Introduction

Livestock housing systems and feed rations have received a substantial boost in the last two decades with emerging technologies (precision technology and genetic engineering) owing to ongoing research and development. Those practices, however, have led to more on-farm organic waste generation. In the USA, the total amount of animal manure and on-farm organic waste, generated in the last decade, was approximately between 6.64 and 7.03 teragrams (Tg), with 884–907 gigagrams (Gg) of the manure solely from dairy farms [1]. Important challenges associated with this considerable volume of manure are further management costs for farmers as well as the environmental concerns resulting from the storage and land application of manure [2, 3]. Greenhouse gas emissions (GHGs, methane-CH4; carbon dioxide—CO2; nitrous oxide—N2O) are one of the most contentious issues concerning farm waste management. For instance, dairy manure from about 1.3 million cows could contribute at least 22.7 million of kg CO2e/year to the atmosphere [4, 5]. Another potential challenge with on-farm waste disposal is that manure transported to the surrounding waterbodies (streams) via surface runoff could facilitate eutrophication (Tiwari et al. 2022). Anaerobic digestion (AD) is a feasible management option to mitigate these challenges. AD is the degradation of organic matter by hermetic microbes (bacteria and archaea) that mainly produces biogas and digestate. During the process, AD reduces odor production while offering the generation of heat and energy at a low cost, as well as the digested organic matter can be used as a source of nutrients [6]. These benefits make AD a preferred waste management technology option compared to other alternatives such as pyrolysis and composting Aguirre-Villegas et al. [7].

Despite AD benefits, low methane yield attributed to low carbon-to-nitrogen ratio (C/N) is a key limitation when only manure, particularly dairy manure, is mono-digested [8]. The low methane yield makes manure AD uneconomical and the environmental benefits unattractive. On the other hand, lignocellulosic residues are rich in carbon but low in nitrogen. Hence, the co-digestion of manure with lignocellulosic residues is a way to improve the C/N ratio and subsequently methane yield [9]. However, to improve the accessibility of anaerobic microbes to the fiber content of harvested lignocellulosic residue, pretreatment, or modification of the fiber structure before AD is beneficial [10, 11].

In addition to co-digestion and pretreatment, studies have shown that the application of nanoparticles, micro-elements, or compounds smaller than 100 nm can further improve the methane yield in AD [12]. For instance, various nanoparticles (Fe3O4, Fe2O3, TiO2, CuO, and ZnO) have been suggested in AD to improve biomethane yield, from an environmental perspective, only few favor the use of AD digestate as biofertilizer [13,14,15,16]. However, magnetite nanoparticles would benefit biofertilizer application due to the trace amount of iron content with barely no environmental concerns unlike ZnO NPs [14]. In addition, magnetite in solid-state AD (solid content > 15%) has been reported to cushion the challenges that prevent rapid utilization and treatment of biomaterial [13], making the process desirable than liquid-state AD (LSAD, solid content is < 10%) where biomaterial use and treatment are limited. Thus, based on the high feedstock to inoculum or water ratio in SSAD, higher volumetric methane would be generated than LSAD [17, 18].

From an environmental perspective, AD can reduce GHG emissions from manure storage in open spaces by at least 23% and potentially decrease marine eutrophication by 8.1% [19]. Even after the completion of manure AD, storage of such digestate in an enclosed container should be discouraged to reduce NH3 emissions Aguirre-Villegas et al. [52]. Instead, further environmental benefits could be derived from composting the digestate [20].

Currently, the environmental impacts of solid-state anaerobic co-digestion (SSAD) of on-farm organic wastes, especially under optimized conditions such as pretreatment and the inclusion of nanoparticles, are not as extensively reported as those of liquid-state anaerobic digestion (AD). Li et al. [21] reported that co-digestion of corn stover, dairy manure, and tomato residue in SSAD reduced acidification, eutrophication, and ecotoxicity potentials by at least 40%, compared to the typical AD processes. Additionally, the anaerobic co-digestion of corn stover with these substrates contributed to global warming potential (GWP) environmental credits demonstrating a potential advantage over incineration approach [21]. However, the study did not consider the effects of pretreatment and the use of lignocellulosic waste. As such, an informed decision cannot be made about the environmental benefits or constraints attributed to the pretreatment or inclusion of nanoparticles in SSAD. A recent study of pretreated corn stover co-digested with dairy manure demonstrated that the addition of nanoparticles to pretreated lignocellulosic feedstock before digestion with animal manure can substantially reduce detention time for digestion and overall improve reactor performance [22]. Similar to the previous study, the environmental impacts were not quantified. Additionally, the lack of operational full-scale solid-state anaerobic digesters in the USA makes it challenging to fully comprehend the environmental implications of this technology on a larger scale.

