CHAPTER 2: Literature review
2.6 Attached growth sulfidogenic bioreactors, bioprecipitate characterization
2.7.2 Membrane based separation for purification of metal sulfide
Membrane separation techniques such as microfiltration (MF) and ultrafiltration (UF) are the most effective methods to overcome the limitations of conventional clarification process. The membrane process has many advantages, such as high yield, fewer processing steps, etc. In addition, they involve less working time, no use of clarifying agents, easy cleaning and maintenance of equipment, and reduction in waste byproducts (Estay et al., 2021). Also, membrane processes are characterized by their high efficiency, simple equipment, and low energy
consumption (Li et al., 2010). Microfiltration is used to filtrate particles ranging from 0.1 to 10 µm and operate at low-pressure conditions. Ultrafiltration operates at higher pressure (2-10 bar) and can separate molecules higher than the molecular weight of 1-1000 kDa (Cassano et al., 2014).
The drawback associated with membrane filtration is the retention of some particles leading to their accumulation on the membrane surface, causing membrane fouling, which results in the reduction of permeate flux. The flow reduction causes loss in efficiency, increase operational costs, and demands maintenance (Saad et al., 2014).
2.7.2.1 Modes of operation in a membrane separation process
There are different operation modes in membrane processes. Fig. 2.4 represents the different modes of operation.
2.7.2.1.1 Dead-end filtration mode
This is the primary mode of membrane separation operation. The feed is forced to pass through the membrane using external pressure, and the filtered matter is accumulated on the top of the membrane surface. This mode of operation is called a batch process because filtered material accumulates on the membrane surface and clogs the pores, ultimately reducing the membrane efficiency and necessitates cleaning of the membrane surface. Nevertheless, dead-end filtration is a highly useful method for concentrating solids (Vasanth et al., 2011).
2.7.2.1.2 Cross-flow filtration mode
The technique is called "cross-flow" because the feed flow is perpendicular to the filtration flow direction. In this mode of operation, primarily tubular configuration of membranes is used. The high feed through the membrane surface act as a driving force to filter the feed, and a high flow rate creates high turbulent conditions. The higher pressure-driven feed flow restrains filtered
particles to retain on the surface of the membrane. The cross-flow mode is generally used for the feed having high filterable matter (Estay et al., 2021).
2.7.2.1.3 Hybrid-flow filtration mode
Hybrid-flow filtration mode combines both dead-end and cross-flow modes, and it consists of two phases: production and flushing. The production phase involves dead-end mode of operation, and the flushing phase consists of cross-flow mode. In flushing mode, both ends of the tubes are open, and the part of the feed that doesn’t pass through the membrane is eliminated to clean the membrane surface. This type of filtration mode is generally used for feed with a high concentration of suspended matter.
Fig. 2.4 Various modes of membrane separation operation (a) dead end (b) cross-flow (c-d) hybrid.
2.7.2.2 Membrane Modules
A membrane module is a way the membrane is incorporated into a filtration device for filtration.
The following modules are commonly used in industries:
Flat sheet modules
Tubular modules
Hollow fiber modules
These are the module design developed by manufacturers for various applications based on the type and characteristics of feed. The flat sheet membranes resemble filter paper. Similarly, tubular membrane resembles the single hollow tube of circular cross-section, in which the wall of the tube works as the membrane. Hollow fiber membranes also look like tubular membranes, but their diameter is very small compared to tubular membranes. The hollow fiber membranes have a typical diameter of 1 mm (Cui and Muralidhara 2010).
2.7.2.3 Membrane materials
The two most common membrane materials used to prepare symmetric membranes for industrial applications are polymeric and ceramic materials. The asymmetric membrane is prepared from symmetric polymeric and symmetric ceramic membrane material.
2.7.2.3.1 Polymeric membranes
Polymeric membranes have a thin film-like structure with a thickness of 10-100 μm. Various types of polymers are used to prepare the polymeric membrane, such as polyacrylonitrile (PAN), polyamide (PA), polyetherimide (PEI), cellulose acetate (CA), polypropylene (PP), polysulphone (PSU), etc. Table 2.10 presents advantages and disadvantages of polymeric membrane.
Table 2.10 Advantages and disadvantages of polymeric membrane
Advantages Disadvantages
Different pore size ranges are available Low life span
Low cost Low solvent resistance
Easy to fabricate Low corrosive resistance
Easy to scale up Temperature sensitive
2.7.2.3.2 Ceramic membranes
Ceramic membranes are made from inorganic materials such as kaolin, alumina, zirconia, silica, titania, etc. The ceramic membrane provides good thermal, chemical, and mechanical stability.
The pore diameter of commercially available ceramic membrane ranges from 10 nm to 10 μm, and thickness ranges from 2 to 5 mm depending on the type of application. Advantages and disadvantages of ceramic membrane are presented in Table 2.11.
Table 2.11 Advantages and disadvantages of ceramic membrane
Advantages Disadvantages
High corrosive resistance Brittle in nature High temperature resistance High cost
High solvent resistance Most ceramic membranes are available in pore diameters within the MF and UF range (0.010 – 10 μm)
High life span
High mechanical strength
2.7.2.4 Polymer vs. Ceramic membranes
Comparing the advantages and disadvantages of polymeric and ceramic membranes, polymeric membrane seem to be more useful for lab-scale application. For industrial applications, cost and life span of the membrane are the two most important factors that need to be considered along with their efficiency. The life span of polymeric membranes varies from 12 to 18 months, and that of ceramic membranes is ten years (Mulder 1991). Therefore, life span of ceramic membranes are on the higher side if we consider their industrial application. Though the separation characteristics of ceramic membranes are similar to the polymeric membrane, they are not widely used in industries due to their higher costs. So the development of low-cost membranes with an increased life span is anticipated to drive the economic competitiveness of ceramic membranes in industries. The membrane filtration process has not been applied for filtration of metal sulfide precipitate formed by biogenic sulfate reduction despite the fact that the membrane filtration process has many advantages over conventional clarification processes.
From the detailed literature review, it is clear that the main focus has been given to treat heavy metal and sulfate-containing metallic wastewater and recovery of heavy metals in the nanopowder form employing IFBR. Although few literature reports are available for metal recovery using sulfidogenic reactors, the influence of specific process parameters such as influent with low pH, hydraulic retention time, low/high metal loading, etc., on metal recovery using IFBR has not been reported so far. Moreover, detailed characterization of metal bioprecipitates obtained from sulfidogenic reactors treating wastewaters for potential industrial application is limited. Also, purification of metal sulfide bioprecipitate recovered from IFBR using indigenous low-cost ceramic membrane has not been reported in the literature. Furthermore, application potential of metal sulfide nanoparticles recovered by sulfate reduction process need to be examined.