Full-scale anaerobic digestion processes for organic solid waste are common in Europe, but generally unaffordable in Canada and the United States because of inadequate regulations to restrict cheaper forms of disposal, particularly landfill. We investigated the viability of solid-state anaerobic digestion (SS-AD) as an alternative that reduces the costs of waste pretreatment and subsequent wastewater treatment. A laboratory SS-AD digester, comprising six 10L leach beds and an upflow anaerobic sludge blanket reactor treating the leachate, was operated continuously for 88 weeks, with a mass balance of 101±2%. The feed was a mixture of cardboard, boxboard, newsprint, and fine paper, and varying amounts of food waste (from 0% to 29% on a COD basis). No process upset or instability was observed.The addition of food waste showed a synergistic effect, raising CH4 production from the fibre mixture from 52.7 L.kg -1 COD fibreadded to 152 L.kg -1 COD fibreadded, an increase of 190%. Substrate COD destruction efficiency reached 65% and a methane yield of 225 L.kg -1 CODadded was achieved at 29% food waste on a COD basis, and a solids retention time of 42 days. This performance was similar to that of a completely stirred tank reactor digesting similar wastes, but with much lower energy input. INTRODUCTIONAnaerobic digestion (AD) is a well-established method for the treatment of organic solid waste and the production of renewable energy. In a comparative study, Hodge et al. (2016) concluded that, among composting, landfilling, combustion with energy recovery and AD, a combination of AD and landfill was the leading alternative in terms of lowering global warming potential. Solid organic waste is heterogeneous, variable and complex. Consequently, conventional anaerobic digesters (De Baere & Mattheeuws, 2013;Guilford, 2009) must be preceded by extensive pretreatment, such as sorting, size reduction, contaminant removal, and water addition, to render the feedstock suitable for processing, all of which add significantly to the cost..

Solid organic waste is a significant source of antibiotic resistance genes (ARGs) and effective treatment strategies are urgently required to limit the spread of antimicrobial resistance. Here, we studied ARG diversity and abundance as well as the relationship between antibiotic resistome and microbial community structure within a lab-scale solid-state anaerobic digester treating a mixture of food waste, paper and cardboard. A total of 10 samples from digester feed and digestion products were collected for microbial community analysis including small subunit rRNA gene sequencing, total community metagenome sequencing and high-throughput quantitative PCR. We observed a significant shift in microbial community composition and a reduction in ARG diversity and abundance after 6 weeks of digestion. ARGs were identified in all samples with multidrug resistance being the most abundant ARG type. Thirty-two per cent of ARGs detected in digester feed were located on plasmids indicating potential for horizontal gene transfer. Using metagenomic assembly and binning, we detected potential bacterial hosts of ARGs in digester feed, which included Erwinia, Bifidobacteriaceae, Lactococcus lactis and Lactobacillus. Our results indicate that the process of sequential solid-state anaerobic digestion of food waste, paper and cardboard tested herein provides a significant reduction in the relative abundance of ARGs per 16S rRNA gene.

A 50 kg-scale, high solids anaerobic digestor (AD) comprising six sequentially-fed leach beds with a leachate recirculation system was operated at 37°C for 88 weeks. The solid feedstock contained a constant fibre fraction (a mix of cardboard, boxboard, newsprint, and fine paper) and varying proportions of food waste. Previously, we reported on the stable operation of this digestion system, where significantly enhanced methane production from the fibre fraction was observed as the proportion of food waste increased. The objective of this study was to identify relationships between process parameters and the microbial community. Increasing food waste led to a large increase in the absolute microbial abundance in the circulating leachate. While 16S rRNA amplicons for Clostridium butyricum were most abundant and correlated with the amount of FW in the system and with the overall methane yield, it was more cryptic Candidatus Roizmanbacteria and Spirochaetaceae that correlated specifically with enhanced methane from the fiber fraction. A faulty batch of bulking agent led to hydraulic channeling, which was reflected in the leachate microbial profiles matching that of the incoming food waste. The system performance and microbial community re-established rapidly after reverting to better bulking agent, illustrating the robustness of the system.

