Tillman U. Gerngross is assistant professor of biochemical engineering, Thayer School of Engineering, Dartmouth College, Hanover, NH 03755 (e-mail: tillman@dartmouth.edu)
A case study of biodegradable polymer production from agricultural feedstocks casts doubt on the premise that alternative biological processes always offer environmental benefits over conventional manufacturing processes.
The sustainability of a society based on finite fossil resources is the subject of ongoing scientific and political debate. One aspect of this debate, besides exploring alternative energy sources, is the challenge to provide chemical commodities (fuels, lubricants, adhesives, solvents, paints, materials, etc.) to an advanced consumer society, without depleting nonrenewable resources. An approach that has recently gained popularity advocates the use of biological (fermentation) processes to produce chemical commodities from agricultural feedstocks1. Fueled by advances in the area of metabolic engineering, an array of products, ranging from polymers to polymer intermediates and industrial dyes, can now be produced by fermentation. With numerous such biological approaches currently under consideration, it is pertinent to analyze whether the proposed processes have the intended effect of sparing nonrenewable resources and benefiting the environment.
Fermentation-based processes offer intuitive advantages, such as aqueous processing environments, nontoxic waste, and most importantly the use of renewable, nonfossil feedstocks. In most cases, however, these benefits have not been critically weighed against an overall inventory of materials and energy required to generate a given product. This article offers a side-by-side comparison of a biological versus a conventional petrochemical plastic manufacturing process to illustrate the complexity of choices confronting society and the commodity biotechnology industry in the coming years.
Weighing the alternatives
Much has been made of the environmental shortcomings of conventional, fossil oil–based polymers, such as polyethylene, polypropylene, and polystyrene. While these polymers offer good material properties at a low price, their environmental impact and manufacture has traditionally been viewed in a negative light. As a result, much effort has been dedicated to developing alternative plastic materials that are both biodegradable and produced from renewable resources, preferably of agricultural origin.
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Of the various alternative polymers developed to date, polyhydroxyalkanoates (PHAs), a class of aliphatic microbial polyesters, have been considered among the leading candidates to replace conventional plastics on a large industrial scale2. Like their petrochemical counterparts, PHAs are moldable, water insoluble, thermoplastic polymers. The most common PHA, poly-3-hydroxybutyrate (PHB), is a stiff, high-melting-point aliphatic polyester similar to many industrial polyolefins. Unlike polyolefins, however, PHAs can be synthesized by microorganisms, which can produce and store the polymer in the form of intracellular inclusions at levels exceeding 80% of the cell dry mass (see Fig. 1). These microbial polymers can be made entirely from glucose in a fermentation process and, in addition to offering favorable material properties, are completely biodegradable. Thus, the replacement of conventional plastics with PHAs has been promulgated as a desirable approach to solid-waste management and sustainable polymer production3, 4.
A cradle-to-grave analysis Several factors contribute to the environmental impact and the degree of sustainability of a given product or material. In many instances, however, environmental impact is the result, not of the product per se, but rather of the consumption of raw materials and the release of waste products generated during manufacture. Thus, a "cradle-to-grave" analysis, which incorporates manufacturing practices, energy input/output, and overall material flows, is a good benchmark for assessing environmental impact and sustainability5, 6.
Contrary to the widespread belief that PHAs are a sustainable alternative to polymer production, a surprisingly high latent energy content is associated with their fermentative production. By considering the utilities required to make glucose from corn, it is possible to estimate that a PHA fermentation process consumes 22% more steam, 19-fold more electricity, and 7-fold more water than a conventional process for producing polystyrene. While only polystyrene is directly derived from fossil oil, both polystyrene and PHAs require energy in their manufacture. As most energy is generated by combusting fossil resources7 (in the US at least), both polymers also have a latent fossil fuel content. This suggests that, despite the use of renewable agricultural feedstocks, fermentative PHA production consumes significantly more energy, releases more net greenhouse gases8, and therefore is no more sustainable than conventional petrochemical polymer production.
The balance of power
By compiling an energy and feedstock inventory of a "theoretical" large-scale PHA process and a conventional polystyrene process, it is possible to determine the environmental benefits of substituting a petrochemical process with a biological process.
