Biomass separation filtration processes

The final system, shown in Figure 30.4D, is the continuous system with a partial (PRF) or complete (RF) cell recycle. It is similar to the continuous system, but cells are returned to the fermentor by means of a biomass separation device. Cross-filtration units, centrifuges, and settling tanks have all been used for biomass separation.22 In the partial cell recycle fermentor, a steady state is achieved as in the continuous system. This process is typically used to increase the productivity of the system and is used commonly in wastewater treatment and ethanol production type applications. 

Most filtration processes used in large-scale biotechnology should not have the potential to generate microbial aerosols. Filtration columns normally use gravitational forces to separate products from impurities and so are low energy processes. The only potential problem can come in either rotary vacuum filtration or with filter presses if a violent method of biomass removal from the filter is used. Wickramanayake has reported that it is often the practice with filter presses to knock the cell mat of the filter with hammers. This practice has been shown to generate microbial aerosols. 

More than 10 years operating experience with full-scale MBRs has shown that the use of membranes in the activated sludge process requires an adaptation of the overall process— consisting of (1) pretreatment, (2) aeration tank, and (3) filtration—to the specific strengths and weaknesses of membranes for biomass separation. As the specific measmes required may differ with the module design, the various options will be discussed in general in the following sections. 

Lime-Sulfuric. Recovery of citric acid by calcium salt precipitation is shown in Figure 3. Although the chemistry is straightforward, the engineering principles, separation techniques, and unit operations employed result in a complex commercial process. The fermentation broth, which has been separated from the insoluble biomass, is treated with a calcium hydroxide (lime) slurry to precipitate calcium citrate. After sufficient reaction time, the calcium citrate slurry is filtered and the filter cake washed free of soluble impurities. The clean calcium citrate cake is reslurried and acidified with sulfuric acid, converting the calcium citrate to soluble citric acid and insoluble calcium sulfate. Both the calcium citrate and calcium sulfate reactions are generally performed in agitated reaction vessels made of 316 stainless steel and filtered on commercially available filtration equipment. 

The first case covers for example flue-gas treatment, which requires the filtration of fly-ash and the reduction of NOx, or gasification processes, where particulates and high-boiling tars have to be removed. An example of the second case is that of combustion processes, where incomplete combustion leads to the emission of carbonaceous particulates. The most relevant topic in this category is the reduction of diesel particulate emissions ( diesel soot ) by catalytic filtration. A more exotic example is the reaction cyclone for the thermal conversion of biomass, which also combines chemical reactions and separation in one apparatus, though its separation mechanism is not filtration. 

After the process of fermentation is over, the exhausted bacteria can be separated from the broth by filtration. This cell mass has a number of names, such as microbial biomass or single cell protein (SCP). Microbial biomass is a side product of all fermentation processes but in some cases it is actually the sole target product. Bacterial cells have a high content of protein, but are low in fat and cholesterol. This explains the names single cell protein (SCP) or microbial protein. SCP is mainly used as an additive in animal feed to enhance protein content. In principle, it is also safe for human food use, but the acceptance has been low until now. 

Yeast separation and concentration. Extraction of ethanol from biomass requires several separation steps, traditionally by centrifuging, sedimentation and cake filtration. First, after the fermentation, the yeast is removed from the fermentation broth and may be recycled. Additionally, after ethanol is stripped from the fermentation broth by steam followed by the removal of solid fractions, the remaining material called the thin stillage can be clarified for reuse upstream as the process water. 

The PAC activated slndge system is a modified activated sludge process. PAC is added to the aeration tank where it is mixed with the biological solids. The mixed liquor solids are settled and separated from the treated effluent in a gravity clarifier. Polyelectrolyte will normally be added prior to the clarification step to enhance solids-liquid separation. If phosphorus removal is necessary, alum is often added at this point. Even with polyelectrolyte addition, tertiary filtration is normally required to reduce the level of effluent suspended solids. The clarifier nnderflow solids are continuously returned to the aeration tank. A portion of the carbon-biomass mixture is wasted periodically to maintain the desired solids inventory in the system. 

The most frequently applied technique for cell disintegration of yeast is autolysis. The cell wall is digested by yeast-bome proteases. The process may be supported by externally added proteolytic enzymes from other sources and/or acid or alkali. Consecutively biomass is separated and the aqueous filtrate further processed. In many cases simple concentration by evaporation and drying is sufficient for the production of the yeast extracts or autolysates. 

The criterion of a good separation is production of a solution of the desired viscosity which does not unacceptably plug formations, without loss of substantial fractions of the polymer in the process. Dlatomaceous earth flltrations have customarily been used to remove plugging constituents. In filtration of fermentation broths, particularly in the field, it is desirable to minimize use of these fllteralds. If the biomass is heavily contaminated with them, possible beneficial use as a cattle-feed supplement may be inhibited, and an expensive waste disposal problem may be incurred a cost adding 20< to 40c/lb of polymer is conceivable. 

Traditionally, the cellular biomass is separated by centrifugation. Ammonium sulfate is then added to the supernatant to precipitate the protein product from the media. This is followed by further centrifugation and dialysis to remove the residual ammonium sulfate from the protein product. Cross-flow filtration (CFF) can replace all of these steps with a significant improvement in recovery and yield. Indeed, CFF appears to offer many advantages over conventional separation processes like centrifugation, vacuum filtration and precipitation/dialysis for this application. 

Recovery of cells can be accomplished using one of several alternative unit operations, such as centrifugation, membrane filtration, or settling. Subscripts are employed in the figure to identify the usual process variables at particular points in the diagram. We employ the recycle ratio (R) to represent the ratio of the volumetric flow rate of the suspension of cells leaving the separation device to the volumetric flow rate of the net product stream. The symbol t / represents the ratio of the concentration of biomass in the recycle stream to that in the effluent from the CSTBR. You may assume that the system is operating at steady state and that the feed is sterile. 

Downstream processing constitutes a key part of the entire PHA production process. After biosynthesis of the polyester and separation of the bacterial biomass (normally by well-established techniques like centrifugation, sedimentation, flocculation or filtration), the process required for PHA recovery from the microbial cell mass must be considered as an important cost factor, especially considering large scale production. Choosing the adequate method for separating PHAs from residual biomass is dependent on several factors the production strain, the required product purity that is determined by the final application of the biopolymer, the... 

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