Process intensification by industrial brine treatment with extreme halophiles in laboratory scale

Abstract

Highly saline industrial fluxes containing organic contaminants occur in large amounts for example during leather or food processing. The untreated release of this brine can not only cause severe environmental damage but is also a great waste of salt and water as valuable resources. Process intensification can be established by recycling these brine streams, however physico-chemical treatments lack efficiency in eliminating the harmful organic contaminants under hypersaline conditions, which is mandatory in terms of brine reuse. Thus, biological systems are required to remove bespoken organic matter. In recent years, halophilic Archaea have attracted a lot of attention in this respect, as they are able to inhabit these extreme environments and metabolise a big variety of organic substrates. However they show low biological activity due to low growth rates, which is an issue for industrial feasibility where maximizing volumetric productivity as well as minimizing organic contaminants is essential. This study aimed at developing an efficient and robust continuous bioprocess for industrial brine treatment with the extreme halophilic Archaeon Haloferax mediterranei (HFX) at laboratory scale. Investigations on an external microfiltration cell retention system with a feed and bleed strategy were conducted to increase volumetric productivity and ensure process controllability. The external cell retention loop was evaluated and optimized in terms of oxygen limitation and damage caused by shear stress. Upstream processing was followed by ultrafiltration to remove and potentially identify halophilic macromolecules. These molecules are relicts from biological treatment and contribute to the undesired Total Organic Carbon (TOC). Complementary studies to serve industrial needs in terms of antifoam and inoculation issues were also addressed. Summarizing, volumetric productivity was increased 10-fold by employing a cell retention system compared to standard continuous process cultivation. Quantification and balancing approaches, moreover demonstrate controllability of the process under these optimized condition. It was further shown that oxygen limitation caused higher protein release while at higher cross flow rates shear stress affected the excretion of DNA related compounds. Hence operating the system at a physiological optimum was found to be essential in order to establish low TOC levels as well as robust process conditions. Via ultrafiltration the TOC level could be reduced indicating the successful removal of extracellular polysaccharides (EPS) and proteins as well as large DNA fragments. For industrial implementation and upscale purposes an inoculation strategy for shake flask volumes up to 1 L was developed. Furthermore, HFX precultures proved to be very robust, allowing advantageous flexibility for inoculation. To counteract foaming AF agent Struktol J673 turned out to be suitable for brine treatment. The results of these studies contribute in a pioneering way on the bioprocess development of extreme halophilic Archaea by optimizing laboratory scale processes regarding robustness, scalability and long-term strategies. Furthermore this work establishes a basis for process intensification, as a result of waste to value conversion, by allowing the creation of linked system between chemical processes recycling highly saline industrial fluxes.