As a start I would try

1. meta-analysis to combine results from all feedstocks

2. If you have results from replicate sample then Non-hierarchical cluster analysis can combine those results into overall average

3. using feedstocks as treatments ANOVA can also give you COMBINED Means

4. without replicate information unable to list other options...

Every single feedstock differs, and the resulting digestate is then different. But on top of it, N, P and K are not found within the same molecular species in digestates and feedstocks. Examples: N is organic macromolecules in feedstocks, while as ammonium cations in digestates.

Standardization seems a bit difficult ...

Typical values for main feedstocks have been reviewed by Felipe Guilayn during his PhD : Article First fertilizing-value typology of digestates: A decision-m...

The results are summed up here (in French) : https://www.infometha.org/effets-agronomiques/digestat/caracteristiques-des-digestats

Azote is N, Phosphore is P, Potassium is K.

Values can vary by more than a ratio of 2 even with the same category of inputs.

You can also check the interesting introductory chapter we proposed (with Gabriel Capson-Tojo ) for the IWA book on digestate management. We've added a complementary critical approach and more extensive data.

https://iwaponline.com/ebooks/book/855/Anaerobic-Digestate-Management

Dear Srinivas Kasulla,

The total (liquid + solids) NPK mass in the AD digestate would be almost equal to the total (liquid + solids) NPK mass in the corresponding AD feedstock except for a small loss of free ammonia NH3 to the biogas and ambient air depending on the pH and temperature of the digestate. This total NPK mass varies a lot with the kind of feedstock. In high N cases such as chicken manure it would be necessary to remove the excess ammonia and lower the pH as to reduce methanogenesis inhibition.

Digestates from biogas digesters exhibit varying NPK (Nitrogen, Phosphorus, Potassium) nutrient contents depending on the feedstock used, typically ranging from 2.3 to 4.2 kg of nitrogen per tonne. While there is considerable variation in nutrient concentrations, digestates can still be effective fertilizers if they meet quality standards. Standardization remains a challenge due to the influence of feedstock type and digestion conditions on nutrient consistency.

Abdelhak Maghchiche Thankyou and yes i do agree with the Nitrogen Part

Bruno Peeters Yes i totally agree with your point of what goes in comes out except the loss of free amonia which will at least ensure that the NPK of the feedstock would be almost similar to what the effluent slurry has

Felipe Guilayn Thankyou for sharing the anaerobic digestion book link and would try to have a look at the book if this is available in my data base, however the link you shared is for the purchase of the book but I can look out for the same thanks

Alice L'Hostis Thankyou for sharing the book in the French Language and yes I do agree with the NPK details you shared, also Dr Felipe Guilayn has already shared a link of the book and I have also requested for the research paper link you have shared in your answer which am sure would be helpful for most of the people interested in the anaerobic digestion based digestate.

Daniel Chateigner Yes to an extent you are right but I don't feel that this is impossible as rightly mentioned by the experts here we can standardise by the input NPK rather than looking out for the effluent slurry NPK details

Mewa Singh Dhanoa The four points you have mentioned about the Meta analysis of the feedstock, Non hierarchical cluster analysis, ANNOVA can give you combined means, and without replicate information unable to list other options is a perfect example of how the effluent can be treated in the laboratory . However the details shared by the other experts are highly recommendable and I shall also look into the thesis and the paper published by Felipe Guilayn as well as the book he shared and will shortly post the outcome in the discussion section as well