ANDREW SCISSON 6 MONTH MEMORIAL

I know you are all tired of vechnaya pamyat, so here is another hymn, done in byzantine style.  The skeleton images are not orthodox but the muxic is beautiful and correct.

Thid is byzantine chant, wich is always acapella.  takes you back to Constantinople before the filthy turks overran it.

Here is a photo of Andrew and I

BIX BEIDERRBECKE AND ANDREW SCISSON

I was listening to Bix Beiderbecke's "In a Mist" last night and thought of my son.  It made mme very sad that he is not here now, and that he missed so much in life, and that the courts took him from me to be poisoned by his mother and then gave him back when I could do nothing despite all me trying.

Who is Bix Beiderbecke?  A cornet player and pianist in the 20's a seminal figure in jazz.  Mostly known as a cornet player, he also composed several very modern sounding piano pieces including "In the Dark", :Candlelight" and "Flashes".  They are all on YouTube if you wan to listen to them.  I have "In a Mist" here.  I think it sounds better played slower, but I am putting the original 78 rpm recording here

WET WEATHER OPERATION - KEEPING YOUR CLARIFIER CLEAR

I have worked at numerous plants that have big flow increases during wet weather operation and have learned by trial ,error and observation how do deal with rainy weather.

The key to good wet weather operation is simple.  YOU NEED TO KEEP THE SURFACE RISE RATE (in feet per hour) SLOWER THAN THE SLUDGE SETTLING RATE. What is the surface rise rate? Its the rate, in feet/hr that water fills the tank if it were empty and the flow is x MGD If the flow doubles, the rise rate doubles.

What is the sludge settling rate?  Take you r settleomeeter and run a settling test.  Take the readings EVERY 30 SECONDS for 10 minutes or so.  Plot the readings on graph paper or on your computer if you can stop watching that internet porn.  The part of the graph that is a straight line is you sludge settling rate.  This is the settling rate when the sludge is not hindered by other particles.

To keep your sludge in the plant durng a storm, you have to either decrease the sludge rise rate (put more tanks on line), or make the sludge settle faaster (yes you can)

I will divide this post into 2 sections:

• Physical plant additions and alterations, and
• Things the plant staff can do themselves, instead of whining about it

Plant Additions and Alterations
The keys to good settling at high flows is, in addition to a good settling sludge, are QUIESCENCE,  DILUTION OF THE MIXED LIQUOR, AND REDUCING CURRENTS/SHORT CIRCUITING

What is quiescence?
Quiescence is quietness, of a non turbulent area in the clarifier where the activated sludge floc can coagulate and begin to settle.  The best way to improve quiescence is to have better clarifiers.  The best clarifiers are peripheral feed-peripheral overflow (PFPO) clarifers.  I discussed this in a previous posting.  PFPO clarifiers have 10 times the quiescent zone that a center feed clarifier does. It is possible to convert center feed clarifiers to PFPO clarifiers.  I'd be glad to help you.
Rectangularr clarifiers also perform well at high flows, according to John Esler, clarifier expert.  The clarifers have to be built a certain way with 2 effluent launders that are parallel to the long sides, influent baffling, and co-current sludge removal (Gould Types 1 and 2).
Extra clarifiers are good insurance for that rainy day. Why is that?  BECAUSE YOU NEED TO KEEP THE SURFACE RISE RATE (in feet per hour) SLOWER THAN THE SLUDGE SETTLING RATE. 

Diluting the Mixed Liquor

Why is diluting the mixed liquor good?

Because it lets you improve the settling rate

Secondary clarifiers can become solids limited - that is, you can push more solids into the clarifier than can settle.  There are  various ways of  diluting the mixed liquor, including step-feed, contct stabilization and RAS adjustment

If you have multiple aeration tanks you can convert your system to step feed or contact stabilization.  Since most plants have the influent agt one end and the clarifier at the other end, your system can be converted to 3-pass, 5-pass or 7-pass , etc.

Going to for example, a 3-pass step feed parks some the the mixed liquor in tanks one and 2, for instance and feeds some of the influent into pass 3, diluting the mixed liquor and lowering the solids loading rate to the clarifier

Step feed conversion can be done by your engineer, or you can do it yourself.  If you want some help, call me.  n Carey, OH, Doug Keller, Roy Johnson and staff did this as a DIY after I suggested it.  They had 5 tanks of 2 different sized and a problem with short circuiting and flow spliting.  They rented a concrete say and cut holes in the tanks to make them plug flow.  The also had to instaqll a submersible pump and a float switch because 2 of their tanks were detache3d from the other 3.  Total cost?  $9000 plus their labor.  The results?  Well worth the money.

