Environmental Life Cycle Assessments of Chicken Manure Compost Using Tobacco Residue, Mushroom Bran, and Biochar as Additives

Abstract

As an environmental management method, the (life cycle assessment) LCA method can be used to compare the differences between various waste treatment processes in order to provide an environmentally friendly and economically feasible method for waste management. This study focused on the reutilization of typical organic waste to produce organic fertilizer in southwest China and used the life cycle assessment method to evaluate three aerobic chicken manure composting scenarios modified with three additives (biochar, mushroom bran, and tobacco residue) from an environmental and economic perspective. The results show that the total environmental loads of the optimized treatments using mushroom bran and biochar mixed with mushroom bran as additives were reduced by 30.0% and 35.1%, respectively, compared to the control treatment (viz. chicken manure composted with tobacco residue). Compared to the control treatment, the optimized composting treatment modified by mushroom bran with and without biochar improved the profit by 23.9% and 35.4%, respectively. This work reflected that the combined composting mode of chicken manure, tobacco residue, mushroom bran, and biochar is an environmentally friendly and economically feasible composting process, which is more suitable for the resource utilization of the typical organic waste in southwest China.

1. Introduction

Chicken manure has become an abundant organic waste with the quick economic development and enhanced demand for food in the world. It is reported that the amount of chicken manure generated will be up to about 457 million tons in 2030 [1]. Because chicken manure is rich in essential and micro-plant nutrients, chicken manure is generally used for increasing soil nutrient content and raising the crop growth by directly returning the chicken manure to the crop [2]. However, serious environmental concerns can be caused by continuous direct applications of chicken manure on farmlands, which include antibiotic pollution, antibiotic resistance gene generation, groundwater pollution, and greenhouse gas emissions [3]. As a green way to overcome the such issues, chicken manure can be processed to produce a stable organic fertilizer applied to soil by aerobic composting with proper additives [4]. It is well known that chicken manure cannot be directly composted due to the high moisture content, the high bulk density, and low C/N, which can hinder the growth of aerobic bacteria [5]. As well as chicken manure, both mushroom bran and tobacco belong to a type of nitrogen (N)-rich organic waste with low C/N. It is noteworthy that about 7 × 10⁷ tons of mushroom bran cannot be reused per year in China [6]. Meanwhile, 2.24 million tons of tobacco was produced in China in 2018, of which 30~35% were abandoned as residue [6]. In order to make chicken manure decompose more rapidly in aerobic compost, it is reasonable to supplement the chicken manure compost with mushroom bran and tobacco powder, which can decrease the steep compost bulk density [7]. Our previous study found that 10% mushroom bran combined with 5% tobacco powder can improve maturity and reduce greenhouse gas emissions in chicken manure composting [1]. However, similar to the results of the C/N shown in the current study, the C/N of the chicken manure compost modified by mushroom bran and tobacco powder still did not reach the optimal level (25~30:1) [1,6].

Biochar is a kind of porous carbonaceous material that can be pyrolyzed from various organic materials under high-temperatures (>300 °C) in an anaerobic or completely oxygen-free environment [8], featuring high porosity [9], large surface area [10], favorable adsorption performance [11], and excellent carbon stability [12]. Chen et al. (2020) reported that chicken manure biochar reduced greenhouse gases and ammonia emissions, and improved the quality of the chicken manure compost product [13]. Sánchez-Monedero et al. (2019) also stated that 3% (w/w) holm oak biochar reduced the volatile organic compounds (VOC) concentration during the thermophilic phase of composting and improved the VOC removal efficiency [14]. Chung et al. (2021) found that the addition of 10% (w/w) rice husk biochar reduced emissions of ammonia and greenhouse gases (GHGs) and pathogens and enhanced nutrient retention and overall compost quality [15]. These recent studies adequately confirm the critical role of biochar in enhancing the economic and environmental values in chicken manure compost. Our previous study investigated the effects of biochar on co-composting chicken manure and spent mushroom substrate, in which the results showed that the addition of 10% (w/w) bamboo biochar in the co-composting of chicken manure and mushroom bran can enhance composting quality and reduce nitrogen loss [6]. Although previous research progress has proved that biochar, mushroom bran, and tobacco powder mixed with chicken manure compost has environmental significance in terms of greenhouse gas emission reduction, improving compost product quality, and reducing pathogenic bacteria from the micro scale of different scientific issues, whether this technology is feasible in practical processes still needs systematic evaluation from environmental and economic perspectives [16,17].

