Effect of Hydraulic Retention Time on MES Operation for Biomethane Production

Anirudh B T Nelabhotla,Neha Chopra,Mahdi Khoshbakhtian,Carlos Dinamarca

doi:10.3389/fenrg.2020.00087

Anirudh B T Nelabhotla, Neha Chopra + Show 2 more

Open Access

https://doi.org/10.3389/fenrg.2020.00087

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Abstract

Hydraulic retention time (HRT) is one of the most important factors to be analysed and optimised in continuous flow operations such as the integrated process of microbial electrosynthesis system (MES) and anaerobic digestion (AD). Highest methane production rate of 12.2±0.1 mmol/L(feed)-d was obtained at 18-hr HRT with reject water feed that was supplemented with acetic acid. Highest amount of COD removal of 23.4% was obtained at 18-hr HRT operation with the reject water feed that was not supplemented with acetic acid. The pH of the effluent was 8.63 and 7.64 at 18-hr HRT for both the feed types respectively. This resulted in net alkalinity reduction implying conversion of bicarbonate to methane at 90% of biogas. It was also observed that the electrochemical methane production rates were higher in feeds that were not supplemented with acetic acid along with additional COD degradation via direct electro-oxidation of organics at anode.

Highlights

Microbial electrosynthesis systems (MES) have long been discussed as a tool for sustainable biochemical and biofuel production
It is important to note that the methane production rate [MPR(feed)] in terms of feed reduces by 50% (10.1–5.0 mmol/L(feed)-d: Table 1, Supplementary Table 1) when the feed rate is doubled from 6- to 3-h hydraulic retention time (HRT)
Considering ∼36–40% of fed chemical oxygen demand (COD) is equivalent to the amount of supplemented acetic acid, it can be said that the lower HRT operations viz., 3, 2, and 1- h (25.6, 18.6, and 8.6% of COD consumed) are limited to produce methane from the available acetic acid

Summary

Introduction

Microbial electrosynthesis systems (MES) have long been discussed as a tool for sustainable biochemical and biofuel production. One such application is to integrate MES with anaerobic digestion (AD) systems as a post-treatment unit for enhanced biomethane production and organic degradation (Nelabhotla and Dinamarca, 2019). One of most important aspects of a continuous flow reactors is the organic loading rate (OLR) which increases with shorter hydraulic retention time (HRT). These parameters have never been explored in the context of biomethane production via MES and integration of MES with AD. In a continuous flow stirred tank reactor (CSTR) the SRT is equal to HRT whereas the presence of a surface within the bioreactor that is able to support biofilm growth the value of SRT can dramatically increase compared to HRT (Metcalf Eddy et al, 2014)

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Conclusion