Use of the methane yield to indicate the metabolic behaviour of methanogenic biofilms

Abstract

Methane yield ($Y_{\text{C}{\text{H}}_4}$) is the result of the balance between the flows of organic carbon to catabolism and anabolism in methanogenic ecosystems. In anaerobic biofilm processes, it can be easily obtained to be used to characterize the behaviour of the biofilm in important transient phases. Thus, at the beginning of the start-up period, $Y_{\text{C}{\text{H}}_4}$ is very low, indicating an important anabolic activity of the microorganisms to build the biofilm. Then, its value increases upto a stable level corresponding to the end of the biofilm formation. The time course of $Y_{\text{C}{\text{H}}_4}$ describes the three phases of biofilm formation with the induction, the growth and the steady state. Therefore, it is possible to compare different start-up strategies and define the best operating conditions to grow the methanogenic biofilm. In steady state operations, $Y_{\text{C}{\text{H}}_4}$ expresses the balance of the dynamic system between the net growth of the fixed cells and the biomass detachment, depending on the shear stress and on the capacity of the solid support to retain biomass. It also reflects the detachment rate evolution in anaerobic fixed film processes.

Introduction

The commonly used methods to characterize fixed biomass in biofilm processes are the determination of dry and wet weights, polysaccharide and protein contents, phospholipid analysis, and microscopic observations. All these techniques are rather time-consuming to be done daily. In some biological reactors, such as anaerobic filters, sampling of the biofilm is excluded. Moreover, data obtained by these methods characterize the biofilm by a single parameter such as the amount of organic matter or cells, biofilm thickness, but never in the terms of biological activity of the anaerobic ecosystem. In addition, the information obtained does not enable to identify the dynamics of the important stages, especially during biofilm formation. Trulear and Characklis [1] indicate that the colonization process proceeds in three consecutive phases: lag phase (primary cellular attachment), biofilm production (bacterial accumulation with production of biopolymer matrix), steady state after the establishment of a mature biofilm. The difference between the transient phases (two first steps) and stable conditions obtained in the last phase can be very important in terms of metabolic activity. These periods characterize two different physiological behaviours in the case of an anaerobic ecosystem [2], which implies different optimum operating conditions for each of them.

Methane yield, defined as the amount of methane produced for a given quantity of organic matter removed, characterizes the metabolic activity of a methanogenic ecosystem. Its value is constant in steady state conditions for a given carbon substrate in anaerobic respiratory conditions (i.e., catabolism) and depends on the fraction of biodegradable matter [3] and on the nature of the organic compounds [4]. Theoretically, the methane yield of a methanogenic consortium is 0.35 ${\text{L}}_{\text{C}{\text{H}}_4}$/g_COD,rem [5]. Its value reflects a balance between anabolic (biological synthesis) and catabolic (methane production) fluxes (Fig. 1).

In studies on the start-up period of biofilm systems, many researchers noted some changes in the biological activities compared to the control but did not bring any clear explanation. Thus, Sanchez et al. [6] described a biofilm formation after a batch incubation of 6 days and did not observe the same methane production increase than in the control vial. As well, Lauwers et al. [7] noticed 5–7 days without any methane production during the start-up of methanogenic fluidized bed reactors after different inoculation procedures. No direct correlation could be established between the inoculation conditions and the time gap between the beginning of colonization and the onset of methane production. This gap can be as long as 2 weeks [8]. At the beginning of the start-up period, bacteria seem to use carbon principally to build the initial biopolymer matrix (i.e., anabolism), hence reducing the methane yield [9]. Therefore, the methane yield could follow the different phases of the biofilm formation [10]. After the start-up period and in steady state conditions, even if exopolymer production still occurs to compensate for biofilm losses as a result of detachment (especially important in fluidized bed systems), organic carbon is mainly converted to biogas and the methane yield is close to the theoretical value. Reaching a stable value for the methane yield in a fixed film reactor would indicate that the carbon fraction used for biofilm synthesis is constant. Therefore, this constant methane yield indicates stable shear stress conditions.

In this work, an attempt has been made to confirm the possibility of using methane yield kinetics to monitor the start-up of an anaerobic biofilm digester. Next, from results obtained by other authors, the relationship between the constant methane yield value and the evolution of the biofilm detachment rate is shown.