Enhancing anaerobic methane production in integrated floating-film activated sludge system filled with novel MWCNTs-modified carriers

Abstract

Conductive materials can enhance anaerobic methane production by accelerating interspecies electron transfer between electroactive bacteria and methanogens. However, the daily loss or less specific surface area of small/big size of conductive materials always limits their application in anaerobic digestion. In this study, the conductive multi-walled carbon nanotubes (MWCNTs) (15 wt% and 20 wt%) were mixed with high-density polyethylene (HDPE) and novel conductive suspended carriers were prepared. Results showed the conductivity of the novel conductive suspended carriers increased by 1–2 orders of magnitude comparing with HDPE carriers, as well as the attached biomass improved from 3.93 g/m2 (HDPE carriers) to 5.82 g/m2 (15 wt% MWCNTs-modified carriers) and 6.67 g/m2 (20 wt% MWCNTs-modified carriers). Integrated floating-film activated sludge (IFFAS) filled with MWCNT-modified carriers showed significant advantages in chemical oxygen demand (COD) removal (removal efficiency increased by 3.6–37.2%) and methanogenic performance (cumulative methane increased by 12.28–62.91%) compared with the control reactor filled with conventional HDPE carriers when treating sodium propionate wastewater at the organic loading rates (OLR) of 11.3–26.3 kg COD/(m³∙d). SEM images and high-throughput sequencing results proved potential direct interspecies electron transfer (DIET) had been established successfully on the MWCNTs-modified carriers. The syntrophic electroactive bacteria (Geobacter, Thauera) and Methanotrix were enriched by 2.28–4.58% and 9.41–16.80% respectively owning to the addition of novel conductive carriers. This study proved IFFAS process filled with novel MWCNTs-modified suspended carriers showed great potential in establishing DIET to enhance anaerobic digestion in practical application.