This study aims to evaluate the environmental impacts of operating a full-scale anaerobic digester fed with corn stover co-digested with dairy manure under three scenarios. These high-methane-yielding scenarios are compared with solid-state and semi-solid-state mono-digestion of dairy manure as the baselines. All three scenarios generate methane for on-farm use and digestate to be used as a soil amendment. Thus, the objectives of this study are to (1) quantitatively evaluate the holistic environmental impact of producing methane from the blend of either corn stover or calcium hydroxide pretreated corn stover and manure through solid-state co-digestion and (2) assess the impact of nanoparticles (magnetite or Fe3O4) on environmental indicators with the highest methane yield treatment in objective 1. The environmental impacts of methane production through the investigated system were compared to solid-state and semi-solid-state mono-digestion of dairy manure.

Materials and Methods

This cradle-to-gate LCA comprises four steps: scope and goal; inventory, system description, and boundary; impact assessment; and the interpretation (ISO, 2006).

Scope and Goal Definition

The approach used in this study is the life cycle assessment (LCA) method following ISO 14040 and 14,044 international standards (ISO, 2006) to investigate the environmental performance of three scenarios of anaerobic solid-state co-digestion of manure with corn stover. Primary data used in this study were obtained from the pilot-scale solid-state anaerobic co-digestion experiments [22,23,24] and weather data from the North Dakota Agricultural Weather Network (NDAWN), and 2). The secondary data were obtained from the Ecoinvent V3 database [25]. Besides these sources, other relevant data were obtained from the literature. To assess the environmental impact, tools for the reduction and assessment of chemical and other environmental impact (TRACI 2.1, Version 1.05) method and SimaPro software (Version 9.0.0.35) were used. Environmental impacts were determined with TRACI and midpoint impact categories of global warming potential (GWP), ozone depletion (OzD), smog, acidification potential (AcP), eutrophication potential (EuP), carcinogenic (Cc), non-carcinogenic (NCc) respiratory effects (RPE), ecotoxicity potential (EcP), and fossil fuel depletion (FFD) were assessed. A time horizon of 100 years was adopted to investigate the environmental impact and the term technosphere in this study describes the inputs and outputs of technological systems such as material consumption, transportation, and energy. The goal of this LCA study was to quantify and compare the environmental impacts of on-farm energy production through solid-state anaerobic co-digestion of corn stover or calcium hydroxide-pretreated corn stover and dairy manure with or without the inclusion of nanoparticles. Biogas generated from this study was considered for both heat generation and cooking while the co-product (digestate) from AD was considered as a commercial substitute as either soil amendment or organic fertilizer. The functional unit used to compare the three scenarios and the baselines for this cradle-to-gate study was 1 MJ of methane produced during solid-state anaerobic digestion and the selected geographic location is North Dakota, USA. Methane yield employed as a functional unit in this study is a uniform output parameter despite different SSAD scenarios.

Chemical Analyses

The feedstocks used in this study were analyzed with standard methods. Precisely, carbon content was quantified following the protocol described by Pella [26], using an Elementar Vario Macro Cube – CNS analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany). Similarly, the nitrogen content was determined using the Kjeldahl method, as described by Isaac and Johnson [27]. Ammonia–nitrogen was measured following the flow analysis method described by ISO – 11,732:2005 protocol.

System Description and Boundary

Three scenarios of anaerobic co-digestion of manure and corn stover were investigated in this study. In all the scenarios, methane produced is considered the main product, while digestate is a co-product of the system. The study was intended for an integrated farming system with the digester located about 4 km from the farmland [28]. The environmental footprint from the feedstock (dairy manure and corn stover) from the point of collection to processing of the feedstock, such as milling, sieving, pretreatment, and drying of the corn stover, was also included within the system boundary of this analysis (Fig. 1). Furthermore, the inoculum applied in this study was at first sourced from a mesophilic liquid-state anaerobic digester operated by City of Fargo Wastewater, Fargo, ND, USA. For subsequent digester runs, the liquid fraction of the digestate from this study was considered as inoculum. The electricity source for heating the digester was assumed from the USA average electricity grid. Furthermore, weather data used for the energy required to maintain the digester was extracted from NDAWN.

