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 3^rd 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.