A continuous stirred tank bioreactor produces 48 kg lysine.day-1. If the volumetric productivity is 2 g.l-1.h-1, the volume of the reactor is:
Answers
Answer:
Microbial polyhydroxyalkanoates (PHAs) are promising biodegradable polymers that may alleviate some of the environmental burden of petroleum-derived polymers. The requirements for carbon substrates and energy for bioreactor operations are major factors contributing to the high production costs and environmental impact of PHAs. Improving the process productivity is an important aspect of cost reduction, which has been attempted using a variety of fed-batch, continuous, and semi-continuous bioreactor systems, with variable results. The purpose of this review is to summarize the bioreactor operations targeting high PHA productivity using pure cultures. The highest volumetric PHA productivity was reported more than 20 years ago for poly(3-hydroxybutryate) (PHB) production from sucrose (5.1 g L−1 h−1). In the time since, similar results have not been achieved on a scale of more than 100 L. More recently, a number fed-batch and semi-continuous (cyclic) bioreactor operation strategies have reported reasonably high productivities (1 g L−1 h−1 to 2 g L−1 h−1) under more realistic conditions for pilot or industrial-scale production, including the utilization of lower-cost waste carbon substrates and atmospheric air as the aeration medium, as well as cultivation under non-sterile conditions. Little development has occurred in the area of fully continuously fed bioreactor systems over the last eight years.
Explanation:
The production of completely biodegradable materials from renewable resources is an urgent challenge for the mitigation of the cradle-to-grave environmental impacts associated with petroleum-based plastics. In 2015, the worldwide production of synthetic plastic materials exceeded 320 million tonnes [1], and at current consumption rates, it is estimated to reach 33 billion tonnes by 2050 [2]. Of the amount currently produced, less than half is recycled or landfilled [2]. An astonishing amount of these materials are unaccounted for, and persist in natural ecosystems due to their high molecular weight and recalcitrance to biodegradation [3,4,5], which can have multiple detrimental impacts on both terrestrial and aquatic ecosystems. It is estimated that 10 to 20 million tonnes of plastic material is discarded into oceans alone [6]. Once in the natural environment, plastics can absorb and concentrate persistent organic pollutants [7], and release toxic additives that are not chemically bound in the polymer [8,9]. Through mechanical abrasion, plastics are broken down into microsized to nanosized particles that bioaccumulate in filter-feeding animals, posing a significant threat to marine ecosystems [6]. Recent evidence suggests that particles <10 μm may pose significant risks to human health through inhalation and the penetration of pulmonary surfaces [10,11].
Microbial polyhydroxyalkanoates (PHAs) are a class of bio-based and biodegradable polymers that may help displace certain applications for traditional plastic materials [12,13,14]. PHAs are intracellular carbon and energy storage polymers that are synthesized (generally under growth-limiting conditions) by more than 300 species of Gram-positive and Gram-negative bacteria [13,15,16], as well as several species of archaea [17]. These R-configuration hydroxyalkanoic acids have properties that are often compared to polyethylene or polypropylene [18,19,20], but can be completely broken down to water, biomass, and CO2 through the enzymatic activity of microorganisms [21]. On the basis of the length of the monomer subunits, PHAs are generally classified as either short-chain length (scl), or medium-chain length (mcl). The scl-PHAs have monomer subunits with carbon chain lengths of three to five carbon atoms. To date, the most common and best characterized PHAs are poly(3-hydroxybutryate) (PHB) and copolymers of PHB with poly(3-hydroxyvalerate) (PHBV) [19,22]. In contrast, mcl-PHAs have carbon chain lengths of six to 14 carbon atoms, and account for 95% of the 150 identified PHA monomers, implying the potential for many polymers with different properties and a diversity of applications [23,24]. For the remainder of this document, all of the abbreviations referring to the PHA composition will imply the 3-hydroxy form, whereas the 4-hydroxy form will be specified explicitly (i.e., P(4-HB)).