Elsevier

Bioresource Technology

Volume 198, December 2015, Pages 237-245
Bioresource Technology

Dairy manure resource recovery utilizing two-stage anaerobic digestion – Implications of solids fractionation

https://doi.org/10.1016/j.biortech.2015.09.017Get rights and content

Highlights

  • Dairy manure AD is economically uncompetitive.

  • Research investigated AD of separate (fine/coarse) vs. combined solids.

  • Combined solids AD generated enhanced VS destruction.

  • Combined solids enriched for a more heterogeneous bacterial/archaeal consortium.

  • Targeted AD of fat-rich solids could be an optimal approach for processing manure.

Abstract

Dairy manure management is increasingly becoming an environmental challenge. In this regard, manure anaerobic digestion (AD) can be applied to address environmental concerns; however, dairy manure AD remains economically uncompetitive. Ongoing research is focused on enhanced resource recovery from manure, including maximizing AD methane yield through a novel multi-stage AD configuration. Research presented herein centered on the hypothesis that separately digesting fine and coarse solids from fermented dairy manure would improve methane production; the hypothesis was disproven. While maximum methane concentration was realized on fine solids, combined solids AD yielded enhanced VS destruction. The diverse combined-solids substrate enriched for a more heterogeneous bacterial/archaeal consortium that balanced fermentation and methanogenesis to yield maximum product (methane). However, results suggest that targeted AD of the fat-rich fine solids could be a more optimal approach for processing manure; alternate (non-AD) methods could then be applied to extract value from the fibrous fraction.

Introduction

Based on the agricultural census, in 2007 there were 71,510 dairy operations in the United States housing 9.158 million cows and producing an estimated 500 billion pounds of wet manure yearly (Betts and Ling, 2009); additional cows have since been added to the working herds. Regarding byproducts management, as a legacy practice dairy manure is predominantly land applied to enhance forage crop production, with high levels of nitrogen, phosphorus, and other nutrients enhancing plant growth (USDA-NRCS, 2012). However, this approach is becoming increasingly problematic due in part to concerns regarding climate change caused by greenhouse gas (GHG) accumulation. Microbial metabolism of land applied manure releases significant amounts of methane and nitrous oxide to the atmosphere (EPA, 2014); both are potent GHGs (EPA, 2014). GHG emissions associated with dairy manure management account for 7.0% of agricultural sector emissions according to the most recent U.S. EPA estimates (EPA, 2014). Recognizing this concern, in January 2009 the Innovation Center (IC) for U.S. dairy announced a voluntary goal to reduce dairy GHG emissions 25% by 2020.

Beyond GHG emissions, challenges associated with conventional manure storage and land application practices include emission of unpleasant odors, potential nutrient migration to surface and ground water, and potential cross-contamination of crops with pathogenic organisms present in land-applied manure (Sahlstrom, 2003). These challenges have been exacerbated in recent years through consolidation of the U.S. dairy industry, which has increased manure densities (geographically). Public opposition to large dairy operations based on offensive odors, waste management, and environmental concerns has made it increasingly difficult for dairy operators to build new facilities or expand existing ones (Sanders et al., 2010).

Anaerobic digestion (AD) is an established waste treatment technology that can be applied to address many of the manure management environmental concerns. AD leverages an interdependent consortium of anaerobic microorganisms to break down complex organic wastes and produce a biogas consisting primarily of methane and carbon dioxide. The methane produced can be combusted to generate heat and electricity, which reduces GHG emissions through conversion of methane to carbon dioxide and by decreasing demand for fossil fuels. AD also provides pathogen reduction and can be used to produce EPA Class A or B biosolids (EPA, 2003). Finally, digestate is rich in ammonium, which is readily assimilated by plants when applied at agronomic rates, and it contains lower concentrations of volatile compounds responsible for objectionable odors than untreated manure (Betts and Ling, 2009, Weiland, 2010). The combination of pathogen and odor reduction, increased nutrient availability, and reduced GHG emissions make anaerobically digested manure a superior choice to untreated manure for land application.

In considering deployment of ADs at dairies, it is estimated that such installations are feasible at over 8000 U.S. dairy, swine, and poultry operations. However, only about 2% of the sites where AD is feasible actually have digesters installed (EPA, 2010). U.S. farmers generally avoid anaerobic digesters for a variety of reasons, with high initial cost and low or negative rate of return on investment being the most common (Faulhaber et al., 2012, Zaks et al., 2011). Indeed, studies have shown new AD projects will probably require some form of government market support in order to be profitable, such as carbon offset credits, low interest loans, or grants to compensate for low energy prices and the high initial cost of digester facilities (Faulhaber et al., 2012, Zaks et al., 2011).