Fig. 1
figure 1

System boundaries for the three studied scenarios (SYM1, SYM2, and SYM3) and the baselines. Note: Baseline 1 represents dairy manure only as influent in solid-state anaerobic digestion (SSAD), baseline 2 denotes dairy manure only as feedstock in semi-solid state anaerobic digestion, SYM1 represents untreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, SYM2 calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, and SYM3 represents calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion with magnetite nanoparticles as an additive

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System Scenarios

Solid-state anaerobic digestion experiments were conducted in the lab for this study and the scenarios chosen were based on the experiments with high methane yield and thus no bioreactor failure. Hence, three scenarios with different solid-state anaerobic co-digestion mixes under a total solid (TS) of 16% were considered. The detailed mix ratios for the three systems are presented in Table 1. Variation in the mix ratio as shown in Table 1 was due to the secondary effect of the pretreatment on the carbon-to-nitrogen ratio (C/N ratio) of the substrate as well as the differences in the moisture content of the pretreated and untreated corn stover. These were catered for in the targeted 20–24:1 carbon to nitrogen ratio computation. Furthermore, the disparity in the hydraulic detention time was considered in the data computation of this study (Table 1).

Table 1 Feedstock mix for the scenarios considered in this solid-state study based on TS
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System 1 (SYM1)

Corn stover was blended with dairy manure in a semi-continuous stirred solid-state-anaerobic reactor. The detention time for the substrate mix in the reactor was 60 days under 35–37 °C mesophilic temperature. As previously stated, co-digestion addresses the challenges of nutrient imbalance [29], as well as helps achieve a TS > 15% required for solid-state digestion [28]. Furthermore, high TS in AD minimizes energy use and results in high volumetric methane production [30].

System 2 (SYM2)

In the second scenario, corn stover was pretreated with 8% Ca(OH)2 concentration for 5 days following wet state procedure prior to co-digestion with dairy manure in a solid-state condition. The choice of Ca(OH)2 was due to high methane yield obtained in our previous studies relative to ammonium hydroxide and sodium hydroxide [23, 24]. This pre-digestate had a hydraulic retention time (HRT) of 76 days under 35–37 °C mesophilic temperature. As previously stated, pretreatment was carried out due to the lignocellulosic nature of the stover, which makes its degradation difficult. Hence, pretreatment was used to enhance the anaerobic microbes’ accessibility to the cellulose and hemicellulose fraction of the stover.

System 3 (SYM3)

Based on the high methane yield and lower retention time with SYM2 [23, 24], pretreated corn stover with 8% Ca(OH)2 concentration was co-digested with dairy manure together with the addition of 20 mg of Fe3O4 nanoparticles (Fe3O4 NPs). This pre-digestate was retained in the digester for 52 days under 35–37 °C mesophilic temperature. The availability of micro-nutrients such as Fe from Fe3O4 could boost methanogen activities [12]. Hence, Fe3O4 NPs were introduced into similar treatments as SYM2 for this purpose.

Baselines

Two baselines considered in this study are dairy manure mono-digested under solid-state (TS = 15%, baseline 1) and dairy manure mono-digested at semi-solid-state (TS = 10%, baseline 2). Notably, baseline 1 was characterized by inhibition during anaerobic digestion and thus low methane yield, while baseline 2 was successful with high methane yield despite prolonged retention time. The baselines help to clearly juxtapose the environmental implications of co-digestion, pretreatment, and the use of nanoparticles in dairy manure management with AD.

For all the scaled-up scenarios, a semi-continuous state was assumed in which the initial and subsequent mass of the influent (41.8 Mg) was subjected to a continuous 300-day digestion period. Outside this startup time, other months (remaining 2 months of the year) represent the digester maintenance period. The digester considered in this study was a “garage-type” batch digester which as previously stated runs for approximately 10 months annually.

