling construction at one
plant, it was necessary to break into a live
drain line that had no valve in it. This was
done by constructing a two-piece collar to fit
around the pipe. The collar was approxi-
mately four feet long and four inches larger in
diameter than the sludge line. A space was left
around the entire diameter of the pipe and
the length of the device. Liquid nitrogen vyas
fed into the space in the collar. This method
froze the sludge in the pipeline in about two
hours, blocking the line. The valve was
inserted in the line below the frozen section.
About eight hours later the frozen sludge had
thawed and began to flow through the newly
installed valve.
TOXICITY
For over a year a plant had had chronic
problems in starting the digester. The cause of
the. problem was "found to be outside the
plant. The digester would show signs of a
good startup with increasing acid production,
but every weekend the digester would quit
working and on Mondays the operator would
find no digestion taking place. This was
repeated week after week.
Because of the regular cycle of the problem, it
was thought that some industry might be
involved. The operator found that a furniture
factory was consistently dumping about
1,500 gallons of paint waste into the sewer
every Friday.
The problem was handled when the operator
reduced mixing to three hours a day. This
allowed the toxic sludge to stay on one side
of the tank and not become thoroughly and
immediately mixed with the digester
contents. The long-term solution for this
problem is to enforce the industrial waste
ordinance and prevent the paint dumps at the
source.
3-26
-------
COLD WEATHER PROBLEMS
.- - —!•*
How to Prevent Freezing of Digester Pressure
Relief Valves
Cold weather problems with gas pressure
relief valves are commoh and one operator
found, the solution by placing a barrel over
the relief valve with a light bulb inside it. The
bulb produced enough heat to keep the valves
from freezing/This type of device should
contain an explosionproof cover over the
bulb.
Another method of solving freezing problems
in digester pressure relief valves is to put a
light grease mixed with salt on the mating
surfaces. This will prevent freezing. However,
it should be cleaned off in the summertime to
prevent corrosion.
DIGESTER DRAINING
Solving a Sludge Removal Problem
Plant operators in one plant needed to empty a
digester for routine cleaning. An area suitable
for sludge storage was found in a lagoon not
connected to the digester. Some method was
needed to transfer the sludge other than the
existing'sludge drawoff line.
It was determined that the city personnel
could do the job less expensively than a
contractor if they had their own pump and
used their own personnel. A pump normally
used for emptying barnyard manure pits was
fitted with an explosionproof motor and
hoisted to the top of the digester.
A tripod was arranged over a large manhole
opening and the pump lowered into the
digester. A discharge hose was attached to an
irrigation pipe to carry the sludge to the
lagoon.
The purrip had -a cutter bar underneath the
impeller which chopped up thick scum, rags,
sticks, etc. When the thick scum was broken
up with high pressure water, it flowed quite
easily^ through the pump. .
As the sludge level dropped, the pump was
lowered to keep it approximately r/a to 2 feet
below the surface'of the sludge.
About two digester volumes of water were
needed to liquify the sludge enough to pump.
A scum layer about three feet thick and a grit
layer about four feet deep were removed from
a 50-foot diameter digester in ten days,
How to Control Odors Using Hydrogen
Peroxide
When it was necessary to.drain a digester
containing partially digested sludge, odors
were a problem. A line was tapped into the
sludge draw-off pipeline and hydrogen per-
oxide solution at 30 percent concentration
was added to the sludge. The concentration
was about one gallon for every 12,000 gallons
of sludge.drawn to the beds. , . "...
PLANT STARTUP
A plant with two digesters, primary and
secondary, found it necessary to empty the
primary for repairs. 'The following startup
procedure was used.
Temp.
Day Deg. pH
1
3
4
7
8
9
10
69 6.7
75 6.1
82
92
93
97
5:4
5.5
5.6
5.9
Comments
Tank being filled with
raw sewage.
Tank full.
Added 10,000 gallons
secondary sludge from
another plant.
Added 250 Ibs. of lime.
3-27
-------
11
12
13
19
20
25
97
97
98
97
98
98
5.7
5.7
5.7
5.7
5.8
5.9
Added 400 Ibs. of lime.
Added 200 Ibs. of lime.
Added 200 Ibs. of lime.
Added 300 Ibs. of lime.
Added 150 Ibs. of lime.
Added 1,000 Ibs. of lime
in last five days.
Added 1,000 Ibs. of lime
in last ten days.
Added 2,000 Ibs. of lime
in last 44 days in 100-lb.
or less increments.
Also added 21/2gal. de-
foaming agent about day
60 to prevent foaming.
Sludge was being added at about 4,000 gpd at
3.3 percent solids and 77 percent volatile. At
the end of about 80 days, the volatile
reduction averaged about 51 percent.
35
79
98
98
6.0
7.1
3-28
-------
DIGESTER GADGETS
Operators have devised several gadgets that
assist in solving problems around their plants.
A few examples are listed on the following
pages showing what can be done with little
expense and some ingenuity.
DIGESTED SLUDGE SAMPLER-This "home-made" sampler is made from materials found
around the plant (some, such as the rubber balls, might even be retrieved off bar screens)..
The lead can be poured around the inner can using a metal container approximately one inch
larger in diameter. The spring support and trip mechanism can be readily fashioned from scrap
materials. The spring is weak enough so that it trips without lifting the device.
A tripod with a reel for raising and lowering can be used to allow selecting samples at the desired
depths. ' ., • .-.-.•-.:-•- -;• -,
Rope or Cable '
Calibrated @ 5 Feet
Trip Mechanism
Spring
Rubber Ball
Chain Attached To
Lifting Rope
Threaded Rod
Lead
Beer Can
Rubber Ball
FIGURE 3-1
DIGESTED SLUDGE SAMPLER
3—29
-------
GAS PRODUCTION ESTIMATOR-When the gas meter is not operating, the following system
may be used as a rough estimate of gas production.
1. Fill the carboy with sludge from the active zone.
2. Turn on heating pad and hold contents at same temperature as digester.
3. Fill a 500 ml graduated cylinder with water and invert it in a 2 liter beaker over the end of
the gas hose, being careful to keep the cylinder filled with water and not admit any air.
4. Allow gas to purge from the carboy for one hour, then set gas tube under lip of cylinder.
5. Note length of time to displace 400-500 ml.
6. Repeat for several consecutive days to get trend of production.
Fill
Thermometer
Glass Tube &
500 Ml Graduated
Cylinder
2 Liter Beaker
FIGURE 3-2
GAS PRODUCTION ESTIMATOR
__________
-------
SCUM BLANKET FINDER-Qne method for finding the depth of Ihe scum blanket in g digester;;
is illustrated here. .'•.,: . , . • :
A one inch pipe marked every foot is attached to a wooden paddle by a hinge,. This can be
pushed between the digester wall and cover in the first position. .
As the finder is raised after passing the bottom of the blanket, the paddle will straighten out and
lock under the scum'blanket. The appropriate depth mark is noted, the paddle pulled back
parallel .with the pole and lifted out of the digester. ^
Nylon Cord
% Marine
Plywood
T'Pipe
Mark Every Foot
Push Down
Between
Wall and Cover
Hinge
Lowering and
Raising Position
Wall
Measuring
Position
Floating
Cover
Scum
Blanket
Measuring
Method
FIGURE 3-3
SCUM BLANKET FINDER
-------
SUPERNATANT LINE PURGE DEVICE-Plugged lines due to scum can cause severe problems
in fixed cover digesters, particularly in cold weather when pressure relief valves may freeze.
A two inch piece of rubber approximately the same size as the I.D. of the line can befitted with
a piece of pipe through the center and secured for moving up and down in the line.
Either water or steam can be used to loosen the scum.
This may be used also in a chronically plugged sludge line if a tee or wye and valve are provided
for access.
Welded
Plate
High Pressure Water
or Steam
Pipe Thread to Hose Thread
Adapter
T'Pipe
2" x 6" Rubber Ring
Supernatant
Line
FIGURE 3-4
SUPERNATANT LINE PURGE DEVICE
3-32
-------
AUTOMATIC PUMP SHUT-OFF COIMTROL-To prevent,damage to the piston, pump,;sludge
piping or valves, a pressure shut-off control can be added to existing systems with a minimum of
expense as described below.
An adjustable pressure switch to be used as a permissive interlock in the pump control circuit can
be installed. When pressures downstream from the pump exceed the switch setting, the pump
shuts off. This effectively prevents damage in the event a downstream valve is unintentionally
closed or if plugging develops in the discharge-line.
The switch is available off the shelf at electrical or control supply firms.
Discharge
Pressure
Gauge
Drive Motor
Positive Displacement
Sludge Pump
X~~N
i i'
... . ,
FIGURE 3-5
PRESSURE SHUT-OFF SYSTEM TO PREVENT DAMAGE TO PUMP
3-33
-------
RAW SLUDGE THICKNESS CONTROL-A rather simple control system was installed at one
plant to prevent pumping excess water to the digester by using the amperage from the piston
pump motor to sense changing sludge thickness.
Amperage readings were recorded at the same time that total solids samples were collected. It was
found that as the total solids decreased, amperage decreased and when the values for the two
were plotted on a graph, the minimum desirable solids content could be matched with an amper-
age reading (see Appendix G for information on graphing).
A load meter that sensed amperage of the motor was installed in conjunction with a one minute
time delay switch. When the pump came on automatically, sludge was cleared out of the line,
then the load switch sensed the sludge thickness and the pump shut off if the sludge thinned out
before the time clock timed out.
Ammeter or
Load Meter
Time Clock
Control
Motor
o
k
t
y
Diston Pump
i
r 'X
o
FIGURE 3-6
RAW SLUDGE THICKNESS CONTROL
3-34
-------
SUPERNATANT SELECTOR-An "operator-made" device was installed in an existing digester
while it was down for repairs that helped draw the besf possible supernatant even though liquid
level varied. :
A hoist was mounted on the tank wall and 1/4" plastic coated boat control cable was attached to a
section of movable supernatant pipe. A swivel joint composed of an ell and street ell allowed the
draw-off point to be changed by operation of the hoist.
Hoist
FIGURES-?
SUPERNATANT SELECTOR BUILT BY OPERATORS
3-35
-------
-------
PART 4
THE BASICS
SUMMARY
INTRODUCTION
WHY DIGEST ORGANIC SOLIDS?
WHAT MATERIALS ARE REMOVED FROM WASTEWATER?
WHAT TYPE OF DIGESTION: AEROBIC OR ANAEROBIC?
WHAT HAPPENS INSIDE AN ANAEROBIC DIGESTER?
WHAT ARE THE PRODUCTS?
HOW IS DIGESTION AFFECTED BY TYPES OF DIGESTERS?
WHAT FACTORS AFFECT SLUDGE DIGESTION?
DIGESTER CONTROL
TYPES OF EQUIPMENT
-------
SUMMARY
Decomposition of organic material in
digesters is a continuous two-step process.
Two different kinds of bacteria are involved.
In the first step, the organic material is
converted into organic acids by acid-forming
bacteria. The organic acids are used as food in
the second step by strictly anaerobic
methane-forming bacteria which convert the
acids into methane and carbon dioxide gases.
During the last step, complete digestion takes
place, bound water is released, and the sludge
is completely stabilized and ready for
dewatering.
The products formed as a result of digestion
are a well stabilized sludge, carbon dioxide,
methane, and water.
Five factors must be in balance to accomplish
digestion: bacteria, food, loading, mixing,
and environment. The operator must control
all of these factors.
o The bacteria (good seed sludge) must be
kept in plentiful supply.
o The food (incoming sludge) should be as
concentrated as possible (4 to 8 percent
solids), and fed continuously or in
frequent small amounts.
o Mixing should be continuous or nearly so
to provide contact of bacteria with the
food.
o Sufficient time must be provided to
permit complete digestion. Digesters must
be kept operating at or near full volu-
metric capacity.
o The environment must be kept within
extremely narrow ranges. The optimum
conditions are:
Anaerobic Conditions
Temperature
PH
No Toxic Material
No oxygen (air)
85-95 deg. F.
6.8-7.2
The type of digester will affect what the
operator can or cannot do to control the
process. The degree of layering, scum forma-
tion, and supernatant clarity are all affected
by type of digester and associated equipment.
Sampling, testing and analysis are the basic
steps in making a good digester control
program. The purpose of a laboratory testing
program is to identify and characterize the
kind of waste being sampled. This means that
a sample .is collected and checked by chemical
procedures to find what is in it and .how it
will act in the digester.
The best and closest digester control is
achieved by monitoring the process with more
than one control parameter. The tightest
control is obtained by monitoring the ratio
between volatile acids and alkalinity. This
method is the one which allows preventive
control action and is not too difficult to
perform. This method is the best type of
control for those plants having other pro-
cesses which are affected by digester
operation.
Various indicators of the progress of digestion
are used for predicting possible trouble. These
are CC>2, pH, gas production, loading, volatile
solids reduction and pounds of solids in the
system.
4-2
-------
ANAEROBIC SLUDGE DIGESTION
INTRODUCTION
Natural decomposition of organic material has
;occurred for 'millions of years. This natural
• decaying process breaks organic material such
as leaves and grass, animal waste, dead car-
casses, rubbish and refuse of all kinds into
simple elements or compounds that return to
the soil as nutrients. These -materials are
broken down biologically by bacteria which
• come in all types, sizes and shapes. Some live
and work in, almost any environment, while
others are extremely sensitive. Some use any
type of organic material as food, while others
are very selective. In a manner of speaking,
- bacteria eat the organic material as food,
digest it, and convert it into end products
consisting of liquids, gases and stabilized
solids.
Bacterial decomposition can occur with or
•without air (oxygen). The bacteria that need
air use it in the same way we do. These are
called aerobes, or aerobic bacteria. Bacteria
that live without oxygen are called anaerobic
. bacteria. Some bacteria can live under either
condition and are called facultative bacteria.
Man is perhaps the greatest producer of waste
materials. Since man is also an inventive
animal, he has discovered that one of the best
and cheapeast ways of getting rid of his waste
is to put these millions upon millions of
bacteria to work for him. All man" needed to
do was collect his waste material in ope place,
put it in a container, create the right environ-
ment, and let nature take its course. Water is
used to carry waste materials from hundreds
of sources to a central point where these
materials can be removed from the water:for
treatment. A typical wastewater treatment
facility has several "container" processes,
each designed to remove or treat these wastes.
WHY DIGEST ORGANIC SOLIDS?
The organic solids removed from the waste-
water produce offensive odors when they
decompose. These sludges also may contain
pathogenic (disease-causing) organisms
harmful to man.
Sludge also contains water held within the
sludge particle which makes it difficult to
dewater. It is, therefore, necessary to contain
and treat these wastes so that:
I.The treated sludge is stabilized, which
means that the sludge is decomposed and
further bacterial activity is minimal.
2. The offensive odors are eliminated.
3. Many pathogenic bacteria are eliminated.
4. The disposal problem of sludge is mini-
mized by converting a large percentage of
sludge to gases and liquid. Gases can be
used as fuel.
" 5. The sludge is easily dewatered and will
dry readily. It has value as a soil condi-
tioner when recycled back to the land.
4-3
-------
WHAT MATERIALS ARE
FROM WASTEWATER?
REMOVED
Figure 4-1 shows that the incoming waste-
water contains two basic types of material
classified as being about 70 percent organic
and 30 percent inorganic..The organic portion
of the sewage is used as food for bacteria,
while the inorganic portion passes on through
the entire treatment process unaffected.
Inorganic material includes rock, grit, rags,
plastic, metal, etc.
These materials are normally removed in
pretreatment units such as bar screens, rock
traps, and grit collectors. Material passing
through these collectors contains solids too
large to be effectively treated. These must be
reduced in size by other pretreatment units
such as comminutors and barminutors, which
shred the material and allow it to pass
through the treatment units. All of these units
must operate continuously at top efficiency if
the subsequent plant processes are to work
properly. The material removed from the
wastewater in this phase is usually sent to a
landfill site.
After pretreatment, the remaining solids are
either settleable, suspended or dissolved.
Settleable solids are those which are heavy
enough to settle out when the wastewater
flow is slowed down and enough time is
allowed for them to settle. These solids are
removed in primary clarifiers and are called
raw primary sludge.
The remaining solids are either suspended in
the water or dissolved just as sugar is dissolved
in coffee. Most of this material passes on to
some form of biological treatment process
where it is converted to biological solids
heavy enough to settle. These solids are
removed from the wastewater flow in second-
ary clarifiers. Figure 4-1 shows that both raw
sludge and biological sludge are sent to the
anaerobic digester for natural decomposition.
Of the materials reaching a digester, only the
organic portion can be decomposed by the
bacteria. The inorganic materials are unaffect-
ed by biological treatment; however, they can
Settleable
Suspended
Dissolved
Disinfection
i
Recycled Water
(Supernatant)
FIGURE 4-1
DIAGRAM OF MATERIALS REMOVED FROM WASTEWATER
4-4
-------
cause severe. problems, such as reducing
digester volume. •^r•;•'•-• ;•,
.Several methods are. used to concentrate
digester feed sludges. Primary clarifiers,
gravity thickeners, flotation .thickeners and
centrifuges are commonly used.
Primary clari.fiers are. the only sludge-
thickening devices in many plants. Biological
sludges, such as activated sludge or trickling
filter humus, are often wasted to the primary
clagfier..and settled with the raw sludge. The
sludge concentration developed in the
.primary clarifier is controlled by the. fre-
quency, duration and rate, of sludge
withdrawal.. .
Gravity-type thickeners 'are used in plants
which remove, raw sludge continuously from
the primary clarifier. They yield 4 to 8
percent solids concentration. Waste-activated
sludges may be thickened with the raw sludge
in these units, but difficulties occur when .the
waste-activated, sjudge .is', young and
slow-settling.
Flotation thickeners are often used to thicken
waste-activated sludges. Tnese lightweight
sludges tend to'float, which makes air flota-
tion a practical means to concentrate them to
about 3 to 4 percent. The use of centrifuges is
another method for thickening waste-activated
sludge. Both units will thicken the waste-
activated sludge to about 3 to 4 percent.
. .Regardless..,ot.the sjudge source, the eoncen-
tration of total so.lids fed to a digester should
be as thick as possible/Normal ranges are
from 3 to 8 percent. This is necessary:.
1. To prevent dilution of the alkaline buffer
which could cause a pH change.'NOTE:
A buffering material is the amount of
alkalinity in the digester"needed to offset
the acids and keep the pH near neutral.
2. To prevent dilution of the .food material.
this,wpij;!d.rpaj
-------
WHAT HAPPENS INSIDE AN ANAEROBIC
DIGESTER?
Anaerobic sludge digestion is a continuous
process. Fresh sewage sludge should be added
continuously or at frequent intervals. The
water separated from the sludge (supernatant)
is normally removed as sludge is added.
Digested sludge is removed at less frequent
intervals but it must be removed. The gas form-
ed during digestion is removed continuously.
