Approaches for adding value to anaerobically digested dairy fiber
Introduction
Dairy farmers in several places worldwide are facing the difficulty of managing large amounts of dairy manure on concentrated animal feeding operations (CAFOs). In the United States (US), the midpoint size of dairies has risen from 275 to 900 cows, resulting in roughly 1800 large CAFO dairies with a wet-cow equivalent (WCE) herd size of 1000 or greater (many on the order of 5000, 10,000 or even 20,000). These large CAFOs produce over 50% of the milk supply [1]. With a production rate of 64–69 kg wet manure per cow per day (14–18% dry weight), these CAFOs, can produce a remarkable amount of manure and manure wastewater.
At such large scales, these dairy CAFOs are capable of becoming bio-refineries, harnessing the manure for heat, power, fuel, chemicals, fertilizers, fiber, wood composites, and chars/carbons, while mitigating climate, air, water and human health concerns associated with the manure [2], [3]. A baseline for many CAFO bio-refinery visions is an anaerobic digestion (AD) operation for production of biogas and its resulting revenues from either combined heat and power (CHP) or renewable, compressed natural gas fuel (CNG), while also yielding significant environmental benefits related to methane capture and conversion, pathogen and odor destruction, and organic matter stabilization [4], [5]. Unfortunately, adoption of even a baseline AD model on CAFOs within the US is currently limited to around 244 farms [6]. While several hurdles exist, a key limit to adoption rests on business economics with revenue from the most traditional biogas off-take, electrical power, simply not enough to supply a preferred return on investment [7], [8]. However, incorporation of additional co-products and their revenues can have profound impacts towards financial viability. These additional revenue items can include tipping fees (as well as additional biogas/power) from off-farm organics, carbon credits, and of importance to this review paper, sales from the digested fibrous solid and/or its value-added products.
A key component within dairy manure is recalcitrant fibrous solids (fiber) surviving the cow's digestion process, which comprises roughly 40–50% of total solids (TS) in the as-produced manure [9], [10]. This fiber remains for the most part intact and undigested after incorporation in typical mesophilic AD units, representing, in its mechanically separated form, approximately 40% of the AD effluent TS [11]. This fiber can be an important raw source for value-added processing and product development. Table 1 presents a summary of some characteristics of AD dairy fiber according to two works. The values suggest that AD characteristics depend on the origin of the material. Although hemicellulose degradation is expected during the AD process, it is seen that an important portion of it remains intact. Nitrogen content in AD dairy fiber is high when compared with other lignocellulosic materials such as wood.
Pretreatment through various biological, chemical, mechanical, and thermal methods [13], [14], [15], and subsequent incorporation of the fiber for a second run at the AD process could allow for greater access to its biogas potential. For example, Biswas et al. [16] reported that a wet explosion process, calculated in conjunction with known US AD and fiber operational data, could increase AD methane production by up to 41%. While an impressive increase in gas productivity as well as potential electrical or CNG sales, net revenues resulting from the pretreatment costs and extra biogas should be compared to other value-added uses for the digested fiber. Recognizing this pretreatment pathway as a viable route (especially if value of the produced biogas and final energy/fuel product increases) is of merit but in addition, continuing down the alternate path of obtaining value to the existing AD fiber appears as a promising pathway for complete use of the fiber and for providing dairy farmers with strategies to increase revenues.
Although recent works [17], [18] review some options for adding value to AD agricultural and food waste, there is a lack of works focusing on AD dairy fiber. The objective of this paper is to summarize the literature regarding numerous existing and potential value-added uses for AD dairy fiber. Some discussions rely on processing and use of the non-digested fiber fraction or the whole dairy manure. It is expected that the review will provide information to those interested in planning strategies for adding value to AD dairy fiber.
Section snippets
Manure and/or AD fiber as feedstock for dairy bedding and soil amendments
Bedding for cows remains a common first choice for using the separated AD fiber [19], [20]. From a mass balance perspective, a WCE produces approximately 7–9 m3/year of wet digested fiber (70–75% moisture content–MC) from the back end of the digester and liquid/solids separation [11]. Simple mechanical screens with scale variable capital ($45–80 per cow) and operating ($8–16 per cow per year) costs can effectively separate the fiber from the manure and/or wastewater [21]. The digestion process
Other options for using AD fiber: composites, biofuels, and products derived from thermochemical processing operations
Other important options that hold potential for supplanting or supplementing value-added use for AD dairy fiber consist of: a) production of composite materials, b) liquid biofuels, and c) products derived from thermochemical processing. Thermochemical processes include: combustion, gasification, pyrolysis, and hydrothermal operations (hydrothermal liquefaction and hydrothermal gasification), as presented in Fig. 1 (which includes the uses discussed in Section 2). While combustion requires
Experiences on thermochemical processing of dairy manure and potential of using these with AD fiber
While the focus of this review paper relates to the use of AD fiber, a discussion of thermochemical processing utilizing both the dairy manure and AD fiber is provided. There is a relatively rich experience on thermochemical processing of the non-digested dairy manure and the fibrous fraction of cow manure. Lessons learned from the whole manure studies can inform on potential uses for the AD fiber as well as conclusions for moving forward.
Opportunities for adopting thermal operations for adding value to AD fiber
The literature review presented in the previous sections shows that there is a relatively rich experience on using thermochemical processing of dairy manure for producing heat, synthesis gases, or biochar. However, the experience on using AD dairy fiber is more limited. Most of the research has been conducted at laboratory scale and only a few works were conducted at pilot scale. Although adapting these experiences in current dairy farms (that have implemented AD) for processing AD fiber does
Conclusion
There exists a rich experience, although mostly at laboratory scale, on using dairy manure and the corresponding anaerobically digested fiber for producing an array of products through non-thermal and thermal processes. The products identified include: peat moss substitute, fertilizers, charcoal (via carbonization), sugars for biofuels, syngas (via gasification), wood composites, bio-oil from both fast pyrolysis and hydrothermal operations. Fertilizers and peat moss substitutes produced form AD
Acknowledgements
This research was supported by funding from the USDA National Institute of Food and Agriculture, Contract #2012-6800219814, and Biomass Research Funds from the Washington State University Agricultural Research Center.
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