To date, some studies have used LCAs to evaluate the composting process from economic and environmental perspectives. For example, Pergola et al. (2018) stated that the lesser impacts of energy and cost requirements occurred when maize straw or pruning residues were used as bulking agents in dairy cattle and buffalo manure compost [18]. Cadena et al. (2009) utilized LCAs to compare the environmental impact of aerobic composting technologies of municipal solid waste [19]. Li et al. (2018) investigated the environmental impacts of different on-farm organic waste treatment strategies, including anaerobic digestion, composting, and anaerobic digestion followed by composting [20]. Oldfield et al. (2018) assessed the potential environmental impact of recycling agricultural organic materials via traditional composting, as well as biochar-amended composting. The above LCA results show that the blending of compost and biochar is favorable from a grave-to-cradle perspective [21]. However, few studies systematically evaluate the chicken manure compost from economic and environmental perspectives.

To fill these knowledge gaps, a comprehensive environmental LCA of the chicken manure compost system using three kinds of widely sourced, local organic matters as additives was conducted. The aims of this study were to (1) quantitatively assess the environmental impact of chicken manure compost using three different additives by evaluating typical LCA environmental indexes, e.g., global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), etc.; (2) compare the changes in environmental performance between original chicken manure compost and modified compost strategies, and (3) examine the effect of the biochar addition on the environmental performance of the chicken manure compost. The LCA method was utilized to quantify environmental impacts and eco-efficiency was used to describe the commercial usage potential of various technologies by taking into account the relationship between economic advantages and LCAs. With this research, we aim to offer a scientific foundation for developing reasonable manure management strategies in the future.

2. Materials and Methods

2.1. Objective and Scope Definition

In this study, the Guizhou Kaiyang Nanjing Modern Agriculture Development Company was the experimental site of this LCA (Figure 1) and three different chicken manure treatment processes were compared and evaluated. This company has an annual average of 400,000 laying hens, with an average daily egg production of 17 Mg, annual egg production of more than 6000 Mg, and annual output of chicken manure of about 12,000 Mg. Processing manure with dry cleaning technology was the current manure pre-treatment method and, afterwards, the achieved accumulated chicken manure was composted with tobacco powder, biochar, and mushroom bran. The annual output of organic fertilizer is about 20,000 Mg. A total of three kinds of composting scenarios are included in

Table 1. Inputs and outputs of diverse composting scenarios.

Scenarios	Input	Output
Scenario 1	Chicken manure, Tobacco residue, Electricity, Diesel fuel	Compost product, Waste gas
Scenario 2	Chicken manure, Tobacco residue, Electricity, Diesel fuel, Mushroom bran	Compost product, Waste gas
Scenario 3	Chicken manure, Tobacco residue, Electricity, Diesel fuel, Biochar	Compost product, Waste gas

. The functional unit (FU) was defined as 1 Mg of chicken manure, which entered the system boundary. In addition, the starting point of the three scenarios in the current LCA was the transfer of the collected manure to the composting plant, and the end point was the formation of the mature compost products. The current LCA was mainly conducted by evaluating the global warming potential (GWP), eutrophication potential (EP), acidification potential (AP), abiotic resource consumption potential (ADP-F), terrestrial ecotoxicity potential (TETP), human toxicity potential (HTP), freshwater aquatic ecotoxicity potential (FAETP), and marine aquatic ecotoxicity potential (MAEP).

2.2. Life Cycle Inventory

The treatment of chicken manure used dry clear manure process technology and no wastewater was produced. The environmental impact of chicken manure transportation is negligible due to the organic fertilizer factory being close to the layer-breeding plant. The environmental data of the public systems for calculating the environmental impact of composting additives during transportation are shown in

Table 2. Environmental data of public systems [22].

Public System	Energy (kg)	CO2 (kg)	CO (kg)	CxHy (kg)	NOx (kg)	SO2 (kg)	Waste (kg)
Electricity (Kw·h)	1.2 × 10−2	0.938	4.0 × 10−5	7.2 × 10−5	5.1 × 10−3	1.1 × 10−3	0.1
Diesel fuel (L)	2.0 × 10−2	3.37	1.0 × 10−2	5.5 × 10−3	2.4 × 10−2	6.8 × 10−4	-
Transportation (t·km)	1.42 × 10−2	0.140	4 × 10−4	2.2 × 10−4	9.4 × 10−4	2.8 × 10−5	-

[22]. The power generation pollutant data in Table 1 were calculated based on the average energy consumption of power generation in China, 0.424 kg standard coal (kWh)⁻¹, which is equal to 24,244 kJ kg⁻¹ coal. The coal ash content was calculated as 20%; the generated amount of NO₂ and SO₂ in power production and transportation systems was calculated according to the corresponding average emission factor [23].