The stabilization of organic wastes by anaer-
obic sludge digestion must always result in the
production of methane gas which is insoluble
in water and escapes as a gas. Thus, if no
methane gas is produced there can be no
waste stabilization.
Anaerobic sludge digestion is considered a
two-stage process as shown in Figure 4-2. This
diagram shows organic material as food is
changed in the first stage by acid-forming
bacteria to simple organic material, chiefly
organic acids. The methane-forming types of
bacteria then use the acids as food and
produce carbon dioxide and methane gas. It is
important to understand that no waste
stabilization occurs in the first stage. Real
stabilization occurs only in the second stage.
Liquid
FIGURE 4-3
TYPICAL ACID-FORMING BACTERIA
One of the major considerations is the type of
food available to the acid-forming bacteria,
Food may be in two forms, soluble and
insoluble. In the soluble form it is readily
removed (like sugar in water). Insoluble
forms, such as fats or complex solids, are more
difficult to use. They must first be broken
down into a soluble form. This is accom-
plished, in part, by enzymes which are pro-
duced by the bacteria. The bacteria can only
directly use the soluble solids as food since-it
must be in this form to pass through the cell
wall and the membrane as shown in
Figure 4-3. The cell wall acts as a sieve to
screen out the large particles, while the
membrane selects and guides material both in
and out of the inner cell.
Acid Forming
Methane Forming
X" "">
Bacteria
Second Stage
Stabilization
FIGURE 4-2
DIAGRAM OF WASTE STABILIZATION
4-6
-------
Not all of the organic solids are completely
•broken down nor does all of the, material pass
into the cell. These materials contribute to
that portion of digested sludge which is not
degradable (poor food for bacteria) and that
fraction called inert solids (not food for
bacteria).
The bacteria use the food for energy and
produce organic acids also called volatile acids
or fatty acids. The production of these acids
completes the first stage of the digestion
process and is commonly known as the acid •
phase. ,-,-.- . --.,;-..
In a normal or healthy digester, acids will be
used as food by the second group at approxi-
mately the same rate as they are produced.
The volatile acid content of the digesting
sludges usually runs in the range of about 50
milligrams per liter (mg/l) to 300 mg/l,
expressed as acetic acid.
If the acid phase was the only step occurring
in digestion, the process would be incomplete,
resulting in a continuing drop in pH caused by
an overproduction of acids. This does occur
for a period of time when a digester is first
started or when a digester has lost a large
amount of its methane formers. Digestion can
only be completed when the second phase is
occurring at the same time as the acid phase. :
The second phase in anaerobic digestion
occurs because of another bacterial group
called the methane formers, which use the
volatile acids produced by the acid formers as
food. The acids are then converted to carbon
dioxide (C02) and methane (CH4) gases as.
major end products. This step completes the
work of the two principal forms of bacteria
and results in stabilizing between 40 and 60
percent of the organic waste in domestic
sludge.
The methane formers, which are responsible
for waste stabilization, grow quite slowly
compared to the acid formers since, they, get
very little energy from their food. This causes
the 'methane formers to be very sensitive to
slight changes in loading, pH and temperature.
Since the methane formers are strictly anaero-
bic bacteria, they are also extremely sensitive
to air (oxygen).
The acid formers have a decided edge over the
methane formers since they are rapid growers
and are not as sensitive to environmental
changes. Thus, the operation of anaerobic
digesters depends largely upon keeping
methane formers happy.
The objective of good digester operation,
then, is to control the food supply, the tem-
perature and the pH, thus keeping the acid
formers and the methane formers in balance.
These subjects are discussed later in this
section. "
WHAT ARE THE PRODUCTS?
Gases
The major gases produced in any anaerobic
condition are methane (CH^J and carbon
dioxide. (CC^)- These gases are usually collect-
ed, and compressed. The methane portion is
used as a fuel gas for boilers, gas engines and
other auxiliary uses.
Most of us have seen bubbles rising to the
surface of a swamp, especially on a warm day.
These bubbles are gases formed by the same
kind of methane group as in a digester. Maybe
you have n9ticed that, as the gases rise to the
surface, they carry small chunks of bottom
sludge and there is a little turbulence in the
water—similar to boiling water. The same
thing happens inside a digester. In fact, the
only mixing in many digesters occurs in this
natural manner.
4-7
-------
Scum
Scum blankets form in digesters as the result
of this upward lift of the gas. The formation
of scum presents a special problem to many
operators who have unmixed tanks. The scum
in these tanks tends to concentrate the food
material; the 'working bacteria are generally
concentrated in the bottom sludge. If mixing
doesn't bring the two together, there will not
be much digestion occurring in the scum
layer. In an unmixed tank, the supernatant
layer provides a physical barrier between the
two.
In unmixed tanks, it is necessary to keep the
scum blanket moist so that the gases can get
through. However, if the'blanket dries, the
operator must break it up so that the gases
can escape. Methods for breaking up scum
blankets are discussed in Part I, Trouble-
shooting Guide No. 9. •
Supernatant
The water or liquid inside the digester comes
from two sources: carrier water entering the
digester and water formed as solids are broken
down. A certain amount of this liquid leaves
the digester as supernatant. In most cases,
supernatant is displaced as fresh sludge enters
the digester. However, in some digesters, it is
taken out before sludge feeding.
Supernatant is often normally recycled
through the plant and is high in suspended
solids and BOD. Many secondary plants have
experienced process problems due to the
addition of supernatant. The major problems
seem to be a buildup of solids in the system
and insufficient aeration .to accomplish the
necessary BOD reduction. Both of these
conditions result in a deterioration of the
plant's final effluent. One common method
practiced by many plants to help correct this
problem is to release supernatant frequently
and in small amounts to prevent shocking the
system. Other suggestions are given in the
Operations Section, Part II, page 2-4.
Supernatant quality is affected by the type of
digester system, the efficiency of digestion
and by the type of waste and its settling
capacity. For example, waste-activated
sludges tend to thin out the digester contents,
resulting in less time for digestion to occur.
Mixers tend to homogenize the sludge,
making supernatant removal difficult if
sufficient settling time is not allowed. Good
settling conditions are a must. The digested
sludge must have enough time under quiet,
undisturbed conditions to be allowed to
settle. This is normally accomplished either
by shutting mixers off or by transferring the
digested sludge to a second settling tank.
Operators generally use the results from two
tests: total solids and volatile ' solids, to
indicate the quality of the supernatant as
described later in Digester Control.
Digested Sludge
The inorganic and volatile solids that are not
easily digested make up the final product-
digested sludge. A well digested stabilized
sludge must drain easily or be dewaterable
and not have a noxious odor. The characteris-
tics of the sludge are:
'''"' 1. Some "of the water in the sludge particles
is released as the particles are broken
down. This makes the sludge easier to
dewater.
2. The amount of the well digested sludge
leaving the digester is less than the
amount of raw sludge entering the
digester because the complex organic
material has been broken down into
simpler substances such as liquid acids,
water^.and gases.
4-8
-------
3. The sludge should have- a lumpy
appearance.
4. The sludge turns black. Light gray streaks
indicate a "green" undigested sludge.
5. The original offensive odor changes To a
less objectionable odor.
6. Volatile solids in the stabilized sludge
should be 40 to 60 percent less -than
the feed sludge.
Stable (digested) sludge can be disposed of on
approved land or landfills after it has been
dewatered.
HOW IS DIGESTION AFFECTED BY
TYPES OF DIGESTERS?
Sludge enters the center of the active zone
where digestion takes place and water is
released to form a supernatant zone. The
decomposed solids are heavier than the liquid
and settle to the bottom. As gases are formed,
they rise to the surface, pass through the
scum layer and escape into the atmosphere.
The rising gases carry the lighter sludge
particles to the surface above the supernatant
and form a dense layer of scum. This scum
layer, in time, can become quite thick and be
tough enough to walk on. .
Figure 4-5 shows the same digester after three
to six years of operation.
Single, Unheated and Unmixed Digesters
The simplest digester is a circular or rectangu-
lar, unheated, open-top tank, whose contents
are mixed naturally by rising gases. This type
of digester includes Imhoff tanks and Clari-
gesters which are two-story units with the top
portion serving as a darifier where the solids
settle and then dpop through a slot into the
lower digester portion. In these unmixed
digesters, the sludge arranges itself in layers as
illustrated in Figure 4-4. ..'•'•.
Scum
Supernatant
Active zone
Digested Sludge
Grit, Etc.
FIGURE 4-5
OPEN-TOP, UNHEATED, UNMIXED DIGESTER
AFTER 3 TO 6 YEARS
Gas
Scum Layer
—Supernatant
Active Zone
— Digested Sludge
FIGURE 4-4
OPEN-TOP, UNHEATED, UNMIXED DIGESTER
Deposits 'of grit and other material on the
bottom and a thicker scum layer greatly
reduce .the effective capacity of the tank.
Problems occur more frequently. It may be
difficult to obtain a well digested sludge, or
the supernatant layer may be hard to.find.
The unit is easily overloaded and has more
frequent upsets.
This type of digester is no longer being built
although several remain in use.
4-9
-------
Single, Heated-Mixed and Covered Digesters
Now, let's take the same digester, add a cover
to collect gas, add a heat exchanger and a
pump to pull sludge from the tank bottom,
pump it through the heat exchanger and
return it to the upper level as shown in
Figure 4-6.
FIGURE 4-6 SINGLE COVERED DIGESTER WITH
RECIRCULATION
What changes have occurred? First, the pump
is acting like a mixer and sets up a current
inside the tank. More bacteria are exposed to
the food, and a faster reaction takes place.
The heat exchanger raises the temperature of
the sludge. The operator can control the
temperature to help the bacteria do a better
job. In addition, the collected gas can be used
as fuel for a hot-water boiler to supply the
heat exchanger. Thus, the addition of heating
and recirculation equipment reduces the
layering effect seen in the unmixed units,
complete digestion occurs in less time and a
smaller digestion tank can be used.
There are many types of mixers, many ways
to heat the sludge, and many ways to collect
gases, but they perform the same functions.
The major differences are mechanical. These
systems are discussed later in this section.
When withdrawing supernatant in these tanks,
it is necessary to shut the mixers off and
allow the solids to settle before withdrawal
begins. The operator should also find the
supernatant zone and carefully select the
clearest liquid.
Two-Tank Systems
Next are those digestion systems using two
tanks: one for active mixing and digestion,
and the other serving as a quiet settling tank
as shown in Figure 4-7.
Gas
FIGURE 4-7 TWO-TANK SYSTEM
4-10
-------
Two-tank systems were designed to shorten
the total digester detention time by utilizing
one mixed tank (the primary) to provide for
active mixing and digestion while the second
tank is used to provide settling. These systems
are referred to as two-stage digesters.
Operationally, the primary tank can be mixed
continuously, since the mixers do not have to
be shut off and the contents allowed to
settle before withdrawing supernatant or
sludge. Generally, more efficient mixing is
obtained in these units. Both tanks are
covered, and the gas system cross-connected
between them. The secondary tank, however,
does not produce much gas because most of
the gas production occurs in the primary
tank.
The secondary tank has another beneficial
use. It contains a large volume of good active
sludge (bacteria) which can be transferred to
the primary when the digestion process is
fouled. This seed sludge can be used to cor-
rect pH and toxicity problems by using
"natural recovery" instead of adding chemicals
such as lime.
Conventional Versus High Rate Digesters
The basic difference between these two
systems is their loading rates:
Conventional—0.03 to 0.10 Ibs. per cubic
foot (0.48 to 1.60 kg/m3) of volatile
solids loaded per day. '
High Rate-0.10 to 0.40 Ibs. per cubic
foot (1.60 to 6.40 kg/m3) of volatile
solids loaded per day.
The high rate units achieve their higher
loadings because their design includes uniform
temperature and greater mixing capacity. The
tanks are also usually deeper. Operationally,
the high rate system will be. fed on a more
nearly continuous basis.
WHAT FACTORS AFFECT SLUDGE
DIGESTION?
Five basic factors affect digestion:, bacteria,
food, loading, contact (mixing) and environ-
ment. The acid and methane formers can only
do their best job as a team when the right
conditions are provided. The,purpose of this
section is to describe'the:factors needed.for
good digestion. All of these affect the process
and all can be monitored and controlled by
the operator. • .: • .
Bacteria ;
The digesting and digested sludge contains all
of the necessary bacteria to stabilize the
sludge. Thus, the operator must keep as much
good digested sludge as possible in his digester
to have enough workers ..onhand to do the
job. Do not remove, any more digested sludge
than is necessary, but some digested sludge
must be removed at regular intervals. A good
guideline is to have about 20 times as much
seed sludge as feed sludge expressed as volatile
solids.
In single tank digesters, "liquid (supernatant) is
displaced as fresh sludge is added: Digested
sludge -is withdrawn under strict control. In
two-tank systems, where the primary tank is
mixed, many of the bacteria are trapped in
the supernatant from the primary tank and
are moved to the secondary tank. The secon-
dary tank contains an abundance of bacteria
often called "seed" sludge. •
.This is good seed material which can be
recycled or transferred :back to the primary
digester. This is a good technique to use when
organic overloading or a potential toxic load
is expected or has already occurred. This
technique is often, used by plant operators
who favor the "natural process" for, recovery.
A simijar technique can be; used for single
•tanks except that seed sludge would have to
, be obtained from another installation, hauled
to the site and then fed into the digester. '_
4-11
-------
Food
Contact (Mixing)
Volatile solids in primary and waste secon-
dary sludges are the food for the bacteria.
Raw primary sludges, compared to biological
sludges, produce the clearest and best super-
natant and the most easily dewaterable sludge
from a digester. When biological sludges are
added, good supernatant quality becomes
more difficult to obtain. In fact, some plants
separately treat biological sludges in aerobic
digesters and primary sludge only in the
anaerobic digesters to improve total plant
performance.
Vegetable fats and oils, such as cooking oils,
are readily decomposed in anaerobic digesters,
but mineral oils such as fuel oil, automotive
oils and greases, and paraffins will cause
toxicity problems.
Loading
Feeding is one of the things under the control
of an operator. Each operator must consider:
o The concentration of the incoming sludge,
which is the amount of solids in a given
volume of water.
o The amount of volatile solids in the
incoming sludge, which tells how much of
the material can be used as food by the
bacteria and indirectly the amount of grit.
o The amount of volatile solids per unit to
digester volume, which is used as a loading
factor in much the same way as the "food
to microorganism ratio" is used in activat-
ed sludge.
o The hydraulic loading (hydraulic deten-
tion time) which is related to the organism
growth and washout.
These are described in detail beginning on
page 4-20.
Sludge stabilization cannot occur unless the
bacteria are brought into contact with the
food.
The goals are to expose the bacteria to the
maximum amount of food and also to reduce
the volume occupied by settled inorganic
material, such as grit, and organic material,
such as scum. The benefits of mixing are
speeding up the process of the volatile solids
breakdown and increasing the amount of gas
production.
This is done two ways:
1. Gas Evolution. As gas is produced, it
forms in pockets, then breaks loose and
rises to the surface. This action creates a
boiling effect resulting in some mixing.
This method is controlled by feeding.
When conditions allow a fairly constant
loading, which may be higher than nor-
mally designed in conventional digesters,
internal mixing will occur. A loading of
about 0.4 pounds cubic foot per day is
needed for natural mixing. As long as
loading can be sustained at this level, no
other mixing may be required; however, if
prolonged periods of low loading are
experienced, mixing may be interrupted
and scum blankets may form. On the
other hand, increased loading may cause
organic overloads with resulting slower gas
production. Conditions which cause nat-
ural mixing are somewhat unstable but do
afford an inexpensive method of mixing if
the operation is closely controlled.
2. By Artificial Means. Many types of mixing
devices are used to stir or mix the
digesting sludge. The amount and
frequency of mixing are controlled by the
operator. These have been described
beginning on page 4-27.
4-12
-------
Environmental Factors Affecting Digestion
The methane bacteria, which cause the final
conversion of digesting sludge into a stable
waste, are very sensitive to conditions in the
digester. Their activity slows down unless
optimum conditions are maintained. The
following table summarizes the best condi-
tions for anaerobic digestion'.
Table IV-1
OPTIMUM CONDITIONS FOR
ANAEROBIC DIGESTION
' No oxygen (air)
85-100 deg. F.
: .(29-37 deg. C.)
6.8-7.2'
Anaerobic Conditions
Temperature*
pH " _.'..
No Toxic Materials
' Temperatures between 85-100 degrees F. (29-37
degrees C.) are in the MESOPHILIC RANGE.
Temperatures between 120-135 degrees F. (48-57
degrees C.) are in the THERMOPHILIC RANGE.
Most digesters operate at mesophilic temperatures.
The operator must understand the basics of
each condition below because they must be
controlled to obtain the most efficient
treatment.
Anaerobic Conditions. No air can be admitted
to the digestion tank if anaerobic conditions
are to be maintained. The methane formers
cannot tolerate even small amounts of oxygen.
Closed tanks with covers designed to collect
the methane gas are used to keep air out. Op-
erators must not allow air into the digesting
sludge since an explosive mixture will result
when aircomesJnto contact with methane gas.
See also Safety section, page 3-6. If air is ad-
mitted into a digester, be extremely cautious
about possible ignition,
Temperature. Temperature controls the activy
of the methane bacteria. They can function
best in the 85-100 degree F, (29-37 degree C.)
range or, in another range, 120-135 degrees F.
(49-57 degrees C.) Outside these ranges, the
bacteria's activity, is severely reduced. For
example, activity is almost nonexistent at
50" degrees F. (10'degrees C.). It should be
noted that, although bacterial actions stop,
the bacteria themselves are not harmed but
are simply inactive until1 the temperature
increases again. ' ' ''""~ ' : ••'•'-
The methane-formers are'affected by changes
in' temperature'of as little asT degree Fahren-
heit per day, but the acid formers are not as
sensitive to temperature changes.'Tempera-
ture changes/g'reater than 2 degrees Fahren-
heit will reduce methane former activity while
acids are still forming. This results'in losing
the buffering capacity and possibly
incapacitating the digester. The best bacterial
activity will occur in digesters operating at a
constant temperature somewhere between 90
and 98 degrees'Fahrenheit (32 and 36 degrees
centigrade). :
Once the best temperature for the individual
digester is found, based on the highest gas
production and abil'ify to hold the pH near
7.0, this temperature should be held within
1 degree Fahrenheit. .If heating capacity is
limited, and it is not possible to hold this
temperature in winter months, it is-better to
drop down from 95 degrees F. (35 degrees C.)
to 90 degrees F. (32 degrees C.) and hold this
value constant'than' to fluctuate between 92
; arid-98_-degrees F. (33 and 36 degrees C.)
over a two- to three-day period in an attempt
• ;to! reach the higher temperature.
V-arious statistics are. available showing the
-value, effects and the necessity of heating the
.digester contents, to allow the most efficient
. use .of. the process. When the ..subject of
temperature is discussed, the element of time
cannot be ignored because solids stabilization
• can be accomplished.-.at low;-temperatures..if
enough time is available. Table-IV-2,relates
time and temperature to illustrate this point.
