Methods for Volatile Fatty Acids (VFA) Separation and Recovery from Complex Effluent Streams
M.P.Zacharof *¹and R.W. Lovitt *²
CWATER
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
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 6.5-7 million tonnes/year, at a price of $600800/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
Electrodialysis
High resin costs, High energy
demand due to resin
regeneration, low adsorption
capacities, separation is not
highly selective
Negatively charged carboxylate Carboxylate is
The products have high
ions move through an anion
concentrated in
impurities, further
exchange membrane towards aqueous solution ,
purification might be
the anode in the electrodialyzer does not require acid required, difficulties in scaling
through electric current
treatment to adjust
up, high energy demand.
pH
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
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
Membrane fouling , clogging,
technology, High
largely untried in complex
product yields,
waste systems.
suitable for a wide
range of applications,
low energy.
economic, easy to
scale up
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..
1.
2.
3.
Fig.3. Price range of Volatile Fatty Acids
The feed needs to be acidified
for efficient extraction,
extractants needs to be
regenerated by distillation or
back extraction.
References
Mostafa, N. A. (1999). "Production and recovery of volatile fatty acids from fermentation broth."
Energy and Consumption Management 40: 1543-1553.
Katikaneni, S. P., Cheryan, M. (2002). "Purification of fermentation-derived acetic acid by liquidliquid extraction and esterification." Industrial Engineering Chemical Resources 41(2745-2752).
Dinsdale, R. M., Premier, G.C., Hawkes, F.R., Hawkes, D.L. (2000). "Two-stage anaerobic codegestion of waste activated sludge and fruit/vegetable waste using inclined tubular digesters."
Bioresource Technology 72: 159-168.