Although not strictly related to the production of succinate by anaerobic microorganisms, the downstream processing of this compound ultimately is of paramount importance for any industrial process, and could indeed influence the selection of microorganism. A few processes have been studied for the recovery of succinate from fermentation media, and each of these has its unique advantages and disadvantages. In general, processes developed heretofor have focused on the characteristic of charge that distinguishes succinate from many other components in fermentation media.

One method to recover succinate is by extraction, and recent articles describe extraction of various organic acids (139-146). Extraction processes for succinate and other negatively charged solutes usually involve the transfer of the solute into an organic phase by the use of a positively charged extractant, such as long-chain tertiary amines. The selectivity of separation is very good with amine extractants, because they have favorable equilibration chemistry with deprotonated acids (140,141). A potential problem is water coextraction (142), which reduces the extraction yield of the acid. Of course, the succinate-amine complex in an envisioned extraction process must still be reextracted (stripped) from the organic phase, a process which might, for example, involve a strong base. An advantage of the process is that extraction is a mature process, with numerous designs and devices available to implement the process. A potential problem is the toxicity of the amine extractant if the extraction is carried out in situ. If a water-soluble volatile tertiary amine such as trimethylamine is used to back-extract the succinate, the solution may be partially evaporated to produce the acid product in crystalline form (143).

Another recovery method is adsorption, whereby an ion exchange resin adsorbs the negatively charged ions from the fermentation media (145,146). To avoid clogging the adsorption bed with cells, a membrane separation is required. By the appropriate selection of ion exchange resin, the succinate along with other organic acids can very effectively be removed from the fermentation media. The next step in the process is removal of the succinate from the ion exchange column and regeneration of the ion exchange material. This process may require the use of both a strong base to strip the succinate and a strong acid both to convert the succinate into succinic acid and to regenerate the column. Alternatively, the acid can be back — extracted into a trimethylamine solution, as described in the previous paragraph.

A related, general extractive means to recover carboxylic acids from fermentation media is through the use of liquid membranes (147), and an example is the recovery of lactic acid (148). In supported liquid membranes, a porous polymeric membrane impregnated with the extractant is situated between the feed and the stripping solution. Often additional compounds such as surfactants are added to enhance the transport rate of the extracted species. Liquid membranes can suffer from instability, relatively high cost and comigration of other components from the fermentation media.

A unique method to recover succinic acid is by electrodialysis. This process has been the subject of patents (84,85,149) and a review article (150). Transportation of other mobile ions (and cells if the media is not ultrafiltered) in the fermentation media such as sodium, potassium, chloride, phosphate, sulfate, etc. can pose a problem for the efficiency of the electrodialysis. Essentially the method utilizes electric charge first by "desalting" electrodialysis to concentrate sodium succinate, and then by "water-splitting" electrodialysis to remove preferentially sodium and hydroxide ions from the salt stream yielding a precipitated succinic acid product. When electrodialysis was applied to a succinate fermentation by A. succiniproducens, a solid product of 99.9% purity was obtained (150).