Data Question

Different C/N ratios were used for the untreated and the pretreated studies, the reason for this variation was that the reactor performance for the untreated and pretreated was optimal at different values despite being between 20 and 30 C/N. For instance, optimal methane yield for the untreated corn stover co-digested with dairy manure was observed at a C/N ratio of 24, while for the pretreated, methane yield was maximum at a C/N of 20 (Table 1). Besides the unique difference in C/N, the retention time for all the scenarios and baselines was dissimilar as the digesters were dismantled once the methane yield from the bioreactor was low (continuous less than 2 L/kg VS within a week at the stationary phase). Hence, SYM2 was dismantled on day 79 and SYM3 on day 52. The baselines were dismantled on day 55 for the dairy manure solid-state and day 100 for the semi-solid mono-digestions. This step is necessary to minimize the input energy required to maintain the digester at mesophilic temperature (Table 1). For the nanoparticles (Table 1), the 20-mg magnetite nanoparticles were based on the outcome of our previous study. In the study, 20 mg had the highest methane yield relative to when 50 and 75 mg of the nanoparticles were added (Ajayi-Banji et al. 2021).

Statistical Analysis

Data means were statistically analyzed with SAS software (Version 9.4, SAS Institute Inc., Cary, NC, USA) and Duncan multiple range tests (DMRT) were adopted to conduct pairwise comparisons of the treatment means with a threshold p-value = 0.05.

Feedstock Constituents

Pretreatment of corn stover with 8% calcium hydroxide solution had a significant impact on volatile solids, it reduced volatile solids by 34% (p < 0.05, Table 2). This suggests that polymeric degradation had occurred during pretreatment and the fiber structure of the stover was modified. For other parameters investigated (Table 2), the differences between the pretreated and untreated corn stover were marginal, except for the carbon content (Table 2).

Table 2 Feedstock composition
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Life Cycle Inventory

Feedstock Transportation

This model considered dairy farms and cornfields as an integrated farming system within a distance of 4 km from the anaerobic digester and corn stover processing units. Water usage of digester (Table 3) was also provided from the farm. The inoculum used in this model was initially sourced from a liquid-state household waste management digester operating under mesophilic temperature, as previously stated in this study.

Table 3 Annual input from technosphere
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Corn Stover Processing

Corn stover used in this study was harvested from the field and crushed with a 3.0-mm mesh size Shuttle Buffalo hammer mill (Model W6H, New York, USA) before grading the crushed stover with a Ran-TAP Testing Sieve Shaker (OH, USA). Corn stover particle size of 0.42–0.84 mm was considered for AD and chemical pretreatment, based on our previous study (Ajayi-Banji et. al., 2020a). The amount of energy required to carry out both the milling and grading processes as presented in Table 3 was calculated using a standard procedure [33].

For scenarios SYM2 and SYM3, corn stover was pretreated with wet-state alkaline approach [34]. Water and pretreatment reagent (8% calcium hydroxide) used during the pretreatment process are presented in Table 3. In addition, the energy utilized to oven-dry the calcium hydroxide-pretreated-corn stover under 40 °C for 24 h was estimated with the standard procedure for hot-air convection drying (Motevali et al. 2011).

Anaerobic Digestion (AD)

In this study, the required volumes of the inoculum, dairy manure, water, and corn stover or calcium hydroxide pretreated corn stover mix, depending on the scenario, were fed into the 250 m3 garage-type digester [28]. The working volume adopted for this digester was 80% of the digester volume with approximately 41.8 Mg of ingestate per run. For scenario 3, an additional 4.9 kg of magnetite was added to the digester to improve reactor performance (Table 3. Gas leak was not suspected in the digester; thus, fugitive emissions due to gas were not accounted for in this model. The amount of energy used to heat the digester and maintain the digester temperature was investigated with standard procedure (Table 3, [35]). However, heat generated by microbes was not considered in this estimation. The energy required to agitate the digester considering a long shaft agitator (Table 3) was equally estimated for 300 days based on the procedure described by Naegele et al. [36]. On the digestate volume, it was assumed that 4–7% of the volume of the digestate was used up for gas production. Hence, the remaining fraction (approximately 40 Mg) was considered digestate or effluent. Before the experiment began, propane was used as fuel to heat the digester to the desired temperature (35–37 °C), and during the experiment, part of the methane energy produced from the systems was used to substitute propane for maintaining the digester temperature (Table 4). Most of the previously stated conditions applied to the baselines except that corn stover and additives were not included in the influent anaerobically digested. For the three scenarios previously stated and baseline, environmental impacts were described with indicators like global warming potential, acidification potential, eutrophication potential, fossil fuel depletion, smog, ozone depletion, carcinogenic, non-carcinogenic respiratory effects, toxicity potential, and smog formation potential (Figs. 2, 3, and 4).