•These data show:-that :evenv at 7-7;degrees F.
'(25 degrees C.), digestion can occur.in about
51/2 weeks. However, approximately 60 per-
cent more digester capacity would be
4-13
-------
Table IV-2
EFFECT OF TEMPERATURE
ON DIGESTION TIME
Temperature
(Deg. F.)
59
68
77
86
95
104
113
122
140
Digestion
Time (Days)
67.8
46.6
37.5
33.3
23.7
22.7
14.4
8.9
12.6
(NOTE: Deg. C. equals deg. F minus 32 times
0.55)
needed to reach the same efficiency as when
the temperature is held at 95 degrees F.
(35 degrees C.).
Operation of the heating system will vary
with geographical location, size of digester,
degree of loading, and in some cases type of
industrial waste mixed with the sludge. The
following discussion considers some of the
important principles of sludge heating.
CONSTANT TEMPERATURE CONTROL.
The optimum temperature range for normal
digestion is cited at 90-98 degrees F. (32-36
degrees C.}. If the digester cannot be heated
to 90-98 degrees F. (32-36 degrees C.), it is
better to maintain control at a lower tempera-
ture (at a constant value) than to fluctuate
between high and low temperatures over a
short period of time.
CHANGES IN TEMPERATURE. Temperature
should not be changed more than 1 degree
Fahrenheit per day once the operating temp-
erature has stabilized. During start-up or after
recovery from other difficulties which cause
the temperature to drop more than 10-15
degrees F. (5-8 degrees C.) the temperature
can be brought up to normal at a faster rate,
but should be stabilized and held when the
normal temperature level is reached. This is
particularly true when starting the digester
because it may be necessary to raise from
60-95 degrees F. (15-35 degrees C.) in seven
or eight days.
THERMOPHILIC TEMPERATURES. Temp-
eratures above approximately 102-110 de-
grees F. (39-43 degrees C.) cause a change in
the major type of methane bacteria and can
result in unstable operation if temperatures
fluctuate in this region. In the northern part
of the nation, where temperature fluctuations
affect heating capabilities of the plant, even
greater problems will be encountered than in
the southern part of the nation. A limited
number of plants have operated units for ex-
tended periods of time in this range, but the
practice is not widespread enough and will
not be discussed in detail in this manual. The
reader is referred to the City of Los Angeles
Hyperion plant article in the WPCF Journal
for more information on the subject, as several
years of experience have been gained at this
plant. See Appendix B, "References."
General methods of sludge heating have been
described in this section beginning on
page 4-25.
pH. One of the most important environ-
mental requirements is the proper pH. For
example, the acid workers can function satis-
factorily at any pH level above 5, but the
methane workers are inhibited when the pH
falls below 6.2. In digester operation, slight
decreases in pH will seriously inhibit the
activity of the methane workers.
Best Operating Range: 6.8 to 7.2
Tolerable: 6.4 to 7.4
Cases are known where efficient digestion
occurs at pH's lower than 6.4, probably due
to development of a strain of bacteria able to
live in this environment.
4-14
-------
The pH of the liquid undergoing anaerobic
digestion is controlled by the amount: of
volatile acids produced and the alkalinity in
the digester.
Volatile Acids. The production of organic
acids is largely dependent upon the volume of
sludge fed to the digester. In a normal or heal-
thy digester, acids will be used as food by the
methane formers at about the same rate as
they are pro'duced. Under these conditions,
the volatile acid content of the digesting sludge
usually runs in the range of 50'mg/l to 300
mg/l, expressed as acetic acid
If the same amount of sludge is fed daily, a
population balance between the acid group
and the methane group will be maintained
easily. On the other hand, if a large amount of
readily digestible organic matter were added
suddenly, excess amounts of acids would be
produced and lower the pH. When this occurs,
the methane formers slow down, can't keep
iup with the acid formers and acids accumu-
late in the digester.
Buffers in the digester keep the process from
becoming upset every time there is
overfeeding. '....'-.
Buffers. Process stability depends largely on
a digester's ability to resist a change in pH.
This is commonly known as its buffering
capacity measured as alkalinity. Buffers are
essential in digesters. During the digestion
process, the methane workers also produce
some buffering material, such as bicarbqnates,
carbonate and ammonia, which goes into solu-
tion. The amount of buffer produced in this
stage is usually enough to balance the acid
produced by the acid workers so that the pH
will remain at a constant level.
Alkaline buffers come from two sources:
1. Those already present in the incoming
sludge,"and
2. Those created as part of the digestion
process.
Incoming sludges in communities with hard
water supplies or with alkaline wastes from
industries have a higher buffering alkalinity,
sometimes as high as 6,000 mg/l. These
digesters can absorb much higher swings in
organic acids before pH is affected. Digesters
operating in areas with very little alkalinity in
the incoming wastes may need to add a
caustic material such as lime, soda ash, or
agricultural ammonia to raise the alkalinity.
Changes in the acid production rate or the
amount of buffering material can cause
changes in pH. Here is what happens when the
digester pH suddenly starts to change.
Assume a digester which usually runs at a pH
of 6.7 or 6.8 suddenly changes to a pH of 6.5.
This means that the "natural alkaline buffer in
the digester has been reduced, that acids are
being .made faster than a neutralizing buffer,
and that the methane formers can't keep up.
First, the operator needs to get the pH back
to normal. This gives him time to find the
cause of the problem and correct it. pH
control continues until the process returns to
normal. Typical causes of downward trends in
pH are:
1. Sudden changes in organic loading, temp-
. erature or type of waste.'
. 2. Lack of pH control.
3. Presence of toxic materials.
4. Slow bacterial growth during start-up.
Volatile acids and alkalinity are measured to
indicate the progress of digestion and to con-
trol the digester. These test results are normal-
ly used as the volatile acids to alkalinity ratio
4-15
-------
(VA/Alk) which is the concentration of vola-
tile acids (VA) divided by the alkalinity (Alk).
The digester works best if the VA/Alk is less
than 0.25, and many operators prefer to keep
the VA/Alk less than 0.15 to be safe. This
means that there is four to ten times more
alkalinity than volatile acids, and the digester
will be well buffered to keep the pH from
changing.
Toxic Materials. It is important to keep toxic
substances out of a digester since they inhibit
bacterial activity and can cause complete
failure. It is also important for operators to
recognize potential toxicity problems and to
apply the right corrective measures. All too
often, operators have treated a toxic problem
as an overload problem and added tons of
lime only find that the problem was still
there. Toxic problems and their cures are
described in detail in Part III of this manual.
DIGESTER CONTROL
No process can be operated without having
adequate control and an indication of its
progress. In digester operation, how are
controls and indicators defined? Controls are
short term and used for correction. They are
tests that can be run to confirm satisfactory
operation or to indicate an action that would
bring about change. Indicators are tests run,
recorded and used for forecasting purposes.
Control examples are shown in Figure 4-8.
DIGESTER CONTROL
Gas
Rate
Quantity
Material
External
FIGURE 4-8
DIGESTER CONTROL TEST DIAGRAM
Recirc.
Temp.
VA
Alk
pH
Temperature
Internal
External Control
External control tests are used to help the
operator control what is coming into the
digester. As an example, in normal operation
the operator should control the concentration
of solids in the feed to avoid diluting the
digester contents. To do this, the operator
takes a composite sample of the incoming
sludge and runs a total solids test. This test
measures all solids and what percent of the
liquid is in solid form. The test and applica-
tion is described under "Indicators" on
page 4-20. Best operation is obtained when the
feed sludge concentration is kept as high as
possible, preferably in the range of from four
to eight percent.
Other external control tests are quantity of
sludge handled in 'pounds per day and
tests which describe the characteristics of the
incoming sludge. In the latter case, the
operator can use this information to tell:
1. Whether the existing grit removal system
is operating as well as it should or whether
new equipment is needed.
2. Whether toxic materials are present.
3. Whether the sludge is fresh or stale.
4. How much heat will be needed and if the
digester operating temperature can be
maintained.
Internal Controls
Internal controls, illustrated in Figure 4-8,
show what is happening inside the digester.
Four tests are recommended for best control:
temperature, volatile acids, alkalinity and pH.
TEMPERATURE. Temperature directly
affects the work of the methane bacteria ;as
explained earlier on page 4-13. Variations in
temperature should never exceed more than
one degree per day. The best temperature
range lies between 85 and 100 degrees Fahren-
4-16
-------
heit (29-37 degrees centigrade). However, the
best temperature for any given digester Js
based on:
1. The highest gas production.
2. Ability to hold the volatile acids to alka-
linity ratio between 0.1 and 0.25.
3. Maintaining the pH near neutral (6.8 to
7.2).
Thermometer locations vary according to the
design of the digester. Some are inserted into
the digester wall, or in the sludge recirculation
line. Many operators must take temperatures
from samples drawn from the supernatant
overflow or from thief holes. -/•
VOLATILE ACIDS/ALKALINITY. The major
internal control combines two lab tests: vola-
tile acids and alkalinity. The alkalinity of a
digester is important because it represents the
ability of the digester to neutralize the acids
formed during digestion or present in the in-
coming waste. The results of these two tests
expressed in mg/l (milligrams per liter) are
combined as a ratio (volatile acids divided by
alkalinity) and expressed as a single number.
For example, 140 mg/l of volatile acids per
2,800 mg/l alkalinity is shown as:
140 mg/l _QQ5
2,800 mg/l 'U°
These two tests are run on sludge sa.mples
from the primary digester. Typical sampling
points are from the recirculated sludge, line
and from special sampling pipes located at
different tank levels. (NOTE: It is important
to let the sludge in the line; run for'a few
minutes in order to obtain a representative
sample.) Other sample points are from flowing
supernatant drawoff tubes or thief holes. Do
not take a sample immediately after adding
sludge to a digester because, of possible short
circuiting. Mix the contents thoroughly first.
The concentrations of volatile acids and alka-
linityare the first measurable changes that
take place when the process of digestion is
becoming upset. As long as the volatile acids
remain low, compared to alkalinity, the
digester can be considered healthy with good
digestion taking place. The volatile acid/alka-
linity relationship can vary from less than 0.1
to about 0.35 without significant changes in
digestion. Each plant will have its own charac-
teristic ratio for good digestion. An increase
in the ratio is the first warning that trouble
is starting in the digester and that serious
changes will occur unless the increase is
stopped. If the ratio increases, the following
changes will occur: -.
1. The C02 content of the gas will increase.
2. The gas production rate will decrease.
3. The pH of the digester will drop and the
digester will go sour.
pH. The pH is one of the simple tests that can
be run to indicate the progress of digestion
and .should be run frequently (at least once
per operating shift). The danger lies in depend-
ing, too much, on pH as a process control.
Because of the alkalinity in the digester, the
pH changes very slowly. In fact, the digester
may be completely upset before the pH
changes.
Frequent monitoring of the volatile acids and
the alkalinity and plotting the VA/Alk ratio
provides the best information for controlling
digesters because these indicators are the first
to show a change when the process begins to
become upset,
The graphs in Figure 4-9 illustrate the sequence
of the change within the digester.
4-17
-------
I Relationship
Of Volatile
Acids To
Alkalinity
Time
MG/L
20004-
1000
600
200
Alkalinity • B
This graph shows a digester operating with a good
buffering capacity (the low volatile acids 200 mg/l
compared to an alkalinity of 2,000 mg/l. At Point A,
something has happened to cause the volatile acids to
increase followed by a decrease in alkalinity at
Point D. At Point G, the digester has become sour,
II
Volatile Acids/
Alkalinity
Ratio
HI Relationship
Of The Change
In Ratio Of CO2
To Methane (CH4>
As A Result Of "I"
100T
90-
80-.
70-.
60-.
50->
40"
30-
201
CH4
CO2
Sludge
Feed
This graph continues the same digester performance
by showing the volatile acids/alkalinity ratio. Notice
that at Points CD, the increase in volatile acids
produces an increase in the ratio from 0.1 to 0.3.
By comparing this graph with Graph II, methane
production begins to drop with a corresponding
increase in CO2 when the ratio in Graph II reaches
about 0.5.
IV Relationship
Of pH Change To
Change In "I"
pH doesn't change in this graph until the digester is
becoming sour at Point G.
FIGURE 4-9 GRAPH OF CHANGE SEQUENCE
IN A DIGESTER
4-18
-------
Digesters respond slowly once they are upset.
Therefore, the best operation is obtained by
preventing upsets. The following general
guidelines are given for best process control.
1. Routine volatile acids and alkalinity deter-
minations during any startup process are a
must in bringing a digester to a state of
satisfactory digestion.
2. Measure the volatile acid/alkalinity ratio
at least twice per week during normal
operation, plot against time and watch for
trends. NOTE: An example of this is
found in Appendix G.
3. Measure the volatile acid/alkalinity ratio
at least daily when a digester is approach-
ing trouble such as an increased solids
load from waste discharges or a storm.
4. C02 and pH tests may be substituted for
volatile acids/alkalinity control in those
cases where the loading is uniform and
predictable and process upsets are infre-
quent. It is important, however, to realize
that failures are costly in terms of both
money and time.
The following suggestions are given for correc-
tive response when the volatile acids/alkalinity
ratio exceeds 0.35.
1. Extend the mixing time of digester
contents.
2. Control heat more evenly.
3. Decrease sludge withdrawal rates.
4. Pump some seed sludge from the second-
ary digester to the primary using the
following guidelines.
a. Draw down the primary digester to
make room for the sludge addition.'
b. Use the volatile acid/alkalinity ratio as
a guide to determine how much seed
sludge should be added. Hold the ratio
to less than 0.25.
Several methods are available for performing
the volatile acids test.
1. Silicic Acid or Chromatographic
Method. This is the preferred method
when high accuracy is required. The test
can identify up to 95 percent of the
organic acids present in the sample. It is
the only test recommended by Standard
Methods for the Examination of Waste-
water. The test requires about one hour to
run. The disadvantages include the need
for more special' equipment and more
chemicals than the other methods.
Appendix E.
Standard Methods, 13th Edition,
page 577
2. Straight Distillation Method. This is one
of the most commonly used tests since
the procedure is fairly straightforward and
does not require any special equipment.
The test is not for accurate work but is
satisfactory for digester control. The
disadvantages are the test's dependency
on lab techniques to obtain good results
and it requires about an hour to run.
Appendix E.
Washington State- Wastewater Plant
Operator's Manual
3. Titration or Nonstandard Method. This
test was originally developed by
R. DiLallo and O'.E. Albertson and is
listed in the -WPCF Manual of Practice.
The test takes about ten minutes to,run
and is reported good when volatile acids
exceed 250 mg/l.
Appendix E.
MOP No. 18, Simplified Laboratory
Procedures for Wastewater
Examination
WPCF Journal, VolumeSS, April 1961,
page 356
4-19
-------
Process Indicators
Process indicators are those tests used for
forecasting purposes rather than control. Test
results are always recorded and best use is
made when they are graphed. Procedures for
graphing are described in Appendix G. The
most common tests used are shown on
Figure 4-10 and described below.
INDICATOR
1—•• Quantity
Retire.
Temp
% Moisture
% Volatile
% Moisture
Rgw % Volatile
pH
Quantity
Bottom
FIGURE 4-10
DIGESTER INDICATOR TEST DIAGRAM
Solids. Several points in the digestion process
are sampled and tested for solids— both total
solids and volatile solids. These points are raw
sludge feed, recirculated sludge, supernatant
and digested sludge.
These tests are needed to gain information on
such things as concentration, loading rate,
pounds of solids handled through the process
and the percent reduction of volatile solids.
TOTAL SOLIDS. Sludge concentration is
determined by drawing a sample from a well
mixed point for each source, such as raw
sludge feed, digester mixed sludge and super-
natant, and then performing a total solids
test. Total solids is obtained by evaporating
all of the water from a weighed sample and
weighing the residue. The results are express-
ed in percent of solids (dry basis). The percent
of solids can be converted to mg/l as follows:
water from a weighed sample and weighing
the residue. The results are expressed in
percent of solids (dry basis). The percent of
solids can be converted to mg/l as follows:
10,000 mg/l equals 1 per-
cent solids
20,000 mg/l equals 2 per-
cent solids
and so on ...
5,000 mg/l equals 0.5
percent solids
.To convert mg/l per million gallons into
pounds:
mg/l times volume in million gallons
times-8.34 equals pounds (OR)
mg/l x MG x 8.34= Ibs.
The effect of sludge concentration on diges-
tion time can be seen in the following example.
Suppose an operator pumps 12,800 gallons
(48400 I) of raw sludge with 2 percent (0.02)
solids and then changes the method of pump-
ing sludge to increase the concentration to 4
percent (0.04). The reduced amount to be
pumped can be found by setting up the fol-
lowing ratio:
0.02
0.04
12,800 gals. X gals.
then
0.02
0.04
x 12,800 = 6400 gpd (24000 I/day)
The same amount of food would be added.
For example:
1. 0.02 x 12,800 gpd x 8.34 Ibs./gal.
= 2,135 Ibs./day (968 kg/day)
2. 0.04 x 6400 gpd x 8.34 Ibs./gal.
= 2,135 Ibs./day (968 kg/day)
VOLATILE SOLIDS. Volatile solids tests are
used to indicate organic loading to the digester
and digester efficiency.
4-20
-------
Digester operators need to know what percent
of the total solids entering the digester is
coming in as food matter to be decomposed
by the bacteria. This is found by a volatile
solids test. The residue from the total solids
test sample is burned at 550 degrees centi-
grade until a white ash remains, in a dish. The
ash is then weighed, and this weight is sub-
tracted from the total solids weight. The dif-
ference between the two weights represents
the volatile or organic portion and the residue
after burning represents the ash or the inor-
ganic portion. This is shown in Figure 4-11.
FIGURE 4-11 SLUDGE FEED DIAGRAM
Volatile solids is usually expressed as a
percent of total solids. The numbers in these
examples were used only to illustrate relative
proportions and may be different than actual
plant conditions, : .
Let's carry the example one more step and
find out what typically happens to the sludge
fed:to a digester as shown in Figure 4-12.
FIGURE 4-12
HOW VOLATILE SOLIDS ARE CONVERTED
TO STABILIZED SLUDGE
For example, assume 100 pounds (45.4 kg) of
total sol ids'coming into a digester. Then-70
pounds (31,8 kg) of this material is volatile
solids and 30 pounds (13.6 kg) is ash or inert
material. As the volatile solids portion under-
goes digestion, it is converted into 40 pounds
(18.1 kg) of gas and water and 30 pounds
(13.6 kg) remain as undigested volatile sol ids.
Notice that the ash portion has been unaf-
fected, by digestion and. remains as 30 pounds
(13.6kg).
It is desirable to feed the digester with the
highest volatile solids content sludge possible.
