A blog from a wastewater guy about various things
Wednesday, August 31, 2011
AEROBIC DIGESTION PART TROIS
Now for some process control. Wake up everybody, and turn off your cell phones.
You can improve your digestion and save a llot of energy by following a few simple rules
Most digesters nitrify. Nitrification makes acid and reduces the pH. pH depression can get as low as 3.5, which then pickles all the organisms and you have a pretty dead digester except for some yeasts, and your girlfriend will tell you about those nasty yeast infections.
pH can be restored by turning off the air and allowing it to denitrify. Denitrification is slower than nitrification, so, for best results, you should turn the air off for twice the time you aerate it, and I'll show you as soon as I can get this pesky blogger to allow me to paste a drawing. In real world terms, this means you can turn the blowers off when you leave for the day at the end of the day shift, and turn them back in the morning. You will then track your pH to see if that is effective. When it's hot out you will have to run the blowers longer. Thats why you have timers on your blowers. Pay attention to whats happening and make changes as needed, instead of doing nothng and then blaming the engineer. You will end up with a sludge tha dewaters/decants better and save energy.
Other indicators
pH is slow to change. It may be that you need to look at ather values to control your operation. You all are out of the dark ages and will have these tests available to you, so quit howling at the darkness and get to work.
Ammonia
Nitrification destroys ammonia, meaning you have aerobic conditions. Septic conditions create ammonia, as it evolves out of protein destruction. Since you are most likely testing your effluent, you can test your digester too. You;ll have to figure out for yourself what ammonia values are too high, but 125 mg/L is a place to start.
Alkalnity
Alkalinity is a measure of the sludge buffering capacity. Depending upon conditions, alkalinity will range from 0 (too low) to 700 or more (too high). Adjust your air accordingly.
Total Solids
If you are thickening your sludge, especially with a mechanical thickener, you need to watch your solids and make sure it does not get too thick, unless you are going to unerated storage.
It can be that easy if you are not so stubborn as to do it different just because.
You can improve your digestion and save a llot of energy by following a few simple rules
Most digesters nitrify. Nitrification makes acid and reduces the pH. pH depression can get as low as 3.5, which then pickles all the organisms and you have a pretty dead digester except for some yeasts, and your girlfriend will tell you about those nasty yeast infections.
pH can be restored by turning off the air and allowing it to denitrify. Denitrification is slower than nitrification, so, for best results, you should turn the air off for twice the time you aerate it, and I'll show you as soon as I can get this pesky blogger to allow me to paste a drawing. In real world terms, this means you can turn the blowers off when you leave for the day at the end of the day shift, and turn them back in the morning. You will then track your pH to see if that is effective. When it's hot out you will have to run the blowers longer. Thats why you have timers on your blowers. Pay attention to whats happening and make changes as needed, instead of doing nothng and then blaming the engineer. You will end up with a sludge tha dewaters/decants better and save energy.
Other indicators
pH is slow to change. It may be that you need to look at ather values to control your operation. You all are out of the dark ages and will have these tests available to you, so quit howling at the darkness and get to work.
Ammonia
Nitrification destroys ammonia, meaning you have aerobic conditions. Septic conditions create ammonia, as it evolves out of protein destruction. Since you are most likely testing your effluent, you can test your digester too. You;ll have to figure out for yourself what ammonia values are too high, but 125 mg/L is a place to start.
Alkalnity
Alkalinity is a measure of the sludge buffering capacity. Depending upon conditions, alkalinity will range from 0 (too low) to 700 or more (too high). Adjust your air accordingly.
Total Solids
If you are thickening your sludge, especially with a mechanical thickener, you need to watch your solids and make sure it does not get too thick, unless you are going to unerated storage.
It can be that easy if you are not so stubborn as to do it different just because.
IDLE MUSINGS
AEROBIC DIGESTION, PART DEUX
This section was going to be on operations but I didn't cover enough on blowers and diffusers, so now I will.
