Integrating anaerobic co-digestion of dairy manure and food waste with cultivation of edible mushrooms for nutrient recovery
Introduction
Anaerobic co-digestion of dairy manure and food waste is occuring in the New England region of the United States as multiple states (VT, MA, ME, CT, RI) have enacted policies to divert organic materials away from landfills (VT Act 148 Universal Recycling Law; MA 310 CMR 19.000: Commercial Material Waste Ban Amendments; ME Maine Solid Waste Management Rules, CMR 410; CT Public Act 13-285 CGS Sec. 22a-226e; RI Title 23 Health and Safety Chapter 23.18.9). Co-digestion of dairy manure and food waste is a desirable method of organics management in the region because infrastructure is already in place (USEPA AgSTAR, 2018), and combined feedstocks can increase biogas production (Zhang et al., 2012). In addition to biogas, anaerobic digestion produces residual effluent which can be separated into solid and liquid fractions called digestates (Tambone et al., 2017). Liquid digestates are applied to the landscape as fertilizer for crops and pasture fields. Screw-press separated solid digestates (SS) consist of lignocellulosic biomass resistant to microbial degradation (Möller and Müller, 2012). SS are commonly used as animal bedding on farms before returning to the digester (Fig. 1), though some may be sold as a soil amendment product. As SS organic matter degrades over repeated digestion cycles, nutrients are released into liquid and ultimately applied to the landscape. Sales of milk and crops, in addition to environmental losses following land application of liquid are the primary export pathways for nutrients leaving this dairy-digester-cropland system (Fig. 1). Recycling digestate materials for fertilization of agricultural lands can serve to maintain soil fertility but may not be sustainable in cases where nearby soils receiving repeated digestate applications have excessive nutrient levels and pose environmental risk (Dahlin et al., 2015, Sheets et al., 2015).
Increasing diversion of non-farm organic wastes to on-farm digesters may prove problematic if land application of reclaimed nutrients is the only management option. Recently applied nutrients and legacy nutrients that have accumulated in soils can contribute to nutrient runoff causing harmful algal blooms and declines in water quality (e.g., USEPA, 2016). Introducing non-farm food wastes to the dairy farm landscape represents a new input of nutrients to systems where, in many cases, managing excess nutrients is already challenging (Cela et al., 2015). Therefore, new strategies are needed to export nutrients from dairy farm landscapes in the form of valuable products (Roy, 2017). While SS materials have shown potential as useful soil amendments (Odlare et al., 2011) and are sometimes marketed commercially as part of a potting soil blend (e.g., Magic Dirt™), developing new markets for these materials is of critical concern to the biogas industry and to farmer/operators seeking to export surplus nutrients to restore landscape nutrient balance. Dahlin et al. (2015) report that the large volume of digestate produced by European biogas plants “may cripple the industry and its potential,” due to disproportionate reliance on agricultural lands for disposal. Management strategies which extract value while diverting material away from immediate land application are essential to the long-term economic and environmental sustainability of the industry (Sheets et al., 2015) and could simultaneously provide new sources of revenue for farms while helping to balance nutrient budgets.
Cultivation of mushrooms is a strategy that combines waste management with food production (Chang et al., 1981). Saprophytic white-rot fungi produce extracellular enzymes to degrade cellulose, hemicellulose, and lignin biopolymers (Manavalan et al., 2015), and can be cultivated using a variety of agricultural and forestry byproducts including sawdust and straw (Sánchez, 2010). Global production of cultivated edible and medicinal fungi has increased more than 30-fold since 1978 and was valued around $34 billion USD in 2013 (Royse et al., 2017). Increasing the amount of mushroom protein in human diets can help offset consumption of animal products and the related negative environmental impacts (World Resources Institute, 2018). Of the many commercially valuable edible and medicinal genera of white-rot fungi, Pleurotus spp., commonly known as oyster mushrooms, are the second most widely cultivated genus worldwide, contributing about 27% of the world’s total mushroom production (Royse, 2014, Sánchez, 2010, Zied and Pardo-Giménez, 2017). In addition to their desirable flavor, mushroom tissues have a high vegetarian protein content on a dry weight basis comparable to eggs, legumes, and milk (Oei and Nieuwenhuijzen, 2005). Mushrooms from the genus Pleurotus are rich sources of carbohydrates, minerals, and vitamins, and have shown promise for a variety of medicinal benefits (Corrêa et al., 2016, Zied and Pardo-Giménez, 2017).
