Non-Energetic Chemical Products by Fermentation of Hydrolyzed Sewage Sludge

Hydrolysis and the solubilization of sewage sludge processes are important tools to obtain small and medium molecules with different application perspectives. Although the production of biomethane and other products such as biohydrogen from sludge as biofuel alternatives has been profusely studied, the current perspectives are mainly focused on the use of the sludge hydrolysate to produce non-energy bioproducts and biomaterials. In this review, the most recent bibliography dealing with the use of sludge hydrolysates as fermentation media for the bioproduction of new non-energetic products with industrial interest is here revised and discussed. In this regard, the main research effort has been focused on the bioproduction of short-chain fatty acids due to their direct use in industrial applications or as a carbon source for polyhydroxyalkanoates-producing microorganisms. The use of sludge hydrolysates as fermentation media using pure cultures to produce more complex biomolecules, such as enzymes or lipids, is gaining interest, but it remains an undervalued topic. The literature has been divided into processes where hydrolysis and fermentation stages took place simultaneously or separately, centering them on the effect of the main operational conditions on the yields and properties of the corresponding metabolites produced. In general, the main limiting step of these kinds of processes is the proper solubilization and hydrolysis of the sludge, which can improve the bioassimilation of nutrients and, subsequently, the productivities and compositions of the metabolites obtained. Biological and/or thermal pretreaments are the options more profusely employed, which are frequently assisted by different promoters such as oxidants, surfactants, or cation exchange resins.