Another good example of how step feed can be effective is Bowling Green.  I redesigned their system from 4, 1-pass tanks to 2. 3-pass tanks.  After the conversion, they were still having problems with clarifier performance at high flow.  They were finally persuaded to try step feed.  The results?  They went from a secondary efflulent TSS of 200 mg/L at 20 MGD to 20 mg/L at 24 MGD.  I know some of you out ther are not impressed and will keep doing it the same old way.

Here are some things that operators can do without much effort and keep playing internet poker.

Adjust the RAS Rate

Adjusting the RAS rate will change the solids loading rate and reduce clarifioer turbulence.  I leaned this lesson long ago as a Chief Operator at Bloomington IN, a 15 MGD AWT plant.  (See I did have a real supervisors job at one time).  We began there when it was a brand new plant.  Control the RAS as a fixed percentage of gthe influent flow.  When we had our firstg real big rain, we kept turning the RAS up until we turned the clarifiers into mixed liquor.  My good friend and co-worker, Bill Bardes, suggested that we reduce gthe RAS frlow since turning it up didn't work so well.  We figured out what to do by trial and error, and kept the sludge in the clarifier even at maximum flow.  This trick has worked for me at several places.

I know there are some famous consultants who poo-poo this, and say that RAS has nothng to do with the solids loading rate, but I say  I'm right, you're wrong and that's that.

Put Extra Tanks on Line

Putting extra tanks on line will give you some breathing space whille they are filling up and will dilute the MLSS

Reducing Aeration Tank Airflow

Reducing aeration tank air flow will promote good flocculation and rapid settling.  You dont need to have a DO of 10 mg/L.  Cut your air flow to the recommended minimum.


Follow these suggestions and your plant will work better.

MY DOG

I don't have a picture of my dog Taylor, but this is a good facsimilie.

How can anyone not like a dog like this?  Gary Goldberg said my traffic would increase if I put kittens on my blog:  there are limits.  So here is a lab.  Taylor has never seen food she woudn't eat except uncooked vegetables   She also eats sausage casings and butter paper and bread wrappers.  I love her.

800 PAGE VIEWS!

Thank you loyal readers.  more articles to come

NEW SERIES ON WET WEATHER OPERATION

My next series will be on how to improve your wet weather operation.  And you thought all I could was sludge.  I have lots of practical experience.  Some people are scared.

OPERATOR FOLLIES #4

Years ago a digester sludge recirculation pump quit working.  One operator said  "Don't blame me I haven't been in that room for 6 months"

Automation is looking better all the time

RUSSIANS!

I see I have a lot of page views from Russia!  Well, dobre dyen gospodin and gasperzha.  Send me an email,  I don't read cyrillic or Russian, just a little bit of Church Slavonic  because I'm pravoslavnie.  I'll be glad to talk to you.

MUSICAL INTERLUDE #10

Here is one from my misspent youth  The Fabulous Thunderbirds and "Can't Tear it up Enuff"

This reminds me of when Bob King and I would get out at lunch, go to the ice house, pick up sausage sandwiches and beer, and go to the San Antonio river below the Rillling Rd WWTP and listen to the Fabulous Thunderbirds and watch the Mexicans swim in the San Antonio River.  at that point at least 85% of the River flow was WWTP effluent.

Why is it that bosses are so uptight about having a drinkie or two at lunch these days.  The personnel department is turning bosses into neo-prohibitionists.  So come and have a beer with me.

DEWATERING FOR BIG AND SMALL

Here is my long-awaited post on dewatering.

To paraphrase Alfred P Sloan, former head of Government Motors, there are dewatering devices "for every purse and purpose"  There are dewatering devices that are useful at package plants, and others that are useful at plants as big as 1 BGD (billion gallons per day).  Lets look at some.

I have yet to receive any figures for vendors.  I will do what I can.


GEOTUBES

A geotube is a special material that passes free water and leaves the solids behind.  It is a tube (go figure) most often used to dewater silt from lagoons and as an erosion barrier, but it has sludge dewatering uses too.  The tube is closed at both ends.

