Validation of Two Theoretically Derived Equations for Predicting pH in CO2 Biomethanisation

Abstract

CO2 biomethanisation is a rapidly emerging technology which can contribute to reducing greenhouse gas emissions through the more sustainable use of organic feedstocks. The major technical limitation for in situ systems is that the reaction causes CO2 depletion which drives up pH, potentially leading to instability and even digestion failure. The study aimed to test fundamentally derived predictive equations as tools to manage H2 addition to anaerobic digesters. The methodology used data from the literature and from experimental digesters operated with excess H2 to a point of failure and subsequent recovery. Two equations were tested: the first relating pH to CO2 partial pressure (pCO2), and the second extending this to include the influence of volatile fatty acids and ammonia. The first equation gave good agreement for data from studies covering a wide range of operating conditions and digester types. Where agreement was not good, this could usually be explained, and in some cases improved, using the second equation, which also showed excellent predictive performance in the experimental study. The results validated the derived equations and identified typical coefficient values for some organic feedstocks. Both equations could provide a basis for process control of CO2 biomethanisation using routine monitoring of pH or pCO2 with additional analysis for volatile fatty acids and total ammonia nitrogen when required.