A Simple and Accurate Approach for Determining the VFA Concentration in Anaerobic Digestion Liquors, Relying on Two Titration Points and an External Inorganic Carbon Analysis

Paz Nativ,Ori Lahav,Yaron Aviezer,Yonatan Gräber

doi:10.3390/chemengineering5020015

Paz Nativ, Ori Lahav + Show 2 more

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https://doi.org/10.3390/chemengineering5020015

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Journal: ChemEngineering	Publication Date: Apr 6, 2021
Citations: 9	License type: CC BY 4.0

Affiliation: Technion – Israel Institute of Technology

Abstract

A new analytic approach is presented for determining the total volatile fatty acids (VFAT) concentration in anaerobic digesters. The approach relies on external determination of the inorganic carbon concentration (CT) in the analyzed solution, along with two strong-acid titration points. The CT concentration can be determined by either a direct analysis (e.g., by using a TOC device) or by estimating it from the recorded partial pressure of CO2(g) in the biogas (often a routine analysis in anaerobic digesters). The titration is carried out to pH 5.25 and then to pH 4.25. The two titration results are plugged into an alkalinity-mass-based equation and then the two terms are subtracted from each other to yield an equation in which VFAT is the sole unknown (since CT is known and the effect of the total orthophosphate and ammonia concentrations is shown to be small at this pH range). The development of the algorithm and its verification on four anaerobic reactor liquors is presented, on both the raw water and on acetic acid-spiked samples. The results show the method to be both accurate (up to 2.5% of the expected value for VFAT/Alkalinity >0.2) and repetitive when the total orthophosphate and ammonia concentrations are known, and fairly accurate (±5% for VFAT >5 mM) when these are completely neglected. PHREEQC-assisted computation of CT from the knowledge of the partial pressure of CO2(g) in the biogas (and pH, EC and temperature in the liquor) resulted in a very good estimation of the CT value (±3%), indicating that this technique is adequate for the purpose of determining VFAT for alarming operators in case of process deterioration and imminent failure.

Highlights

The method differs from previous techniques by the fact that the total inorganic carbon concentration is analyzed on the same sample, but by an external method, preferably using a TOC analyzer
The buffer capacity area between these pH values is dominated by the volatile fatty acids (VFAT) and carbonate weak-acid systems, with only a minor effect of the ammonia and orthophosphate systems, these weak-acid systems can be neglected in the analysis without significant loss of accuracy, when the VFAT concentration is high (VFAT > 8 mM)
A PHREEQC-based procedure was developed for estimating the total inorganic carbon concentration based on the measured partial pressure of CO2 in the biogas, for possible use in the case that a direct CT analysis is not available

Summary

Introduction

The control of anaerobic reactors via titrimetric analysis, applied to determine the sum of the volatile fatty acids concentrations (VFAT , namely the sum of the acetate, propionate and butyrate systems in solution), has been addressed in many publications [1,2,3,4,5,6,7,8,9,10,11,12,13,14].More than a few algorithms have been developed in the past to interpret titration results aimed at VFAT concentration determination [5], ranging from methods relying on two [4,7,14,15] through eight [11] titration points, and almost anything in between [8,9].Some of the methods entail (external) analytical knowledge of the concentrations of other, dominant, weak-acid systems present in the water, such as orthophosphate (PT ) and ammonia (NT ) [8,11,13], while others [1,2,6,7,10,15] ignore the presence of these species, a practice that often yields only approximate results. The major difficulty in accurately determining the VFAT concentration through titration lies in the fact that the buffer capacity curves of the carbonate system (pKC1 = 6.375) and the VFA system (pKa = 4.75) overlap close to the pH range where the buffering capacity of the VFA system is dominant. This is true under the water composition that develops in intensive anaerobic digesters, which is often characterized by a very high total inorganic carbon concentration (CT ), such as, e.g., ChemEngineering 2021, 5, 15. Each of these methods has its advantages and disadvantages, elaboration on which can be (partly) found in [12,17]

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