Process simulation
In this study, a process for bacterial P(3HB) production from OFMSW was economically and environmentally evaluated. Mass and energy requirements for the P(3HB) production process were estimated by performing a simulation with the aid of the commercial software SuperPro Designer 10®. For this simulation, the following conditions and specifications were used: (1) the P(3HB) production plant would be built in The Basque Country (Spain); (2) its lifetime would be 20 years; (3) the construction plus start-up phase would take one year; (4) the waste processing capacity of the P(3HB) production plant would be 1 ton day−1; and (5) the plant would operate for 330 days year−1 (the rest of the time, up to 365 days, the plant would be stopped for maintenance and cleaning works).
According to the Spanish Ministry for the Ecological Transition (MITECO 2017), each year 877 thousand tons of OFMSW are generated in Spain, of which 20% (175.4 thousand tons) are produced in The Basque Country. In Spain, OFMSW is selectively collected and, subsequently, 70% is derived to produce compost and biogas, while the remaining 30% is disposed of in landfills or incinerated.
Simulation scenarios
The industrial plant simulated in this study is based on a bacterial P(3HB) production process using Burkholderia sacchari DSM 17165, carried out in the fed-batch mode, which was developed and reported in previous works (Izaguirre et al. 2019, 2020). Here, two scenarios, based on the fermentation medium, were considered to assess the economic and environmental feasibility of the bacterial P(3HB) production process: (1) in Scenario 1 (Additional file 1: Fig. S1A), those sugars present in OFMSW were initially released by the combination of a thermo-chemical pre-treatment and an enzymatic hydrolysis. Subsequently, the enzymatic hydrolysate was used as culture medium for the fermentative production of P(3HB). During the cultivations, glucose (a lower amount compared to Scenario 2) and sugar-rich plum waste juice were added as feed to enhance productivity; (2) in Scenario 2 (Additional file 2: Fig. S2B), the enzymatic hydrolysate from OFMSW was not used as fermentation medium. Instead, fermentation was initiated with a basal medium which contains salts and glucose (for a detailed description of its composition, see Izaguirre et al. 2019). Plum waste juice was added as feed solution after the batch period. Finally, in each scenario, the P(3HB) produced was extracted. By comparing both scenarios (Scenarios 1 and 2 with and without OFMSW hydrolysate, respectively), one can evaluate to what extent the use of OFMSW, as source of nutrients for the fermentation process, contributed to the performance of the P(3HB) production process represented in Additional file 1: Fig. S1. In this Additional file 1: Fig. S1, for simplification purposes, pieces such as valves and piping are omitted from the flowsheet. Nonetheless, they were taken into account for the economic assessment.
Description of the P3HB production process
The P(3HB) production process simulated here included three main steps: (i) thermo-chemical pre-treatment and enzymatic hydrolysis of OFMSW; (ii) fermentation; and (iii) extraction–separation (Izaguirre et al. 2019, 2020).
The OFMSW, kindly provided by a local composting company (EPELE, Gipuzkoa, Spain) was composed of pre-screened domestic and garden wastes (Izaguirre et al. 2019). After removing impurities (i.e. stones, plastics, glass, etc.), OFMSV was ground, using a coffee grinder (Moulinex AR100), and subsequently mixed with a 1% H2SO4 solution at a solid-to-liquid ratio of 13.5% (w/v). Afterwards, the waste was pre-treated at 121 °C for 60 min in a blending tank (V-101). This thermo-chemical pre-treatment is intended to break up the lignocellulosic structure of the biomass, so that it becomes more accessible to enzymatic hydrolysis. The resulting mixture was cooled and then neutralized with NaOH, following Izaguirre et al. (2019). After neutralization, the mixture was enzymatically hydrolyzed in a reactor (R-101) at 50 °C for 24 h using a blend of 90 mg g−1 Pentopan 500 BG and 150 mg g−1 Celluclast BG. Non-hydrolyzed solids were separated by centrifugation in a disk-tank centrifuge (DS-101), and the supernatant (hydrolysate) was used for the next step, i.e. the bacterial cultivation. As mentioned above, the thermo-chemical pre-treatment and the enzymatic hydrolysis described here were only contemplated in Scenario 1.