Sludge is pumped to the tube.  polymer is injected just before entering the tube.  The polymer coagulates the solids and separates them from the water.  free water drains through the tube.  Sludge is pumped in until the tube is full.  Water continues to drain away.  When drainage is complete, further dewatering occurs by leaving the tube at the site for dewatering by evaporation.  At some sites geotubes are stacked together for air drying.
When it is time to remove the sludge the tube is slit and the dewatered sludge removed by a front end loader.  In some cases the tube is placed in a roll-off container prior to use and the sludge, tube and all taken away to a landfill.

The dewatered sludge can be landfilled or land applied.  Most of the material is landfilled.  Cake solids range from mid teens to mid 20s percent. 
The advantages of geotubes are that they are

• relatively inexpensive
• can be used at small, medium and large sized plants
• are easy to use

They are a bit labor intensive for sludge removal and cake solids can be limited.  Collecting all the run off water can be a problem and the EPA does not want this water flowing into the waters of the state.
My friend Doug Brookhart likes these for small and package plants and says that they have been a godsend at some places.

IMPROVED SAND DRYING BED TECHNIQUES

Although sand drying beds are severely limited by the weather, a few simple and inexpensive improvement, and a different operating technique (free) can increase greatly the number of times the bed can be cycled. These improvements and techniques are:

Stir up the Sand Before Each Pour

Drying beds remove water by percolation (drainage) and evaporation. Percolation is rapid. Evaporation is slow and absolutely not controlled by the operator. Increasing percolation greatly speeds up sludge drying.

Stirring the drying bed before pouring a bed :

• Breaks up any crusts in the sand that would blind the bed
• Fluffs up the bed. This increases air voids in the sand, which increases the filtration rate

If the maintenance man or someone at the city garage is handy, a rake that will fit on the forks or toothed bucket of a tractor or bobcat loader that will rake the sand to a depth of 1-2 feet can be made from scrap. The tines should be 2-3 inches apart

Pour Shallow Beds

Evaporation only occurs at sludge surfaces. Sludge in the middle of the pour does not dry until air reaches it when the sludge cracks. The sooner air can get to the sludge the quicker it can dry. By pouring a shallow (10 inch) bed, the bed will crack sooner and allow all the sludge to dry.

Take it Out Wet if You Must. That’s Why Those Concrete Strips are in the Bed

Today’s drying beds usually have concrete strips for the tractor or bobcat tires so the bed can be mechanically cleaned. The operator is not shoveling the wet sludge into a wheelbarrow as in the bad old days. The wet sludge can be stockpiled, and the bed fluffed up with a rake

Don’t Be Afraid to Take Out Some Sand

Some operators wait until the sludge is very dry to avoid losing sand. DRYING BED SAND COSTS $10/TON (ABOUT $27/CUBIC YARD). Get the sludge off the bed and put on some new sand. Sand is cheap.

How Well Does It Work?

These small improvements work quite well, At a small plant in northeast Indiana aerobically digested sludge at about 1.5% TS is poured onto a prepared bed about 10” deep. The sludge will crack within 4 days if the weather is dry.

WEDGEWIRE BEDS

Wedgewire beds are made of plastic tiles with small slotted openings (figure 1). Sludge, conditioned with polymer is poured onto the beds. The polymer binds the sludge solids together, releasing free water, which drains through the tiles. When the bed is full, the sludge is left to set for a few days to continue dewatering. The dried sludge is taken off the tiles with a small tractor and taken to a stockpile. The cake solids depend on the sludge, but range from 15-22% TS.

The beds can be loaded at 2 lbs/sq.ft. for aerobically digested sludge, and 4 lbs/sq. ft. for anaerobically digested sludge.

The drying beds come as a package and include the tiles, a lightweight building to cover the beds (sometimes the building is translucent to speed drying), and a polymer makeup and dosing system. The beds are priced at $50-55/square foot, plus $7500 for the polymer feed system.

The advantages of wedgewire beds are:

• Simple technology. Not much can go wrong.
• Easy to size, price and engineer
• Fair to good solids capture.
• Relatively inexpensive to build. NOTE: the cost depends upon how often the beds are to be cleaned. For example, if the beds are sized to be used twice/week, the cost is about half that of beds sized to be cleaned once/week
• Cake that can be handled with a tractor and a bucket. NOTE: bobcat loaders are not recommended, as the steering system of braking the back wheels is too stressful for the tiles and tends to pop them out

The disadvantages of the wedgewire beds are:

Polymer costs may be high

• Labor intensive. The superintendent at Ada Ohio estimates that is takes 8 man-hours to pour and clean a 70’ x 20’ bed that dewaters 20,000 gallons of sludge for each pour. (But at Upper Sandusky, OH, it only takes 4 hours).

