The Surprising Buoyancy of Biomass

Driven by the goal of mitigating climate change, we’ve developed an innovative solution for removing CO₂ from the atmosphere: anoxic marine biomass storage. One common question we receive is how do we get the biomass to sink and stay at  the bottom of the sea. Read on to find out!

Understanding Density and Weight 

Understanding whether an object sinks or floats comes down to its density relative to the density of the fluid it’s in. Density is defined as the mass of an object per unit of its volume. Imagine a block of wood and a block of steel of the same size. We know that the steel block is significantly heavier as it has a higher density. In a scenario where each object is placed in water, the steel block sinks while the wood block floats. This happens because the density of steel is greater than the density of water, whereas the density of wood is less. Thus, the steel will sink because the gravitational force pulling it down is stronger than the buoyant force pushing it up (applied by the water beneath it).

This basic principle is the foundation of our approach to sinking biomass!

Biomass Types and their Density

Fresh water has a density of approximately 1.0 g/cm³ (not a coincidence, this is by design of the metric system), so any material with a density less than this will float. Different types of biomass have varying densities, which, in turn, affect their respective buoyancies. As you may have guessed, most biomass types are less dense than water:  

• Corn stems: 0.4 g/cm³
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• Wheat straw: 0.3 g/cm³
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• Vine branches: 0.6 g/cm³
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• Oak wood: 0.75 g/cm³
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• Ebony wood: 1.1 g/cm³    

So how does it end up sinking and staying on the bottom of the sea?

From Floating to Sinking

Biomass “shrinks” under pressure. →

When biomass is subjected to heat, pressure, or extended submersion in water, the internal air compartments within the biomass get compressed or filled with water, significantly increasing its density. As the density increases, the biomass transitions from floating to sinking.

This process is similar to how a scuba diver uses a balancing vest to adjust buoyancy by adding or releasing air. The deeper one dives, the “smaller” the vest becomes (due to the water pressure), requiring the diver to fill more air in the vest to prevent uncontrolled sinking. This process can also be demonstrated by a common kids toy, composed of a bottle of water with a floating air capsule inside it. When the bottle is squeezed, the increased pressure forces water into the capsule, shrinking the volume of the air and causing the capsule to sink. See if for yourself in the video below.

The same principle applies to biomass: when exposed to certain conditions, like heat, pressure and prolonged submersion, its buoyancy flips from floating to sinking.

Natural & Accelerated Water Logging →

When you think about it, since most types of plants should float in the water, how come the oceans aren’t filled with plant debris? This is because the surprising buoyancy flip we just described happens naturally and is called waterlogging. In nature, it takes a week or two for a plant to soak up water and sink. Different types of wood with varying densities, as well as different aquatic environments with varying salinity (salt water is denser than fresh water), will affect the time it takes for plant matter to become waterlogged and start sinking. 

Luckily waterlogging can be accelerated with pressure, and just like with scuba diving, when submerging to certain depths such as 10 meters, 20 meters, or even 50 meters, the plant matter soaks up water much more quickly and starts sinking instantly. The “flip depth” for each type of biomass will vary according to the biomass density, moisture content, water salinity and many other factors. From our experience, 50 meters is enough for most cases.  

Seeing is Believing

To provide concrete evidence of this surprising effect, we conducted several experiments that illustrate the buoyancy change in action.

Experiment 1: Pressure Cooking Biomass

In an initial experiment performed by Peter, our Co-Chief Scientist, in his kitchen, we observed the effect of temperature and pressure on biomass buoyancy. Fresh leek was placed in water and was floating initially. However, after it was pressure cooked, the leek sank to the bottom. Peter then repeated the experiment with a block of wood. While he was unable to sink it completely, the wood floated less and was way deeper than before being pressure cooked. This was our simplest, cheapest experiment suggesting that adding pressure can increase the density of biomass, causing it to sink. 

Experiment 2: Scuba Diving with Wood

In a series of scuba diving experiments performed by Eitan, our COO, in the Mediterranean Sea, we carried pieces of wood and recorded the depth at which the wood started to sink. Using various types of wood, some started sinking at 17 meters depth, others at 23, and some types stayed afloat even at 30 meters, which was the maximum depth for this experiment due to simple scuba diving limitations. This experiment demonstrated that the “flip depth” isn’t too deep, but we felt we needed to plan a more sophisticated and comprehensive experiment, leaving no room for any type of wood to stay afloat. 

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Experiment 3: Caged Submersion to Measure Buoyancy Flip Depth

To make sure we reach the flip depth for any type of wood we plan to work with, we conducted an experiment based on caged, photographed submersion. In this experiment, a metal cage which was open at its bottom and had a live camera attached to it, was used to submerge the biomass. Using knots on a rope we measured the depth to which the cage with the biomass was lowered, and using the camera, we observed the buoyancy flip occuring. Since the cage was open on the bottom, once the biomass bag started sinking, it fell out of the cage. Don’t worry, the bag was attached to the cage with another rope, allowing us to recover it back to the boat. We tested different types of wood chips (pine, apple, and olive) to answer the question of at what depth does the biomass sink. Here are the results: 

This experiment allowed us to set the flip depth at 50 meters, with a high likelihood of compressing any type of biomass and causing it to sink freely. 

Experiment 4: Leveraging the Flip Depth to Minimize Ballast

In our final experiment, we tested the sinking behavior of biomass bags chained together with a rope. When the bags are chained together, then a relatively small sinker can be used to drag only the first 50 meters of the chain, and then any bag that passes the “flip depth” starts adding more mass to the sinker, continuing to drag the rest of the chain down. We conducted this experiment in the Mediterranean Sea at 80 meter depth, and recorded the sinking using a camera and an ROV. 

Despite the fact that a long chain needed to sink, a relatively small sinker was all that was required to initiate the sinking process. Once the whole chain of biomass bags was down, we were able to verify that it stayed on the sediment without any kind of floatation, and also to retrieve it back out and not leave anything on site. 

Key Takeaways

At Rewind, our approach makes biomass sinking efficient and safe, enabling us to scale sustainably with minimal use of sinking ballasts, if any at all. We hope this post covered all you need to know about biomass buoyancy, including proof  from pressure cooking, scuba diving, and field trials, showing how biomass transitions from floating to sinking. Here are the key points to remember:

1. Water-logging Effect: With time, biomass absorbs water, increases in density, and starts sinking
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2. Long-Term Stability: Once waterlogged, the biomass remains dense and on the ocean floor. This is why our oceans are not covered with floating plant debris.
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3. Biomass Density: Different types of biomass, like corn stems and oak wood, have varying densities and require different levels of pressure to accelerate water-logging. And yes, some types of wood, like ebony, are denser than water and sink immediately.

If you have any questions or thoughts, we'd love to hear from you. Follow us on LinkedIn to stay up to date with our latest adventures and innovation in marine carbon removal.