Scenario 1 is the current operation scenario of the composting plant. According to the survey data, the plant has a trough composting facility with a capacity of approximately 3000 Mg·year⁻¹, the plant covers an area of 12,500 square meters with an annual power consumption of 96,000 kWh·year⁻¹, and the diesel consumption of the forklift is 8550 L (the composting cycle is 35 d, and the frequency of mechanical turning is 4 times·week⁻¹). In addition to about 12,000 Mg·year⁻¹ of chicken manure per year, the current composting raw materials of the composting plant also need to add about 3000 Mg·year⁻¹ of tobacco dust per year (260 yuan·Mg⁻¹). The tobacco powder was provided by the cigarette factory 45 km away from the composting factory. The nutrient content of compost products in the output list of the composting plant was measured in the experiment and the emission of waste gases (NH₃, N₂O, and CH₄) were estimated according to our previous studies [1,6]. The CH₄ emission was 1.52 kg and the emissions of NH₃ and N₂O were 3.70 kg and 0.13 kg, respectively, during the composting process of mixed materials. The input and output list details for Scenario 1 is shown in

Table 3. Input and output inventory of the composting plant in Scenario 1 (data collected in 2018).

Input		Output
Chicken manure (Mg·year−1)	12,000	Compost products (Mg·year−1)	1786
Dry matter (Mg·year−1)	1200	Dry matter (Mg·year−1)	1268
N (Mg·year−1)	45.7	N (Mg·year−1)	22.5
P (Mg·year−1)	19.8	P (Mg·year−1)	21.3
K (Mg·year−1)	17.3	K (Mg·year−1)	28.3
Tobacco dust (Mg·year−1)	3000	Exhaust gas emissions
Dry matter (Mg·year−1)	2512	N2O (Mg·year−1)	1.95
N (Mg·year−1)	81.6	CH4 (Mg·year−1)	22.8
P (Mg·year−1)	18.8	NH3 (Mg·year−1)	55.5
K (Mg·year−1)	44.0
Energy consumption
Electricity (kWh·year−1)	96,000
Diesel fuel (L)	8550
Transportation (Mg km·year−1)	135,000

.

Scenario 2 is the optimized composting treatment of Scenario 1. Overall, the existing compost materials in the composting plant feature high moisture content and fine particles and the pile body heats up slowly in the composting process. In addition, it is difficult to remove water and the energy consumption is high. In this scenario, mushroom bran was used instead of tobacco residue, based on our experimental results [1,6]. Through this strategy, the air permeability of compost material could be increased and the frequency of turning the heap and amount of energy consumption could be reduced. Because the actual energy consumption data of the composting plant facilities were not available, it was calculated based on the laboratory energy consumption that the electricity consumption per 1 Mg of mixed material processed in this scenario was 3.20 kWh·Mg⁻¹. The diesel consumption was 2.85 L·Mg⁻¹ (the composting cycle was 35 d and the frequency of turning the heap was 2·week⁻¹). According to the optimized composting plan, in addition to about 12,000 Mg·year⁻¹ of chicken manure and 1500 Mg·year⁻¹ of tobacco residue, about 1500 Mg·year⁻¹ of mushroom bran should be added every year. The tobacco residue was provided by a cigarette factory 45 km away from the composting plant, and the mushroom bran was provided by a mushroom plantation 26 km away from the composting plant (120 yuan·Mg⁻¹). The nutrient content of the compost products in the output list of the composting plant was the measured value of the compost samples in the laboratory. The emission of exhaust gases (NH₃, N₂O, and CH₄) was estimated according to the test results based on our previous studies [1,6]. The CH₄ emission was 0.84 kg and the emissions of NH₃ and N₂O were 2.78 kg and 0.12 kg, respectively, during the composting process of 1 Mg of mixed materials. A detailed list of inputs and outputs corresponding to Scenario 3 is shown in

Table 4. Input and output inventory of the composting plant in Scenario 2 (data collected in 2018).