This is done, with an efficient grit removal
system. The volatile content is an indirect
way of measuring the amount of grit material
in the incoming .sludge as well as directly
measuring the amount of food available to the
bacteria. It has been found that the volatile
content of incoming sludges should be above
70 percent. This means that the plant's grit
removal facilities must always operate at top
efficiency to prevent filling a digester with
grit and reducing its capacity. .Figure 4-5, on
page 4-9 , illustrates an inefficient digester
condition resulting from grit accumulation.
Typical grit removal equipment includes grit
channels, detritus tank and cyclonic grit
separators.
Now that we've discussed solids-and what
happens to them in a digester, let's look at
loading and efficiency.
Two different loading factors, organic load
and hydraulic load, are important process
indicators. . -.
ORGANIC LOADING. Organic load is the
amount of food (volatile solids) fed to the
digester each day and is normally calculated
as pounds of volatile solids fed per day per
cubic foot of active digester volume.
The method most commonly used to express
loading is to relate the amount of volatile
solids in the feed sludge to the active volume.
in the digester. .This figure is calculated by:
(1) averaging the volatile-solids content of the
4-21
-------
raw sludge; (2) knowing the total pounds of
sludge pumped into the digester in a given
period; (3) measuring or calculating the
volume of the digester; (4) dividing the
digester volume into pounds of volatile solids.
Ibs. raw sludge/day x % volatile content
available digester volume
The number that results can be expressed as
pounds of volatiles per cubic foot of digester
capacity, or as pounds of volatile solids per
100 or per 1,000 cubic feet (2.83-28.3 m3) of
capacity. This number is similar to the
expression used in activated sludge known as
the F/M ratio, except that this is an expres-
sion of the amount of food to the volume of
the digester.
The amount of active volume before digestion
takes place is affected by both the amount of
scum that is on the surface of the tank and
the amount of grit and inorganic material on
the bottom. When the tank starts out in a
clean condition, the active volume is
essentially equivalent to the total volume of
the tank. As time progresses, this active zone
is reduced more and more, causing a higher
loading ratio. Looking at it another way, less
volume is available to treat the same or an
increased amount of solids compared with
what was available originally.
The following example will help to illustrate
changes in loading. The volume when a diges-
ter is first put into operation is compared with
the volume four years later without cleaning
the tank.
Needed information:
Digester volume (available volume)
Pounds of raw sludge feed
Volatile content
Example:
Assume the available volume of a new
50-foot diameter (15.2 m) digester is
50,000 cubic feet (1416 m3), raw sludge
is 8,000 (3630 kg) pounds per day, vola-
1 tile content is 74 percent.
Then:
8,000 pounds/day x 0.74
= 5,920 Ibs. (2687 kg) VS per day
Loading: ;
5,920 pounds/day n,,,. wo
r-n nnfT—77 = 0.11 Ibs. VS
50,000 cu.ft.
per cu. ft. per day (1.76 kg/m3/day)
Let's continue the example to see what
changes have occurred to the same digester
four years later with the same loading rate.
The operator measures the scum blanket and
grit layer (as described in Part III, page 3-31),
and finds the scum averaging 5 feet (1.5 m)
deep and the grit 3 feet (0.9 m) deep. The
available volume has been reduced by a total
of 8 feet (2.43 m) which represents a loss of
8 ft. x ir x dia.2 _ 8 x 3.14 x (50)2
4 -4
= 15,700 cu.ft. (445m3)
Loading now is
5,920 Ibs./day
(50,000-15,700 cu.ft.)
= 0.17 Ibs. VS/cu.ft./day
(2.72 kg/m3/day)
This change in loading from 0.11 to 0.17
pounds of VS per cubic foot per day (1.76 to
2.72 kg/m3/day) will make the digester harder
to operate and may cause more frequent
upsets.
H Y D R A U LIC LO A DIN G. The hydraulic load-
ing is the average time in days that the liquid
stays in the digester and is related to digester
capacity. Hydraulic loading is calculated as
follows:
Hydraulic loading equals
Digester Volume/Feed Volume
For example, at an average pumping rate of
12,800 gallons per day into a 250,000-gallon
digester, the detention time would be:
4-22
-------
250,000 gal Ions ,nc- ,
1 o ono i /^—=19.5 days
12,800 gal./day
There is a minimum time required by a di-
gester to convert the solids into an acceptable
sludge. The minimum hydraulic loading varies
with the type of digester and the type of
solids (up to six months for a single unheated
unit to as low as ten daysfor a high-rate system).
If the time is too short, the methane formers
will not have enough time to convert the acids
produced by the acid formers to methane gas.
Some wastes need a longer time. For example,
a purely domestic waste needs a fairly short
time to complete the decomposition of solids,
but the same kind of municipal waste with
cellulose added by an industry would need a
much longer period.
*
Treatment plants located in agricultural
communities where food processors operate
on a seasonal basis have other problems
because the amount of sludge produced
suddenly, increases when the food processing
plant starts up! The increased volume of
sludge produces an immediate overload on the
digester. The operator must watch the
digester and add lime or other caustic to keep
the buffering capacity high. Often the good
sludge in a secondary digester is used to
accomplish the same purpose.
The hydraulic loading time can be increased
by prethickening the feed to reduce the
amount of water fed. Too much water in the
feed causes a hydraulic washout of both feed
and organisms. :.'.'"(.'"
QUANTITY OF RAW SLUDGE. Amounts of
raw sludge pumped may be found by several
means, but should be recorded even if it is an
educated guess as to the amount. The amount
may be measured by reading a magnetic flow
meter output, by measuring the volume of a
piston pump barrel and counting the number
of strokes per minute, by calculating the
volume of a sump or pit from which sludge is
pumped and recording the ..number of pits
full or sumps full pumped 'in a day, or by
measuring the distance traveled by a floating
cover on a primary digester. One or more of
these methods may be available to the opera-
tor; even an educated guess as to amount is
better than no information at all.
When more than one means exists for making
this determination, it is a good idea to com-
pare two different methods. For instance,
if measurements are made by estimating the
amount pumped out of a pit and into a diges-
ter with floating cover, estimate the volume
of each and compare results over several days
to see how close they are.
DIGESTER EFFICIENCY. The "In-Out" test
using volatile solids indicates digester effi-
ciency. Normally samples are drawn from the
digester feed sludge and from the digested or
bottom sludge. However, the active digester
sludge can be substituted for the bottom
sludge.
Digester efficiency can be calculated using the
volatile solids test results in the following
formula: • , .
p_ (In-Put) 1f.nq/
r — : -j- ———. X 1 UU /o
In - (In x Out)
Where In represents the percent of volatile
solids entering the digester and Out
represents the percent of volatile
solids leaving the digester and P is the
percent'reduction of volatile solids
Example:
Assume that the volatile solids entering a
digester is 70 percent and that a test
.showed 50 percent volatile solids leaving
the digester. , ', '.'
In =70% = 0:70
Out =50% = 0.50;
Then,
P =
(.70-.50)
.70 - (.70 x .50)
x100% = -20
.70-.35
= ^xTOO%
.3D
P = .57x 100% = 57%
4-23
-------
The In-Out tests can be used for indication
purposes. For example, if the trend shows a
decrease in percent reduction, then, this
might mean that the:
1. Volume of the digester has decreased.
2. Throughput has increased.
3. Temperature is not high enough.
4. An inhibitory or toxic material has
entered the digester.
CO2- This is a most easily measured major
component in the digester gas. Because the
sum of the C02 and CH4 (methane) is
approximately 100 percent, the amount of
CH4 can be roughly estimated by measuring
the C02. The CO2 content of the gas in
well-operating digesters ranges from 25 to 35
percent. The percent of CO2 can be an early
indicator of problems. When the percent of
C02 begins increasing, trouble may be on the
way. It is important, however, to realize that
the percent of C02 will increase soon after
feeding if sludge is fed into the digester
intermittently. If sludge is fed to the digester
only two or three times a day, information
should be obtained at different times during
the day to find normal values for the plant.
The best procedure for taking the C02 test,is
to take it the same time after feeding.
Gas Production. Gas production from a diges-
ter should be fairly constant if the feed is
constant. Gas production should range be-
tween 7 and 12 cubic feet for each pound of
volatile matter destroyed.
pH. pH run on raw feed may indicate the
presence of toxic material and whether the
incoming sludge is septic or fresh. Tests on
the digester contents indicate the balance of
neutralizing buffer. Changes show the need
for making caustic additions. The normal
range for pH in digesting sludge is from 6.8 to
7.2.
Summary: Process Control Indicators. As a
summary, some early indications of problems
are given by graphing the following param-
eters: pH, C02, alkalinity and volatile acid
ratio and gas production. Direction of the
parameter compared to time is more impor-
tant than absolute numbers. Table IV-3 below
illustrates how direction of several of these
parameters can indicate possible problems.
Table IV-3
DIRECTION OF PROCESS INDICATORS OCCURRING
SIMULTANEOUSLY INDICATE POSSIBLE DIGESTER PROBLEMS
Indicator
PH
CH4
(amount)
C02
(percent)
Alk.
Vol. Acid
Trend
of
graph
down
down
up
down
up
pH
down
down
CH4
down
down
down
CO2
up
Alk.
down
Vol. Acid
up
4-24
-------
TYPES OF EQUIPMENT
Sludge Heating
Submerged Burners. There are two general
types of these devices, one of which dis-
charges hot gas and flame directly into the
sludge (see Figure 4-13).' The other type has a
burner that is enclosed in a ductwork tubing
arrangement which allows the flame to heat
the interior of the tube, while the tube passes
through the contents of the digester. Hot
gases are exhausted through an opening in
the roof (see Figure 4-14).
Gas & Air
FIGURE 4-13 INTERNAL SUBMERGED BURNER
Hot Gases
Compressor &
Carburetor
FIGURE 4-14 EXTERNAL SUBMERGED BURNER
Steam Injection Into the Digester. Digester
gas or other fuel is used to fire boilers which
'.- JjdutasMfjTj**,^,+•=', ;-,'-•-'!*'' ''"*£•$ • " ' . '
supply steam to be injected into the digester.
Generally, multiple steam feed points,are
. used—either as pipes that extend to some
point below the sludge surface or are connect-
ed into sludge feed lines. Several companies
make steam injection devices that are mount-
ed in the sludge pipeline downstream of any
valves.
This type of system adds water to the tank
contents and, because boilers must be con-
tinuously fed, boiler water conditioning is an
important operational consideration.
Equipment that is standard with steam-
injection systems includes:
1. Boiler water conditioning.
2. Steam . lines which require safety
precautions.
3. Check valves which prevent sludge back-
ing up in the lines.
4. Pressure gauges and steam line controls.
Particular care must be taken that check
valves function correctly in lines that connect
with sludge under pressure. A check valve
failure in one plant with this type of heating
system caused sludge to be transferred into
the boiler. Fortunately, the boiler was not hot
at the time of the discharge or a serious
explosion could have occurred. After this
accident" occurred a second check valve was
placed in the line backing up the first one as
an added safety measure.
Steam Injection Into Preheat Tank. A system
similar to that described for "Steam Injection
Into The Digester" allows raw sludge to be
heated in a separate tank before being
pumped to the digester. Steam is injected into
the tank until a preset temperature is reached.
4-25
-------
Digester sludge may also be recirculated
through the tank, or both raw and digested
sludge may be introduced simultaneously.
Hot Water Transferred to Internal Digester
Heat Exchanger. Hot water from gas-fired
boilers or cooling water from gas-driven
engines may be pumped through several dif-
ferent types of devices located inside the
digester. Each type is described below:
1. Coils of pipe placed inside the tank and
normally secured to the wall allow hot
water to circulate around the walls and
return to the heat source. Various types
of mixers cause the sludge to move past
the hot water pipes. In some older instal-
lations, natural convection currents
caused by rising hot sludge may provide
the only mixing available (see
Figure 4-15).
Mixer
Expansion
FIGURE 4-15 INTERNAL HEAT EXCHANGER
5. Water meters.
6. Air relief valve.
7. Temperature gauges for monitoring the
water and sludge systems.
8. Temperature gauges for controlling the
Mixer
%- Out Return Water
±- In Hot Water
FIGURE 4-16 INTERNAL HEAT EXCHANGERS
2. Several different systems with heat
exchangers surrounding draft tubes are
used. The principle is to move sludge to
pass heated surfaces to increase heat
transfer (see Figure 4-16).
Common equipment is listed as follows:
1. Hot water source (boiler or engine jacket).
2. Heat exchanger circulation pump.
3. Heat exchanger submerged in tank.
4. Expansion tank.
4-26
heater and circulation pump.
External Heat Exchanger. Many of the major
pieces of equipment are the same as those
discussed'for internal heat exchangers. In this
system, sludge is circulated from the digester
through the exchanger and back to the tank
rather than transporting water to a point
inside the tank. Raw sludge may also be
heated in this system, either separately or.
with the digester recirculated sludge.
-------
It is common "practice to locate the heat
. z^*^'1' '• -
exchanger near the boiler to reduce heat loss
• A. :.
-------
Digester Covers
Motor
| Gear Reducer
Shaft Seal
Cover
Motor
Shaft Seal
—•••
.Cover
Propeller
Pump
Cutter
Impeller
FIGURE 4-19
INTERNAL MOVING MIXERS
An important factor related to this method of
mixing is the exposure of moving surfaces to
grit and debris. Material that collects around
shafts and impellers can cause vibration due
to imbalance, and grit wears away impellers.
Generally, as the diameter of the rotor
increases, the debris problem increases, but
wear from grit decreases. On the other hand,
small diameter pump impellers or mixing
propellers are subject to rapid wear in heavy
grit conditions.
Recirculation. Recirculation demands the use
of a pump, which may be centrifugal or pis-
ton type, and which is located externally
from the digester. The total capacity of the
pump is generally less than the circulating
capacity of mixers. However, some plants use
recirculation as the only means of mixing.
The top surface or cover of a digester has
some'unique features which merit discussion.
Personnel must be aware of how variation in
pressure, contents and level inside the tank
may affect the cover. Three major types are
discussed; fixed, floating and gas holder.
General Comments. The cover on the digester
serves several purposes: superstructure for
mixing equipment, access to the tank, support
for safety devices, space for accumulation and
collection of gas and variable storage space for
the gas produced. The operator should recog-
nize that precautions must be taken to
prevent damage due to excess pressure which
may occur when the sludge lines plug and/or
gas control devices fail. Damage may also
result when vacuum devices fail and movable
covers are on the corbels. Fixed covers are
vulnerable when the rate of sludge being
drawn out exceeds the feed rate, or vacuum
devices fail.
Fixed Covers. The biggest hazard in fixed
cover operation is encountered when the
pressure relief device fails, supernatant over-
flow line plugs and the liquid level continues
to rise. The pressure inside the tank can lift
the fixed cover off the walls causing serious
damage. The covers may either be concrete
structure rigidly fixed to the walls, or metal
with anchor bolts securing the cover in one
position (see Figure 4-20). In either case, sep-
aration from the top of the wall will require
Fixed Cover
Pressure Vacuum
Relief
(Alternate Design)
Supernatant
Overflow
Metal
Cover
Anchor
Sealing
Material
FIGURE 4-20 FIXED COVER
4-28
-------
tank draining and expensive repairs.
Floating Covers. Various types of floating
covers are in use; however, they have many of
the same characteristics and operational prob-
lems. Figure 4-21 shows one type. Generally,
the cover floats on the surface of the sludge
which varies as feeding and supernatant
removal rates change.
Gas Holder
Cover
Gas
P-V Relief Valve
-Floating
Cover ^-Weights
Supernatant
FIGURE 4-21 FLOATING COVER
Maintaining cover guides in smooth operating
condition and keeping the cover level are the
two main operational concerns with floating
covers. For covers with wooden super-
structures, replacing the deck and repairing or
recovering roofing may also be necessary.
Care must be taken not to pump in excessive
amounts of sludge, particularly if plugging of
the overflow line is a problem. There have
been instances where covers have floated over
the wall because of high sludge levels. Covers
have also been known to collapse when the
vacuum relief failed and the covers were
setting on the corbels. Failures of this type
are most prevalent during freezing weather.
Gas Holder Covers. The third major type of
cover is used to store gas as it is produced.
The pressure developed inside the tank causes
the cover to lift as much .as six feet or more
above the minimum height. The cover has a
much longer skirt than the floating type (see
Figure 4-22).
Stiff metal guides and rollers are.mounted
between the cover and the wall superstructure
Supernatant
FIGURE 4-22 GAS HOLDER COVER
to allow the cover to travel .up and down
without binding. Accumulation of heavy
scum around the edge between the cover and
the walls can cause excessive friction and pre-
vent free travel. Pressure and vacuum relief
valves must also be kept in good operating
condition to maintain the desired pressure.
Gas Handling Equipment and Control Devices
Figure 4-23, Typical Flow and Installation
Diagram—Digester Gas System, shows many
of the major pieces of equipment in a gas
collection and distribution system.
1. Pressure Relief Device. The pressure relief
device allows excess pressure to escape
from the digester in the'event that a
blockage occurs and pressures build up
above a safe level. Troubleshooting infor-
mation has been given in Part I, Trouble-
shooting Guides 10 and 11 on digester
covers.
2. Vacuum Breakers. The vacuum breaker
functions opposite to the pressure relief
valves and allows air to enter the tank in
the event that sludge is drawn out of the
tank too rapidly or the level of the sludge
changes suddenly with relation to the
floating cover. Troubleshooting informa-
tion is given in Troubleshooting Guides
10 and 11.
4-29
-------
Digester
(Fixed or
||j Floating Roof)
I
||| Digester
||| (Fixed or
FIGURE 4-23
TYPICAL FLOW AND INSTALLATION
DIAGRAM
<«*
3. Sediment and Drip Trap Assembly. Water
that condenses in gas lines is normally
taken out at the low points in drip traps.
This water should be removed daily or
more frequently when condensation rates*
are high. Gas seals sometimes leak due to
drying out of the material; therefore,
these should be checked monthly and the
entire unit disassembled and inspected
annually.
4. Flame Trap Assembly. Flame traps are
installed in lines to prevent flames travel ing
up the gas line and reaching the digester.
The trap consists of a metal grid which
allows the gas to cool down below the
combustion point as it passes through the
metal grid. If a high amount of impurities
is in the gas, the metal grid may become
fouled and prevent gas passing through.
These units should be disassembled
annually and washed out in a safe solvent.
Refer to the manufacturer's instructions
for specific details.
5. Pressure Regulator. When gas is used, a
lower pressure than the system operating
pressure may be needed. Regulators are
installed to maintain a constant pressure
at the point of use. The pressure regulator
can be adjusted to values less than system
pressures; however, adjustments should be
made following the manufacturer's
instructions.
6. Gas Meter. Several types of gas meters are
in use in treatment plants. These can be a
useful tool or an exasperating headache
depending on where they are installed and
the difficulties in keeping them operating.
Several of these are discussed below:
4-30
-------
a. Bellows Type Meter. The bellows '
meter is most similar loathe, jde,y^
that the old fashioned blacksmith
used to provide air for his fire. • • .. .
b; Shunt-Flow Meter. This meter, which
has a propeller in it, allows a certain
amount of gas to bypass the main
section of the meter while a measured
amount passes through the meter.
c. Positive Displacement Type Meter.