1Full-scale anaerobic digestion processes for organic solid waste are common in Europe, but generally 2 unaffordable in Canada and the United States because of inadequate regulations to restrict cheaper 3 forms of disposal, particularly landfill. We investigated the viability of solid-state anaerobic digestion 4 (SS-AD) as an alternative that reduces the costs of waste pretreatment and subsequent wastewater 5 treatment. A laboratory SS-AD digester, comprising six 10L leach beds and an upflow anaerobic sludge 6 blanket reactor treating the leachate, was operated continuously for 88 weeks, with a mass balance of 7 101±2%. The feed was a mixture of cardboard, boxboard, newsprint, and fine paper, and varying 8 amounts of food waste (from 0% to 29% on a COD basis). No process upset or instability was observed. 9The addition of food waste showed a synergistic effect, raising CH4 production from the fibre mixture 10 from 52.7 L.kg -1 COD fibreadded to 152 L.kg -1 COD fibreadded, an increase of 190%. Substrate COD destruction 11 efficiency reached 65% and a methane yield of 225 L.kg -1 CODadded was achieved at 29% food waste on a 12 COD basis, and a solids retention time of 42 days. This performance was similar to that of a completely 13 stirred tank reactor digesting similar wastes, but with much lower energy input. 14 15 1.production of renewable energy. In a comparative study, Hodge et al. (2016) concluded that, among 18 composting, landfilling, combustion with energy recovery and AD, a combination of AD and landfill was 19 the leading alternative in terms of lowering global warming potential. Solid organic waste is 20 heterogeneous, variable and complex. Consequently, conventional anaerobic digesters (De Baere & 21 Mattheeuws, 2013;Guilford, 2009) must be preceded by extensive pretreatment, such as sorting, size 22 reduction, contaminant removal, and water addition, to render the feedstock suitable for processing, all 23 of which add significantly to the cost. 24

16Antimicrobial resistance is a globally recognized public health risk. High incidence of 17 antibiotic resistant bacteria and antibiotic resistance genes (ARGs) in solid organic waste 18 necessitates the development of effective treatment strategies. The objective of this study 19 was to assess ARG diversity and abundance as well as the relationship between resistome 20 and microbial community structure during anaerobic co-digestion (AD) of food waste, 21 paper and cardboard. A lab-scale solid-state AD system consisting of six sequentially fed 22 leach beds (each with a solids retention time of 42 days) and an upflow anaerobic sludge 23 blanket (UASB) reactor was operated under mesophilic conditions continuously for 88 24 weeks to successfully treat municipal organic waste and produce biogas. A total of ten 25 samples from digester feed and digestion products were collected for microbial 26 community analysis including SSU rRNA gene sequencing, total community 27 metagenome sequencing and quantitative PCR. Taxonomic analyses revealed that AD 28 changed the taxonomic profile of the microbial community: digester feed was dominated 29 by bacterial and eukaryotic taxa while anaerobic digestate possessed a large proportion of 30 archaea mainly belonging to the methanogenic genus Methanosaeta. ARGs were 31 identified in all samples with significantly higher richness and relative abundance per 16S 32 rRNA gene in digester feed compared to digestion products. Multidrug resistance was the 33 most abundant ARG type. AD was not able to completely remove ARGs as shown by 34ARGs detected in digestion products. Using metagenomic assembly and binning we 35 detected potential bacterial hosts of ARGs in digester feed, that included Erwinia, 36Bifidobacteriaceae, Lactococcus lactis and Lactobacillus. 37 38 3 IMPORTANCE 39Solid organic waste is a significant source of antibiotic resistance genes (ARGs) (1) and 40 effective treatment strategies are urgently required to limit the spread of antimicrobial 41 resistance. Here we studied the antibiotic resistome and microbial community structure 42 within an anaerobic digester treating a mixture of food waste, paper and cardboard. We 43 observed a significant shift in microbial community composition and a reduction in ARG 44 diversity and abundance after 6 weeks of digestion. We identified the host organisms of 45 some of the ARGs including potentially pathogenic as well as non-pathogenic bacteria, 46 and we detected mobile genetic elements required for horizontal gene transfer. Our 47 results indicate that the process of sequential solid-state anaerobic digestion of food 48waste, paper and cardboard tested herein provides a significant reduction in the relative 49 abundance of ARGs per 16S rRNA gene. 50 51 4 INTRODUCTION 52