Input		Output
Chicken manure (Mg·year−1)	12,000	Compost products (Mg·year−1)	1873
Dry matter (Mg·year−1)	1200	Dry matter (Mg·year−1)	1330
N (Mg·year−1)	45.7	N (Mg·year−1)	24.0
P (Mg·year−1)	19.8	P (Mg·year−1)	25.0
K (Mg·year−1)	17.3	K (Mg·year−1)	34.0
Tobacco dust (Mg·year−1)	1500	Exhaust gas emissions
Dry matter (Mg·year−1)	1256	N2O (Mg·year−1)	1.73
N (Mg·year−1)	40.8	CH4 (Mg·year−1)	12.6
P (Mg·year−1)	9.40	NH3 (Mg·year−1)	41.6
K (Mg·year−1)	22.0
Mushroom bran (Mg·year−1)	1500
Dry matter (Mg·year−1)	1221
N (Mg·year−1)	11.6
P (Mg·year−1)	12.3
K (Mg·year−1)	24.2
Energy consumption
Electricity (kWh·year−1)	48,000
Diesel fuel (L)	4275
Transportation (Mg km·year−1)	106,500

.

Scenario 3 is optimized based on Scenario 2. Scenario 3 reduced carbon and nitrogen loss during the composting process and improved compost quality with the addition of biochar. The source and dosage of chicken manure, tobacco dust, and mushroom bran as raw materials for composting are consistent with Scenario 1, the energy consumption data of the composting plant is consistent with Scenario 2, and the biochar was provided by the fertilizer plant of the Guizhou Academy of Agricultural Sciences, which is 50 km away from the composting plant (3000 yuan·Mg⁻¹). The nutrient content of the compost products in the output list of the composting plant was the measured value of the laboratory compost samples, the emission of exhaust gases (NH₃, N₂O, and CH₄) are estimated with reference to the test results from our previous studies [1,6]. The CH₄ emission was 0.74 kg and the emissions of NH₃ and N₂O were 2.45 kg and 0.05 kg, respectively, during the composting process of 1 Mg of mixed materials. A detailed list of inputs and outputs corresponding to Scenario 3 is shown in

Table 5. Input and output inventory of the composting plant in Scenario 3 (data collected in 2018).

Input		Output
Chicken manure (Mg·year−1)	12,000	Compost products (Mg·year−1)	2118
Dry matter (Mg·year−1)	1200	Dry matter (Mg·year−1)	1503
N (Mg·year−1)	45.7	N (Mg·year−1)	27
P (Mg·year−1)	19.80	P (Mg·year−1)	29
K (Mg·year−1)	17.28	K (Mg·year−1)	38
Tobacco residue (Mg·year−1)	1500	Exhaust gas emissions
Dry matter (Mg·year−1)	1256	N2O (Mg·year−1)	0.78
N (Mg·year−1)	40.8	CH4 (Mg·year−1)	11.1
P (Mg·year−1)	9.42	NH3 (Mg·year−1)	36.7
K (Mg·year−1)	22.0
Mushroom bran (Mg·year−1)	1500
Dry matter (Mg·year−1)	1221
N (Mg·year−1)	11.6
P (Mg·year−1)	12.3
K (Mg·year−1)	24.2
Biochar (Mg·year−1)	480
Energy consumption
Electricity (kWh·year−1)	48,000
Diesel fuel (L)	4275
Transportation (Mg km·year−1)	130,500

.

2.3. Environmental Impact Assessment

The environmental assessment of this study only involves three processes: compost material transportation, energy consumption, and pollutant discharge. This assessment was divided into three steps: characterization, standardization, and weighted assessment.

At the same time, eight factors with greater impact on the environment, such as GWP, AP, EP, etc., were selected for quantitative calculation of the total environmental impact assessment potential. The environmental reference value and weight coefficient are shown in Table 5 and the calculation formula (Equation (1)) is as follows:

$$\mathrm{EIL}={\displaystyle \sum }\frac{{\displaystyle \sum }\mathrm{Q}{\left(\mathrm{j}\right)}_{\mathrm{i}}\times \mathrm{PF}{\left(\mathrm{j}\right)}_{\mathrm{i}}}{\mathrm{ER}\left(\mathrm{j}\right)}$$

(1)

where Q(j)_i is the emission amount of the ith substance, PF(j)_i is the equivalent factor of the environmental impact, and ER(j) is the environmental reference value.