P. ostreatus is often viewed as one of the easiest and most cost-effective species to cultivate at different commercial or experimental scales because the C:N ratio of substrates can reportedly range from 30 to 300:1 (Zied and Pardo-Giménez, 2017), and substrates require only pasteurization instead of sterilization, reducing energy inputs. Substrate recipes generally consist of a base material high in lignocellulose to which N supplements are added to increase mushroom yields.
Biogas residues can be an effective ingredient in substrate recipes in mushroom cultivation (Table 1). Yields of P. ostreatus increased when grown on wheat straw and millet supplemented with solid digestate from combined broiler chicken litter and wood chip bedding compared to recipes without digestate (Isikhuemhen and Mikiashvilli, 2009). Similarly, P. ostreatus has been grown successfully on separated solid (SS) digestate materials derived from corn and grass silage, cattle manure, poultry litter, jute caddis, municipal solid waste, and broiler hen bedding feedstocks (Santi et al., 2015, Banik and Nandi, 2004). However, few studies have examined use of dairy manure and/or food waste digestates for mushroom cultivation, and those have been limited to Agaricus spp. (Table 1). Additionally, no studies to our knowledge have quantified nitrogen and phosphorus mass balances through the process to quantify the efficacy of harvesting mushrooms from nutrient-rich recipes as a strategy for nutrient recovery.
Mushroom cultivation produces residual spent mushroom substrate (SMS) in addition to saleable mushrooms (Fig. 1). SMS can be a compost bulking agent or valuable soil amendment product to aid in the cultivation of certain vegetables, as well as serve numerous other purposes (Rinker, 2017, Singh, 2006, Degenkolb and Vilcinskas, 2016). A growing organics recycling industry may find SMS useful. However, as with digestate and food waste, properties of SMS likely vary with regards to input feedstocks and this variability needs to be better understood.
The specific objectives of this study were to:
- (1)
Test the effectiveness of using SS’s derived from dairy manure and combined dairy manure-food waste digester feedstocks as components within substrate recipes used to cultivate Pleurotus ostreatus.
- (2)
Measure the recovery of nutrients in mushroom tissue across different substrate recipes.
- (3)
Characterize SMS materials to elucidate their potential utility as a compost ingredient, soil amendment, or digester feedstock.
Section snippets
Materials collection
Screw-press separated solids (SS) were collected from two full-scale mesophilic (37–40 °C) dairy manure digesters accepting 0% and 35% food waste as percent of total annual feedstock volume. The feedstock for SS material A was 100% dairy manure. SS material B was collected from a digester fed by 53% dairy manure, 35% food waste, 6% fats, oils, and grease (FOG), 4% dissolved air flotation (DAF) sludge, 1% glycerin, and <1% other by volume on an annual basis for 2017. These two materials
Analysis of substrate recipe ingredients
Separated solid digestates derived from dairy manure only (SS-A) had the greatest amount of total N of the four substrate ingredients tested, occurring primarily in organic forms (Table 2). Inorganic N was greatest in SS-B, due to high levels of NO3-N. Nitrogen in soyhull and sawdust was present as organic N and NH4-N. Phosphorus was most abundant in SS-B, which contained approximately 2.8 times the mass of P per dry kg in SS-A, while soyhull and sawdust were comparatively low in P. SS-B also
Conclusions
This study suggests a range of separated solid digestate materials from dairy manure and food waste feedstocks can be useful ingredients for commercial cultivation of P. ostreatus. Optimal proportions in substrate recipes will vary depending on physicochemical properties of the digestate material and other ingredients. Recipes can be designed to sequester a substantial fraction of nutrients in mushrooms, while producing spent mushroom substrate materials with reduced recalcitrant organic matter
Acknowledgements
We acknowledge Adrian Wiegman and Lauren Bomeisl for assistance with sample collection and laboratory work. We also thank Drs. Carol Adair and Deborah Neher for comments on drafts of this manuscript. This work was supported by Casella Waste Systems, Inc.
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