LIGHT DUTY BELT PRESS

The light duty belt press is a one-belt press with an extended gravity zone and one or two nip rollers to provide the pressure for dewatering (figure 3). The units are generally sold skid mounted and ready to operate with a polymer system, flocculation tank, sludge feed pump, washwater pump, sludge conveyor or cake pump and a control panel. 

The press will produce cake solids at 14-21% TS depending upon the type of sludge dewatered. Anaerobically digested sludge generally dewaters to higher cake solids content, but may not be as easy to handle in the field.

The advantages of a light-duty belt press are:

• Comes ready to use. No assembly required except connection to sludge, water and power
• Modest capital cost ($80,000 for 1.5 meter unit and 33 gpm capacity)
• Easy to use. NOT labor intensive
• Compact: Can fit in a space of a 1-car garage.
• Good cake solids: 13-21% depending upon the sludge
• Good solids capture
• Fair polymer consumption

The disadvantages of a light-duty belt press are:

• Higher capital cost, especially when the building cost is added in
• Noisy
• Lower cake solids that a 2-belt filter press
• Low output: about 33 gpm for 1.5-meter unit (One manufacturer has a unit with a drum pre-thickener ahead of the gravity drainage zone who claims an output of 100 gpm, but there are not many of these units sold).

REGULAR (2 BELT) BELT PRESSES

A regular belt press has 2 belts and come in widths ranging from 0.5 mdters to 3 meters.  It is possible to get these machines skid mounted, with the machine, polymer system, control panel and sludge feed pump on one skid.  The most common size is the 2.2 meter width.  Machine throughput ranges from 50 gpm to 300 gpm through these size ranges.  Throughput for the common 2.2 meter machine can range from 50 gpm to 180 gpm depending upon sludge type, condition and operator philosophy.

Belt press cake solids depend upon the type of sludge dewatered.  Aerobically digested sludges will range from 14-22%  Anaerobically digested sludge ranges from 18 to 30%, and raw sludge ranges from 20 to 32%

The advantages of a belt press are:

• Moderate price and fair ot good cake solids
• lower polymer dose
• easy to clean
• the operator can watch the sludge on the machine and make adjustments based on his observations
• relatively low maintenance requirements and costs
• low power costs

The disadvantages of the belt press are:

•  Odors, especially with anaerobically digested sludge and raw sludge.  Sometimes the machine is completely enclosed due to odors.
• Cake solids are not as high as a centrifuge
• High water rrequirements 
• Operators tend to turn down the sludge flow and turn up the polymer so they don't have to do as much work I have over 20 years experience observing this so don't complain that I am a meanie

SCREW PRESSES

A screw press is usually an auger inside a taperd screen.  sludge mixed iwth polymer is fed into the machine.  As the sludge passes thru the machinewater drains out through the screen.  The screen's taper provides osme pressure to aid dewatering.  This type of machine is very common for dewatering cow manure, both raw and digested, and works well bacause there is much undigested vegetable matter to provide structure and prevent solids from leading out through the screen.  New polymers called cross linked polymers have allowed these machines to be used for municipal sludges.

The advantages of these machines are

• Moderate equipment cost
• Low operator attention
• cake solids comparable to a belt press
• no washwater required
• less odor than a belt press
• Low power requirements

The disadvantgages are:

• lower throughputs than some othr methods
• higher polymer cost than a belt press, about double the cost

North Baltimore has a type of screw press if you want to go look at one


ROTARY PRESS

Rotary presses are interesting to my feeble mind.  They are sort of a cross bwtween a hose pump and a screw press and a geotube.The press is hose that allows water to pass.  The hose is wrapped in a u-shape sround a cam.  Sludge and polymer are pumped into the machine.  The can revolves slowly, pressing sludge into the hose, and flushing water out of the sludge and moving it through the hose, where it is discharged at the end

Photo Thanks to Fournier

These presses are getting popular, especially beause the manufaturers have convinced owners that they can be run unattended 24 hrs/day

The advantages of this machine are

• Fair to great cake solds.   great for ATAD and primary sludge, about like a belt press fro WAS and aerobically digested sludge
• Low power requirements
• quiet
• Low maintenance
• Medium polymer requirements
• Can be run round the clock without attention

the disadvantages are

• Low throughput/dollar cost
• Each channel can dewater about 50 gpm For large plants the presses canbe ganged, but the equipment costs are high

CENTRIFUGES

A centrifuge is a spinning cylindrical bowl that uses centrifugal force to separate water and solids.  Sludege mixed with polymer is injected into the bowl.  The solids migrate to the outside, where they are removes by a screw called a scroll.  The liquid passes through a weir and leave the bowl.

