FOS/TAC levels in the digester are an efficient way of assessing the health of the digester. This is because pH alone is not suitable to evaluate biological health. Although at first, FOS/TAC levels can seem quite confusing, they aren’t as complicated as they may seem.
What are they?
FOS/TAC = Fluchtige Organische Sauren/Total Anorganic Carbon
In more simple terms, FOS refers to the volatile organic acids within the digester. TAC refers to the alkaline buffer present. Most AD plants will have a machine that calculates this value automatically. However, there are manual ways of conducting this test.
Why are they important?
Buffering capacity is the amount that can be “soaked up” of the acid or base in the system as if it is a sponge. But, once this sponge becomes saturated, it can no longer soak anymore up. In our case, this is when the digester can start to become sub-optimal.
With a strong buffer in the digester, the pH will remain stable during acidification. This means that the biology will stay healthy and within the correct pH range. If the digester has a small or non-existent buffer when the digester goes through the acidification process, the pH will fall, and the biology will be unhealthy. Potentially leading to souring.
Is the FOS/TAC ratio the same for all systems?
There are many types of AD systems all of which have varying feedstocks. Typically, AD that uses manure as its primary feedstock tends to have a good buffering capacity. Whereas food waste AD processes will potentially need bicarbonate to provide alkalinity.
The ratio we get from calculating the FOS/TAC levels can then tell us how healthy our system is. If the level is higher, then the chance of acidification is also higher. The optimal level is between 0.3-0.4. However, each plant has its optimal ratio.
Advantages
- Easy and fast to test
- A useful parameter that tells us the state of the biology
- Allows us to see problems before it’s too late
Disadvantages
- Relies on accurate pH measurement
- Absolute value depends on the preparation of samples
How to test the FOS/TAC levels
As mentioned previously the FOS/TAC levels can be measured with specialised equipment that will do all the work for you. However, using the Norman method, you can do it manually.
- Take 20ml sample & add 180ml demineralised water
- Agitate and connect to pH meter
- Titrate with H2SO4 (0.05mol/l)
- Note the volume of H2SO4 needed to reach pH 5 (V1)
- Note the volume of H2SO4 needed to get from pH 5 to pH 4.4 (V2)
FOS = (V2x1.66-0.15) x 500
/
TAC = V1 x 250
References
Roots Organics