Only a minor fraction (less than 4%) of crude oil is refined into intermediates that serve as feedstocks for the polymer industry (the rest being used for production of transportation fuels and heating oil). Analysis of the polystyrene production process reveals an energy requirement that is equivalent to the consumption of 1 kg of fossil oil for each kilogram of polystyrene6. In other words, the synthesis of 1 kg of polystyrene requires a total of 2.26 kg of fossil oil (1 kg of fossil fuel to generate energy and 1.26 kg of fossil fuel to serve as feedstocks for polymer production; see Table 1 below footnote).
Production of PHAs, on the other hand, is based on corn, which is one of the more energy-intensive agricultural crops. Corn production in the US accounts for about 44%, 44%, and 55% of total fertilizer, insecticide, and herbicide use, respectively9. Several researchers have attempted to quantify the energy required to till, irrigate, fertilize, and harvest corn, estimating that the process consumes between 2,226 and 6,722 kJ per kilogram of corn grain9-11. By incorporating primary energy usage patterns of US farms, this translates into the combustion of between 50 and 160 kg of fossil fuels per ton of corn grain12.
Subsequent processing of the corn also involves energy expenditure. On transfer to a wet mill, it must be fractionated and processed to yield gluten meal, oil, starch, and sugars (mainly glucose and fructose). In 1991, the average energy needed to process 1 kg of corn grain was about 4,375 kJ (see Table 2), which equates to the combustion of 130 kg of fossil fuels per ton of processed grain13.
Several fermentation-based PHA processes have been described14-16. Although they differ in the choice of microorganism and other process characteristics, all share the following features: they can yield very high PHA concentrations, commonly above 100 g/L; biological polymer synthesis is an exothermic process and therefore requires external cooling; 2.4 kg of carbon dioxide are emitted per kilogram of polymer produced during the fermentation; all processes are aerobic and therefore require considerable aeration and agitation; cell wall disruption is required to release the polymer from the microorganism; between 0.15 and 0.3 kg of biological waste are generated per kilogram of polymer; and approximately 3.33 kg of glucose are required to produce 1 kg of PHAs (see Table 1).
Using an optimistic high-cell-density PHA fermentation as a basis for analysis, a 48 h culture would produce an impressive 190 g/L biomass, of which 150 g/L would be PHA (79% polymer content). Assuming recovery of 100% of the polymer produced in the fermentation, this would require 2.39 kg of fossil resources (gas, oil, and coal) to produce 1 kg of PHAs.
The hard facts
As the analysis above indicates, the amount of fossil fuel (2.39 kg) required to produce 1 kg of PHAs exceeds that (2.26 kg) required to produce an equal amount of polystyrene (see Tables 3 and 4). While the consumption of fossil resources does not differ greatly between the two processes, the emissions reveal a discouraging fact: PHA production requires the combustion of the entire 2.39 kg for energy production, whereas polystyrene production combusts only 1 kg of the 2.26 kg fossil fuel required for its manufacture, the balance being used as a feedstock.
The energy consumption estimates used in the analysis above are very conservative and far below those of other researchers who have assigned energy requirements to PHA fermentation processes15. In fact, van Wegen and co-workers15, who analyzed the fermentation process alone and assigned energy values that were 57% and 467% greater than those used in the present analysis for electricity and steam, respectively. If their values were used, the net effect would be fairly drastic, resulting in an overall fossil fuel consumption of 3.73 kg per kilogram of PHAs.
Conclusions
For the particular system studied, the replacement of conventional polymers with fermentation-derived PHAs does not appear to be a useful approach if a sustainable production of polymers is the desired outcome. Other benefits such as the biodegradability and biocompatibility of PHAs could justify the expense of considerable fossil resources; however, those benefits would have to be quantified and evaluated separately. For example, the usefulness of biodegradability itself has been put into question because the degradation of biodegradable materials, such as paper in landfills, is not only slow17, but also results in greenhouse gas emissions (methane and carbon dioxide) as well as leachates with increased biological oxygen demand. Proper incineration, on the other hand, produces less harmful greenhouse gases and, more importantly, allows the partial recovery of energy expended during manufacture.
