Engineers design mixers for methane digesters. Watch out! Immediate benefits hide long-term consequences. And the bacteria are laughing.
The first effect of mixing is positive for inoculation. However, the negatives add up over time. The initial bump-up becomes a long slide-down.
The Bacteria’s Engine
- The Forced Escape: Mixers agitate the slurry. With good gas-producing slurry, the bubbles are agitated to pass around solids and escape.
- The Lost Engine: The bacteria laugh because, as the producers of those bubbles, they know that those bubbles are the engine of their Natural Mixing.
- The Anti-Natural Device: That mixer is now an Anti-Natural Mixing Device. Also, it bugs the bacterial associations.
🔬 HOW IT WORKS
When you understand how things work, you can work with things. Otherwise, you work around and among things.
So, the bacteria and the bubbles, how do they work?
The Non-Motile Bacteria
- The Need for Motion: Bacteria are not motile. They depend on the motion of their environment to move around. This motion is essential for inoculating fresh manure for efficient digestion.
- The Slurry Matrix: Digesting cow manure is full of solids that delay the bubbles’ gravitational upward motion.
- The Density Drop: Those delayed bubbles reduce the density of the slurry. That is, until the bubbles grow large enough to reach the tipping point of escaping around the solids.
- The Cascade: Agitation now easily releases those bubbles. As they move up, they disturb more slurry, cascading a channel of rising bubbles.
The Degassing Plunge
Now be careful here. It is often said that rising bubbles “lift” the slurry up. But too often, the bubbles escaping just leave behind a degassed, heavier channel of slurry that now moves down.
The sum of many bubbles in a slurry retained by the solids can lift. But agitate them to leave, and the result is the opposite. Calm is now necessary to restore the gravity balance for the special condition of natural continuous motion.
🌡️ The Complexity of Heat
This gets more complicated with heat. Heaters are said to cause thermal uplifting currents. But remember those bubbles, what do they do?
- The Expansion: First, their size grows. More bubbles pass the release tipping point and travel up, causing more agitation and release.
- The Chemistry Analog: Dissolved gasses of oxygen and carbon dioxide form bubbles sooner and faster than boiling water converts to water vapor.
- The Counter-Current: The bubbles exit, rising, while the heavier slurry left behind goes down!
Proved by the Plexiglass
I know this. I have seen it. I put plexiglass windows in my bench and pilot-scale digesters. But watch out, it’s still more complicated:
- Weak Gas Production: The thermal force dominates. The weak Natural Mixing currents allow the heat to lift the fluid.
- Strong Gas Production: Improved gas production and the resulting degassing meet, then override, the thermal currents of a heater. And down the slurry goes.
And so the bacteria laugh harder.
🚜 Working with Nature, Not Against It
With design and operation, there are many options to improve the environmental condition of continuous motion for Natural Mixing. All we need to do is:
- Observe the slurry.
- Log the conditions.
- Find the boundary of that special condition of free, superior mixing with less harm to the bacteria.
If a mixer is thrown in when stratification is seen or expected, only more stratification will result. That mistake must then be overcome with more parasitic energy.
Find the options that increase the natural mixing, and we should have an expanding natural mixing condition. Now, do we have the respect of the bacteria?
To view the original 1979 thermal strip charts, the 1991 Bill’s Dairy pilot rig metrics, and the full development of this concept, see the complete documented report on my permanent page: Read the Full Report: Biology Driven Physics the History.