This meter operates like a gear motor
or in reverse of a lobe type blower. In-
ternal moving parts turn in direct
ratio to the amount of gas passing
through them. Some operators maintain
a spare unit for emergencies. ;
7. Check Valve. When dual gas systems are
used (such as a dual fuel engine), check
' valves are installed to prevent flow of the
higher pressure gas back to the digester.
These are built to allow flow in one
direction only and should be inspected
and cleaned annually to assure .that all
moving parts are free of corrosion and
debris. • ' , ' .' . .
8. Manometers. Gas pressure is measured by
a glass column, which contains a special
oil or water. These columns are called
manometers. The measured pressure is '
• read in inches'of water column.
Several sources. Of information are available
for discussions on gas systems. The reader is
referred to the Operation of Wastewater
Treatment Plants—A Field Study, Chapter 8,
pages8-17 to 8-40, for' more detailed
discussion and pictures of the devices
described above.
4-31
-------
-------
APPENDIX
A-GLOSSARY
B- REFERENCES
C-PLANTS VISITED
D - METRIC CONVERSION EQUIVALENTS
E - DIGESTER TEST PROCEDURES
F-FORMULAS
G - DATA REVIEW AND GRAPHING
H-WORK SHEETS
-------
APPENDIX A
GLOSSARY
Acid Forming Bacteria-The group of bacteria
in a digester that produce volatile acids as one
of the by-products of their metabolism. The
acids are used as a food source by the
methane forming bacteria.
Aerobic—A condition in which "free" or
dissolved oxygen is present in the aquatic
environment.
Aerobic Bacteria—Bacteria which live and
reproduce only in an environment containing
oxygen which is available for their respiration
(breathing), such as atmospheric oxygen or
oxygen dissolved in water. Oxygen combined
chemically, such as in water molecules, H^O,
cannot be used for respiration by aerobic
bacteria.
Alkaline—The condition of water, wastewater,
or soil which contains a sufficient amount of
alkali substances to raise the pH above 7.0.
Anaerobic—A condition in which "free" or
dissolved oxygen is not present in the aquatic
environment.
Anaerobic Bacteria—Bacteria that live and
reproduce in an environment containing no
"free" or dissolved oxygen. Anaerobic
bacteria obtain their oxygen supply by
breaking down chemical compounds which
contain oxygen, such as sulfates (SO4).
Anaerobic Decomposition—Decomposition
and decay of organic material in an
environment containing no "free" or
dissolved oxygen.
Anaerobic Digestion—Wastewater solids and
water (about 5% solids, 95% water) are placed
in a large tank where bacteria decompose the
solids in the absence of dissolved oxygen. At
least two general groups of bacteria act in
balance: (1) Saprophytic (acid forming)
bacteria break down complex solids to
volatile acids, and (2) Methane Fermenters
break down the acids to methane, carbon
dioxide, and water.
Antagonistic Compounds—Materials that are
added to a digester usually in a solution form
that counteract or nullify the toxic effect of
certain metals. An example is adding ferric
sulfate to counteract copper salts.
Available Volume—The actual volume avail-
able in a digester for bacterial action. It is
calculated by subtracting the volume
occupied by grit and scum from the total
digester volume.
Buffer—A measure of the ability or capacity
of a solution or liquid to neutralize acids or
bases. This is a measure of the capacity of
water or wastewater for offering a resistance
to changes in the pH.
Concentration—(1) The amount of a given
substance dissolved in a unit volume of
solution. (2) The process of increasing the
dissolved solids per unit volume of solution,
usually by evaporation of the liquid.
Contact—The action occurring in the digester
whereby food and bacteria are intermixed,
allowing the food to be taken into the cell.
A-2
-------
Degradation—The breakdown of substances
by biological action.
Detention Time—The theoretical time
required to displace the contents of a tank or
unit at a given rate of discharge (volume
divided by rate of discharge).
Digester—A tank in which sludge"is placed to
allow sludge digestion to occur. Digestion
may occur under anaerobic (more common)
or aerobic conditions.
Draft Tube—An extension of the impeller
passage in a hydraulic turbine from the point
where the sludge leaves such passages down to
the bottom of the tube as part of the mixing
system in a high-rate sludge digestion tank.
Enzyme—A catalyst produced by living cells.
All enzymes are proteins, but not all proteins
are enzymes.
Facultative— Facultative bacteria can use
either molecular (dissolved) oxygen or oxygen
obtained from food 'materials. In other words,
facultative bacteria can live under aerobic or
anaerobic conditions.
Grit—The heavy mineral material present in
wastewater such as sand, eggshells, gravel, and
cinders. :
Imhoff Cone—A clear, cone-shaped container
marked with^graduations used to measure the
volumetric ^concentration of settleable solids
in wastewater.
Inhibitory Toxicity—Any demonstrable
inhibitory action of a substance on the rate of
general metabolism (including rate of
reproduction) of living organisms.
Inorganic Waste—Waste material such as sand,
salt, iron, calcium, and other mineral
materials which are not converted in large
quantities by organism action. Inorganic
wastes are chemical substances of mineral
origin and may contain Carbon and oxygen,
whereas organic wastes are chemicai
substances of animal or vegetable origin and
contain mainly carbon and hydrogen along
with other elements!
Load Factor—The ratio of the average load
carried by an operation to the maximum load
carried, during a given period of time,
expressed as a percentage. The load may
consist of almost anything; examples are
electrical power, number of persons served/or
amount of volatile solids added in proportion
to solids in a digester.
Manometer— Usually a 'glass tube filled with a
liquid and used to measure the difference in
pressure across a flow measuring device such
as an orifice or venturi meter.
Mesophilic Bacteria—Medium temperature: a
group of bacteria that thrive in a temperature
range between 68 and 113 degrees Fahrenheit.
Methane Forming Bacteria-Jhe group of
bacteria in a digester that use volatile acids as
a food source and produce methane as a
by-product.
Muffle Furnace—A small oven capable of
temperatures up to 550 degrees Celsius
(centigrade) and used in laboratories for
burning or incinerating samples to determine
their loss on ignition (volatile) or fixed solids
(ash),content ... '. ...
Organic Waste—Waste material which comes
from animal or vegetable sources. Organic
waste generally can be consumed by bacteria
and :other small organisms. Inorganic wastes
are chemical substances of mineral origin and
may contain carbon and oxygen, whereas
organic wastes contain mainly carbon and
hydrogen along with other elements.
A-3
-------
Pathogenic Organisms—Bacteria or viruses
which can cause disease (typhoid, cholera,
dysentery). There are many types of bacteria
which do not cause disease and which are not
called pathogenic. Many beneficial bacteria
are found in wastewater treatment processes
actively cleaning up organic wastes.
Septic—A condition produced by the growth
of anaerobic organisms. If severe, the
wastewater turns black, giving off foul odors
and creating a heavy oxygen demand.
Settleable Solids—That matter in wastewater
which will not stay in suspension during a
preselected settling period, such as one hour,
and settles to the bottom. In the Imhoff
cone test, the volume of matter that settles
to the bottom of the cone in one hour.
Sludge—The settleable solids separated from
liquids during processing or deposits on
bottoms of streams or other bodies of water.
Sludge Digestion—A process by which organic
matter in sludge is gasified, liquified, miner-
alized, or converted to a more stable form by
anaerobic (more common) or aerobic
organisms.
Sludge Gasification—A process in which
soluble and suspended organic matter is
converted into gas. Sludge gasification will
form bubbles of gas in the sludge and cause
large clumps of sludge to rise and float on the
water surf ace.
Supernatant— Liquid removed from settled
sludge. Supernatant commonly refers to the
liquid between the sludge on the bottom and
the scum on the surface of an anaerobic
digester. This liquid is usually returned to the
influent wet well or the primary clarifier.
Suspended So/ids—Solids that either float on
the surface of, or are in suspension in, water,
wastewater, or other liquids, and which are
largely removable by laboratory filtering.
Thief Hole—A digester sampling well.
Toxicity—A condition that may exist in
wastes that will inhibit or destroy the growth
or function of any organism.
Total Solids—The sum of dissolved and sus-
pended constituents in water or wastewater,
usually stated in milligrams per liter.
Volatile Acids—Fatty acids which are produced
by acid forming bacteria and which are
soluble in water. They can be steam-distilled
at atmospheric pressure. Volatile acids are
commonly reported as equivalent to acetic
acid.
Volatile Matter—Apparent loss of matter from
a residue ignited at 550 degrees plus or minus
25 degrees Celsius (centigrade) for a period ,of
time sufficient to reach constant weight of
residue, usually 10-15 minutes.
Volatile So/ids—The quantity of solids in
water, wastewater, or other liquids, lost on
ignition of the dry solids at 550 degrees
Celsius (centigrade).
A-4
-------
APPENDIX B
SUGGESTED READING AND
REFERENCES
BASICS
*Kerri, Kenneth D.,'et aL, "A^ Field Study
Training Program," Operation of Waste-
water Treatment Plants. (Chapter 8)
Sacramento State College Department of
Civil Engineering. .".'...
McCarty, P.L., "Anaerobic Waste'Treatment
Fundamentals," Public Works. September
. 1964. , - . .." .
Dague, R.R., "Application of Digester Theory
to Digester Control," Journal of Water
Pollution Control Federation. Vol. 40,
page 2021, December 1968.
**"Safety in Wastewater Works,"
tion Control Federation
Water Pollu-
Manual of
Practice No. 1.
**;'.'Operation of Wastewater Treatment Plants,"
Water Pollution Control Federation Man-
• ual of Practice No. 11.
**"Anaerobic Sludge Digestion," Water Pollu-
tion Control Federation Manual of Prac-
tice No. 16.
**"Simplified Laboratory Practices for Waste-
water Examination," Water Pollution
Control Federation Manual of Practice
No. 18.
DIGESTER OPERATION
Torpey, W.N. & Melbinger, N.R., "Digested
Sludge Recirculation," Journal of Water
Pollution Control Federation. Vol. 39,
page -1464, September 1967.
.Masseli, J.W., et al., "Sulfide Saturation for
Better Digester Performance^" Journal
of Water Pollution Control Federation.
Vol. 39, page 1369, August 1967.
Pfeffer, J.T., "Increased Loading on Digesters
with Recycle of Digested Solids," Journal
of Water Pollution Control Federation.
Vol. 40, page 1920, November 1968.
Cameron, M.S. "Grease Burner Stops Digester
Scum," American City. December 1974.
Andrews, J.F., "Control Strategies for the
. Anaerobic Digestion Process," Water and
Sewage Works. March 1975.
Garber, W.F., et al., "Energy Assessments of
Certain Wastewater Treatment and Solids
Disposal Processes," American Society of
Civil Engineers, January 1974.
Garber, W.F."Plant Scale Studies,of Thermo-
philic Digestion at Los Angeles," Journal
of Water Pollution Control Federation.
Vol. 26, page 1202,4954.;
Garber, W.F.,'et al., "Thermophilic Digestion
.:... at the Hyperion Treatment Plant," Journal
of Water Pollution Control Federation.
Vol. 47, page 950, May 1975.
B-1
-------
DIGESTER RECOVERY
Dague, R.R., et al., "Digestion Fundamentals
Applied to Digester Recovery—Two Case
Studies," Journal of Water Pollution Con-
trol Federation. Vol. 42, page 1666, Sep-
tember 1970.
Cooper, et al., "Agricultural Ammonia for
Stuck Digesters," 20th Purdue industrial
Waste Conference, May 5, 1965.
TOXICITY
McCarty, P.L, & McKinney, R.E., "Volatile
Acid Toxicity in Anaerobic Digestion,"
Journal of Water Pollution Control Fed-
eration. Vol. 33, page 223, March 1961.
McCarty, P.L, & Lawrence, A.W., "The Role
of Sulfide in Preventing Heavy Metal
Toxicity in Anaerobic Treatment," Jour-
nal of Water Pollution Control Federation.
Vol. 37, page 392, March 1965.
DIGESTER CLEANING
Garno, R.A., "Cleaning Digesters at Niles,
Michigan," Journal of Water Pollution
Control Federation. Vol. 33, page 996,
September 1961.
DIGESTER START-UP
Cassel, E.A., & Sawyer, C.N., "A Method for
Starting High-Rate Digesters," Journal of
Water Pollution Control Federation.
Vol. 31, page 123, February 1959.
Lynam, B., et al., "Start-up and Operation of
Two New High-Rate Digestion Systems,"
Journal of Water Pollution Control
Federation. Vol. 39, page 518, April 1967.
GENERAL
Environmental Protection. Agency, "Estimat-
ing Laboratory Needs for Municipal
Wastewater Treatment Facilities," EPA
430/9-74-002.
Environmental Protection Agency, "Estimat-
ing Staffing for Municipal Wastewater
Treatment Facilities," 68-01-0328.
* Available for purchase from Dr. Kenneth D.
Kerri, Department of Civil Engineering,
California State University—Sacramento,
6000 Jay Street, Sacramento, California
45819.
** Available for purchase from the Water
Pollution Control Federation, 3900 Wis-
consin Avenue, Washington, D.C. 20016.
B-2
-------
APPENDIX C
PLANTS VISITED
Plant •
- •
Vermont
Burlington
Vergenhes
'•' Rutland -
Massachusetts
Gardner
Clinton
Brockton' :
Connecticut .
Danbury
Ohio. v
Rocky River
Brecksville
Solon
Bedford
Twinsburg
Wooster
Wellington
Lurain
Cleveland
Georgia
St. Simons Is
Jesup
Statesboro
Atlanta
So. Carolina
Orangeburg
Greenville
California
Hyperion
Primary
Secondary
Oregon
Portland
Hillsboro
W.Side
Hillsboro
R. Cr.
Albany
Washington
Walla Walla
Size
I
3-.0 '
'•0.2
6.2
3.8
1.6
•7.4'
12.5
10' '
1
2
2
1
5
0.5
10
100
1
2
2
68
3
20
350
100
200
2 .
2
6
6
Type of
Plant
— ~~| T ~ "
<0
£
• x
X
X
X
to <>>
If
X
X v
X
X
X
X
X
X
X
X
X
X
X
X
X
li
'x
X
X
X
X
X
X
X
X
0
Number i
Digesters
2
1
2
2
2
"2 '
2 :
3
•2
2
,2
r
- 4
1
2
24
1
1
1
7
2
6
18
, 4
.. 2
2
".-*.
3
Type Cover
-j ]
Q)
O
X
,r
X
6
X
•
"S
X
il
1 .
1
2
1
2
•2.
2
1
:.1
r
2
1
1
7
1
2
is
2
1
O)
4=
1
U-
1
9
1
; 1
' 1
2
1
1
2
1
18
1
1
4
4
2
2
3
2
Type of
Mixing
External
X
X.
X
X
X
X
X
X
C3
X
X '
X
. ' '
X
,x
X,-' ;
X
X
X
X
X
X
o>
.Q
Draft Tu
X
\/
X
X ;
S--
0
X
X
X
X
Heating
External
X
x
X
X
X
X .
X
x
X
X
X
X
X
.x
X
X
X,
X,
Internal
X
X
: „ \
'A
\/
X
X
X
o
X
X
X
X-
-------
-------
APPENDIX D
METRIC EQUIVALENTS
METRIC CONVERSION TABLES
Recommended Units
Description
Length
Area
Volume
Mass
Time
Force
Unit
meter
kilometer
millimeter
centimeter
micrometer
square meter
squire kilometer
square centimeter
square millimeter
hectare
cubic meter
cubic centimeter
liter
kilogram .
gram
milligram
tonne
second
day
year
newton
Symbol
m
km
mm
cm
//m.
m*
km*
£&
ha
m3
cm3
1
kg
g
mg
t
s
day
yr or
a
N
Comments
Basic SI unit
The hectare (10.000
m2) is a recognized
multiple unit and
will remain in inter-
national use.
The liter is now
recognized as the
special name fof
the cubic decimeter
Basic SI unit'
1 tonne = 1,000kg
Basic SI unit
Neither the day nor
tha year is an SI unit
but both are impor-
tant.
The newton is that
force that produces
an acceleration of
1 m/s2 in a mess
of 1 kg.
English,-.
Equivalents
39.37 in. = 3.28 ft =;
1.09 yd
0.62 mi ...
0.03937 in.
0.3937 in.
3537 X 10-3 =103A
10.744 sq ft
= 1.196sqyd
6.384 sq mi =
247 acres
0.155 sq in:
0.00155 sq in.
2.471 acres
35.314 cu ft =
1.3079cuyd
0.061 cu in.
1.057 qt* 0.2 64 gal
= 0.81X10-* acre-
ft
2.205 Ib
0.035 oz= 15.43 gr
0.01 543 gr
0.984 ton (long) *
1.1 023 ton (short)
0.22481 Ib (weight)
= 7.5 poundals
*
Description
Velocity. .
linear
angular
Flow (volumetric)
., Viscosity
Pressure
Temperature
Work, energy,
quantity of heat
Power
Application of Units
Description
Precipitation,
run-off,
evaporation
River flow
Flow in pipes.
conduits, chan-
nels, over weirs.
pumping
Discharges or
abstractions.
yields
Usage of water
Density
Unit
millimeter
cubic meter
per second
cubic meter per
second
liter per second
cubic mater
per day
cubic meter
per year
liter per person
per day
kilogram per
cubic meter
.Symbol
mm
m3/s
m3/s
l/s
m3/day
m3/yr
I/person
day
kg/m3
Comments
For meteorological
purposes it may be
convenient to meas-
ure precipitation in
terms of. mass/ unit
area I kg/m 3).
1 mm of rain =
1 kg/sq m
Commonly called
the cumec
1 l/s = 86.4 nvVday
The density of
water under stand-
ard conditions is
1,QOQkg/m3or
1.0QOg/l
English
Equivalents
35.314 cfs
15.85gpm
1.83X10-3gpm
0.264 gcpd
0.0624 Ib/cu ft
Description
Concentration .
BOD loading
Hydraulic load
per unit area;
e.g. filtration
rates
Hydraulic load
par unit volume;
e.g. biological
filters, lagoons
Air supply
Pipes
diameter
length
Optical units
Recommended Unitjs
Unit
meter per
second
millimeter
par second
kilometers
par second
radians per
second
, cubic meter
per second
liter per second
poise
newton per
square meter
kilonawton per
square meter
kilogram (force)
per square
centimeter
degree Kelvin
degree Celsius
joule
kitojoule
watt
kilowatt
joule per second
Symbol
m/s
mm/s
km/s
rad/s
m3/s
l/s
poise
~ . •
N/m2
kN/m2
kgf/cm2
K
C
J
kJ
W
kW
J/i
Comments
Commonly called
the cumec
The newton is not
yet well-known as
the unit of force
and kgf cm2 will.
clearly be used for
some time. In this
field the hydraulic.
head expressed in
maters is an accept-
able alternative.
Basic SI unit
The Kelvin and
Celsius degrees
are identical.
