Permeation characteristics of volatile fatty acids solution by forward osmosis

Separation of nutrients from SCFAs with a dynamic membrane in a sludge anaerobic fermenter

The complexity and high cost to separate and recover short chain fatty acids (SCFAs), ammonium ions, and phosphates in the sludge fermentation liquid hinder the application of sludge anaerobic fermentation. In this study, an interesting phenomenon was found in a sludge anaerobic fermenter with a dynamic membrane (DM) which could not only enhance SCFAs production but also retain most SCFAs in fermenter. The separation factor of DM for NH3-N/SCFAs and PO43-/SCFAs throughout the DM development were about 40 and 80, respectively. Analysis reveals that rejection of SCFAs by DM could not be simply correlated to molecular weight or membrane pore size. The rejection mechanisms might be dominated by Donnan rejection. In addition, biodegradation in the DM may also have contribution. Findings of this study suggest the potential of DM as an economical technology for nutrients and SCFAs recover.

Recovery of volatile fatty acids using forward osmosis: Influence of solution chemistry, temperature, and membrane orientation

This study investigates the suitability of forward osmosis (FO) for recovery of volatile fatty acids (VFAs) from anaerobic digesters (ADs) and identifies the conditions favorable for commercially viable maximum recovery of VFAs. The recovery efficiency of VFAs is evaluated using a polyamide (PA)-based thin-film composite (TFC) membrane. The pH (3, 5, 7, and 9), temperature (20 °C and 40 °C), and membrane orientation (active-layer [AL]-facing FS and AL facingDS) were changed, and water flux, reverse salt flux (RSF), rejection rate, and concentration factor (CF) were evaluated for five VFAs. The water flux and RSF were higher at a higher pH, temperature and in AL-DS mode. A low rejection rate of 23-36% and a CF of 0.20-1.90 were observed at a pH below the pKa due to the solubility of molecular VFAs, while rejection rates was 80-97% and concentration increase by 1 to 4.8-fold at a pH above the pKa values were achieved due to deprotonation of VFAs and changes in membrane surface charges. With an equal increase in temperature of FS and DS from 20 to 40 °C, the rejection rate decreased by almost 20%. While with a transmembrane temperature change, a decrease in rejection rate of 20% was observed compared with baseline experiments due to decreases in viscosity and high diffusivity. In AL-DS mode, VFAs were rejected at a rate of almost 20% lower than that in AL-FS mode due to internal concentration polarization and membrane properties. These findings provide useful information on the factors that can influence optimal recovery rates of VFAs.

Factors influencing pressure-driven membrane-assisted volatile fatty acids recovery and purification-A review

Volatile fatty acids (VFAs) are building block chemicals that can be produced through bioconversion of organic waste streams via anaerobic digestion as intermediate products. Purified VFAs are applicable in a wide range of industrial applications such as food, textiles, cosmetics, pharmaceuticals etc. production. The present review focuses on VFAs recovery methods and technologies such as adsorption, distillation, extraction, gas stripping, esterification and membrane based techniques etc., while presenting a discussion of their pros and cons. Moreover, a great attention has been given to the recovery of VFAs through membrane filtration as a promising sustainable clarification, fractionation and concentration approach. In this regard, a thorough overview of factors affecting membrane filtration performance for VFAs recovery has been presented. Filtration techniques such as nanofiltration and reverse osmosis have shown to be capable of recovering over 90% of VFAs content from organic effluent steams, proving the direct effect of membrane materials/surface chemistry, pore size and solution pH in recovery success level. Overall, this review presents a new insight into challenges and potentials of membrane filtration for VFAs recovery based on the effects of factors such as operational parameters, membrane properties and effluent characteristics.

Forward Osmosis as Concentration Process: Review of Opportunities and Challenges

In the past few years, osmotic membrane systems, such as forward osmosis (FO), have gained popularity as "soft" concentration processes. FO has unique properties by combining high rejection rate and low fouling propensity and can be operated without significant pressure or temperature gradient, and therefore can be considered as a potential candidate for a broad range of concentration applications where current technologies still suffer from critical limitations. This review extensively compiles and critically assesses recent considerations of FO as a concentration process for applications, including food and beverages, organics value added compounds, water reuse and nutrients recovery, treatment of waste streams and brine management. Specific requirements for the concentration process regarding the evaluation of concentration factor, modules and design and process operation, draw selection and fouling aspects are also described. Encouraging potential is demonstrated to concentrate streams more than 20-fold with high rejection rate of most compounds and preservation of added value products. For applications dealing with highly concentrated or complex streams, FO still features lower propensity to fouling compared to other membranes technologies along with good versatility and robustness. However, further assessments on lab and pilot scales are expected to better define the achievable concentration factor, rejection and effective concentration of valuable compounds and to clearly demonstrate process limitations (such as fouling or clogging) when reaching high concentration rate. Another important consideration is the draw solution selection and its recovery that should be in line with application needs (i.e., food compatible draw for food and beverage applications, high osmotic pressure for brine management, etc.) and be economically competitive.

Separation of Volatile Fatty Acids from Model Anaerobic Effluents Using Various Membrane Technologies

Effluents of anaerobic processes still contain valuable components, among which volatile fatty acids (VFAs) can be regarded and should be recovered and/or used further in applications such as microbial electrochemical technology to generate energy/energy carriers. To accomplish the separation of VFAs from waste liquors, various membrane-based solutions applying different transport mechanisms and traits are available, including pressure-driven nanofiltration (NF) and reverse osmosis (RO) which are capable to clarify, fractionate and concentrate salts and organics. Besides, emerging techniques using a membrane such as forward osmosis (FO) and supported liquid membrane (SILM) technology can be taken into consideration for VFA separation. In this work, we evaluate these four various downstream methods (NF, RO, FO and SILM) to determine the best one, comparatively, for enriching VFAs from pH-varied model solutions composed of acetic, butyric and propionic acids in different concentrations. The assessment of the separation experiments was supported by statistical examination to draw more solid conclusions. Accordingly, it turned out that all methods can separate VFAs from the model solution. The highest average retention was achieved by RO (84% at the applied transmembrane pressure of 6 bar), while NF provided the highest permeance (6.5 L/m2hbar) and a high selectivity between different VFAs.

Recent Advances and Challenges towards Sustainable Polyhydroxyalkanoate (PHA) Production

Sustainable biofuels, biomaterials, and fine chemicals production is a critical matter that research teams around the globe are focusing on nowadays. Polyhydroxyalkanoates represent one of the biomaterials of the future due to their physicochemical properties, biodegradability, and biocompatibility. Designing efficient and economic bioprocesses, combined with the respective social and environmental benefits, has brought together scientists from different backgrounds highlighting the multidisciplinary character of such a venture. In the current review, challenges and opportunities regarding polyhydroxyalkanoate production are presented and discussed, covering key steps of their overall production process by applying pure and mixed culture biotechnology, from raw bioprocess development to downstream processing.