Exploring biorefinery alternatives for biowaste valorization: a techno-economic assessment of enzymatic hydrolysis coupled with anaerobic digestion or solid-state fermentation for high-value bioproducts

Figures & data

Table 1. Examples of published works on SSF for enzyme production.

Enzymes	Inoculum	Substrate	SSF scale	Solid load	Air flow	SSF Temperature	SSF time	Enzyme yield	Reference
Pectinases	Aspergillus oryzae	Citrus waste & sugarcane bagasse	400 L	15 kg	0.15 L h^−1	28–30°C	24 h	37 U g^−1	[26]
Cellulases and xylanases	Specialized consortium from compost	Coffee husk	50 L	15 kg	NM	25–60°C (NC)	24 h	47 U g^−1 DM	[27]
Cellulases and xylanases	Aspergillus niger	Brewer’s spent grain	0.5 L	0.1 kg	0.05 L h^−1	30°C	96 h	268 U g^−1 DM	[28]
Amylases	Bacillus sp.	Agroindustrial waste	0.25 L	0.05 kg	-	40°C	48	82 U g^−1 DM	[29]
Cellulases, proteases and glucoamylases	Aspergillus awamori	OFMSW	0.25 L	0.05 kg	-	30°C	96 h	36 U g^−1 DM	[13]

NM, not mentioned. NC, not controlled. DM, dry matter.

Figure 1. Process flowchart of the different biorefinery scenarios for the application of enzymatic hydrolysis in OFMSW treatment. (a) Scenario I, commercial enzymes and solid hydrolysate valorization through SSF. (b) Scenario II, commercial enzymes and solid hydrolysate valorization through anaerobic digestion. (c) Scenario III, in situ produced enzymes and solid hydrolysate valorization through SSF. (d) Scenario IV, in situ produced enzymes and solid hydrolyzate valorization through anaerobic digestion.

Table 2. Parameters for the economic evaluation.

Parameter	Estimation assumption
General considerations
Plant location	Province of Barcelona
Plant capacity	100,000 t of source selected OFMSW year^−1
Plant availability	333 days year^−1
Capital cost	Capital cost (€) = FCI + working capital
Fixed capital cost (FCI)
• Direct field cost (DFC)	DFC (€) = equipment cost + installation cost
o Installation cost	Equipment cost × 3.2
•Off-site cost	DFC × 0.4
• Engineering cost	(DFC + off-site cost) × 0.25
• Contingency cost	(DFC + off-site cost) × 0.15
Working capital	(DFC + off-site cost) × 0.15
Annual operating cost	Operating cost (€ year^−1) = FOC + VOC
Fixed operating cost (FOC)
• Labor cost	15.72 € t^−1 of OFMSW
• Maintenance cost	DFC × 0.06
• Insurance cost	FCI × 0.01
Variable operating cost (VOC)
• Utilities cost	Electricity: 0.2213 € kWh^−1 Water: 2.415 € m^−3 Demineralized water = water × 2
• Raw materials cost	Viscozyme L®: 13.8 € kg^−1 Rich media: 200 € m^−3
• Wastewater cost	4.94 € t^−1 of OFMSW
• Laboratory & analysis cost	0.72 € t^−1 of OFMSW
Revenues
Treatment fee	34 € t^−1 of OFMSW
Mixed sugar syrup	365 € t^−1
Solid biopesticide	400 € t^−1
Fertilizer (solid digestate)	100 € t^−1

Table 3. Overall component balance in a year for each scenario.

Item	Scenario I	Scenario II	Scenario III	Scenario IV
Inputs
OFMSW (t year^−1)	100,000	100,000	100,000	100,000
Enzyme cocktail (t year^−1)	720	720	0	0
Rich media (m^3 year^−1)	1,289	0	1,358	20
Water (m^3 year^−1)	49,350	61,185	49,350	55,577
Demineralized water (m^3 year^−1)	46,289	45,000	10,764	9,465
Electricity (MWh year^−1)	10,614	0	0	0
Outputs
Sugar syrup (t year^−1)	5,614	5,614	2,954	2,954
Biopesticide (t year^−1)	42,138	0	32,207	0
Fertilizer (t year^−1)	0	25,551	0	22,890
Electricity (MWh year^−1)	0	2,305	10,258	17,561

Figure 2. Total energy consumption and production per ton of processed OFMSW in the different biorefinery processes for each scenario.

Table 4. Overall economic evaluation of the four OFMSW biorefinery scenarios.

	Scenario I	Scenario II	Scenario III	Scenario IV
Capital cost
Total capital cost (Million €)	68.6	51.0	73.9	56.2
Operating cost
VOC (€ t^−1 OFMSW)	135	109	9	7
FOC (€ t^−1 OFMSW)	39	33	41	35
Net production cost (€ t^−1 OFMSW)	174	142	50	42
Revenues
Treatment fee (€ t^−1 OFMSW)	34	34	34	34
Sugar syrup (€ t^−1 OFMSW)	20	20	11	11
Biopesticide (€ t^−1 OFMSW)	169	0	129	0
Fertilizer (€ t^−1 OFMSW)	0	26	0	23
Electricity (€ t^−1 OFMSW)	0	5	23	39
Total income (€ t^−1 OFMSW)	223	87	196	107
Profitability analysis
Gross profit margin (%)	22	−66	74	60
Pay-back time (years)	15	−10	6.1	10
ROI (%)	5	−10	13	8
NPV (Million €)	−22.6	−97.0	41.2	−2.0

VOC, variable operating cost. FOC, fixed operating cost. ROI, return on investment. NPV, net present value.

Figure 3. Investment cost, operating cost, revenues and cumulative net present value (NPV) for the four OFMSW biorefinery scenarios. FOC, fixed operating cost. VOC, variable operating cost.

Figure 4. Static tornado diagram for each scenario showing the sensitivity of net present value (NPV) to the variation (±25%) in each variable while other variables are held constant. The nominal value is displayed as a vertical line.

Figure 5. (a) Cumulative cash flow diagram at different discount rates (0%, 6% and 12.8%) for scenario III. (b) Breakeven chart of scenario III.

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Data availability statement

The data supporting the findings of this study are available within the article and its supplementary material.