The production of PHAs using corn as a feedstock with current fermentation technology is thus of questionable environmental benefit, even under rather favorable assumptions. Although biological processes that use renewable resources certainly have the potential to conserve fossil resources, this case study demonstrates that such an approach can also have the reverse effect. Therefore, future assessments of biological processes must not only incorporate the use of raw materials (which are mostly renewable), but also address the indirect consumption of nonrenewable energy sources required for the process.
REFERENCES
Tables
Table 1: Direct raw material requirements for the production of polystyrene versus PHA.
Raw material requirements in (per kilogram polymer) Item Polystyrene PHA Glucose none 3.33 kilogram a petroleum fractions 1.78 kg b none inorganic assaults 20 g b 149 g c water 4 L b 26 L
a The yield of PHA on glucose and bacterial
fermentations is 30 percent.
b Petroleum fractions serve as a feedstock and as an energy source in the
manufacture of polystyrene. The production of one kilogram of
polystyrene requires a total of 2.26 kilograms of fossil fuels, of which 0.48
kilograms can be directly assigned to the production of steam and
electricity. Of the remaining 1.78 kilograms, which serve as feedstock's,
a fraction is recycled and also used as an energy source. For
details see references 5, 6.
c Inorganic salts are required for the fermentation process in the form
of potassium phosphate, sodium phosphate, ammonium, phosphoric acid,
and trace elements 18. In subsequent energy analysis, only the energy to
produce ammonia is taken into account (which is about 109 g/kg polymer).
d Water is required to fill the fermenter and for four washes. It
does not include cooling water, which is expected to be recycled through a
chiller or a cooling tower.
Table 2: Energy required to produce raw materials for the production of PHA.
Item energy (kJ) required requirement (kJ/per kg of raw material) Glucose 8129 a Inorganic salts 30,135 b Water 5.6 c
a Energy to produce 1.52 kilograms of corn
grain (60 percent starch) and process the grain to yield one kilogram of glucose
minus energy for coproducts (33 percent of corn milling energy).
Calculation: 1.52 kilograms x 2442 kJ/kg (reference 11) + 4417 kJ/kg (corn what
milling) = 8,129 kJ. The entire corn what milling industry consumed 147
trillion kJ of energy by combusting externally produced fossil fuels or burning
external electrical power 13. In the same year, a total of 33.66 million
tons of corn grain or processed 19. Thus, the average energy input can be
estimated to be about 4375 kJ/kg processed corn grain, and therefore the
production of 1 kg of dextrose from 1.52 kg of corn grain (starch content 60
percent) consumes about 6650 kJ (reference of a 20). Of that energy, 33
percent (2194.5 kJ) is reallocated to production coproducts (corn meal, etc.)
leaving 4417 kJ/kg for glucose. In addition, to engineering firms (Nofsinger,
Kansas City, MO, and process systems, Memphis, TN) active in the construction of
corn what males are contacted to confirm these estimates. We obtained
values ranging from 3718 kJ to 5631 kJ for the production of one kilogram of
glucose from corn grain.
b Energy to produce 1 kg ammonia, which is used in the fermentation to
control pH and provide nitrogen 21.
c Energy to filter and palm water to point of use 22.
Table 3: Energy requirement for fermentative production of PHA from glucose.
Energy Requirement (per Kg PHA) Item Electricity (KWh) Steam (Kg) Fossil Fuel Equivalent a (Kg) Fermentation Media Sterilization b None 0.45 0.02 Agitation c 0.32 None 0.09 Aeration c 1.27 None 0.35 Cooling d 0.76 None 0.21 DOWNSTREAM Centrifugation and Washing e 0.50 None 0.14 High-Pressure Homogenization f 1.97 None 0.54 Centrifugation and Washing e 0.50 None 0.14 Evaporation g None 0.33 0.02 Spray Drying None 2.00 0.10 Total 5.32 2.78 1.59
a Amount of fossil fuel (kg) consumes to
generate electricity and steam listed in the same row. In 1997, the U.S.