For the bacterial production of P(3HB), the enzymatic hydrolysate was heat-sterilized and transferred to a fermenter (FR-101), which was inoculated with Burkholderia sacchari DSM 17165 (10% v/v) previously grown for 12 h in Luria–Bertani (LB) broth in a 2000-mL Erlenmeyer stirred in an orbital shaker at 170 rpm, 30 ºC (Izaguirre et al. 2019). The fermentation process was carried out aerobically at pH = 6.8 and 32 °C in a stirred tank bioreactor for 50 h. The solid fraction was separated from the liquid fraction by centrifugation in a disk-tank centrifuge (DS-102). Finally, the P(3HB)-enriched bacterial biomass was lyophilized in a freeze dryer (FDR-101).
For the extraction–separation step, we followed the procedure reported by Rosengart et al. (2015). Briefly, the lyophilized biomass was transferred to a blending tank (V-102), which contained anisole at 120 °C, and mixed for 30 min. The extraction solvent (anisole) alters cell permeability and then dissolves the released P(3HB). After extraction, the remaining biomass was separated by filtration in a rotary vacuum filter (RVF-101). Finally, the separation of P(3HB) from the extraction solvent (anisole) was carried out by crystallization in a continuous crystallizer (CR-101). For the economic and environment assessment presented here, it was assumed that the evaporated solvent was collected to then be re-used in subsequent extractions.
Economic analysis
The economic evaluation of both scenarios was performed using SuperPro Designer 10® software, through which the total capital cost, annual production cost and revenue generation can be estimated.
The total capital cost of the P(3HB) production plant is dependent on three different parameters: direct fixed capital, working capital, and start-up validation cost. The direct fixed capital includes equipment purchase costs, as well as other direct and indirect costs related to the construction of the plant, such as piping, insulation and engineering, among others. The contribution (%) of each component was estimated, based on total equipment purchase cost, using several multipliers (Petrides 2015). For the economic analysis, the price of the equipment was gathered from reputable websites. In this study, and since the local government (Basque Government) strongly promotes and enables the creation of new industries and facilities, the cost of the land was not taken into account in the economic evaluation. The contribution of the working capital and start-up validation cost to the total capital cost was 1.5 and 5% of the direct fixed capital, respectively.
Concerning annual production costs, raw materials (namely, enzymes, solvents, salts, carbon sources, etc.) usually have a most important contribution to these costs, but they also include other costs derived from maintenance and repair, labour, utilities, quality control, consumables, waste disposal and so on. For the economic assessment, the cost of raw materials and consumables was obtained from reputable suppliers of laboratory equipment, reagents, etc. In our specific case, OFMSW (and its transportation to the P3HB production plant) was kindly provided by a local composting company (EPELE, Gipuzkoa, Spain). Likewise, the plum waste juice was freely provided by CATAR-CRITT Agro Ressources (France) and, thus, its cost was not included in the economic evaluation. The so-called “facility-dependent costs” correspond to depreciation of the fixed capital investment, equipment maintenance costs, insurance, taxes and other general expenses. Our P(3HB) production plant was designed to operate for 20 years and, according to this, the straight-line method was used to calculate capital depreciation. Equipment maintenance and repair cost were estimated to be 1% of the direct fixed capital. In accordance with the local legislation, insurance and taxes were estimated to be 0.04 and 1.38% of the direct fixed capital, respectively. Labour costs basically consist of the salaries of operators and engineers, to which corresponding taxes must be added. For the correct operation of the envisioned plant, in Scenario 1, six operators and two engineers were considered necessary, while four operators and two engineers were estimated for Scenario 2. The estimated salary of an operator and an engineer was US$ 26,000 and US$ 41,000 year−1, respectively. The cost of the quality control was estimated to be 15% of total labour cost. The cost of waste disposal and treatment of gaseous emissions was estimated to be 0.013 and 0.002 US$ kg−1, respectively. Finally, the level of consumption of materials and energy was estimated according to the mass and energy balance calculated by the simulation software (unit costs were obtained from supplier companies).