The centrifuge is the machine to have if maximum cake solids is important.  Centrifuge cake solids will be 5-10% drier than a belt press. The centrifuge has several advantages over other mahcines, including

• The driest cake solids
• No odors or aerosols
• unattended operation,including start-up and shutdown
• vey good solids capture, up to 99.5% (this may require and excessive polymer dose)

And of course some disadvantages

• Higher capital cost
• Polymer dose 150-200% of a belt press, but equal to some other forms of dewatering
• Noise - the machines rotate at 2000+ rpm
• Very high wear when dewatering gritty sludge
• For some applications, like lime stabilization and multiple hearth furnace operation, it is possible to get the sludge too dry unless the operator watches the process.

CONCLUSION

I suppose I should make up a table  I'llge tto that later

Which type of dewatering device is best for you?  It all depends upon the type of sludge you have, how much money you  have and how dry you deen it to be and how often you have to (want to) dewater.

Musical interlude #9

My dewatering post is delayed due to vendors who promised to send me images did not.  Go figure.  The post will be on line Monday.  Meanwhile, some more music from The Band.  This will be a favorite of the men. 

700 PAGE VIEWS

Thanks, loyal readers.  guess I better finish that dewatering series.  I'm having trouboe finding jpeg images.  If you want you can send me some at jscisson@aol.com

COMING UP

A new posting on dewatering.  Meanwhile read my existing articles

600 PAGE VIEWS

My traffic is picking up.  Thank you, loyal readers!

MUSICAL INTERLUDE #8

This song has been a favorite of mine for over 35 years.  I hope you like it too.  The horn section really makes it good.  It's ovedue on this site

If my heart was made of glass, you could surely see......

THE DOG ATE MY HOMEWORK

W while back I interviewed for a job with a private operating company around Bedford OH.  They made the owner's ancestral home into an office, lab and storage space.  Pretty neat.

We talked and then they asked me to fill out an application, but hey, they didn't have an appllication, so they were going to mail it to me.  It never arrived.  What a scummy trick.  I'll have to remember that one for when I become a manager.

500 PAGE VIEWS

I am now over 500 page views.  Thank you mty loyal readers and keep  reading!

A SELECTION NOT A BID

Engineers and their customers often get hung up on bidding everything, and are suspicious of sole sources (selections) as being unethical, or unfair, or something.  I will show you today that in the case of blowers, the energy considerations are so great that you should almost always select the most efficient blower.

Back in 2007 I was redesigning the aeration system for Monroe MI.  They had 3, 1000-hp blowers with synchronous motors, and were using 700-some horsepower to run the system.  They had canvas sock diffusers (aka elephant prophylactics).  they even had a washing machine with which to clean them.  I redesigned the sysgtem with efficient fine bubble diffusers and new, smaller blowers,  I simulated the variations in air flow for a typical day, and asked blower manufacturers to give me shaft horsepower at the various points.  I then calculated the energy use and cost of the year 2007 and the year 2027.  The results are below. 

COST COMPARISION OF VARIOUS BLOWERS, $/YEAR PER SIMULATION

	Spencer 6000	Spencer 7000	Hoffman 7000	Hoffman 8000	National Turbine	Turblex KA10 single point 7500	Turblex KA10 dual point	Turblex 8550 KA10 single pt
2007	108424	108424	98418	104491	111112	82040	74572	79321
2027	123010	129183	108026	115891	127739	96726	89999	94126
delta from lowest
Year 2007	-29103	-29103	-19097	-25170	-31791	-2719	4749	0
Year 2027	-28884	-35057	-13900	-21765	-33613	-2600	4127	0
Average	-28994	-32080	-16499	-23468	-32702	-2659	4438	0
mulltistage only 2007	-10006	-10006	0	-6073	-12694
2027	-14984	-21157	0	-7865	-19713
average	-12495	-15582	0	-6969	-16204

shows that there is a considerable difference in energy cost between the most efficient and least efficient machines.  The single impeller machnes were more efficient.  I selected the second most efficient blower, because the outlet diffusers and the computer algorithm needed to operate them cost an additional $43,000/machine, which pushed the payback for the additional cost from 5 years to  15 years or so, and I thought that was too long (Private industry always wants a payback in 1 year).  The difference between the most and least efficient machin was over $30,000/year, or $600,000 in 2007 dollars and no increase in the cost of electricity. Vendors, don't complain because these are the data you gave to me.  Even among the multistage machines, there was a considerable difference in energy costs over a 20 or 30 year equipment life, would far outstrip the diffence in machine costs at bid time.

