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.

AN ENERGY PRIMER

Everyone is talking about "green" "sustainable" "bioenergy" these days.  One of those alternatives is an anaerobic digester and an engine generator set and then heating the digester with the waste heat from the engine, or even running a drier (partially).

How much power is there in biosolids?
Turning volatile solids into methane yields about 17 cf digester gas/lb volatile solids. Digester gas has a wet value of about 600 BTU/cf.  Thats 10,200 BTU/Lb.  The most efficient engines are 35-37%  efficient.  This yields about 1 kilowatt-hour of electricity per pound of volatile solids destroyed. With low grade heat recovery from the engine, your total efficiency can be 75-80%, enough to heat the digester, and enough for other uses as well, including building heat, and , in theory, an air conditioner.

I like the idea of running a dryer with it, though there is not enough enery to run and engine and a dryer without additional gas to close the loop.

How much does an engine generator set and appurtanences cost?

 A rule of thumb is at least $1,100/megawatt (1000 kilowatts), plus a building plus installation. In addition, you have gas conditioning equipment ahead of the engine.  You need to remove water vapor, particulates, and siloxanes.

Jim what are these siloxanes?

Siloxanes are teeny weenie silicon-based particles used in beauty products becuase it make them flow better, etc.  They asre light enough to flow outof the digesgter wsith the gas.  When they get burned in an ingine or turbine, they create a ceramic coating on the moving parts (not a good thing).  They can be removed by running the digester gas through a gel media, or through a pressure-swing absorption machine.  The pressure swing absorption machine removes EVERYTHING inculding carbon dioxide, and produces pipeline quality gas, but is (of course) expensive.  The gel is much less expensive.  water is removed by refrigerating the gas and having it drop out as water.

So Jim, are we going to make a mint off biogas?

Probably not.  There are firms that will put an engine generator seet at your plant and sell you electricity for 9 cents/kWh.  In the midwest, one can buy electricity for much less than that.  Toledo pays 4.2 cents/kWh.  Sidney pays 8 cents, but Brian Shultz says that half of that is demand charge.  So, no you are not going to make a lot of money unless you are subsidized?

But Jim, what about all these dairy operations that are making a mint from cow power?
I went to a lecture about Bridgewater dairy out in Montpelier, OH. The dairy has a cow power digester and generator set.   They have 3 revenue streams:  A methane destruction credit, an alternative energy credit, and cost avoidance/selling electricity to someone.  No one said how much the credits were, but I'll bet that Bridgewater gets more revenue from credits (that is subsidies) than from selling or usng their homemade electricity.  No one is making a mint without subsidies.  Another advantage for the dairies is that the manure is in a more acceptable form and may be easier to use.  Fecal bacteria are much reduced as is that "fresh country air" smell

What if we take in food waste?
If you take in food waste you can make more gas, especially if you have lots of fats in it.  Fats have twice the energy value of carbohydrates and protein.

How much does it cost to maintain an engine?
I asked someone who keeps good records of engine maintanance, and he said it varies between $3 and $4 per operating hour, plus 75 cents per operating hour for the engine rebuild, usually after 60,000 hours. If you kept one engine on line for a whole year, 8,000 ish hours, your maintenance costs will be $32,000 to $40,000/year.  A hat tip to Dapper Dan.

What other hidden problems are you going to warn me about?
Air Permitting  You are installing a stationary power source.  It will be treated just like a generator running on diesel or natural gas.  Depending on where you are and how big your generator set, permitting can be difficult.