for getting knowledge about stability of the process. This information may useful to control the process.
pH values for anaerobic digestion processes follow in the range 6.5 – 7.5 but very different values can be found in stable anaerobic digestion processes perfectly working: this is because pH is the result of the presence several compounds in the bulk ammonia, VFA, phosphates.
In anaerobic digestion processes pHis strongly influenced by the presence of carbonate and bicarbonate
pH 5.75 partial alkalinity (PA)
pH 4.3 or total alkalinity (TA)
A high VFA concentration would have to form in order to cause a detectable drop in pH.
Yes, there is an optimal range of VFA to alkalinity for a digester to work smoothly. The VFA to Alkalinity ratio should be in between 0.05 to 0.15. If the value goes up to 0.15, it will be an alarming situation that the VFAs are increasing and a proper buffering is required.
I agree with Denny Pop. The best practise is to measure regularly the VFA/alkalinity. Each case might be quite different. Measuring regurally the VFA/alkalinity ratio and obsering also all the other parameters regularly, you will get indeed the "feeling" for stable reactor operation. However, I would start a ratio of < 0.3.
@ Denny Popp
I find many articles mentioning the ratio to be within 0.5, however which of the scientific articles mention for higher values of 0.8?
Would you be willing to share?
Denny Popp, Dimitri Georgiou etc , I think you all misunderstood his question. He is after the optimal range for VFA/TA for stable digesters, and not the IA/PA ratio. Intermediate alkalinity (IA): Partial alkalinity (PA) ratios are digester specific and the value can go all the way to 0.8 for stable digesters (as you pointed out), certainly IA/PA ratios < 0.3 are generally considered stable. On the other hand, digesters with VFA/TA ratios less than 0.3 - 0.4 is considered to be operating favorably without the risk of acidification. There's a need to point out that these are two different process parameters, that can easily be mistaken. cheers
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