The lesson is:  You, the engineer or customer, should give the manufacturer a typical daily air flow variation  at your plant, and ask him to give you shaft horsepower at each flow point, and then select the most or 2nd most efficient machine and have it sole-sourced. You will save a lot of money for the customer.

How did the job work out?  Monroe reduced it electric bills by $50,000/month.

This same exersize works for diffusers, too.  You are better off to buy more diffusers and use less air than to cheap out with the least expensive diffuser layout.  I'll try to make a presentation on that later.

Now, If I can do this calculation, some of you superintendents should be able to do it too.  After all, superintendents in Wood and Lucas counties feel that I am too dumb to be an operator it can't be alll that diffucult.

IDLE THOUGHTS

My dad served a term as vestryman (trustee)for St. Paul's Episcopal Church in Greenville, OH.  Aftetr that he never attended church again, ever.

I understand how he felt.

Any time one stands up against wrongdoing, one learns who one's friends arent.

6 MONTH MEMORIAL

anuary 26th is Andrew's 6 month memorial.  It's early yet, but I am attachinga Vechnaya Pamyat (Memory Eternal) video.  I miss him, and sorrow that he did not get to live the normal life he deserved.

TROUBLESHOOTING WITH AN AMMONIA PROBE

This is my ever famous troubleshooting with an ammonia probe paper.  See I can do things owther than sludge.  I have to convert all the figures into jpeg so you can see then.  Probably tomorrow

USING AN AMMONIA PROBE FOR PROCESS CONTROL TROUBLESHOOTING

INTRODUCTION

An ammonia probe is a handy tool for troubleshooting process control problems. Now that most plants nitrify plants, even small ones, have ammonia probes. The probes themselves cost less than $400, and the selective ion meter (a fancy pH meter) used to measure the ammonia probe output may cost as little as $250.

Why is an ammonia probe a good troubleshooting tool?

1. Many plant labs will have an ammonia probe and staff trained in its’ use.
2. The nitrification reaction occurs at a steady rate throughout the secondary treatment process. This allows the operator to track the ammonia depletion with the ammonia probe.
3. The nitrification reaction is sensitive to many operating problems, such as lack of DO; recycle side streams and lack of detention time.
4. The ammonia probe is easy to use and can measure ammonia in all types of samples, The samples can be from the refrigerator or one grabbed only minutes ago.

Thes qualities allow the alert operator to detect many problems that may otherwise go unfound. Examples of problems that can be sniffed out by an ammonia probe are:

• Short-circuiting in an aeration tank.
• Poor influent or RAS distribution among multiple aeration tanks
• Lack of DO in one tank, or in part of a tank
• RAS going septic in final settling tanks
• Poor air distribution among aeration tanks
• Supernatant or dewatering filtrate bleeding through the plant partially treated.
• Trickling filter media or snail problems
• Solids decay in “tertiary” lagoons

This paper will demonstrate how the ammonia probe can be used for troubleshooting and include real-world problems detected by ammonia probes.

CHARACTERISTICS OF NITRIFICATION

Nitrification is a two-stage biological process which oxidizes the ammonia to nitrate ion. Compared to carbonaceous BOD consuming organisms, nitrifiers are relatively slow growing, strict aerobes that consume ammonia at a steady rate in the aeration tank. This difference in food consumption rate is shown in Figure 1.

FIGURE 1

BOD AND NITRIFICATION DEPLETION RATES

IN A PLUG-FLOW AERATION TANK

This steady rate of ammonia depletion can be seen in the real world by doing a “bucket test”. In a bucket test, RAS with nitrifiers present and secondary influent wastewater are combined at a ratio to approximate the MLSS concentration, poured into a large bucket and aerated to maintain aerobic conditions. Samples are taken from the bucket at regular intervals and analyzed for ammonia concentration. A typical bucket test result is shown in Figure 2.