If improvements could be made to the AD process, both in terms of improving digester methane yield and in developing complementary processes capable of producing additional revenue streams, its benefits could be more broadly realized. With this goal in mind, ongoing research has been exploring the potential for diversifying the commodity portfolio from manure waste streams (Coats et al., 2013). Central to the proposed integrated suite of processes is a novel two-stage anaerobic digestion process, which utilizes a fermenter and digester operated in series to separate manure hydrolysis and fermentation from methanogenesis in order to optimize the environment for the microbes responsible for each process (Coats et al., 2012). Within this integrated system, a portion of the volatile fatty acid (VFA)-containing liquid fraction generated in the fermenter is diverted from the digester and used as substrate in the production of polyhydroxyalkanoates (PHAs; a high-value bioplastic) by mixed microbial cultures (Coats et al., 2007, Wei et al., 2014), while the remaining liquid and residual solids are directed to an AD to produce methane (Coats et al., 2012).

With ongoing efforts to continuously enhance resource recovery from the proposed integrated suite of processes, investigations of this novel AD operation highlighted potentially useful process improvements associated with solids fractionation. Specifically, screening was applied to separate the carboxylate-rich liquid fraction from coarse, lignocellulosic residual solids (referred to hereafter as “coarse solids”). Applying this solids separation method, particles small enough to pass through the 1 mm mesh screen could be separately isolated by subsequent centrifugation; the residual solids material present in the screened effluent is hereinafter referred to as “fine solids.” Preliminary investigations indicated the fine solids exhibited the potential to yield higher methane content biogas when digested separately from the coarse solids (∼69% vs. ∼55% (by volume)). Moreover, microbial population analyses demonstrated the ADs would select and enrich for fermenting bacterial consortia specialized in degrading the unique substrates (fine vs. coarse solids), thereby ensuring process stability (Briones et al., 2014). Building upon these investigations, the research presented and discussed herein focused on detailed interrogation of enhanced solids phase-separation AD. Research was driven by the hypothesis that separate AD of the two distinct solids streams (fine vs. coarse) would increase system-level methane production (both in concentration and yield) because the ADs would be microbially tailored to the respective substrates. Research objectives were to (i) comprehensively assess methane production potential of the separated fine and coarse residual solids fractions, (ii) assess overall performance of the contrasting respective AD configurations, (iii) evaluate the microbial populations in the AD systems, and (iv) make a final determination for combined vs. separate solids AD.

Section snippets

Experimental design

Two phase-separated AD systems, referred to as Systems 1 and 2, were operated to conduct the investigations (illustrated in Fig. 1a, Fig. 1b). Each “system” received the same volatile solids (VS) loading rate, with identical total operational volumes (20 L fermenter, 40 L digester, 60 L total). Reactor SRTs were also equivalent in each system (4 day fermenter, 20 day digester). Thus, the dominant factor which varied between the systems was the substrate. The substrate for System 2 was separated into

Results

Two integrated two-stage AD systems were deployed for this study (Fig. 1a, Fig. 1b); as shown, the comparative systems ultimately included three separate ADs. The principle goal of this research was to develop an enhanced understanding of the potential and viability of separating fine and coarse residual solids from the fermenter for targeted, and enhanced overall, methanogenesis. Results presented and discussed herein represent data collected over an 85-day steady state operational period.

Conclusions

Research presented herein centered on the hypothesis that separately digesting fine and coarse solids (vs. combined) from fermented dairy manure would improve methane production; the hypothesis was disproven. While maximum biogas methane concentration was realized on fine solids, combined solids AD realized enhanced VS destruction. The diverse combined-solids substrate enriched for a more heterogeneous bacterial/archaeal consortium that balanced fermentation and methanogenesis to yield maximum

Acknowledgements

The material presented and discussed herein is based upon work supported by (i) the U.S. Department of Agriculture under Grant Number NIFA#2012-68002-19952, and (ii) the National Science Foundation under Grant Number CBET-1235885. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the funding agencies.

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    At the time of the research, was a graduate student in the Department of Civil Engineering, University of Idaho, Moscow, ID, USA.

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