Tha use of the
Celsius scale is
recommended as
it is the former
centigrade scale.
1 joule* 1 N-m
1 watt = 1 J/s
English
Equivalents
3.28 f pi
0.00328 fps
2.230 mph
15,850gpm
= 2.120 elm
15.85 gpm
0.0672/lb/
sec-ft
0.00014 psi .
0.145 psi
14.223 pii
* - "•"
2.778 X 10-'
fcwht =
3.725X10-'
hp-hr = 0.73756
ft-lb-9.48X
10-" Btu
2.778 kw-hr
Application of Units
Unit
milligram per
liter
kilogram per
.cubic meter
per day
cubic meter
per square meter
per day
cubic meter
per cubic meter
per day
cubic meter or
liter of free air
par sacond
millimeter
meter
lumen per
square meter
Symbol
mg/l
kg/m3 day
m3/m2 day
m3/m3day
m3/$
l/s
mm
m
luman/m2
Comments
If this is con-
verted to a
velocity.it
should be ex-
pressed in mm/s
(1 mm/s - 86.4
m3/m2day).
' >','".
Engliih
Equivalanti
1 ppm
0.0624 Ib/cu-ft
day
3.28 cu ft/sq It
m*
3.28 ft
0.092 ft
candlt/iq ft
D-1
-------
-------
APPENDIX E
DIGESTER TEST PROCEDURES
LIST OF TESTS
Determination of Volatile Acids in Waste-
water Sludge
Alkalinity of Wastewater and Sludge
Sludge Solids (Total Solids and Volatile
Solids)
Settleable Matter (I mhoff Cone Test)
Sludge (Digested) Dewatering Characteristics
Supernatant Graduate Evaluation
Gas Analysis
ALKALINITY OF WASTEWATER AND
SLUDGE
Reference:
p. 370)
(StandardMethods, 13th Edition,
All samples must be settled so; that a liquid
free of solids is available for the test. Tests
cannot be calculated correctly if solids are in
the sample. All samples must be kept cool and
analyzed as soon as possible.
What is Tested?
Sample
Recirculated sludge
Common Range
2-10 times volatile acids
Apparatus
1. Centrifuge and centrifuge tubes, or set-
tling cylinder. .
2. Graduated cylinders (25 ml and 100 ml).
3. 50 ml Burette.
4. 250 ml Erlenmeyer flask or 250 ml beaker.
5. pH meter or a methyl orange chemical
color indicator may be 'used (see
Procedure).
Reagents
For preparation consult Standard Methods or .
purchase prepared.
•j.Sulfuric acid, 0.1 N, which is sufficient
for alkalinities ranging from 500-6,000
mg/l. Cautiously add 2.8 ml of concen-
trated sulfuric acid (H2S04) to 300 ml
of distilled water. Dilute to one liter
with boiled ~and cooled distilled water.
Standardize against 0.10 N sodium carbo-
nate (Step 2).
2. Sodium Carbonate, 0.10 N. Dry in oven
before weighing. Dissolve 5.30 Og of
anhydrous sodium carbonate (Iv^COg) in
boiled and cooled distilled water and di-
lute to one liter with distilled water.
3. Methyl orange chemical color indicator.
Dissolve 0.05 g methyl orange in 100
• milliliters of distilled water.
Procedure
This procedure is followed to measure the
alkalinity of a sample and also the alkalinity
of a distilled water blank.
1. Take a clean 250 ml beaker and add 100
- ml or less of clear supernatant (in case of
water or distilled water, use 100 ml sam-
ple). Select a sample volume that will give
a usable titration volume. If the liquid
will not separate from the sludge by
standing and a centrifuge is not available,
use the top portion of the sample. This
same sample and filtrate should be used
for both the total alkalinity test and the
volatile acids test.
E-1
-------
2. If digester alkalinity tends to be above
3,000 mg/l, adjust sample size to between
25 and 50 ml.
3. Place the electrodes of pH meter into the
250 ml beaker containing the sample.
4. Titrate to a pH of 4.5 with 0.10 N sulfuric
acid. (In case of a lack of pH meter, add
five drops of methyl orange indicator. In
this case, titrate to the first permanent
change of color to a red-orange color.
Care must be exercised in determining the
change of color and your ability to detect
the change will improve with experience.)
5. Calculate alkalinity as mg/l CaCOg.
Calculation Example
Where: B = 38.0 mis
N = 0.10
Sample size = 100 mis
_ 38.0x0.1 x 50,000
100
4. Titrate
1. Centrifuge or settle
Sludge Sample
3. Place electrodes of
pH meter in beaker
2. Pour 100 mis of
supernatant into
beaker
,-Rs
OR 3. Add 2 drops of
methyl orange
Formula:
_Bx N x 50,000
Alkalinity (mg/l) = m,s ot samp|e
Where B = mis of h^SC^ required to titrate sample to pH 4.5
N = Normality of H2S04, i.e., 0.1 N
E-2
-------
DETERMINATION OF VOLATILE ACIDS
IN WASTEWATER SLUDGE ,
What is Tested?
Sample
Recirculated sludge
Desired Range
150-600 mg/l (expect trouble
if alkalinity less than two times
volatile.acids)
Method A (Silicic Acid Method)
(StandardMethods, 13th Edition, p. 577)
If an aqueous sample containing volatile.acids
is adsorbed on a silicic acid column and an
organic solvent is passed through the column,
the volatile acids will be extracted from the
aqueous sample. The extracted acids then can
be determined by titration with a base dis-
solved in methyl alcohol.
Water J
Drain
Apparatus.
1. Centrifuge or
filtering
apparatus
2. Two 50 ml
graduated
cylinders
3. Two medicine
droppers
4. Crucibles,
Gooch or fritted
glass
5. Filter flask .-
6. Vacuum source
7. One 50 ml
beaker
8. Two 5 ml
pipettes,
9. Burette
10.IL separatory
funnel
Reagents.
Silicic acid, solids, 100-mesh. Remove
fines from solid portion of acid by slurry-
ing the acid in distilled water and remov-
ing the supernatant after allowing settling
Centrifuge or vacuum filter 50 ml of sludge.
for 15 minutes. Repeat the process several
times. Dry the washed acid solids in an
oven at 103°C. until absolutely dry and
then store in a desiccator.
2. Chloroform-butanol reagent. Mix 300 ml
chloroform, 100 ml n-butanoi, and 80 ml
0.5 N ^504 in separatory funnel and
allow the water and organic layers 'to
separate.-Drain off the lower organic layer
.through filter paper into a dry bottle.
3. Thymol blue indicator solution. Dissolve
80 mg thymol blue in 100 ml absolute
..methanol.
4. Phenolphthalein indicator solution. Dis-
solve 80 mg phenolphthalein in 100 ml
•absolute methanol.
5.^Sulfuric acid, concentrated.
6. Standard sodium hydroxide titrant,
0.02 N. Dilute 20 ml 1,0 N NaOH stock
solution to 1 liter with absolute, methanol.
The stock is prepared in water and stan-
dardized against 50 mis of 0.1 N h^SCV).
as prepared in the alkalinity test, using
five drops of phenolphthalein indicator as
an endpoint detection. It will require
approximately 5.0 mis of 1.0 N NaOH
stock to titrate and concentrations shoujd
be adjusted so that it takes exactly 5.0 mis.
E-3
-------
A fritted-glass
filtering crucible
is used in the
volatile acid
determination.
Procedure.
1. Centrifuge or filter enough sludge to
obtain a sample of 10 to 15 ml. This same
sample and filtrate should be used for
both the volatile acids test and the total
alkalinity test.
2. Measure volume (10 to 15 ml) of sample
and place in a beaker.
Volume of sample, B =
ml
3. Add a few drops of thymol blue indicator
solution.
4. Add concentrated H2SO4, dropwise, until
sample just turns red or use pH paper to
a pH of 1.0 to 1.2.
5. Place 12 grams of silicic acid (solid acid)
in crucible and apply suction. This will
pack the acid material and the packed ma-
terial is sometimes called a column.
6. With a pipette, distribute 5.0 ml acidified
sample (from Step 4) as uniformly as
possible over the column. Apply suction
briefly to draw the acidified sample into
the silicic acid column. Release the vac-
uum as soon as the sample enters the
column.
7. Quickly add 65 ml chloroform-butanol
reagent to the column.
8. Apply suction and stop just before the
last of the reagent enters the column.
9. Remove the filter flask from the crucible.
10. Add a few drops of phenolphthalein indi-
cator solution to the liquid in the filter
flask.
11. Titrate with 0.02 N NaOH titrant in
absolute methanol, taking care to avoid
aerating the sample (discard when white
precipitate forms). Nitrogen gas or C02—
free air delivered through a small glass
tube may be used both to mix the sample
and to prevent contact with atmospheric
C02 during titration (C02—free air may
be obtained by passing air through ascar-
ite or equivalent).
Volume of NaOH used in sample titration,
a = ml
12 Repeat the above procedure using a blank
consisting of 5.0 mis of acidified distilled
water, extract with reagent and titrate in a
similar manner.
Volume of NaOH used in blank titration,
b= ml
Precautions.
1. The sludge sample must be representative
of the digester. The sample line should be
allowed to run for a few minutes before
the sample is taken. The sample tempera-
ture should be as warm as the digester
itself.
2. The sample for the volatile acids test
should not be taken immediately after
charging the digester with raw sludge.
Should this be done, the raw sludge may
E-4
-------
3. Add a few
drops of thymol
blue.
1. Separate solids by
centrifuging or
filtering sample.
5. Place 12 g \
of silicic acid ^
in the fritted- ^
glass crucible, ^
and apply suc-
tion to the flask.
2. Measure 10-15 ml of
sample into beaker.
6. Add 5 ml acidified
sample.
7. Add 65 ml
chloroform-butanol.
8. Apply suction until
all of reagent has
entered solid acid
column.
4. Add concentrated
H2SO4 drop-wise
until thymol blue
turns red.
9. Remove filter flask.
short-circuit to the withdrawal point and
result, in the withdrawal of raw sludge
rather than digested sludge. Therefore,
after the raw sludge has been fed into the
tank, the tank should be well mixed by
recirculation or other means before a
sample is taken.
3. If a. digester is performing well with low
volatile ,acids and then if one sample
should Unexpectedly arid suddenly give a
high value, say over 1,000 mg/l of volatile
acids, do not become alarmed. The high
resiilt maV be caused by a poor, nonrepre-
sentative sample of raw sludge instead of
digested sludge. Resample and retest. The
11
10. Add a few drops of
phenolphthalein
Titrate the solution
with 0.02 N NaOH
until the pink color
of phenolphthalein
first appears.
second test may give a more typical value.
When increasing volatile acids and de-
creasing alkalinity are observed, this is a
definite warning of approaching control
problems. Corrective action should be
taken immediately, such as reducing the
feed rate, reseedirig from another digester,
maintaining optimum temperatures, im-
proving digester mixing, decreasing sludge
withdrawal rate, or cleaning the tank of
grit and scum.
E-5
-------
Example:
Volume of sample, B = 5 mis
Normality of NaOH titrant, N = 0.02 N
Volume of NaOH used in sample
titration, a = 1.4 ml
Volume of NaOH used in blank
titration, b = 0.5 ml
Calculation.
Volatile Acids, mg/l (as acetic acid)
Formula:
(a - b) x N x 60,000
mis of sample
Example:
(1.4 - 0.5} x.02x60,000
Method B (Nonstandard Titration Method)
Volatile Acid Alkalinity
Apparatus.
1. One pH meter
2. One adjustable hot plate
3. Two Burettes and stand
4. One 100ml beaker
Reagents.
1. pH 7.0 buffer solution
2. pH 4.0 buffer solution
3. Standard acid
4. Standard base
Procedure.
1. Standardize the pH meter with 7.0 buffer
and check pH before treatment of sample
1. Separate solids by
centrifuging or
removing water
above settled sample.
sssw
2. Measure
50ml&
place in
beaker.
3. Titrate with
sulfuric acid
to a pH of 4.0.
o oo
4. Note acid used and'
continue titrating to
pH 3.5 to 3.3.
5. L
s
^
5
.ightly boil
ample for
i minutes.
'• !• ". •' '
>-
6. Cool in water bath.
i ' ' ' ^
—I
7. Titrate to pH of 4.0,
with 0.05 N NaOH, note
burette reading, and
complete titration to a
pH of 7.0.
E-6
-------
to remove the solids. Filtration is not nec-
essary. Decanting (removing water above
settled material) or centrifuging sample is
satisfactory. Do not add any coagulant
aids. .
2. Titrate 50 ml of the sample in a 100 ml ;
beaker to pH 4.0; with the appropriate
strength sulfuric acid (.depends on alka-
linity), note acid used, A = ______ ml,
a.nd continue to pH 3.5 to 3.3. A mag-
netic mixer is extremely useful for this
titration.
3. Carefully buffer'pH meter at 4.00 while
lightly boiling'the sample a miriimuna of
three minutes. Cool in cold water bath to
original temperature. • .•
4. Titrate .sample : with standard 0.050: N
sodium hydroxide up to pH 4.00, and
note burette reading, a = ____1_ mi-
Complete the titration at pH 7.0, b =
,,______ ml. (If this titration consis-
tently takes more than 10 ml" of the
standard hydroxide, use 0.100 N NaOH.)
5. Calculate total alkalinity,- Take answer
in mis from Step 2, titration to' pH 4.0
and calculate alkalinity according to for-
mula on Alkalinity of ,;Wastewater and
Sludge, page E-2, remembering that there
will be a discrepancy between titration of
sample to pH 4.5 and this titration tp_4.0.
6. Calculate volatile acid alkalinity (alka-
linity between pH 4.0 and 7.0).
Volatile Acid Alkalinity, mg/l (as CaC03)
7. Calculate volatile acids.
Steps 1 and 2 will give the analyst the,pH and
total alkalinity, two control tests normally run
on digesters. The. difference between.the. total
and the volatile acid alkalinity is bicarbonate
alkalinity. The time required for Steps 3 and 4 is
about 10 minutes. - '
8. Calculate bicarbonate alkalinity. Volatile
• acid alkalinity in mg/l (Step 6)—Total
Alkalinity in mg/l (Step 5) = Bicarbonate
Alkalinity in mg/l. •
This- is an acceptable method^for digester control
to determine the volatile acid/alkalinity relation-
ship, but not of sufficient accuracy ;for research
work.
Example and Calculation. Titration from pH
4.0 to 7.0 of a 50 ml sample required 8 ml of.
0.05 N NaOH (a= 1.1 ml, b = 9.1 ml). • :
Step .5: Calculate volatile acjd alkalinity
(alkalinity between pH 4.0 and 7.0).
.^Volatile Acid Alkalinity, mg/l
.- = (b--:a) x 2,500/50 : '.,- '. ;" '..
. _8ml x 2,500 -...-..--,.;-
50ml -.-'""/ .
= 400 mg/l
Step 6: Calculate volatile acids. .
V.Casel: 400 mg/l > 180 mg/l. Therefore, ' ,.
--"-'- Volatile. Acids, mg/l .-••'•- :
= Volatile acid alkalinity x 1.50
= 400 mg/l x 1.50
= 600 mg/l ..... ; :.--':, '
Case 1: >180 mg/l volatile acid alkalinity.
Volatile Acids = volatile acid alkalinity times
-1.50
Case 2: < 180 mg/l volatile acid alkalinity.
Volatile Acids = volatile acid alkalinity times
' 1.00 ' .'. ' •
E-7
-------
Distillation Method.
This method is quite empirical and should be
carried out exactly as described. It is not
intended for accurate work but satisfactorily
serves the purpose of digester control.
1. Centrifuge 200 ml of sample for 5 min-
utes or allow the sludge sample to settle
for approximately one-half hour.
5. Titrate with 0.117 N sodium hydroxide,
using 4-6 drops of phenolphthalein as an
indicator.
6. Titrate to the first general appearance of
the pink color.
Volatile Acids = ml sodium hydroxide x 100.
2. Pour off 100 ml of the supernatant liquor
in a 500 ml distillation flask.
3. Add 100 ml of water and 4-5 clay chips or
similar material to prevent bumping. Add
5 ml concentrated sulfuric acid
then mix.
4. Distill off 150 ml at a rate of 5 ml/
minute.
Rubber Tubing & Clamp
for Pressure Relief
Glass Tubing
'wo Hole Rubber Stopper
ml. Boiling
isk w/water
Glass Beads
Bunsen Burner
or Equal Heat
Source
.Graduate Cylinder
VOLATILE ACIDS SET-UP WITH STEAM GENERATOR
NOTE:
The diagram shows the Volatile Acids setup with a steam generator. This method will give more
consistent results as the temperature of the sample remains constant and the rate of distillation is then
controlled by the rate at which the steam is produced. The alternate method would be to eliminate the
steam generator and apply heat directly to the sample flask.
E-8
-------
SLUDGE SOLIDS
Reference: (Standard Methods, 13th Edition,
p. 535)
Total Solids (Sludge)
Definition. Total solids in sludge is a measure
of all material present in sludge, both in
suspension and in solution. This test is accom-
plished by evaporating a weighed sample in a
drying oven. All the material remaining in a
sample after the water has been evaporated is
considered as the total solids.
Unlike total solids in wastewater which is
expressed in mg/l, total solids in sludge is
expressed in terms of percent by weight of
the total amount of solids.
Significance. This test is used for wastewater
sludges or where the solids can be expressed
in percentages by weight, 'an'd the weight can
be measured on an inexpensive beam balance
to the nearest .01 of a gram, The total solids
are composed of two components, volatile
and fixed solids. Volatile solids are composed
of organic compounds which are of either
plant or animal origin. Fixed solids are
inorganic compounds such as sand, gravel,
.minerals, or salts.
Volatile Solids
Definition. The volatile solids are that por-
tion of the total solids which will ignite at
550°C. Total volatile solids are usually ex-
pressed as a percent of the total solids.
What is Tested?
Common Range, % by Weight
Sample
Raw sludge
Raw sludge + waste
activated sludge
Recircu lated sludge
Total
-6% to 9%
2% to 5%
1.5% to 3%
Vol.
75%
80%
75%"
Fixed
25% ±6% -
. 20% ±5%
25% ±5%
Supernatant:
Good quality, has
suspended solids
Poor quality
Digested sludge to
air dry
1%
5%
3% too thin
to 8% :
50%
50%+ 10%
50%+ 10%
Apparatus.
1. Evaporating dish, , '
2. Analytical balance
/' 3. Drying oven, 103° -105°C. '
4; Measuring device—graduated cylinder
5.: Muffle furnace, 550°C.
Precautions.
1. Be sure that the sample is thoroughly
mixed and is representative of the sludge
being pumped. Generally, where sludge
pumping is intermittent, "sludge is much
r heavier at the beginning and is less dense
' toward the end of pumping.'Take several
: 'equal portions of sludge at regular inter-
>a|s and mix for a go.qd sample. :
2. Take a large sample (at least 1 I). Measure
•:• a 50 or 100ml sample which approximates
50 or 100 grams into an evaporating dish
that has been preweighed. Since this ma-
"terial is so heterogeneous (nonuniform), it
E-9
-------
is difficult to obtain a good representative
sample with less volume. Smaller volumes
will show greater variations in answers due
to the uneven and lumpy nature of the
material.