FIGURE 2

BUCKET TEST AMMONIA PROFILE

Time, Minutes

This test shows what happens in a plug flow aeration tank as the mixed liquor flows through the tank. (Note: results are site-specific and will vary due to temperature, mixed liquor dissolved oxygen (DO) concentration and other factors. The rate of depletion will remain fairly constant during the test)

WHAT CAUSES NITRIFICATION PROBLEMS IN ACTIVATED SLUDGE?

The most common causes of incomplete nitrification are:

• Poor flow splitting between aeration tanks,
• Lack of sufficient DO in some tanks, parts of tanks, or at certain times of the day,
• Plant side streams high in ammonia concentration, such as anaerobic digester supernatant, filtrate or centrate,
• Short circuiting caused by tank geometry, or
• A combination of all or some of the above.

WHAT MAKES THE AMMONIA TEST A VALUABLE TROUBLESHOOTING TOOL?

Because nitrification removes ammonia at a constant rate through an aeration tank allows the ammonia concentration to be used as a “tracer”, just like a dye tracer.

Ammonia depletion will reveal patterns of flow through aeration tanks. Ammonia removal, or lack thereof, can identify less-than ideal conditions, such as low DO or side streams.

WHERE CAN THE TEST BE USED?

The ammonia probe can be used to analyze refrigerated grab or composite samples, and fresh grab samples. Samples from all plant processes can be analyzed, including:

• Influent wastewater
• Primary effluent
• Mixed liquor
• Return activated sludge (RAS)
• Waste activated sludge (WAS)
• Aerobic and anaerobic supernatant
• Belt press filtrate and centrifuge centrate
• Gravity thickener (GT), gravity belt thickener (GBT) and dissolved air floatation (DAF) thickener overflows
• Secondary effluent
• Final effluent
• Water from any other plant process

HOW TO TAKE AN AERATION TANK PROFILE

Taking an ammonia profile of an aeration tank is just the same as taking a DO profile of an aeration tank. Grab samples of mixed liquor are taken at the aeration tank inlet, outlet and points in between. For a single pass tank, take a total of 3 or 4 samples. For a multiple-pass tank, take at least two samples in each pass. . The samples do not have to be settled and supernated before analysis. Grab samples of mixed liquor should be analyzed within 30 minutes of being taken, and preferably sooner than that. The operator should calibrate the ammonia probe before collecting the sample to reduce analysis time.

Figure 3 shows typical plug flow tank sample locations

FIGURE 3

AMMONIA PROFILE SAMPLE LOCATIONS

DIFFERENCES BETWEEN PLUG-FLOW AND COMPLETE MIX PLANTS

The ammonia probe is best used in a plug-flow tank, because a plug-flow tank tends to have an ammonia depletion profile similar to a bucket test, as shown in figure 4.

FIGURE 4

AMMONIA PROFILE IN A PLUG-FLOW TANK

A complete mix reactor tends to have uniform ammonia concentration, such as the one in this oxidation ditch in figure 5.

FIGURE 5

AMMONIA CONCENTRATION IN AN OXIDATION DITCH

EXAMPLES OF HOW TO USE THE TEST

The following examples are real-world examples of how use of an ammonia probe revealed process problems.

POOR FLOW SPLITTING

Plants that have multiple aeration tanks operating in parallel always have the potential for unequal flow splits. Tanks that receive more flow will have less detention time. All other things being equal, an aeration tank receiving more flow will have more ammonia at the tank effluent than one receiving less flow. An example of this is shown in figure 6.

FIGURE 6

POOR FLOW DISTRIBUTION, CASE 1

The tank effluent ammonia results show that tanks 1 and 8 receive more flow than tanks 2-7. In this instance, the poor distribution did not cause a problem because effluent limits were met.

SHORT-CIRCUITING (TANK GEOMETRY)

Tank geometry affects aeration tank detention time. In a complete mix tank, or rectangular “plug-flow” tanks with a length to width ratio or 4:1 or less, the average aeration tank detention time may be less than 25% of the theoretical detention time. Switching a multiple-tank system where the tanks operate in parallel to one where the tanks operate in series flow will reduce short-circuiting and improve actual detention times. Figure 7 shows the difference in results between parallel and series operation

