Chapter 4: A novel ceramic membrane assembly for the separation of
4.1 Introduction
Polyhydroxybutyrate (PHB) is a green polymer, and unlike petroleum-based polymer, it is of natural origin, particularly from microorganisms (Raza et al., 2018). Properties of PHB are similar to that of petroleum-based plastics, and therefore it finds numerous applications in various industrial fields. Owing to its biodegradable, biocompatible, thermoplastic and nontoxic nature, it serves as the best alternative to the petroleum-derived polymers (Khanna and Srivastava, 2005; Raza et al., 2018). During fermentation and under nitrogen-limited conditions, the excess carbon source is converted to PHB by certain microorganisms. And these are stored as insoluble inclusion bodies in the cytoplasm of these microorganisms.
Some microorganisms are capable of producing PHB, which includes Ralstonia eutropha and Cupriavidus necator. Fermentative PHB production is a two-stage process: with the individual growth and PHB accumulation inside as the first phase or upstream processing in PHB production. During the second phase or the downstream processing, the biomass produced during fermentation is separated from the culture broth and processed further to extract PHB out of it (Aramvash et al., 2015; Khanna and Srivastava, 2005; Raza et al., 2018). The initial separation of biomass from the culture broth during the downstream processing phase is very crucial for PHB production. This is mainly because the R.
eutropha, a typical rod-shaped PHB producing bacteria, is small in size (0.4 x 0.7 μm), and it poses a significant problem during its separation. Furthermore, in relation to culture volume, the microbial biomass produced is very less. Consequently, a large volume of culture broth needs to be processed in order to recover the whole (intact) biomass from the broth (Elcik et al., 2016). In general, 20-30% of the total biomass production cost is utilized for the separation of biomass from the culture broth (Pragya et al., 2013).
In this scenario, membrane separation seems to be more attractive and suitable for industrial applications, particularly in the field of biotechnology (Piry et al., 2008; Pragya
et al., 2013; Roshanida et al., 2018; Vasanth et al., 2011). By integrating membrane with a bioreactor, separation of microbial biomass from culture broth can be carried out under batch or continuous mode based on the operation mode of the bioreactor. Microfiltration is generally employed for the separation of microorganisms from the liquid as the pore size of these membranes is in the range of 0.1 - 10 μm, which matches well with the size range of microorganisms (Piry et al., 2008; Vasanth et al., 2011). In microfiltration, cross-flow filtration is mostly preferred, because the flow in this mode is parallel to the membrane and perpendicular to the permeate, thereby reducing the membrane fouling (Piry et al., 2008).
Membrane filtration offers numerous advantages over the conventional separation processes, such as sedimentation, filtration, flocculation-flotation and centrifugation processes. Some of these advantages are isothermal operation, zero addition of chemical agents. Furthermore, the microfiltration process is regarded as safe for shear-sensitive bacterial cells with intracellular products (Kumar et al., 2016; Piry et al., 2012; Pragya et al., 2013). Membrane separation is also relatively cheaper in comparison with centrifugation. This is because the major cost, including capital cost, operation and maintenance cost and depreciation cost, associated with centrifugation is reduced in cross- flow microfiltration just with membrane replacement and pumping cost (Barros et al., 2015;
Pragya et al., 2013; Zhao et al., 2017). This has led to great interest among researchers and industrialists to develop a novel low cost as well as porous ceramic membranes exhibiting good filtration performance with excellent chemical and thermal stability, good mechanical strength and extended period of use without any environment pollution (Issaoui et al., 2017). Most of the research is focused on the development of ceramic membranes with starting material such as alumina, titania, silicon carbide, cordierite, and zirconia. However, the cost of these starting materials is very high, which increases the final cost of the prepared membranes (Issaoui et al., 2017; Medjemem et al., 2016; Vinoth Kumar et al.,
2015). In order to address the cost aspect, recent research interest is to explore low-cost starting materials for membrane preparation and its performance evaluation (Issaoui et al., 2017). Some of these materials include natural raw clay, dolomite, apatite powder, bauxite, and kaolin (Issaoui et al., 2017). Among these starting materials, kaolin seems to be a cost- effective and suitable material that is available abundantly in the earth (Emani et al., 2014;
Issaoui et al., 2017; Vasanth et al., 2011; Vinoth Kumar et al., 2015). Previously, ceramic membrane prepared using a mixture of kaolin, feldspar, pyrophyllite, quartz, ball clay, and calcium carbonate was reported with an estimated cost of 69 $/m2 (Vinoth Kumar et al., 2015). Therefore, this study was focused on the application of the low-cost ceramic membrane for bioseparation of PHB rich biomass from the culture broth. This is mainly because an increase in membrane resistance due to fouling is a serious issue with membrane separation processes. Several studies have been carried out in the literature to address this problem by either varying the pore size and porosity of such membranes (Dizge et al., 2011;
Pragya et al., 2013), or changing the pore shape/geometry from circular to rectangular pore (Bromley et al., 2002), or the length of the membrane (Piry et al., 2012, 2008). In the present study, water and broth flux along with the biomass and PHB recovery were examined using single, double and four membrane assembly as a function of applied pressure. To the best of our knowledge there is no previous literature available on application of ceramic membranes for PHB recovery and there is no study is reported on enhancing the flux by increasing the number of membranes using a modified membrane assembly. Therefore, this is the first study which reported the application of ceramic membranes for biomass and PHB recovery from the culture broth in a novel tubular ceramic membrane assembly.
Furthermore, the resistance due to membrane was overcome without any change in pore size or pore geometry or length of the membrane, but with a change in the number of membranes.
4.2. Materials and methods