1. Methods of Production of Volatile Fatty Acids
Methods for Volatile Fatty Acids (VFA) Separation and Recovery from Complex Effluent Streams
M.P.Zacharof *¹and R.W. Lovitt *²
College of Engineering, Centre for Water Advanced Technologies and Environmental Research Swansea University (CWATER)SA2 8PP, UK*¹
College of Engineering, Multidisciplinary Nanotechnology Centre (MNC), Center of Complex Fluids Processing (CCFP), Centre for Water Advanced Technologies and Environmental Research (CWATER) Swansea University , Swansea, SA2 8PP, UK*²
CWATER
Methods of Production of Volatile Fatty Acids
Carboxylic acids are heavily involved with organic carbon cycling on the planet. They are extensively used in the industry and are typically produced of oil based chemical processing. On the other hand most organic carbon in environments goes through fatty acid intermediates as they are ultimately metabolized either to carbon dioxide (CO2) and methane anaerobically or CO2 and water in oxidative systems [1].
The removal of Volatile Fatty Acids (VFA), from wastewater from numerous sources such as chemical plants has been an area of great research interest. With the global petroleum resources facing scarcity and the constantly rising awareness of the environmental impact the carbon based economy has created, research on alternative methods of their production, such as anaerobic fermentation (Fig.1) and digestion.
Commercial Importance of Volatile Fatty Acids
Other uses of VFA do include their usage in the food industry as flavouring and antimicrobial agents and in the pharmaceutical and cosmetics industry as raw materials. Certainly one of the most important VFA commercially is acetic acid, consumed worldwide, with about one third of its consumption occurring in United States. Global production of acetic acid is approximately million tonnes/year, at a price of $ /t. Of the global demand of acetic acid,1.5 mt/year) is met by recycling; the remainder is manufactured from petrochemical feedstock, produced by oxidation of acetaldehyde, the oxidation of liquid phase hydrocarbons, the carbonisation of methanol or from biological sources [2].
Separation Methods of Volatile Fatty Acids
Separation of volatile fatty acids can be achieved using a variety of physical and chemical methods, the most important tabulated below.
Methods
Description
Advantages
Disadvantages
Precipitation
Calcium salts are added in the medium, to neutralize the acids. The resulting calcium carboxylate solutions, can be concentrated by evaporation, then crystallized and separated of the mother liquor
Well established. Higher product yields, low capital costs, products of high purities
Generating solid wastes as sulfuric acid is used to release carboxylic acids from the calcium carboxylates.
Distillation
Ammonia is used to neutralize the acids reacting to form ammonia carboxylate, which is then mixed with alcohol to form esters , to be separated by distillation
Well established Highly pure products, byproducts can be used as fertilizer
High energy and capital costs related to distillation that is used to separate the alcohol from carboxylic acids after the formed esters are hydrolyzed.
Adsorption
Ion exchange resins used to exchange to adsorb carboxylate ions of the feeds
Well established. Easily operable
High resin costs, High energy demand due to resin regeneration, low adsorption capacities, separation is not highly selective
Electrodialysis
Negatively charged carboxylate ions move through an anion exchange membrane towards the anode in the electrodialyzer through electric current
Carboxylate is concentrated in aqueous solution , does not require acid treatment to adjust pH
The products have high impurities, further purification might be required, difficulties in scaling up, high energy demand. Prone to fouling
Solvent Extraction
Organic acids use to extract carboxylic acids from the stream
Higher product yields, suitable for carboxylate salt production, lower costs
The feed needs to be acidified for efficient extraction, extractants needs to be regenerated by distillation or back extraction.
Membrane Separations
Use of membrane filters of various pore sizes to treat the mixed effluents for solids removal and fractionate the desired substances for recovery
Developing technology, High product yields, suitable for a wide range of applications, low energy. economic, easy to scale up
Membrane fouling , clogging, largely untried in complex waste systems.
Fig.1 Diagram of Fermentation Process
In anaerobic digestion (Fig.2) the hydrolysis of target solid wastes followed by the microbial conversion of them to biodegradable organic content results in the production of intermediate organic acids, specifically VFA. VFA are often detected at high concentrations in the effluent streams and mixed liquors of anaerobic membrane reactor systems, because of sudden variations in hydraulic and organic loading rates [3].
Table 1 Suitable Separation methods for Volatile fatty acids from complex streams
Fig.2 Diagram of Anaerobic Digestion
Commercial Importance and Value of Volatile Fatty Acids
Acetic, propionic and butyric acid are also attracting attention as potential candidates for synthesis of high value biodegradable plastics production replacing petrochemical feedstock (Fig. 3).
Conclusions
VFA are substances of great value and wide use in the industry nowadays and further application VFA from waste can substitute for fossil carbon sources.
Anaerobic Fermentation of liquid or solid media have potential for economical production from waste directing carbon away from methane combustion so reducing the carbon foot print while creating a source of valuable carbon materials.
A review of the recovery processes shows that they are a significant barrier to implementing such strategies however, membrane technology in particularly nanofiltration, possibly used in combination with other processes, offers a relatively cost-effective recovery method..
References
Mostafa, N. A. (1999). "Production and recovery of volatile fatty acids from fermentation broth." Energy and Consumption Management 40:
Katikaneni, S. P., Cheryan, M. (2002). "Purification of fermentation-derived acetic acid by liquid-liquid extraction and esterification." Industrial Engineering Chemical Resources 41( ).
Dinsdale, R. M., Premier, G.C., Hawkes, F.R., Hawkes, D.L. (2000). "Two-stage anaerobic co-degestion of waste activated sludge and fruit/vegetable waste using inclined tubular digesters." Bioresource Technology 72:
Fig.3. Price range of Volatile Fatty Acids