Table 4 Annual outputs of the studied systems to the technosphere
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Fig. 2
figure 2

Environmental indicator for the three solid-state anaerobic digestion scenarios. Note: Baseline 1 represents dairy manure only as influent in solid-state anaerobic digestion, baseline 2 denotes dairy manure only as feedstock in semi-solid state anaerobic digestion, SYM1 represents untreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, SYM2 calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, and SYM3 represents calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion with magnetite nanoparticles as an additive

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Fig. 3
figure 3

Normalized environmental indicators for the three scenarios and baselines. Note: Baseline 1 represents dairy manure only as influent in solid-state anaerobic digestion, baseline 2 denotes dairy manure only as feedstock in semi-solid state anaerobic digestion, SYM1 represents untreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, SYM2 calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, and SYM3 represents calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion with magnetite nanoparticles as an additive

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Fig. 4
figure 4

Corn stover contribution to some indicators for the three scenarios. Note: SYM1 represents untreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, SYM2 calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion, and SYM3 represents calcium hydroxide pretreated corn stover co-digested with dairy manure in solid-state anaerobic digestion with magnetite nanoparticles as an additive

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Results and Discussion

Relating the Environmental Impact of SSAD

Ozone Depletion

Ozone depletion is expressed as kg CFC-11 eq. In this study, ozone depletion was less than 4.8 × 10−9 kg CFC-11 eq/MJ Methane irrespective of scenarios and baselines (Fig. 2a). Among the scenarios, SYM3 had the least ozone depletion potential (8.36 × 10−10 kg CFC-11 eq/MJ Methane), while SYM1 had the highest stratospheric ozone depletion potential (3.74 × 10−9 kg CFC-11 eq/MJ Methane, Figs. 2a and 3). Prolonged pretreatment time (10 days) during wet-state calcium hydroxide pretreatment has been previously reported to enhance ozone layer depletion [37, 38]. Contrarily, in this study, calcium hydroxide as a pretreatment reagent with a 5-day pretreatment time showed no substantive impact on ozone depletion potential, as ozone depletions in SYM2 and SYM3 were at least 70% less than in SYM1. The possible reason for this might be the shorter pretreatment time in our study (5 days) relative to the 10 days in the previous study. However, unlike the chemical pretreatment in this study, environmental savings were reported for non-chemical pretreatments such as ionization radiation-pretreated, ultrasonic-pretreated, and thermally-pretreated influents in a full-scale study [39, 40]. An indication that chemical pretreatment has a considerable ozone depletion impact relative to other non-chemical pretreatment options in AD, possibly due to the energy required to produce the reagent and to dry the influent after pretreatment. Van Fan et al. [38] led credence to this observation of the huge energy requirement in chemical pretreatment. Interestingly, the inclusion of nanoparticles and the wet-state pretreatment of the corn stover with calcium hydroxide solution before co-digestion with dairy manure in SSAD achieved a slight ozone depletion reduction of 15%, which is in tandem with Hijazi et al. [41] view. The possible explanation for the slight improvement in ozone layer depletion in SYM3 could be owing to the higher methane yield in SYM3, compared to SYM2. Unlike some previous studies, environmental gains from ozone depletion potential in this study were not observed for the untreated scenario (SYM1, [42]) as the measured potential was close to the solid-state baseline (baseline 1 = 5.8 × 10−9 kg CFC-11 eq/MJ Methane, Figs. 2a and 3). There could be two reasons for this huge environmental impact, the first being that in this study, biomass was co-digested with dairy manure relative to mono-digestion of lignocellulosic waste digestion from the cited literature [42],thus, co-digestion with livestock manure might have contributed to the substantial ozone depletion. Another viewpoint is the high mass of corn stover co-digested in this study, the biomass co-digested in SYM1 was almost fivefold of SYM2 and SYM3. Hence, the production (planting and harvesting) and processing (milling) of the huge mass of corn stover needed in SYM1 could have possibly impacted ozone depletion potential with a value of 5.82 × 10−10 kg CFC-11 eq/MJ Methane reported for corn stover processing and storage, though the impact is insignificant relative to some of the environmental indicators (Fig. 4). Notably, ozone depletion potential in the semi-solid baseline was the least (3.91.0 × 10−10 kg CFC-11 eq/MJ Methane, Fig. 2a), a trend attributed to avoidance of production and processing of corn stover as well as high methane yield from baseline 2. In summary, ozone depletion is primarily facilitated by the production and processing chain of corn stover. Furthermore, the role of calcium hydroxide during pretreatment with less than 6-day retention time is insignificant in ozone layer depletion.

