Application of Microfiltration Membrane in Fermentation Broth Pre-Treatment

Introduction

Biological fermentation products include amino acids, antibiotics, biological enzymes, microbial polysaccharides and probiotic preparations. After fermentation, the broth is turbid, with a large amount of bacterial cells and insoluble residues. The quality of the first solid-liquid separation step directly affects the load of subsequent ultrafiltration, nanofiltration, resin separation and crystallization units.

Traditional separation methods mainly include plate-and-frame filter press, vacuum drum filter and disc centrifuge. Filter press requires frequent cloth replacement and manual slag removal, and the filtrate often contains residual fine bacteria, which easily pollutes the subsequent membrane and resin. Centrifugation has high investment and energy consumption, and it is difficult to separate tiny microbial thalli with low density.

Cross-flow microfiltration membrane technology is widely used as the primary clarification unit for fermentation broth. It realizes continuous automatic separation at normal temperature, reduces the breakage of heat-sensitive active substances, and provides stable and clear feed liquid for the downstream purification process. It has become a standard pre-treatment unit for many biological fermentation factories.

Working Principle of Microfiltration Membrane

Microfiltration belongs to the pressure-driven membrane separation technology, with the largest pore size among common pressure-driven membranes (MF, UF, NF, RO). Its separation mechanism is mainly sieve effect. Under the action of feed pressure, liquid and small soluble substances pass through the membrane pore to form permeate; particles, mycelia and flocs larger than the pore size are trapped on the feed side.

The system adopts cross-flow mode instead of dead-end filtration. The material liquid flows tangentially along the membrane surface, and the shear force generated by the flow continuously scours the membrane surface, reduces the accumulation of intercepted bacteria and sludge, and delays the rapid rise of transmembrane pressure. Common microfiltration membrane materials include PVDF, PES, PP, ceramic and stainless steel. Ceramic and stainless steel microfiltration membranes have excellent chemical resistance and high temperature resistance, suitable for fermentation broth with high viscosity and high solid content. PVDF microfiltration is widely used in biological products with mild cleaning requirements.

Complete Process of Fermentation Broth Microfiltration

Typical process flow: Fermentation broth → pre-filter (coarse filter, remove large agglomerates) → feed tank → feed pump → security filter → cross-flow microfiltration membrane module

Permeate: clear fermentation supernatant, containing target metabolites, sent to subsequent purification Concentrate: mycelium concentrate, can be subjected to wall-breaking extraction, drying or harmless treatment Supporting units: circulating pump, online turbidity detector, TMP monitoring, CIP cleaning system

According to different production purposes, the process can be divided into two modes: full clarification and concentration washing. In the concentration washing mode, adding washing water into the concentrated mycelium liquid can elute the target products adsorbed on the surface of thalli, improve the total yield of the product, which is an obvious advantage compared with a single centrifugation process.

Core Advantages of Microfiltration for Fermentation Broth

First, stable clarification effect. Microfiltration can effectively remove most thalli and suspended solids, and the turbidity of permeate is far lower than that of filter press filtrate, which greatly protects the downstream ultrafiltration and nanofiltration membrane elements and reduces the frequency of cleaning and replacement. Second, continuous automatic operation. The skid can run continuously for a long time, with automatic slag concentration and liquid level interlock, greatly reducing manual operation and labor cost. Third, mild separation condition. It runs at room temperature and low pressure, avoiding the thermal degradation and shear inactivation of biological enzymes and antibiotics. Fourth, flexible material selection. Organic polymer microfiltration membranes have low investment cost; ceramic microfiltration membranes can withstand strong acid, strong alkali and high-temperature cleaning, suitable for difficult-to-handle fermentation broth with high viscosity and easy caking. Fifth, high product recovery rate. Combined with diafiltration, the target product wrapped in mycelium can be washed out, reducing the loss of effective components in the bacterial residue.

Membrane Fouling and Cleaning Control

The main pollutants of microfiltration membrane for fermentation broth are mycelium cake layer, intracellular protein, polysaccharide mucus and culture medium residue. The cake layer formed by thalli is the most common pollution, which will cause flux attenuation quickly. The conventional CIP cleaning sequence:

1. Permeate water backwash and forward flushing to remove loose mycelium cake layer

2. Alkaline cleaning (food-grade sodium hydroxide, matched with surfactant) to decompose protein and polysaccharide organic pollutants

3. Rinse to neutral

4. Acid cleaning (citric acid or dilute nitric acid) to remove inorganic salt scaling from culture medium

5. Pure water rinsing and standby

Backwashing is a special and important operation for microfiltration systems. Regular backwashing reverses the flow direction of the liquid, blows away the thallus deposited on the membrane surface, and is the key to maintain stable flux during continuous operation.

Summary and Prospect

As the first barrier of biological fermentation purification, microfiltration membrane replaces the traditional filter press and serves as the primary clarification unit. It improves the automation level of the workshop, stabilizes the quality of supernatant and increases the product yield. At present, cross-flow microfiltration has been widely used in amino acid fermentation, enzyme preparation, microbial polysaccharide, yeast extract and vaccine fermentation. In the future, with the development of anti-fouling modified membrane materials and intelligent flux control system, microfiltration technology will further reduce cleaning frequency and energy consumption, and become an indispensable core unit in the biological manufacturing industry chain.