Under both scenarios, the main revenue comes from the sale of the produced P(3HB). The market for green, bio-derived, biodegradable bioplastics, such as P(3HB) and PLA, is very promising, specially taking into consideration the increasing awareness of the negative impact of petroleum-derived, non-biodegradable plastics on the environment (Dhaman and Ugwu 2013). Poly(3)hydroxybutyrate is probably the most studied polyhydroxyalkanoate due to a variety of promising characteristics (Dhaman and Ugwu 2013): (i) its material properties are comparable to those of polypropylene; (ii) it can be synthesized from renewable low-cost feedstocks; (iii) its synthesis can be operated under mild process conditions with minimal environmental impact; (iv) many different microbial strains are known to produce P3HB; (v) it can be degraded aerobically and anaerobically without forming toxic products; and (vi) it can be used as biomaterial for medical applications and packaging, among other uses. At present, the market price of P3HB is 4000 US$ ton−1 (Ramos et al. 2019; Stavroula et al. 2020).
In addition, in Scenario 1, after the enzymatic hydrolysis, the undigested OFMSW was sold as biofertilizer at a price of 0.01 US$ kg−1. Also, a waste management remuneration (0.077 US$ kg−1) was received from the Basque Government, as it is currently promoting and encouraging Circular Economy initiatives.
The economic feasibility of the bacterial P(3HB) production process was evaluated according to several indicators calculated by SuperPro Designer 10® software: gross and net profit, gross margin, return on investment, net present value, and payback time. Gross profit is the revenue from which the annual operating cost has been subtracted, while net profit also considers the depreciation, income tax (20% in The Basque Country), and similar costs. The return on investment (ROI) evaluates the viability of the investment, according to the following equation:
$$ {\text{ROI }}\left( \% \right) = \frac{{\text{Net profit Total investment}}}\times 100. $$
The net present value (NPV) measures the profitability of the production process in absolute net terms (thus, it allows one to know whether the investment will bring profits or not). A positive NPV value means that, a priori, the planned investment should make a profit. The NPV can be calculated according to the following equation (Van Dael et al. 2015):
$$ {\text{NPV}} = \mathop \sum \limits_{n - 1}^{T} \frac{{{\text{CF}}_{n} }}{{\left( {1 + i} \right)^{n} }} - I_{0} , $$
where T = lifetime of the investment; CFn = difference between revenues and costs in year n; I0 = initial investment; and i = discount rate.
The payback time, or time required to recover the capital investment, is calculated as follows:
$$ {\text{Payback time}} \left( {{\text{years}}} \right) = \frac{{\text{Total investment}}}{{\text{Net profit per year}}}. $$
Finally, in order to determine cash flow patterns during the lifetime of the P3HB production plant, cumulative cash flows were calculated. The patterns were plotted using Microsoft Excel 2010. Similarly, various discount rates (see below) were considered to assess their possible effect on profitability.
Environmental assessment
In order to estimate the potential environmental impact (PEI) of the bacterial P3HB production process, an algorithm developed by the U.S. Environmental Protection Agency was used (i.e. WAR tool). This algorithm, based on the calculation of PEI, is divided into eight impact categories: human toxicity potential by ingestion (HTPI); human toxicity potential by dermal exposure and inhalation (HTPE); aquatic toxicity potential (ATP); acidification or acid rain potential (AP); terrestrial toxicity potential (TTP); photochemical oxidation potential (PCOP); global warming potential (GWP); and ozone depletion potential (ODP).