Article pubs.acs.org/est
Two-Stage vs Single-Stage Thermophilic Anaerobic Digestion: Comparison of Energy Production and Biodegradation Efficiencies Andrea Schievano,*,† Alberto Tenca,‡ Barbara Scaglia,† Giuseppe Merlino,§ Aurora Rizzi,§ Daniele Daffonchio,§ Roberto Oberti,‡ and Fabrizio Adani*,† †
Ricicla Group, Dipartimento di Produzione Vegetale, Università degli Studi di Milano, Via Celoria 2, 20133 Milano, Italy Dipartimento di Ingegneria Agraria, Università degli Studi di Milano, Via Celoria 2, 20133 Milano, Italy § Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, DISTAM, via Celoria 2, 20133 Milano, Italy ‡
S Supporting Information *
ABSTRACT: Two-stage anaerobic digestion (AD) for integrated biohydrogen and biomethane production from organic materials has been reported to promise higher process efficiency and energy recoveries as compared to traditional one-stage AD. This work presents a comparison between two-stage (reactors R1 and R2) and one-stage (reactor R3) AD systems, fed with identical organic substrates and loading rates, focusing the attention on chemical and microbiological aspects. Contrary to previous experiences, no significant differences in overall energy recovery were found for the two-stage and one-stage AD systems. However, an accumulation in R2 of undegraded intermediate metabolites (volatile fatty acids, ketones, amines, amino acids, and phenols) was observed by GC-MS. These compounds were thought to be both cause and effect of this partial inefficiency of the two-stage system, as confirmed also by the less diverse, and thereby less efficient, population of fermentative bacteria observed (by PCRDGGE) in R2. The extreme environment of R1 (low pH and high metabolites concentrations) probably acted as selector of metabolic pathways, favoring H2-producing bacteria able to degrade such a wide variability of intermediate metabolites while limiting other strains. Therefore, if two-stage AD may potentially lead to higher energy recoveries, further efforts should be directed to ensure process efficiency and stability.
1. INTRODUCTION The two-stage anaerobic digestion (AD) process has been reported as a viable biotechnology to coproduce hydrogen and methane (in two separated bioreactors in series) from a variety of organic materials.1,2 Compared to the traditional single-stage AD process, the two-stage approach has been proposed by several authors as a possible solution to improve the overall process efficiency, in terms of biodegradation rates/yields and overall energy productivity. Some authors reported that splitting and separately optimizing hydrolysis/acidogenesis and methanogenesis could enhance the overall reaction rate, maximize biogas yields, and make the process easier to control, both in meso- and thermophilic conditions.3,4 Some other authors stated that enriching different microorganisms in each anaerobic digester, the two-stage AD should extend the possibility of processing different biomass species, enhance substrate conversion, improve the chemical oxygen demand (COD) reduction, and upgrade percent energy recovery.5,6 Moreover, the two-stage solution could increase the stability of the overall process: a controlled acidification process in the first digester should help in maintaining a constant composition of the methanogenic digester feed and, thus, in avoiding © 2012 American Chemical Society
overloading and/or inhibition of the methanogenic population.7 Furthermore, as typically only 15% of the energy contained in the organic substrate is obtained from the first stage in the form of H2, with relatively short retention times (RT = 1−4 d), while 80−90% of the initial chemical oxygen demand (COD) remains in the liquid phase, many efforts for enhancing the overall process performances have been focused on optimizing and speeding up the second stage process, i.e. the methanogenic reactor.8 Many authors relied on the possibility of enhancing methane production rates in the second stage, taking advantage of the fact that hydrolyzed and prefermented organic matter (OM) is more available to methanogens, as compared to the untreated substrate.7,9 This would mean more efficient biodegradation with the same RTs or similar biodegradation yields with lower RTs. Unfortunately, methanogenic communities were widely reported to be very sensitive to volatile fatty acids (VFA) Received: Revised: Accepted: Published: 8502
April 11, 2012 May 30, 2012 June 14, 2012 June 14, 2012 dx.doi.org/10.1021/es301376n | Environ. Sci. Technol. 2012, 46, 8502−8510
