Accurate Estimation of Bicarbonate and Acetic Acid Concentrations with Wider Ranges in Anaerobic Media Using Classical FOS/TAC Titration Method

Xiaojun Liu,Thierry Ribeiro,Jean-Marie Grosmaître,André Pauss,Maël Mercier-Huat,Laura André

doi:10.3390/app112411843

Abstract

The determination of a volatile fatty acid content (FOS) and total alkalinity (TAC) can be carried out using Nordmann’s FOS/TAC titration method developed in the 1970s. This two-point titration (pH = 5 and 4.4) can be simply implemented and is widely employed by both the academic and industrial worlds. However, the present study proves that Nordmann’s method is only valid in limited ranges, since the titration of one FOS and TAC has an impact on the determination of the other, especially in extreme conditions. The present work develops a numerical tool with Scilab simulating the acid–base equilibria of titration. The program is efficient in predicting the experimental equivalent volumes obtained from Nordmann’s method with different combinations of sodium acetate and sodium bicarbonate contents. The mean absolute percentage errors (MAPE) between the simulation and experiment are below 7%. Two new formulas are developed, considering both equivalent volumes at pH = 5 and 4.4 to calibrate FOS and TAC values. The proposed formulas show their good performance in predicting various combinations of FOS and TAC contents in an anaerobic digestate at TAC ranging from 0 to 20,000 mg CaCO3·L−1 and FOS ranging from 0 to 31,000 mg HAc·L−1.

Highlights

Anaerobic digestion (AD) is attracting increasing attention due to its environmental benefits such as the production of renewable energy and the valorization of organic biomass
The acidic nature of volatile fatty acids (VFAs) leads to the acidification of reactors by lowering the pH, which is a crucial parameter for the success of AD, for methanogenic activities
High loading in FOS could greatly impact the determination of TAC and vice versa

Summary

Introduction

Anaerobic digestion (AD) is attracting increasing attention due to its environmental benefits such as the production of renewable energy (biogas) and the valorization of organic biomass. The molecules produced at the end of the hydrolysis stage are degraded by acidogenic microorganisms to produce volatile fatty acids (VFAs) such as acetic, propionic and butyric acids. The VFAs are consumed by acetogenic and methanogenic microorganisms for the production of biogas and, methane. For this reason, the VFA concentration is often expressed in an equivalent acetic acid content (usually expressed in g HAc·L−1 ). Depending on the digestion method and the nature of substrates, the VFA concentration can reach 3 g HAc·L−1 or much more [1]

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