average for producing 1 kWh
electrical power, from all power sources (including geothermal, hydroelectric,
nuclear, and alternative power generation), required to direct
combustion of 0.272 kilograms of fossil fuel resources (83 percent coal, 13
percent natural gas, and 3.5 percent petroleum) 8. Thus, the
conversion of electricity to fossil fuel equivalent was carried out as follows:
electrical energy (kWh) x 0.272 (kg/kWh) = fossil fuel equivalent
(kg). The conversion of steam to fossil fuel equivalent was as follows:
steam (kg) x heat of evaporation of water (2400 kilojoules/kg)/heat of
combustion of natural gas (47,219 kilojoules/kg) = fossil fuel equivalent
(kg). Calculation assumes 100 percent efficiency, use of natural gas
for steam generation, and no heat loss.
b Medium is continuously sterilized to 143 degrees centigrade for 30
seconds; 68 percent of energy is recaptured through a heat exchange or an
used to prewarm the incoming medium 23.
c Agitation and aeration for this type of aerobic fermentation with very
high cell densities is estimated to require 5 W/L of power input. Power,
delivered by mechanical agitation and compressed air, and kWh/kg of PHA 24.
d Cooling assumes the use of a fully jacketed 114,000 L fermenter (type
to diameter ratio of 3:1) that has additional cooling coils on the inside,
providing a total cooling area of 266 m2 or 2.32 square meters per cubic
meters. Under the assumed production schedule (a 48-hour culture
that yields 190 g per liter of biomass), approximately 17.6 W./L. or a total of
2 millionJ/s (reference 23) must be removed. Coolant is provided
through cooling towers and chillers (coefficient of performance of 5.0), which
deliver about 48,700 kilojoules and 18,000 kilojoules of
cooling per kilowatt-hour input, respectively 22.
e For centrifugation, C. reference 15. Energy input was reduced by
50 percent to reflect the energy savings from our higher solids content.
f For cell disruption, C. reference 15.
g Evaporation is performed with a triple-effect evaporator. A
preconcentrated slurry containing 30 percent solids is concentrated to 50
percent
solids.
h The final polymer slurry is spray dried to yield a powder of the
polymer. The energy required for spray drying is generally about twice the
amount of the water that has to be evaporated from the slurry. Thus, about
2 kilograms of steam are required to remove one kilogram of water,
leading one kilogram of dry polymer powder.
Table 4: Energy and fossil fuel equivalents (FFEs) required in the production of polystyrene versus PHA.
Energy and FFEa (per kg polymer) Item Polystyrene PHA energy FFE energy FFEa production of raw materials see below b 1.78 kg b 31,218 kJ c 0.80 kg UTILITIES steam d 7.0 kg 0.4 kg 2.78 kg 0.14 kg electricity e 0.30 kWh 0.08 kg 5.32 kWh 1.45 kg TOTAL 2.26 kg 2.39 kg
a Fossil fuels required to produce the
energy and raw material in the corresponding energy column, taking into account
the primary fuel usage patterns for each industry. For corn wet milling,
energy is generated from natural gas (37 percent), coal (48 percent), and
petroleum (10 percent) 11. For each megajoule of energy currently consumed
by the corn wet milling industry, 370,000 kilojoules are generated by combusting
7.84 kg of natural gas, etc. Petroleum, coal, and gas had a heat of
combustion of 47,000 kilojoules per kg, 25,788 kilojoules per kg, and 47,218
kilojoules per kg, respectively. For other conversions, see table 3.
b Feedstock required to produce 1 kg of polystyrene from crude oil.
The feedstock for direct polymer synthesis is included, as well as a fraction
that is lost in the process and recycled for energy generation. As this
type of process does not allow a clear allocation of feedstocks, energy numbers
include both feedstock and process energy. The analysis does not, however,
include the itemized steam and electricity energy
listed below (see also reference 6).
c Energy including steam and electrical power) to produce glucose,
ammonia, and water. Calculations: 3.33 (yield of PHA on glucose) x 8128
kilojoules (see table 2) + 4028 kilojoules (energy to produce 0.109 kg of
ammonia) + 120 kilojoules (energy to provide 26 L of process water) = 31,218
kilojoules.
d The production of one kilogram of steam requires the combustion of
0.058 kg of residual fuel oil during PS production. Fermentation for PHA
most likely will use natural gas for steam generation and therefore requires
only 0.0508 kg of gas to produce the same amount of steam.
e For conversion of kWh to FFE, see table 3
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