3. Control oven temperature closely at 103°
to 105°C. Some solids are lost at any
drying temperature. Close control of oven
temperatures decreases the losses of vola-
tile solids and evaporated water.
4. Heat dish long enough to insure evapora-
tion of water, usually about 3-4 hours. If
heat drying and weighing are repeated,
stop when the weight change becomes
small per unit of drying time. The oxida-
tion, dehydration, and degradation of the
volatile fraction won't completely stabi-
lize until it is carbonized or becomes ash.
5. Since sludge is so nonuniform, weighing
on the analytical balance should probably
be made only to the nearest 0.01 grams or
10 milligrams.
Procedure.
1. Dry the dish by ignition in a muffle
furnace at 550°C. for one hour. Cool dish
in desiccator.
2. Tare the evaporating dish to the nearest
10 milligrams, or 0.01 g on the analytical
single pan balance. Record the weight as
Tare Weight equals g.
3. Weigh dish plus 50 to 100 ml of well
mixed sludge sample. Record total weight
to nearest 0.01 gram as Gross Weight
equals g.
4. Evaporate the sludge sample to dryness in
the 103°C. drying oven and place in des-
iccator to cool to room temperature.
5. Weigh the dried residue in the evaporating
dish to the nearest 10 milligrams, or
0.01 g. Record the weight as Dry Sample
and Dish equals g.
6. Compute the net weight of the residue by
subtracting the tare weight of the dish
from the dry sample and dish.
1. Ignite empty dish in
muffle furnace
4. Measure out sludge
6. Cool dish + residue
5. Evaporate water at 103-105°C
E-10
-------
Outline of Procedure for Volatile Solids
(continue from total solids test).
1. Determine the total solids as-previously--
described.
2. Ignite the dish and residue from total
solids test at 550°C. for one hour or until
a white ash remains.
3. Cool in desiccator to room temperature,
4. Weigh and record weight of Dish Plus Ash
equals g.
2. Cool
1. Ignite dried solids at 550°C
3. Weigh fixed solids
Total Solids, Volatile Total Solids and Mois-
ture in Sludge
A sample of sludge was tested for solids
content. The dish used weighed 8.62 grams
when empty and dry; the dish and wet sludge
weighed 21.82 grams. After drying the dish
overnight at 103-105°C., the dried sludge
weighed 9,.28 grams. After ignition for one
hour at 550°C., the dish plus ash weighed
8.85 grams. What was the percent of solids
in the sample, calculate the percent volatile
matter and percent moisture?
Type of form to be used:
Mo istu re = 100% - % sol ids = 100% - 5% = 95%
Weight of sample & dish, g
Weight of dish, g
Difference
Formula:
wt. of dried sludge
wt. of wet sludge
0.66 g
Wet Dry Ash
21.82 9.28 8.85
R.62 8.62 8.62
13.20 0.66
x 100 = % solids
0.23
13.20g
x 100% = 5%
Fixed matter (dry basis)
wt. of ash
x 100
wt. of dried sludge
0 23
= 066X10°=35%
Volatile matter (dry basis)
= 100%-% fixed solids • • \.
= 100%-35%
= 65%
Or another way of determining volatile, matter
.is to subtract weight of ash from weight of
dried .sludge and divide this difference by
weight of dried sludge.
% volatile matter equals .
wt. of dried sludge - wt. of ash
wt. of dried sludge
x 100
E-11
-------
SETTLEABLE MATTER (IMHOFF CONE TEST)
Mix well and
pour 1 liter
into Imhoff
cone
Settle
45 Min.
Reference: (Standard Methods, 13th Edition,
p. 539)
This simple test can be made to show quickly,
visibly and qualitatively if the primary and
secondary processes are functioning properly.
The volume of settleable solids in the raw
waste and effluents is seen readily, and the
turbidity removal due to secondary purifi-
cation processes is evident immediately to the
eye. The results are not quantitative but are
very illuminating to both the untrained and
trained operator.
Apparatus
An Imhoff cone, made either of Pyrex glass or
clear plastic material, a cone support, and a
long glass stirring rod.
Procedure
1. Fill the Imhoff cone to the one-liter mark
with a well-mixed sample.
2. Settle for 45 minutes and then gently stir
the sample with the glass rod to dislodge
suspended matter clinging to the tapered
sides of the cone.
Gently stir
sides
1 Liter
Settle
15 Min.
Read sludge
Volume
3. Settle 15 minutes longer; then read the
volume of settleable matter in ml/I.
Record the settleable solids as ml/l or milli-
liters per liter.
Settleable solids, influent =
Settleable solids, effluent =
Settleable solids, removal =
.ml/I
.ml/l
.ml/l
Example: Samples were collected from the
influent and effluent of a primary clarifier.
After one hour, the following results were
recorded:
Settleable
Solids, ml/l
Influent
Effluent
12.0
0.2
E-12
-------
Calculations. Calculate the efficiency or per-
cent removal of the above prirrjary clarifier in
removing settleable solids. <«•-<•-• ~««.«f|j
Percent Removal of Settleable Solids
(infl. set. sol., ml/I - effl. set. sol., ml/1)
. influent set. sol., ml/1
12 ml/1-0.2 ml/I
x 100%
100% =98%
11.8
12
-x100%
= 98%:
Estimate the gallons per day of sludge
pumped to a digester from the above primary
clarifier if the flow is 1 mgd (1 million gallons
per day). In your plant, the Imhoff cone may
not measure or indicate the exact perfor-
mance of your clarifier or sedimentation tank,
but with some experience you should be able
to relate or compare your lab tests with actual
performance.
Sludge removed by clarifier, ml/l
-(influent set. sol., ml/l) minus (effluent
set. sol., ml/I)
= 12 ml/l minus 0.2 ml/l
= 11.8 ml/I
To estimate the gpd (gallons per day) of
sludge pumped to a digester, use the following
formula:
Sludge to digester, gpd :, ?-"..•--;," - ?• '>
= total set. sol. rerrioved, ml/I times 1,000
^times flow; mgd (assume for illustra-
tion-a 1.0 mgd flow)
- = 11.8 x1,000 x. 1 = 1 -1,800 gpd
This value may be reduced-by 30 to 75% due
to compaction of the sludge in the clarifier.
If you figure sludge removed as a percentage
(1:18%), the sludge pumped to the digester
would be calculated as follows:
1.18%
100%
Sludge to
digester, gpd
sludge to digester, gpd.
flow of 1,000,000 gpd
1.18%'x 1,000,000gpd
=' 100%
= 11,800 gpd
E-13
-------
SLUDGE (DIGESTED) DEWATERING
CHARACTERISTICS
Discussion
The dewatering characteristics of digested
sludge are very important. The better the
dewatering characteristics or drainability of
the sludge, the quicker it will dry and the less
area will be required for sludge drying beds.
What is Tested?
Sample
Digested Sludge
Preferred Range
Method A Method B
Depends on
appearance
100-200 ml
Procedure
Two methods are presented in this section.
Method A relies on a visual observation and is
quick and simple. The only problem is that
operators on different shifts might record the
same sludge draining characteristics different-
ly. Method B requires 24 hours, buttheresults
are recorded by measuring the volume of
liquid that passed through the sand. Method B
would be indicative of what would happen if
you had sand drying .beds.
Apparatus
Method A. 1,000 ml graduated cylinder.
1. Add digested sludge to
1,000 ml graduate.
2. Pour sample from graduate
back into container.
Sample Container
3. Watch solids
adhere to
cylinder walls.
I.Add sample of digested sludge to
1,000 ml graduate.
2. Pour sample back into sample container.
Set graduated cylinder down.
3. Watch graduate. If solids adhere to cylin-
der wall and water leaves solids in form of
small streams, this is a good dewatering
sludge on a sand drying bed.
E-14
-------
Method B. .
1.1 mhoff cone with tip removed
2. Sand from drying bed
3.500 ml beaker • : _ :
Reagents
None.
1. Pour digested sludge
on top of sand in Imhoff
cone'.
2. Place beaker under
tip and wait 24 hrs.
Broken Tip
3. Measure liquid that
has passed through
the sand.
1. Broken glass Jmhoff cone that has tip
removed and a glass wool plug in the end
to hold the sand in the cone.
2. Fill halfway with sand from sand drying :
. bed. ' .."• '
.3. Fill remainder to one liter with digested
• sludge.
4. Place 500 ml beaker under cone tjp and
wait 24 hours. : ''":._.iV"rb
5. Record liquid that has passed through
sand in ml. If less than 100 ml has passed
through sand, you have poor sludge
. drainability.
E-15
-------
Supernatant Graduate Evaluation
Discussion. The digester supernatant solids
test measures the percent of settleable solids
being returned to the plant headworks. The
settleable solids falling to the bottom of a
graduate should not exceed the bottom 5% of
the graduate in most secondary plants. When
this happens, you are imposing a load on the
primary settling tanks that they were not
designed to handle. If the solids exceed 5%
you should run a suspended solids Gooch
crucible test on the sample and calculate the
recycle load on the plant that is originating
from the digester.
What is Tested?
Sample Common Values
Supernatant % Solids should be < 5%
Apparatus. 100 ml graduated cylinder.
Reagents. None.
Procedure
1.F1II a 100 ml graduated cylinder with
supernatant sample.
2. After 60 minutes, read the ml of solids
that have settled to the bottom.
3. Calculate supernatant solids, %.
Supernatant solids, % = ml of solids
Example. Solids on bottom of cylinder,
10ml.
Calculations.
Supernatant Solids, %
= ml of solids
= 10 ml
= 10% solids (high) by volume
1. Fill 100ml graduate
with supernatant.
2. After 60 min., read
ml of solids at bottom.
Supernatant Sample —
J_
10ml
100 ml Graduate
E-16
-------
GAS ANALYSIS
Definition
The digester gas analysis is the carbon dioxide
content of the digester gas expressed in
percent.
Significance
Digester gas production is a direct indication
of the activities taking place in a digester. The
gas generally analyzes at 30% carbon dioxide
and 70% methane. Each plant must develop a
history of analysis results. Deviations from
the normal trend indicate changes in activities
within the digesting sludge. When the opera-
tor detects these changes through changing
gas analysis, he may perform further and
more intensive study of the sludge. The CC>2
content of a properly operating digester will
range from 25-35% by volume. If the percent
is above 35%, the gas will not burn. The easiest
test procedure for determining this change is
with a CC>2 analyzer.
Method A (Orsat). The Orsat gas analyzer can
measure the concentrations of carbon diox-
ide, oxygen and methane by volume in diges-
ter gas. To analyze digester gas by the.Orsat
method, follow equipment manufacturer's in-
structions. This procedure is not recommended
for the inexperienced operator.
Method B.
APPARATUS
1. One Bunsen burner
2. Plastic tubing- '- '
3. 100 ml graduated cylinder
4.250 ml beaker
REAGENTS. C02 Absorbent (KOH), Add
500 g potassium hydroxide (KOH) per liter of
water.
PROCEDURE.
1. Measure total volume of a 100 ml grad-
uate by filling it to the top with water
(approximately 125 ml)/ Record this
volume. •."..-..;
2. Pour approximately 125 ml of C02 ab-
sorbent in a 250 ml beaker.
CAUTION: Do not get any, of this chem-
ical on your skin or clothes. Wash im-
mediately with running water until slip-
pery feeling is gone or severe burns can
occur.
3. Collect a representative sample of gas
from the gas dome on "the digester, a hot
water heater using digester gas to heat the
sludge, or any other gas outlet. Before
collecting the sample for the test, attach
one end of a gas hose to the gas outlet and
the other end to a Bunsen burner. Turn
on the gas, ignite the burner, and allow it
to burn digester gas for a sufficient length
of time to insure collecting a representa-
" :~ tive gas sample. ,
4. With gas running .through hose from gas
sampling outlet, place hose inside inverted
calibrated graduated cylinder and allow
digester gas to displace air in graduate.
Turn off gas.
CAUTION: The proper mixture of di-
gester gas and air is explosive when
exposed to a flame.
5. Place graduate full of digester gas upside
down In
sorbent.
beaker containing C02 ab-
6. Insert gas hose inside upside down
graduate.
t~— I /
-------
7. Turn on gas, but do not blow out liquid.
Run gas for at least 60 seconds.
8. Carefully remove hose from graduate with
gas still running.
9. Immediately turn off gas.
10. Wait for ten minutes and shake gently. If
liquid continues to rise, wait until it stops.
11. Read gas remaining in graduate to near-
est ml.
Example:
Total volume of graduate equals 126 ml
Gas remaining in graduate equals 80 ml :
CALCULATION
Percent C62
_ (total vol., ml - gas remaining, ml)
total volume, ml
(126 ml-80 ml)
x 100%
126ml
• x 100%
46
= i26x100%
= 37%
1. Clean out sampling
line by allowing gas
from sampling outlet
to burn until line is
full of gas from digester.
2. Displace air in
graduated cylinder.
3. Place graduate
upside down in
beaker containing
CO2 absorbent.
4. Insert hose in graduate
and run gas for 60 sec.
5. Remove hose from
graduate and then
turn off gas. Wait
10 min.
"^
o
0
0
6. Read volume of gas
remaining to nearest ml.
PRECAUTIONS:
1. Avoid any open flames near the digester.
2. Work in a well ventilated area to avoid the formation of explosive mixtures of methane gas.
3. If your gas sampling outlet is on top of your digester, turn on outlet and vent the gas to the
atmosphere for several minutes to clear the line of old gas. Start with step 2, displace air in
graduated cylinder. NEVER ALLOW ANY SMOKING OR FLAMES NEAR THE DIGESTER
AT ANY TIME.
E-18
-------
APPENDIX F
FORMULAS AND CALCULATIONS USED
IN DIGESTER OPERATION AND
CONTROL
These are some of the more commonly used formulas and calculations for digester operation and
control. They are set up with captions showing general use and the metric conversion is shown
after the English unit form of the answer. If no metric equivalent is shown, expression is the same
in both systems.
1 . Total Pounds Fed
Find total pounds of solids fed to the digester per day.
TS(%)/100 x 8.34 (Ibs./gal.) x raw sludge (gal./day) = TS(lbs/day) [x 0.454 = kg/day] •
2. Volatile Pounds Fed
Find pounds of volatile solids fed to the digester per day.
Vol.(%)/1 00 x TS(lbs./day) = VS (Ibs./day) [x 0.454 = kg/day]
3. Volatile Solids Loading '
Find pounds volatile solids per cubic foot of digester capacity per day.
VS (Ibs./day) _ VS(l.bs./day) rx 16 Q2 = kg/day ]
Digester Vol. (cu. ft.) (cu.ft.) • ' m^
4. Ash or Inorganic Percent
Find percent ash in sludge sample when percent volatile is known.
! . 100%-VS(%) = ash(%)
5. Ash or Inorganic Solids Fed
Find pounds of ash if total solids and percent volatile are known.
TS(lbsVday) x (100% - Vol. %)/100 = ash(lbs./day)
6. Hydraulic Detention Time
Find time in days for digester volume to be displaced.
Tank VoUcu.ft.) x 7.5(gal./cu.ft.) = h drau|jc detention time, days
raw sludge (gal./day)
7. Solids Detention Time (SRT)
Find average time in days that solids remain in the digester (NOTE: Use information cover-
ing at least a month's averages).
TS in dig, ibs.) - supernatant (Ibs.) =
TS in raw sludge fed (Ibs./day)
retentjon t- days
F-1
-------
8. Volatile Solids Reduction
Find percent volatile reduction between feed and sludge disposed of (NOTE: Use percent
expressed in decimal equivalent).
VS in (%) - VS out (%)'
x 100% = VS reduction (%)
VS in (%) - [VS in (%) x VS out (%)]
9. Volatile Solids Converted
Find pounds of volatile converted to other forms per cubic foot digester volume per day.
Sludge pumped (gal./day) x TS(%) x Vol.(%) x VS red.(%) x 8.34(lbs./gal.)
Volume Digester (cu.ft.)
= VS converted
cu.ft.
[x 16.02=kg/d.f ]
10. Gas Produced From Volatile Solids
Find volume gas produced per pound volatile solids converted.
produced (cu.ft /^ay) x Djgester vo| (cu ft } = Qgs cu.ftyib. [x a062 = m3/kg]
Vol. solids converted - ,./
cu.ft.
1 1 . Gas Produced in Pounds per Day
Find pounds of gas produced per day if pounds of raw sludge volatile solids are known.
a. Net volatile available for conversion = raw Ibs. in - (dig. vol. remaining + Ibs. lost in super-
natant. NVS = VS raw (Ib./day) - (dig. VS Ib./day + super. VS (Ib./day).
b. NVS (Ib./day) x VS reduction (%) = Gas (Ibs./day) [x 16.02 = kg/m3] .
1 2. Volume of Sludge Pumped (Approximate From Piston Pump Strokes)
Find volume of sludge to digester from piston pump operation.
a. Volume pump cylinder per inch times stroke in inches
o
Vol. cu.in./in. x stroke (in.) = Vol. sludge (cu.in.) [x 16.4 = cm05]
b. Gallons per stroke
Vol sludge (cu.in.) = , /stroke [x 3 79 = |/stroke]
231 cu.m./gal.
c. Gallons sludge per day
No. strokes (stroke/day) x (gal./stroke) = sludge gal./day [x 3.79 = I/day]
F-2
-------
13. Volatil? Acids/Alkalinity Ratio (VA/Alk)
Find volatile acids/alkalinity ratio with both expressed as mg/l.
• "- -..-;- .-•••m. • m-: •• •' - -• • •,
Vol. Acids (mg/l)=VA/A|kratio
Alk (mg/l)
14. Volatile Solids Ratio
Find the ratio of total solids in the digester to volatile solids in the feed.
,-...'., TS in digester (Ibs.) = TS (should be at least
VS in raw sludge (Ibs./day) VS/day 10 for good digestion)
(NOTE: See Metric Conversion Tables in Appendix D.) - -.
F-3
-------
-------
APPENDIX G
DATA REVIEW AND GRAPHING
A number of tests have been recommended in
this manual to provide process control infor-
mation for digesters/The amount of informa-
tion collected is of particular value if it is put
to use for:
1. Day-to-day process control and routine
preventive maintenance.
2. Making decisions to drain tanks.
3. Making decisions to repair equipment.
4. Providing consulting engineers with future
design information.
Several books are available that assist the
operator in methods for collecting and
recording data. It is not the intent of this
section to present those procedures. These
publications include:
1. Operation of Wastewater Treatment
Plants, prepared by Sacramento State
College, Chapter 16.
2. Manual of Wastewater Operations, Texas
Water Utilities Association (4th Edition),
Chapters 28 and 29.