Blowers
Every book you read says you must use PD blowers for aerobic digesters, and cannot use centrifugal blowers. This is a lie. (I'd use stronger words, but someone would be offended). Centrifugal blowers can be used, and can supply multiple tanks. All you, the operator, has to do is use the air flow control valve on the drop header to each tank, and pinch back the tank with the lowest level so all the air does not flow into that tank. Stop watching the TV and do a little work. Why use centrifugals? They are more efficient than PD blowers and much quieter. The PD blower manufacturers are now offering PD blowers with VFD for speed control and nice enclosures to get the noise level down to 70 dB or even less. Aerzen, for one has a nice package for that, and easy access for maintenance as well.
In addition to air flow control valves, each tank drop should have a flow meter. Why? BECAUSE IF YOU CANT MEASURE IT YOU CAN'T CONTROL IT! I know, I'm a slave driver, spoiling your day making you write down all these data boo hoo.
Diffusers
There are numersou diffusers, both fine bubble and coarse bubble that can be used. I think that some are better than others. Mooers flex cap diffusers work well in digesters and are better at resisting fouling that some.
Fine bubble diffusers can be used for aerating digesters, and require less air than coarse bubble diffusers. They are more prone to fouling, but EDI promotes fine bubble diffusers for aerobic digesters.
As I said earlier, the kind of diffuser you use limits the reactor total solids. With fine bubble diffusers, the tanks solids should be less than 3% TS. For coarse bubble duffusers, less thatn 4% TS. If you want to see proof, write to me and I'll bore you with all the calculations.
ORP meters.
To know the conditions inside the tank, an ORP (oxidation-reduction potential) meter is better than a DO meter. ORP tells you how and how to cycle the air flow to good effect in part trois.
Blowers
Every book you read says you must use PD blowers for aerobic digesters, and cannot use centrifugal blowers. This is a lie. (I'd use stronger words, but someone would be offended). Centrifugal blowers can be used, and can supply multiple tanks. All you, the operator, has to do is use the air flow control valve on the drop header to each tank, and pinch back the tank with the lowest level so all the air does not flow into that tank. Stop watching the TV and do a little work. Why use centrifugals? They are more efficient than PD blowers and much quieter. The PD blower manufacturers are now offering PD blowers with VFD for speed control and nice enclosures to get the noise level down to 70 dB or even less. Aerzen, for one has a nice package for that, and easy access for maintenance as well.
In addition to air flow control valves, each tank drop should have a flow meter. Why? BECAUSE IF YOU CANT MEASURE IT YOU CAN'T CONTROL IT! I know, I'm a slave driver, spoiling your day making you write down all these data boo hoo.
Diffusers
There are numersou diffusers, both fine bubble and coarse bubble that can be used. I think that some are better than others. Mooers flex cap diffusers work well in digesters and are better at resisting fouling that some.
Fine bubble diffusers can be used for aerating digesters, and require less air than coarse bubble diffusers. They are more prone to fouling, but EDI promotes fine bubble diffusers for aerobic digesters.
As I said earlier, the kind of diffuser you use limits the reactor total solids. With fine bubble diffusers, the tanks solids should be less than 3% TS. For coarse bubble duffusers, less thatn 4% TS. If you want to see proof, write to me and I'll bore you with all the calculations.
ORP meters.
To know the conditions inside the tank, an ORP (oxidation-reduction potential) meter is better than a DO meter. ORP tells you how and how to cycle the air flow to good effect in part trois.
Tuesday, August 30, 2011
WHAT YOU NEED TO KNOW ABOUT AEROBIC DIGESTION ,PART ONE
People think I’m an aerobic digestion expert, and I was an author on the latests EPA solids handling manual, so I’ll give you a boiled down (not sleep inducing) version of what I think you should do.
What is aerobic digestion?