Global Warming Potential

Another important environmental indicator is global warming potential (GWP), which is compared with the global warming of a century time horizon and expressed in terms of CO2 eq. Of the known waste management practices for biomass and livestock manure, AD generates lower GWP relative to incineration or composting [43], as a result of methane captured in AD [44]. Thus, aside from the baseline, all of the scenarios in this study had environmental gain with respect to GWP and hence could be classed as being environmentally friendly. This trend is not surprising as low GWP has been ascribed to co-digestion in AD [45, 46]. Nonetheless, SYM2 and SYM3 had higher GWP (> − 0.02 kg CO2 eq/MJ Methane, Fig. 2b) compared with SYM1, as pretreatment often increases GWP [47]. The low GWP value reported for SYM1 (− 0.119 kg CO2 eq/MJ Methane, Fig. 2b), despite the high energy consumption (2.6 × 10−4 and 4.94 × 10−3 kg CO2 eq/MJ Methane), could be linked to carbon credit gained during corn stover cultivation (− 0.125 kg CO2 eq/MJ Methane, Fig. 4) as this system uses a higher corn stover to manure ratio as feedstock than the other scenarios.

The GWP values stated in this study for the three scenarios are considerably lower than the values (0.1 × 10−1 kg–1.4 kg CO2 eq/MJ methane) reported for municipal solid waste management in literature [48], the baselines (0.004 and 0.07 kg CO2 eq/MJ Methane, Fig. 2b) as well as fossil-based methane (1.8 kg CO2 eq/MJ methane, data not shown). The substantial difference between the GWP values in the literature and the scenarios in our study could be attributed to complex treatment methods in the literature and possibly differing waste streams. While for the baselines, co-digestion has previously been mentioned with lignocellulosic biomass with the ability to mitigate GWP. Hence, SSAD co-digestion of dairy manure with corn stover carried out in mesophilic environs does not negatively impact the environment. The baseline GWP values also aligned with the 100% GWP reduction documented when dairy manure was only mono-digested [21]. A further environmental gain in GWP could be achieved in this study with more efficient use of energy by minimizing agitation frequency, a 20–30% reduction in GWP potential could be attained.

Smog

Smog often called photochemical oxidation is an environmental impact that is majorly linked to transportation, combined heat and power (CHP) engines, and the emission of air pollutants [49]. Smog formation in this study could be linked to the supply chain of the inputs to the digestion process and the small-distance transportation and is expressed as kg O3 eq. Smog potential for the three scenarios in this study ranges between 7.88 × 10−4 kg and 3.32 × 10−3 kg O3 eq/MJ Methane, with SYM2 and SYM3 having a 99% reduction relative to SYM1 (Fig. 2c). The substantial smog potential from SYM1 (3.31 × 10−3 kg O3 eq/MJ Methane, Fig. 2, could be linked majorly to fuel consumption (such as diesel) during corn stover cultivation and harvest (“Global Warming Potential” section, Table 3) as corn stover is used in a higher proportion in this scenario relative to the other scenarios. As expected, all the scenarios had high smog potential relative to baseline 2, due to corn stover production, processing, and storage previously mentioned. While for baseline 1, the huge smog formation (4.08 × 10−3 kg O3 eq/MJ Methane, Fig. 2c) is possibly a result of the energy required to manage the liquid fraction of the manure after digestion, since a huge proportion of dairy manure is utilized for baseline 1 (2.03 × 10−3 kg O3 eq/MJ Methane, data not shown). Hence, SSAD of dairy manure only is a major atmospheric polluter and it would be environmentally beneficial to complement such with lignocellulosic wastes. Nonetheless, some fraction of the smog formed in the three scenarios (SYM1, SYM2, and SYM3) and baselines could be attributed to the propane initially used to heat the digesters and partially maintained mesophilic temperature. Hassanein et al. [50] suggested the use of propane as an energy source could increase this environmental impact.