3. Manual of Practice No. 11, WPCF,
Chapters 19 and 22. -
4. Simplified Lab Procedures for Wastewater
Examination, WPCF MOP No. 18.
There are several helpful systems developed
by other operators that may be of value and
these include methods of averaging, construc-
tion of graphs, use of graphs and use of solids
balances.
MOVING AVERAGES
With some sets of figures that may change
widely from day-to-day (such as sludge pump-
ing'or gas production), long-term moving aver-
ages are needed to establish trends. An exam-
ple is given in Table G-1 to calculate a seven
day moving average (DMA). When lab.results
are plotted on a graph as a 7 DMA, trends are
easier to detect. .."-.-
Table G-1
GAS PRODUCTION AVERAGE
2/1
2/2
2/3
2/4
2/5
2/6
2/7
12,300
14,700
11,000
1 1 ,500
14,600
15,800
12,500
92,400/7
13,200 = 7 DMA
Similarly, the 7 DMA for the next day 2/8 is
the average of the previous six days starting
with 2/2 and dropping 2/1 as shown in
Table G-2.
Table G-2
GAS PRODUCTION AVERAGE
2/2 14,700
2/3 11,000
2/4 11,500
2/5 14,600
2/6 15,800
2/7 12,500
2/8 13,700
, 93,800/7
13,400 = next 7 DM A
:"• ;"" ' • ". •"•. "•'' G-i
-------
The 7 DMA for 2/8 could also be calculated
easily from the previous day's calculations by
subtracting the data for 2/1 (12,300) from
the subtotal on Table G-3 (92,400) adding
the value of 2/8 (13,700) and dividing by
seven.
Table G-3
7-DAY MOVING AVERAGE
Previous 7-day
total (2/1-2/7)
Subtract day 2/1
Six-day total
(2/2-2/7)
Add day 2/8
New 7-day total
(2/2-2/8)
Divide by 7 to get
average
New 7 DMA
92,400
12,300
80,100
13,700
93,800
n
13,400 = 7 DMA
This method may be particularly useful in
putting data together for percent volatile
sol ids converted.
CONSTRUCTION OF GRAPHS
Placing columns of figures on graphs can show
trends in information that will never be
spotted when looking at monthly report
forms.
The following example describes a plant that
went through a period of high loading result-
ing in the need for corrective action. Previous
episodes had occurred which resulted in long
periods without adequate digestion and no
gas production. By spotting the problem early,
the corrections were made and failure was
avoided.
As an example of how graphs are constructed,
the information on Table G-4 is plotted on
the graphs shown on Figure G-1. As this
example shows, the operator was able to
prevent an extended upset by corrective
action taken early, before the VA/Alk ratio
went beyond 0.32. This was accomplished by
adding 200 Ibs. (91 kg) of soda ash on 2/10,
500 Ibs. (227kg) on 2/12 and 600 Ibs.
(272kg) on 2/18.
One of the best uses of this type of a graph is
to provide a history of what happened before,
during and after a problem was corrected.
Future operators at the plant can refer to the
incident and use the action taken to prevent
repeated upsets. The "Comments" column is
very important. When anything unusual hap-
pens, it should be noted: without notes you
won't know what happened, why, or what
you did to correct it.
A blank form of this graph is also included in
Appendix H.
Table G-4
PLANT MONTHLY REPORT-EXAMPLE
Sludge
Vol.
G-2
2/1
2/3
2/8
2/10
2/12
2/15
2/18
2/22
2/25
2/29
VA
mg/l
120
180
450
620
900
700
950
450
200
180
ALK
mg/l
2400
2300
2250
2200
2800
3200
3400
3700
3300
3000
Temp CO2
V/A
0.05
0.08
0.20
0.28
0.32
0.22
0.28
0.12
0.06
0.06
°F.
97
97
97
98
98
97
97
98
98
98
%
30
30
31
32
34
33
33
33
32
30
PH
6.9
6.9
6.9
6.85
6.8
6.85
6.85
6.9
6.9
7.0
Pumped
gpd
10,000
9,300
1 0,200
1 1 ,200
12,000
8,200
10,500
1 1 ,600
10,200
10,200
/Q '
Co
65
62
58
50
42
42
50
59
61
66
-------
COMMENTS
0.8
V.A. ALK.O-J
RATIO o'.4
0,3
O.2
O.I
FEB.
98
97
96
95
94
93
92
9 I
9O
89
88
87
86
TEMP.
COZ
%
35
34
33
32
31
3O
29
28
v
•
•«•
«
^
is
•
*•!
^
^
NO
_j
BURNABL
E
GAS
*•
•>
X
S
s
N
i
SLUDGE
PUMPED
(GAL./DAY)
7.4
7.3
7,2
6,9'
6.6
t.5
.4
6.3
6.2
I2OOO
II ppp
IOOPCK
9000
8000
7OOO
6OOO
5OOO
M
«i
••
•>
••
»— •
•*
w
~*
^m
••
^
l»
£
^
•^
,»*
^
^
s
a
L
1
^
/
^
/
••
/
^
^
•M
^
^H
X
••
J
^"
t~
^
^
|
^
0
^
^i
^
"to*
%v,s,
CONVERTED
5O -
4O
FIGURE G-1
DIGESTER DATA GRAPHING EXAMPLE
G-3
-------
SOLIDS BALANCE
F. J. Ludzack writes in the Operation of
Wastewater Treatment Plants, "What comes
into a treatment plant must go out. This is the
basis of the solids balance concept. If you
measure what comes into your plant and can
account for at least 90 percent of this
material leaving your plant as a solid (sludge),
liquid (effluent), or gas (digester gas), then
you have control of your plant and know
what's going on in the treatment processes.
This approach provides a good check on your
metering devices, sampling procedures, and
analytical techniques. It is an eye opener
when tried for the first time and advanced
operators are urged to calculate the solids
balance for their plant."
The results of the balance give the operator
an idea of what is happening in his digester. If
annual averages are used, the comparison
between the calculated and actual should be
within 10-15% of each other. If it. is more
than this, the accuracy of flow meters and lab
results should be reviewed.
An example of the solids flow for the entire
plant is shown on Figure G-2. Data for an
individual plant could be shown on a similar
drawing to give the operators visual record of
the overall operation.
Solids balance should be calculated once a
year with the calculated results compared to
the actual digester inputs and outputs.
Settleable
Suspended
Dissolved
FIGURE G-2
EXAMPLE OF A SLUDGE SOLIDS BALANCE
G-4
-------
If the information shown in Table G-5-is
known, a solids balance can be calculated.
Averages over several months, or better, over
the entire year, will give the most accurate
results. The steps in making the calculation
are shown on Table G-5 and a summary of
the calculations appears on Table G-6. Figures
shown in parentheses represent the number of
the formula from pages F-1 and F-2.
Table G-5
INITIAL SOLIDS BALANCE DATA
Di- Super-
Raw gested natant Gas
Quantity, gal ./day 1,200 430 .
Total Solids
Ibs.
"4.0 '6.0
(1) (D
Volatile Sol ids
Ibs.
70 45
(2) (2)
60
(21
Ash
Ibs.
Gas, Ibs.
Note: gal. x 3.785 = liters (I).
(4) (4) (4)
(5) (5) (5)
(11)
Using the information above, the following
nine steps are used to fill in the missing
information on Table G-6..
Step 1. Calculate the pounds of total solids in
the raw sludge.
Step 2. Calculate the pounds of volatile solids
in the raw sludge.
Step 3. Calculate the pounds of ash in the raw
sludge.
Step 4. Calculate the pounds of total solids in
the digested sludge drawn out. . : ...
Step 5. Calculate the pounds of volatile solids
in the digested,sludge drawn out..
Step 6. Calculate the pounds of volatile solids
converted to other forms.
Step 7. Calculate the quantity of supernatant.
Step 8. Calculate the pounds of solids (total,
volatile and ash) in supernatant. .
Step 9. Calculate the pounds of gas produced..
The calculations are made using the formulas
beginning on page F-1. These are summarized
below and the answers. are filled in on
, Table G-6, .;.;•. , : , . :
1. Use Formula 1 : .
• 1,200 x 0.04 x 8:34 = 400 Ibs. (182 kg)
2. Use Formula 2
400 x 0.7 = 280 Ibs. (127 kg)
3. Use Formula 5
400 - 280 = 120 Ibs. (55 kg),
4. Use Formula 1
430 x 0.06 ,x 8.34 = 215 Ibs. (98 kg)
5. Use Formula 2 ;
. . 215x0.45 = 97 Ibs. (44kg)1
6. Use Formula 8
0.7 - 0.45
a.
x 100% = 65%
0.7 - (0.7 x 0.45)
b. 280x0.65 =182 Ibs.
7. Difference between raw in and digested
out
.= 770 gals, (2900 I) :
8. Use Formulas 2,4; and 5 :
25, 15 and 10 Ibs. (11, 7 and 4 kg)
9. Use Formula 11
280-(97+ 15) = 168 (77 kg)
G-5
-------
Table G-6
FINAL SOLIDS BALANCE DATA
Di- Super-
Raw gested natant Gas
Quantity, gal./day 1,200 430 770
Total Solids
Ibs.
Volatile Sol ids
Ibs.
Ash
4.0 6.0
400 215
70 45
280 97
0.4
25
60
15
Ibs.
Gas
Ibs.
Not e:
30 55
120 118
40
10
168
gal. x 3. 785 = I
Ibs. x 0.454 = kg
The practical use of the information in
Table G-6 would be to compare the results
obtained in the solids balance with the aver-
age for three or four winter months or a
period of the year when industrial wastes may
affect digester operation.
For example, one plant that received high
amounts of vegetable wastes during the sum-
mer found that raw sludge dry solids content
reduced while total volatile solids to the di-
gester increased. Gas production increased,
but volatile reduction decreased because de-
tention time in the digester decreased. The
plant had to install sludge thickening facilities
to solve the problem.
Table G-7 summarizes the information from
this plant.
Table G-7
PLANT DATA SUMMARY
Di- Super-
Raw gested natant Gas
Quantity, gal./day 2,600 930 1,670
Total Solids
Ibs.
Volatile Solids
Ibs.
Ash
2.5 3.8
542 294
78 62
423 182
0.6
84
65
55
Ibs.
22 38
119 112
35
29
Gas
Ibs.
186
G-6
-------
APPENDIX H
WORKSHEETS
COMMENTS
0.8
V.A. ALK,°'«
RATIO el*
O.3
o.a
O.I
98
97
96
98
94
TEMP. J«
9O
89
88
§;
CO,
%
35
34
33
32
31
30
29
?B
NO
BURNABL
i
GAS
PH
7.4
7.3
7.2
7il
7.O
6,9
618
6.7
6.6
6.5
6.4
6.3
6.2
SLUDGE
PUMPED
(GAL/DAY)
12000
IIOOO
IOOOO
9OOO
8OOO
7OOO
6000
nnnn
%v,s,
CONVERTED
80
70
6O
SO
Days
-------
u.
Ul
O
O
m
I
f °
5 „ z
1$
ll,
ill
< i-
O u
\l
z
J £r
's.'s
ll
^
S
>
§ *
1-1 *
111 igg
o S <« vi i 6 §
fel? Ills
« f S S -§ o 1
•UtiUti
&efgl|l$
illl* §§£€
g| 1-s.s *.sx |
•£ 8 S S,.2-S><^ 1
illfii^ll
g i -s. i ^ i 1 1 1 i
0 t.§eo'SOQ-oiuu.
F H-ilggSgl
g 1 *l Ss | |0 | |
E§ 5 SS SS
§ »-' c\i oj rf Hi
-------
Recirculation
CNJ
"ro
m
, Sludge withdri
CO
.— QJ
D c
udge Heating E
, Submerged bu
CO i-
. V}
,o
^
<.M.
tf
cu
O
ro
1
v^
in
fcl:
ra S
r Internal hot vv
; External hot v
"•*''ia
-1
<,• £
ludge Mixers
^Internal-fixed
w JT^J
X
E. .." :
31 •
.-Internal movir
;• Recirculation
CN CQ •
-
S2
-------
=
f-
cc
Q.
1
13
u
UJ
_L
U
?•
UJ
s
t-
H
0.
?
1 1
i !
§ s
rio/vs
mn A - An X in the Yes column sho
something or answers a question.
mn B — Indicate from engineering da
information.
033
§ a a
II.
UJ
O
CO
&
c
(D
Is An Alternate
Available
Yes No
~ i S
o-S 51
^ o*
tg
•a g
o £
11
E S
S «*
i*
f*
Q
0
z
a
i a
"5 E "8
•S .5 c
mn C - Write in actual average data.
mn D — Compare Column C with ini
and mark an X in appropriate D col
mn E— Indicate whether plant has a
available.
mn F — Comments
aa a a
2 if 1 1 %
O o t~ ~"~ "^ O O)
S oo If S S
m O *^ Q QJ f2 O
1 e'|| |l T^-l | 11
„? * if! 11 !|ll l? fi!
p ! Sf! ISttKI II II!
§1 fill! ^PHSIIf .isi
DCS Sa.tico ffio^^accic1" S,2£o>
°-3 i£--E^ i2 = S n o — !5 °> 3 3 »2"5E
Infill! iijllllll! fill
<"o5Wr-CN COQ^Q.^-CMCO'* ID C/)*- CM
cc •£ t;
S £ 1 .
O < '-5 < m o
History of Excessive Loadings
1. Flow
2, Organic
3. Inorganic
4. Toxic
Extreme Weather Conditions
d ai
H-4
-------
-
IIV. DIGESTER CONTROL
Note: Norms! ranges and frequencies are noted.
A. Tests
• • .....-»-... ,..,.,..., ..-, , , .. ..,,... . _., ..,,.„... ... ... .,,, ..,,..•
' . • -: ' '
• - - - '
- • ••-•-.•. - - -.- • • ...-...- ...„.„.. .,..,:-. •„ .,.-,. ,; ...
8 .S g m -.,••-"..-.
- o 1 « ' ' -- ' '''I'' .
3 ' > I I . - 5 . ...
J, ^ "S .E c -;- _^.. _" --1,^,§§. 1 - E:
i|s* £••=•••; i-s o is ^f-isssss -- . - ?.. |; '.^. ...
!§&§«! ii|s la s?lJfs|>-l 5^1 . . .1 •
llll«i IfSS fl . il:l'5-«tl5- -1 1 1 . . . | .-
l,g|M lilies? >l3s^sisiilsi • i-| f §:. .•§ _
li-ii!^°llls- n&fal&eiiii.: . .i I • -« -E -f s .. siis:
ss =52-* s S | £. f£i T£5 NQ.^O as^Qb : •/ -5 s | '- ^ o ' • 2 | f : | § § 3 5. .| £ ^;-
^ ^j co' -^f 'v ID CD K CD -: • 0 - :< — ^ <3 — cvi '.«.'-•
• . . - : en '"' • "Z- -- -: • . ' •-;. .
UJ . . . ; "5.
Q. < m o ; H < . m '.
' . .'. ; . . . ..'..' .... ..... .'.- > ..''.>.' - -.
'•
• •
C. . .Contents . ' ,
1. Thick scum layer .
i. Thick grit layer
3. Foaming
H-5
-------
-------
BIBLIOGRAPHIC DATA
SHEET
1. Report No.
EPA 430/9-76-001
3. Recipient's Accession No.
4. Title and Subtitle
Operations Manual Anaerobic Sludge Digestion
5. Report Date
Feb. 1976; Preparation date
6.
7. Author(s)
Chuck Zickefoose and R.B. Joe Hayes
8. Performing Organization Kept.
No.
>. Performing Organization Name and Address
Stevens, Thompson & Runyan, Inc.
5505 S.E. Milwaukie Ave.
Portland, Oregon 97202
10. Project/Task/Work Unit No.
11.
68-01-1706
12. Sponsoring Organization Name and Address
Municipal Operations Branch
Municipal Permits and Operation Division
U.S. Environmental Protection Agency
Washington, D.C. 20460
13. Type of Report & Period
Covered
Final Report: 10/74-2/76
14.
15. Supplementary Notes
16. Abstracts
The subject of the operation of anaerobic digesters in municipal wastewater treatment plants is presented covering the areas of
troubleshooting, general operation, safety, start-up of units, basic theory, sampling and laboratory testing, and other subjects
related to day-to-day operation. The intended audience is plant operators who are operating treatment plants with anaerobic
digesters. The format is set up to allow individuals to choose the portion of the manual of most interest and use that portion
without the necessity of reading all the material sequentially. Information for the contents was obtained by visits to a number
of plants, literature research and discussions with experienced digester operators.
17. Key Words and Document Analysis. 17a. Descriptors
Anaerobic Digestion, Sludge, Waste Treatment, Methane, Anaerobic Bacteria, Toxicity, Supernatant, Volatile Solids,
Volatile Acids.
17b. Identifiers /Open-Ended Terms
Troubleshooting, Gadgets.
17c. COSATI Field/Group 13
18. Availability Statement
Release Unlimited
19.. Security Class (This
Report)
UNCLASSIFIED
UNCLASSIFIED
:urity Class (This
20. Security Class (This
Page
UNCLASSIFIED
21. No. of Pages
198
22. Price
FORM NTIS-3B (REV. 10-73) ENDORSED BY ANSI AND UNESCO.
THIS FORM MAY BE REPRODUCED USCOMM-DC 826S-PT4
. U. S. GOVERNMENT PRINTING OFFICE 1981 - 777-000/1105 Reg. 8
-------
-------
SUBJECT INDEX
CAPACITY
CHEMICAL USAGE
CLEANING
CONTROL
COVERS
DATA REVIEW
FEEDING
FORMULAS
GAS
HEATING
INDICATORS
LAB TEST
LOADING
MAINTENANCE
MANPOWER
MIXING
pH CONTROL
SAFETY
SCUM
SLUDGE WITHDRAWAL
STARTUP
SUPERNATANT
TOXICITY
Parti
Trouble-
shooting
Guides
1-16 to 1-18
1-4
1-14
1-9,1-10
1-4
1-11,1-12
1-13
1-7
1-5
1-19
Part 2
Digester
Operation
2-21,2-29
2-33
2-27
2-11
2-18
2-13.2 14
2-22
2-27
2-27
2-10,2-20
2-9 to 2-1 6
2-32
2-12,2-22
2-33
2-16,2-24
2-33
PartS
Potpourri
3-15
3-30
3-14
3-14,3-25
3-34
3-13,3-25
3-2
3-26
3-6,3-19
3-31
3-29
3-11,3-27
3-32,3-35
3-21,3-26
Part 4
Basics
4-9,4-21
4-16
4-28
4-5,4-20
4=23
4-7,4-24
4-29
4-13,4-25
4-20
4-16,4-20
4-11,4-12
4-20,4-23
4-12,4-27
4-14,4-17
4-8
4-8,4-1 1
4-8
Appendix
G
F
E
-------
3
03 O
-5;
S'
C?
f
Sl o
6^
OS
—• O O o
s-'iM
,i Si
. o a «>
w*
Wg °
-2"
H
-------