Aerobic digestion is dieting. Waste activated sludge is put in a reactor and aerated. The organisms run out of food and die, and the cells lyse, providing food for some of the remaining organisms. There is nitrification going on too, and, hopefully, denitrification as well. The chemical formulas for all this are:
C5H7O2N + 5O2 => 4CO2 + H2O + NH4HCO3
Destruction of biomass in aerobic digestion
NH4+ + 2O2 => NO3- + 2H+ + H2O
Nitrification of released ammonia-nitrogen
C5H7O2N +7O2 => 5CO2 + 3H2O + HNO3
Complete nitrification
2 C5H7O2N + 12O2 => 10CO2 + 5H2O + NH4+ + NO3-
With partial nitrification
C5H7O2N + 4NO3- + H2O => NH4+ + 5HCO3- + 2N2
Denitrification using nitrate nitrogen as electron acceptor
A PRACTICAL EXAMPLE OF DIETING
C5H7O2N +5.75O2 => 5CO2 + 0.5N2 +3.5H2O
With complete nitrification and denitrification
Biomass gets turned into carbon dioxide and water. Ammonia gets turned into nitrate and hydrogen ion, Nitrate gets turned in to nitrogen and oxygen and half of the hydrogen ions are taken up into hydroxide.
How to design a digester
A digester should not be a wide spot in the pipe where you play around with it when you feel like it. An aerobic digester is a reactor, and should be treated as such.
How big should it be?
In the Midwest, 45 days SRT is enough, based on the chart below .The 45 days is for the reactor. Liquid storage is separate.
How may stages should it have?
I think 3. 2 Stages gets you a 25% reduction in SRT needed, and 3 stages lets you take 1 tank out of service for cleaning or whatever without adversely effecting your operation.
Should it have covers?
In places where it gets cold, one tank or more should have a cover for heat retention. NOTE WELL: In the summertime a covered tank will autoheat, stink and foam up (not good). You need to be able to take part of the cover off, or have lots of hatches, or use the covered tank as the 3rd stage or liquid storage. Of course, you operators can do what you want and then blame the consultants when it doesn’t work.
What kind of diffusers should I use?
You can use either fine bubble or coarse bubble diffusers. Both have different qualities and limitations. See the section below on how thick your sludge should be
Should I thicken the sludge
There are several methods for thickening the sludge, including
- Supernating. If possible you should get an SBR decanter for supernating instead of a telescoping valve. It will save a lot of time and allow a cleaner supernatant to be decanted. Another option is to tie a submersible pump to an inner tube and float it in the digester on a long hose and electrical cord. I designed a system like that in Bowling Green. Supernating usually thickens the sludge up to 2.5%. Your results may vary
- Belt thickeners and Drum thickeners. Mechanical thickeners can thicken the sludge up to 10% or more if you want to use high doses of polymer. Not a good idea. You aeration system will not handle more than 3 or 4 % solids (more on this later) and you will end up with a putrid, quivering mess. Limit your thickening to about 5% TS out of the machine.
- Membrane thickeners (MBT). This is the most elegant way to thiken aerobic digester solids, especially if you do it as a recuperative thickener. What is a recuperative thickener? Its one where you pump think sludge from your digester to the thickener, usually at about 200 gpm. The membrane underflow pumps are set to pull out a small amount of permeate – as little as 10-15 gpm, but maybe as much as 25-50 gpm depending upon the capacity. This thickens the sludge up a little, and it falls by gravity back into a digester. You run the MBT until youget it as thick as you want it usually 3 to 3.5% TS. What’s so elegant about this? You don’t have to pay much attention to it. You turn the system on and it can run for days before it needs attention. In addition, you don’t have to worry about phosphorus recycle as you do from supernating. When you supernate a bio-P sludge, you can get phosphorus concentrations as high as 70 mg/L, which will ruin your day when it recycles to the aeration tank. The permeate can be discharged to the plant effluent
An example is shown below
Sludge Thickeness
Sludge thickness effects oxygen transfer efficiency. Oxygen transfer is less efficient as the sludge gets thicker. The point where process air exceeds mixing air is about 3% TS for fine bubble diffusers, and about 4% TS for coarse bubble diffusers. I’ll show you all the boring calculations another time because this article is getting long. Trust me.