This study mainly considers three types of environmental impacts, including GWP, acidification potential (AP), and eutrophication potential (EP). GWP is measured by a CO₂ equivalent; the equivalence coefficients of CH₄ and N₂O are 21 and 310, respectively [24]. AP uses SO₂ as a reference equivalent; the equivalence coefficients of NH₃ and NO_x are 1.88 and 0.7, respectively [25]. EP takes PO₄³⁻ as the reference equivalent, and the equivalence coefficients of NO_x, NO₃⁻, and NH₃ are 0.1, 0.42, and 0.33, respectively [16].

The standardization process establishes a standardized benchmark so that various environmental impact types have a relatively comparable standard. This evaluation used the research results of Stranddorf as the environmental impact benchmark [26]. The weighted evaluation is based on the weighting coefficient set by Ji et al. (2012) [27]. The specific environmental benchmark values and weight coefficients are shown in

Table 6. Normalized values and weight coefficients for world environmental impact.

Type of Environmental Impact	Unit	Weight Coefficients	Normalized Values
Energy consumption	MJ·year−1	0.28	4.74
Global warming potential (GWP)	g (CO2-eq)·t−1	0.23	725
Environmental acidification potential (AP)	g (SO2-eq)·t−1	0.26	2.92
Eutrophication potential (EP)	g (PO4-eq)·t−1	0.23	4.92

.

3. Results and Discussion

3.1. Specific Environmental Impact Potential

The environmental impact analysis list of the typical life cycle of organic waste composting management in Southwest China is shown in

Table 7. Environmental life cycle assessment of diverse compost scenarios based on 1 Mg chicken manure input.

Environmental Influence Factors	Unit	Scenario 1	Scenario 2	Scenario 3
Abiotic resource depletion (ADP-f; in terms of fossil energy)	MJ	146	79.2	251
Acidification potential (AP)	kg SO2-eq	7.47	5.59	4.96
Eutrophication potential (EP)	kg phosphate-eq	1.67	1.26	1.11
Freshwater aquatic ecotoxicity potential (FAETP)	kg DCB-eq	0.406	0.283	2.73
Global warming potential (GWP; 100 years)	kg CO2-eq	106	74.8	54.4
Human toxicity potential (HTP)	kg DCB-eq	4.58	2.67	9.25
Marine aquatic ecotoxicity Potential (MAEP)	Mg DCB-eq	2.37	1.39	6.97
Terrestrial ecotoxicity potential (TETP)	kg DCB-eq	0.091	0.048	0.105

. The combined composting process of chicken manure, tobacco residue, and mushroom bran will emit large amounts of CH₄ and N₂O gases into the environment, the environmental load that leads to the potential greenhouse effect is the main environmental impact type of the three composting processes, and the global warming impact is in the order of Scenario 1 > Scenario 2 > Scenario 3. Among them, compared to the current composting process (Scenario 1), the optimized composting processes (Scenarios 1, 2, and 3) can effectively reduce greenhouse gas (CH₄ and N₂O) emissions; the greenhouse effect potential decreased by 29.6% and 48.7%, respectively. Compared to the current composting process (Scenario 1), each environmental impact of the optimized composting process (Scenario 2) has a decreasing trend to varying degrees. While the environmental impacts of the optimized composting process (Scenario 3) in terms of abiotic resource depletion (ADP-f), freshwater aquatic ecotoxicity potential, human toxicity potential, marine aquatic ecotoxicity potential, and terrestrial ecotoxicity potential increased, the increase was between 13.49% and 85.15%, which is higher than that of the optimized composting process (Scenario 2). This difference is due to the addition of biochar to the optimized composting process (Scenario 3), which increased the energy consumption for transportation. On the whole, the total environmental loads of the optimized composting processes (Scenarios 2 and 3) were reduced by 30.0% and 35.1%, respectively, compared to the current composting process (Scenario 1). Thus, it can be understood that the combined composting mode of chicken manure, tobacco residue, mushroom bran, and biochar is more suitable for waste treatment in the southwest.

3.2. Total Environmental Impact Potential

After analysis and after standardizing and weighing the greenhouse effect potential, acidification effect potential, and eutrophication potential, the environmental impacts of the three composting management scenarios are shown in

Table 8. Comprehensive environmental impact potential of different scenarios based on 1 Mg chicken manure input.