Environmental Science & Technology
Article
concentrations in the liquid medium,9 so that the more substrates are soluble, easily fermentable, and rich in VFA, the lower can be their loading rate (LR) into digesters, as recently suggested by Schievano et al.10 This would limit possible advantages of two-stage, compared to single-stage AD. Furthermore, it remains unclear how the delicate equilibrium among the wide variety of biochemical reactions involved in AD should work in separated stages. Conventional AD involves an incalculable number of different biochemical pathways driven by different microbial consortia, grouped into three main categories: hydrolytic bacteria allow complex organic molecules to be solubilized in the form of simple sugars, amino acids, organic acids, etc.; acidogenic bacteria break down monomers mainly into H2, CO2, and a variety of short-chain organic acids (VFA), such as acetic, propionic, butyric and lactic acids; alcohols and ketones are also commonly formed during the breakdown of organic substrates by the acidogens, but in a welloperating process these products should be mostly converted to acetic acid and H2 by acetogenic bacteria. Contemporarily, a number of methanogenic microorganisms convert mainly H2, acetic acid and/or simple alcohols directly to CH4 and CO2.11,12 As volatile organic compounds (VOCs) represent the main intermediates (VFA, ketones, alcohols, etc.) of the variety of biochemical reactions involved in AD and fermentations, their investigation can be useful to understand the biochemical process in depth. For instance, high concentrations of specific groups of VOCs may be the result of process parameters such as feedstock composition,13 hydraulic retention time (HRT) and organic LR adopted, operational temperature and pH14,15 and could be indicative for specific metabolic pathways. Many VOCs formed as intermediate metabolites by fermentative anaerobes (i.e., phenols, alcohols, ketones, halogenated compounds, long chain fatty acids, alkenes) were indicated in the literature16 to be possibly recalcitrant to further biodegradations and/or possible inhibitors of methanogenic populations. Many of those compounds are formed differently when different pH, redox potential, other chemical environments, and predominant microbial populations are present in fermentation broths.16 For this reason, the separation of fermentative and methanogenic environments, imposed in two-stage anaerobic digestion systems, may drive substantial changes in biochemical pathways and fermentation metabolites formation. Also, microbial species/subpopulations selected and the relative abundance of the same microorganisms in the consortium can differ depending on process operating conditions and strategies.16 It remains unclear whether all the combination of these different biochemical pathways involved in AD can be either optimized or, on the other side, hampered by the separation of acidogenic from methanogenic microbial consortia in a welloperated two-stage process. Even if these different microbial groups differ in terms of physiology, nutritional needs, growth kinetics, and sensitivity to environmental conditions,17,18 in conventional AD they live altogether in the same environment and the separation of acido/acetogenesis from methanogenesis may affect negatively syntrophic associations, above all by preventing interspecies hydrogen transfer.19 To our knowledge, such chemical and microbiological aspects have not yet been clarified and there is the need of deeper efforts in comparing two- and single-stage AD processes. In this work, two AD lab-scale continuous systems (one-stage and two-stage) were identically fed and operated in parallel, registering their overall biogas production and energy
recovery and monitoring some interesting chemical/microbiological parameters. A mixture of manure and fruit/vegetable waste was chosen as feeding substrate, as it was before shown to have interesting H2 production potentials (120−150 Sdm3 H2 kg−1 TS and 300−400 Sdm3 CH4 kg−1 TS).20 The aim was to more closely observe the biochemical processes, the metabolite compounds produced, and the main microbial communities involved in the two AD systems and to give a contribution to our knowledge about the possible conveniences of adopting one approach instead of the other one.
2. EXPERIMENTAL SECTION 2.1. Apparatus and Process Operation. Three continuous flow stirred tank reactors (CSTR), in “wet” AD conditions (total solids