Tank height
Some people advocate building really tall tanks (22’) for improved oxygen transfer efficiency, This is true, but the increased pressure on the diffuser at the tank bottom requires extra energy. With cheap energy, it’s a wash. With expensive energy, it costs you more to have a deep tank than a conventional tank.
Liquid storage
LIQUID STORAGE DOES NOT NEED TO BE AERATED! One you have stabilized it, you do not need to keep aerating it, If you do, you just make pH problems for yourself by nitrification, and if you get it really thick, you blow the stink up in the air.
To recap, the digester should be
- A reactor, not just a wide spot in the tank
- You should have 45 days SRT in 3 switchable stages
- Storage is separate from a reactor
- Don’t make the sludge too thick
- MBTs are great
- Liquid storage does not need to be aerated.
The next posting will talk about operation and troubleshooting.
WHY IS MY BLOG CALLED WHAT IT IS?
You can thank Mark Buchenic for that. We were making a presentation to a client, and we used that photo at the top of my blog. Mark insterted a cartoon balloon that said "Let's talk about poo". So I am using it.
INTRODUCTION
I am a wastewater treatment operator, wastewater plant designer, process "expert" textbook author, and writer of articles on wastewater treatment. I have 33 years experience and have decided to put my thoughts down here for others to read, if you want to.
I have lived and/or worked in Caledonia OH, Lewisburg OH, Ketteriing OH, Bloomington IN , San Antonio TX, Houston TX, Sarasota, FL, Fernandina Beach FL, Toledo OH, Battle Creek, MI, Zinzinnati, OH, Streetsboro OH. Ravenna OH, Akron OH, New Bedford MA, Jeddah, Saudi Arabia, Santiago Chile, Dundee, MI, Deerfield, MI, Carmel, IN and Kalamazoo, MI to name a few. I can even sing the Kalamazoo song.
I like baseball, golf, girls' broomball and lacrosse. I played lacrosse in college. I used to watch football but no longer do. I like TV shows like Suits, Inspector Morse, Inspector Lewis, Arthur, and stuff on the Discovery Channel and the History Channel that most of you consider extremely boring. I read lots of books without pictures, and some with pictures.
I love my kids, but there is only one left.
I used to be a Republican but now I'm an anarchist.
I am an Orthodox Christian. For those of you who don't know what that is, that means that I kiss pichurs and cross myself backwards. If that offends you, too bad.
I am going to blog about things that interest me, and try to teach some of you something as well. I am preparing a series of articles that won't be so long that your eyes glaze over. You will probably get mad at me for something I publish, maybe all of it. I don't care.
I have lived and/or worked in Caledonia OH, Lewisburg OH, Ketteriing OH, Bloomington IN , San Antonio TX, Houston TX, Sarasota, FL, Fernandina Beach FL, Toledo OH, Battle Creek, MI, Zinzinnati, OH, Streetsboro OH. Ravenna OH, Akron OH, New Bedford MA, Jeddah, Saudi Arabia, Santiago Chile, Dundee, MI, Deerfield, MI, Carmel, IN and Kalamazoo, MI to name a few. I can even sing the Kalamazoo song.
I like baseball, golf, girls' broomball and lacrosse. I played lacrosse in college. I used to watch football but no longer do. I like TV shows like Suits, Inspector Morse, Inspector Lewis, Arthur, and stuff on the Discovery Channel and the History Channel that most of you consider extremely boring. I read lots of books without pictures, and some with pictures.
I love my kids, but there is only one left.
I used to be a Republican but now I'm an anarchist.
I am an Orthodox Christian. For those of you who don't know what that is, that means that I kiss pichurs and cross myself backwards. If that offends you, too bad.
I am going to blog about things that interest me, and try to teach some of you something as well. I am preparing a series of articles that won't be so long that your eyes glaze over. You will probably get mad at me for something I publish, maybe all of it. I don't care.
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