FIGURE 7

PARALLEL VS. SERIES FLOW

SIDE STREAMS

Side streams with high ammonia concentrations can affect effluent ammonia concentration due to poor flow distribution, irregular flows and overloading an aeration system. High strength side streams that are not distributed evenly across multiple aeration tanks can overload one section of the plant and cause an increase in effluent ammonia concentration. Figure 8 shows an example of this. In this case, belt filter press (BFP) filtrate with an ammonia concentration of 300-400 mg/L was recycled back to only half of the plant, Two sequential samplers were set up to sample the mixed liquor leaving one tank affected by the recycle and one not receiving any BFP filtrate flow. The results are shown in Figure 8. Additional evidence was found by measuring the ammonia concentration going to the different tanks. The primary effluent going to aeration tanks numbers 3-6 was 19 mg/L; the ammonia concentration to tanks numbers 7-10 was 27 mg/L. As a result of this and other research, the belt press filtrate was re-routed to the primary settling tank influent so it would be distributed evenly to all tanks.

FIGURE 8

EFFECT OF SIDE STREAMS, CASE 1

AERATION TANK DO

Nitrification problems caused by low DO can be very difficult to find, especially in multiple-tank systems, plug flow systems, and in plants with poor DO control and/or monitoring. Nitrifiying bacteria are strict aerobes, and will not nitrify unless there is adequate DO.

What is an “adequate” DO? There is no magic number for an adequate DO. The aeration tank DO at which nitrification begins is site specific, and will change throughout the year. Nitrification will usually begin in a plug-flow aeration tanks when the DO is somewhere between 0.5 and 2.0 mg/L. Variables affecting the minimum DO for nitrification are:

• Aeration tank detention time
• Wastewater septicity
• Wastewater concentration
• Temperature
• MLSS concentration

Low DO can affect an aeration tank at certain times of the day when the flow and/or organic loading is higher. Figure 10 shows the effluent ammonia variation caused by low DO during some parts of the day in some aeration tanks.

FIGURE 10

EFFLUENT AMMONIA VARIATIONS

CAUSED BY PERIODIC LOW DO

In the case above, the cyclic trend of the ammonia concentration in the “south” aeration tanks corresponds with wide variations in mixed liquor DO The “north” tanks did not have low DO and were less affected by changes in load

Low DO problems can be very difficult to detect due to limited monitoring. Many plants do not monitor aeration tank DO, or, if they do, only take grab samples from the end of each aeration tank twice a day. Such sampling is of little value for troubleshooting and can be misleading because it does not reveal anything about the DO in the aeration tank at all the other times of the day, or in the other parts of the aeration tank. Low DO in the portions of an aeration tank can inhibit nitrification as well,, because there may be a minimum DO required to start nitrification, especially in long, plug flow tanks where the initial section is heavily loaded.

Still another use of an ammonia probe is to determine the difference in efficiency, in a practical way, between diffusers in two different tanks. Figure 11 shows the ammonia profiles in two aeration tanks. One tank has new diffusers; the other, seven year old diffusers from another manufacturer. The tank with new diffusers requires about 35% less air to nitrify approximately the same amount of ammonia as the tank with the old diffusers.

FIGURE 11

DIFFUSER EFFICIENCY

AEROBIC DIGESTER OPERATION

Aerobic digester ammonia concentration is a good indicator of aerobic digester health. Most aerobic digesters nitrify, converting the ammonia liberated from cellular destruction into nitrate and acid. The digester will generally have a site-specific baseline ammonia concentration. Rising ammonia concentration is an indicator that the digester air supply is not sufficient to nitrify all the ammonia released by digestion. This test is especially useful when the aerobic digester feed sludge is thickened before entering the digester. When the sludge is thickened an increasing ammonia concentration can indicate that the sludge is too thick and should be thinned down. In these cases the digester may soon develop odors and, if the digester temperature is above 90^o F, foaming may occur. The ammonia concentration can be used as a process control tool, taken at least twice a week and tracked for changes.

SUMMARY

An ammonia probe is a handy tool for tracking and troubleshooting the activated sludge process, and aerobic digesters. The ammonia probe and selective ion meter are inexpensive and easy to use, and will yield results in a few minutes.

ACKNOWLEDGEMENTS

I would like to thank Steve Hallett, Mike Carson and Chris McGibbeney from the City of Toledo Bay View Water Reclamation Plant for helping develop this troubleshooting process, and for allowing me to use examples from plant operations spanning over 10 years’ time. I would like to thank Doug Keller from the Village of Carey, Ohio the use of his plant data and turning an offhanded remark I made one day into a successful project. Last, I would like to thank Angelo Klousiadis from the City of Mansfield OH and Lynnius Maximus Marshall for the use of their data to illustrate problems.