Acidification Potential (AcP) and Eutrophication Potential (EuP)

Acidification and eutrophication potentials are other environmental indicators in AD and have been partially linked to ammonia emission, and these indicators are expressed as kg SO2 eq and kg N eq respectively. Studies have demonstrated that ammonia emission accounts for at least 94% of both acidification and eutrophication potentials from the AD of agricultural and food wastes [51]. In this study, we suspected a similar trend due to feedstock composition, particularly for dairy manure (Table 2). Thus, of the three scenarios, SYM1 had the highest acidification (2.05 × 10−4 kg SO2 eq/MJ Methane) and eutrophication (3.08 × 10−5 kg N eq/MJ Methane) potentials, respectively (Fig. 2d). From another perspective, high eutrophication potential could be also linked with biomass production [46]. This could further explain the high eutrophication potential in SYM1, since about a fivefold proportion of corn stover was added to the digester relative to the other scenarios (Table 1). Interestingly among the scenarios, the previously mentioned environmental indicators did not show an extensive disparity between the SYM2 and SYM3 scenarios (Figs. 2d and 3), except for a slight increase in acidification and a slight decrease in eutrophication potentials for SYM3 relative to SYM2, possibly due to the nanoparticles added.

Relative to literature, the AcP value reported for liquid-state digestion of dairy manure and the co-digestion with plant waste (800–2300 kg × 104 kg SO2 eq, [52, 53]), for the year-round average over a time horizon of 1 month, were higher than the ones reported in this study. Comparatively, with this study, the outcome suggests time horizon duration substantially impacts AcP and EuP estimation. In another study, the AcP of dairy manure was close to 0.1 kg SO2 eq, this further suggests that time horizon duration is an important threshold in describing the impact of environmental contributors [21]. However, similar to the smog potential previously reported in the study, corn stover production was the main contributor to AcP due to fertilizer use. Thoughtfully, substantial contributors to eutrophication potential were beyond fertilizer use in corn stover production, liquid manure application and energy use were inclusive (data not shown).

Carcinogenic, Non-carcinogenic, Respiratory Effects, and Ecotoxicity Potential

In anaerobic digestion, alkaline pretreatment and nanoparticles are reportedly significant contributors to ecotoxicity and carcinogenicity; on the contrary, the environmental impact of co-digestion on the respective environmental indicators is minimal [46]. As expected, calcium hydroxide pretreatment did not negatively impact carcinogenicity in this study. For instance, in scenario SYM1, in which pretreatment was not present, higher carcinogenicity impacts were observed (> 3.0 × 10−10 CTUh eq/MJ Methane) relative to the pretreated (SYM2 and SYM3; 1.8 × 10−10–2.2 × 10−10 CTUh eq/MJ Methane, Fig. 2e, f). This trend suggests the application of calcium hydroxide in SSAD is not a huge contributor to carcinogenicity. Rather, carcinogenicity is principally influenced by corn stover production, possibly through fertilizer and herbicide application. Importantly, secondary toxic products considered carcinogenic and produced as a result of the chemical pretreatment in SSAD were not accounted for in this cradle-to-gate study [54]. On non-carcinogenic, though the alkaline pretreatment did not negatively impact carcinogenicity, the substantive non-carcinogenic impact could be caused by calcium hydroxide in this study. For instance, the non-carcinogenic risk increased by over 99% in this study with calcium hydroxide pretreatment relative to the untreated (SYM1, Fig. 2e, f), this is despite the benefit of calcium to inhibiting non-carcinogenic active ingredients, such as fluoride, release to the environment [55]. In addition, mono-digestion of dairy manure resulted in environmental gain by employing the non-carcinogenic index. Therefore, dairy manure management with AD poses no non-carcinogenic risk to the environment.

