Structure of Straw – Water Absorbtion

Straw consists of air-filled pores from nano-to-meso-scale and to macroscale. Straw is constructed to withstand the ambient conditions in the fields and attach by pests, microorganisms, and enzymes; and it is built very well. For these reasons, straw cannot directly be used for anaerobic digestion.


In order to effectively being able to utilize straw for biogas it is necessary to provide access for water and enzymes to the surfaces of straw. Here it is essential to note, that 80% of the surface area of straw is found in the smallest of pores, the nano-pores, and virtually all surface areas in pores as such. The picture below gives a good impression of the porous structure of straw.

Mechanical briquetting is a technique by which a reciprocating piston compresses the straw at high pressures in matter of split seconds – literally. The straw is compressed to a very high specific density of e.g. 1,0, where lose falling straw has a density of e.g. 0,05. In other words, lose falling straw weights 50 kg per m3, while briquettes weigh up to 500 kg per m3.

The briquetting process also alters to the properties of straw. The picture below shows a test  with a 70 g straw briquette and 70 g shredded straw emerged in water. After just 15 min you can see that the briquettes have absorbed most of the water, whereas the shredded straw has not absorbed much water. The same effect is obtained in a biogas tank and you can see the result of this under data.