Many „new“ procedures to increase the biogas yield from the substrates are developed and propagated particularly in economically difficult times. The best known techniques at the market are
- use of enzymes for decomposition and for decrease of viscosity
- mechanical decomposition by a crusher or an extruder
- ultrasonic decomposition
- thermal decomposition by heating up.
I have reviewed these techniques and also evaluated. Partly I have carried out own tests, have consulted colleagues and found that it is rewarding to have a close look at the different techniques. I would like to start with the enzymes.
Enzymes have the charm, that no investments will become necessary. Some enzymes into the digester and much more biogas will be produced immediately, less mixing energy will be needed and all will be fine – promises the advertising.
When using enzymes firstly in 2007 in order to eliminate the floating cover in a plant, which processed very much whole crop silage, the advertising statements seemed to confirm. The power consumption of the mixers actually decreased and the floating cover became more controllable. However, after a view weeks it became apparent, that the effect was not sustainable. To become an effect, more and more enzymes had to be added, so as if the digester biology had become addicted.
Meanwhile fairly extensive papers on the use of enzymes are available. Therein is proven, that enzymes do not make more biogas. This is logical. Why should bacteria waste energy to produce enzymes, if someone else makes this for them and pays an expensive price.
The problem in adding enzymes lies in the by and by ocurring adaptation of the digester biology. Only the bacteria being able to process the feed prefabricated by the enzymes, remain. If the feed changes problems arise, because the digester biology is no longer as robust as with a well-trained bacteria team.
„Enzymes make bacteria lazy”
Enzymes are suitable for short-time use to decrease viscosity and possibly for the control of floating covers. However they should be used only in case of emergency. As an operator you can have both (decrease of viscosity and avoiding floating covers) also when using a separator, sufficiently separating the digester discharge and leading back the process water into the digester. The separated fibres are mostly inert and make nearly no biogas, but are also unable to form floating covers and to increase viscosity when they are eliminated. But the separated solid material is a good fertilizer.
Enzymes are highly specialized tools, which have to be used accordingly. Used intelligently at the right place enzymes can work wonders. But in the digester externally produced enzymes have no place. From my point of view there are better methods than throwing enzymes into the digester, because enzymes
- make the bacteria lazy and addicted
- create nearly no additional biogas yield
- the other benefits as easier mixing and control of floating covers can be reached better
- and for the operator the use of enzymes normally is not profitable.
If you have any questions, please write me or leave a comment.