Ecotoxicity has been sub-classified into terrestrial, freshwater, and marine [46],however, in this study, the sum of these subunits was considered and referred to as ecotoxicity. Similar to the trend reported for carcinogenic in this study, other human health factors such as respiratory and ecotoxicity potential were high in SYM1 (> 70%, Figs. 2i and 3) relative to scenarios with chemically pretreated corn stover and nanoparticles (SYM2 and SYM3). Though this contradicts previous studies where toxicity has been solely linked with chemicals [46], Ramirez-Arpide et al. [56], the possible reason could be the mild and beneficial impact of calcium on soil which was the main ingredient of the pretreatment reagent. Thus, the application of calcium hydroxide for pretreatment and magnetite nanoparticles as an additive in SSAD has energy, health, and environmental benefits over untreated corn stover. Ugwu et al. [46] assertion led credence to the view that additives such as magnetite nanoparticles did not substantially contribute to ecotoxicity in this study, rather an ecotoxicity potential in the scenario with the magnetite (SYM3) was reduced by > 10% relative to the SYM2 (Fig. 2i).

Fossil Fuel Depletion

The major contributors to fossil fuel depletion are diesel consumed in feedstock haulage and the fraction of the fuel used for other farming activities [45]. This is simply the reason for the considerable high impact on fossil fuel depletion in SYM1, as there is more corn stover application in the scenario (Table 1). Thus, high fossil fuel depletion was directly related to the high value of the ratio of corn stover to dairy manure in terms of quantity. Furthermore, the impact on fossil fuel depletion in SYM1 was > 75% of the values obtained for SYM2 and SYM3 (Fig. 2j). Summarily, transportation activities, such as diesel consumption which are key in corn stover production, considerably impact fossil fuel depletion in this study (data not shown). This trend is also in line with Li et al.’s [21] observation. Therefore, corn stover production such as the use of diesel to fuel tractors and equipment is the major driver for fossil fuel depletion in this study.

In summary, this LCA indicates scenarios with chemically pretreated corn stover co-digested with dairy manure and blended with magnetite nanoparticles (SYM2 and SYM3) show lower ozone depletion, smog, ecotoxicity, carcinogenic, non-carcinogenic, eutrophication, and fossil fuel depletion potential than the untreated corn stover co-digested with dairy manure (SYM1). However, SYM1 shows the highest carbon credit in terms of global warming potential. This demonstrates that having a higher proportion of corn stover to manure in digester has a more dominating effect on these environmental indicators than the use of additives and mild chemicals such as calcium hydroxide for pretreatment. However, the SYM1 scenario has the highest methane yield to manure input compared to the other tested scenarios. The material and energy used for corn stover production and harvest cause a higher negative impact in most categories, while the carbon content of the corn stover generates a higher carbon credit for SYM1 relative to SYM2 and SYM3. Pretreatment and inclusion of nanoparticles have both energy production and some environmental benefits. However, minimal environmental impact benefits when only co-digestion is employed should be considered in holistic decision-making. Cost analysis would also be instrumental in making informed decisions considering the cost of the pretreatment facilities and the nanoparticles.

Conclusion

Methane harnessed from solid-state anaerobic digestion could mitigate environmental impacts. Thus, in this study, solid-state anaerobic co-digestion of dairy manure and pretreated corn residue substantially reduce 73% ozone depletion, 76% smog, 44% respiratory effects, 64% carcinogenic, 83% non-carcinogenic, 76% fossil fuel depletion, 68% acidification, 72% ecotoxicity, and 23% eutrophication potentials. Furthermore, the addition of magnetite nanoparticles to these combined feedstocks led to at least 12% environmental benefit for all the indicators except the global warming, acidification, and respiratory effect. Interestingly, for the co-digestion scenario without pretreatment, corn stover cultivation, processing, and storage led to over 85% environmental gain considering the global warming potential, though detrimentally, the cultivation strongly influenced fossil fuel depletion. Overall, this study demonstrates that the application of calcium hydroxide pretreatment coupled with magnetite nanoparticles (additives) in a solid-state co-digestion study of livestock manure and lignocellulosic waste is a sustainable waste management practice with minimal environmental consequences. The result in this study can guide tradeoffs between magnetite nanoparticle application and pretreatment in enhancing green energy generation in solid-state anaerobic digestion in terms of environmental impact. A future study with a different substrate co-digested with manure is warranted. Similarly, future studies should consider resource allocation to the digestate as a functional unit. In addition, a comparison of the environmental impact of this pretreated SSAD with pretreated liquid-state AD should be investigated. The impact of post-digestion activities such as composting on the presented scenarios and the life cycle costing analysis could also be investigated to provide a full life cycle perspective.