Scenario	Environmental Impact	Benchmark Value (kg-eq (Person Year)−1)	Environmental Impact Potential (kg-eq)	Weights	Total Impact Potential (g CO2-eq. (Person·Year)−1)
1	Global warming potential	8700	106	0.23	64.8
	Acidification potential	35	7.47	0.26
	Eutrophication potential	59	1.67	0.23
2	Global warming potential	8700	74.8	0.23	48.4
	Acidification potential	35	5.58	0.26
	Eutrophication potential	59	1.26	0.23
3	Global warming potential	8700	54.4	0.23	42.6
	Acidification potential	35	4.96	0.26
	Eutrophication potential	59	1.11	0.23

. It can be seen in Table 7 that the current composting process (Scenario 1) has the largest and most comprehensive environmental impact potential (64.8 kg CO₂-eq. (person·yr)⁻¹), followed by Scenario 2 (48.4 kg CO₂-eq. (person·yr)⁻¹) and the most optimized composting process (Scenario 3 at 42.6 kg CO₂-eq. (person·yr)⁻¹). The reason for this is that the pile porosity improved and the energy consumption of turning the pile reduced after the mushroom bran replaced the tobacco residue. Furthermore, the addition of biochar can reduce the loss of carbon and nitrogen during the composting process, so the global warming potential of the optimized composting process (Scenario 3) is the lowest. Therefore, the optimal composting process (Scenario 3) has the lowest emissions of pollutants. The environmental impacts of the current composting process (Scenario 1) and optimized composting processes (Scenarios 2 and 3) are all mainly attributed to the global warming potential (accounting for more than 90%), which is related to higher energy consumption and the loss of carbon and nitrogen in the composting system. It is noteworthy that, regarding the effects of composting plants on diverse environment aspects, it is not only needed to consider direct environmental impacts but also marginal impacts induced by the composting plants [28]. However, in composting LCAs, the credits replacing the mineral fertilizer on the basis of the N, P, and K in compost should be avoided. Firstly, since the compost is frequently applied to soils that were amended with organic fertilizers for several years, most agricultural soils do not require fertilization with N, P, and K [29]. Secondly, the efficiency of mineral fertilizers is considerably higher than that of compost [28].

3.3. Economic Evaluation of Different Composting Scenarios

This study evaluated the economic benefits of three chicken manure composting scenarios by analyzing the cost and output of each composting treatment. The specific cost and income parameters are shown in

Table 9. Economic analysis of chicken manure composting in different scenarios based on 1 Mg chicken manure input.

Scenarios	Cost of Compost Material (Yuan·Year−1)			Fertilizer Income (Yuan·Year −1)	Profit (Yuan·Year −1)
	Tobacco Dust	Mushroom Bran	Biochar
Scenario 1	65.0	0.00	0.00	178	113
Scenario 2	32.5	15.0	0.00	188	140
Scenario 3	32.5	15.0	12.0	212	153

Note: There is no cost for chicken manure; the cost of transporting tobacco residue, mushroom bran, and biochar to the composting plant is 260 yuan Mg⁻¹, 120 yuan Mg⁻¹, and 3000 yuan Mg⁻¹, respectively, and the market price of organic fertilizer is 1200 yuan·Mg⁻¹.

. Without considering energy consumption and human resources required for each composting process, the composting economic analysis of diverse scenarios was estimated and the results are shown in Table 9. The optimized composting processes (Scenarios 2 and 3), which use mushroom bran to replace 50% of the tobacco residue, reduce the cost of auxiliary materials. Although the addition of biochar in Scenario 3 increases the cost of composting materials, the yield of compost products is higher than costs in Scenario 1 and Scenario 2. Therefore, profits are also higher than Scenario 1 and Scenario 2 by 34.6% and 8.93%, respectively. In general, improving the current composting process can improve the yield and quality of organic fertilizers and the environmental impact is also lower than the original process. This is conducive to improving the comprehensive benefits of composting typical materials in southwest China and has application and promotion prospects in practical production.

4. Conclusions

In this LCA study, the total environmental impact of the optimized composting process (Scenarios 2 and 3) was reduced by 30.0% and 35.1%, respectively, compared to the current composting process (Scenario 1). The total environmental impacts of the three composting scenarios are all mainly attributed to the composting greenhouse effect, accounting for 86.4%, 86.9%, and 68.2% of the three total environmental impact potentials, respectively. Overall, the chicken manure composted with chicken manure, tobacco residue, mushroom bran, and biochar is more suitable for the resource utilization of the typical organic waste in southwest China. This optimized composting process increased the yield of organic fertilizer by 331 Mg year⁻¹ and the increased profit was 464,400 yuan·year⁻¹. Improving the current composting process is conducive to improving the environmental and economic benefits of chicken manure composting in southwest China.