an example, the fermentation process is a complex biochemical process during which yeast converts sugars to ethanol, carbon dioxide, and other metabolic by-products that contribute to the chemical composition and sensorial properties of the fermented foodstuffs. Control of the fermentation process is generally considered as a prerequisite to determine the quality of the final product. In this context, fermentation monitoring is a growing need, which calls for fast, low-cost, and nondestructive methods providing real-time or online information in order to assure an effective control at all stages of the process.
The production of the biological alcohol is accomplished by yeast, certain types of bacteria, as well as other microorganisms that result in the conversion of sugar derivatives into ethyl alcohol and carbon dioxide. In the process, yeast is mostly used as the biological agent and the yeast generally carries out the aerobic fermentation process, but it may also ferment the raw materials under anaerobic conditions. The process commences with the breakdown of sugars by yeasts to form pyruvate molecules (glycolysis) which, in the case of glucose, produces two molecules of pyruvic acid. The two molecules of pyruvic acid are then reduced to two molecules of ethanol and carbon dioxide.
Under anaerobic conditions, the pyruvate can be transformed to ethanol, where it first converts into an intermediate product (acetaldehyde, CH3CHO) which further releases carbon dioxide and is converted into ethanol (CH3CH2OH).
See also: Bioalcohol, Bioethanol, Fermentation, Fermentation Chemistry.
Biological Conversion
Biological conversion (also referred to as biochemical conversion) involves breaking down biomass to make the carbohydrates available for processing into sugars, which can then be converted into biofuels and bioproducts through the use of microorganisms and catalysts. Potential fuel blend stocks and other bioproducts include the following: (i) renewable gasoline, (ii) ethanol and other alcohols, (iii) renewable chemical products, and (iv) renewable diesel. Biochemical conversion uses biocatalysts, such as enzymes, in addition to heat and other chemicals, to convert the suitable portions of biomass (hemicellulose and cellulose) into an intermediate sugar stream. These sugars are intermediate building blocks that can then be fermented or chemically catalyzed into a range of advanced biofuels and value-added chemicals.
Bioconversion processes generally take place in bioreactors, which may be operated in batch, continuous, or semi-continuous mode, among others. Moreover, different bioreactor configurations may be suitable depending on the specific application. The technology may range from solid-phase bioconversion processes to gas-phase ones, besides aqueous phase bioprocesses. In any case, a given amount of moisture is generally needed, as this is required, in most cases, for optimal microbial activity. For any given feedstock, biocatalyst and bioreactor configuration and operating conditions will need to be optimized, in terms of aspects such as residence time in continuous processes, pH, or media composition (such as, for example, the carbon-nitrogen ratio).
Thus, bioconversion processes and biorefineries are environmentally friendly alternatives to common chemical processes and conventional oil refineries. They allow the production of a wide range of products with cheap biocatalysts, usually under mild conditions.
See also: Biohydrogen, Biological Action, Biological Alcohol.
Biological Conversion – Aerobic Digestion
Aerobic digestion is the biochemical oxidative stabilization of wastewater sludge in open or closed tanks that are separate from the liquid process system. Aerobic digestion is a solids stabilization process that provides a limited supply of oxygen to microorganisms in order to facilitate oxidation of organic matter and convert it into carbon dioxide and water. The microorganisms utilized in aerobic digestion processes are mesophilic facultative bacteria which mean they thrive in temperatures between 20 and 37°C (68 and 100°F) and live under aerobic (presence of oxygen), anoxic (no oxygen but with presence of nitrates), and anaerobic (no presence of oxygen or nitrates) conditions. Thus, the aerobic digester operates on the principle that as the food is depleted, the microbes, the endogenous phase, and the cell tissue are aerobically oxidized to carbon dioxide, water, ammonium ions (NH4+), nitrite ions (NO2−), and nitrate ions NO3−).
In general, aerobic digestion is a relatively simple process; there are many design and operational parameters such as temperature control, oxygen transfer and mixing, nitrification and denitrification, solids retention time, pH control, sludge loading characteristics, and tank configuration that must be considered in order to achieve a sustainable process.
Typically, an aerobic digestion system consists of two or more aerated tanks used to process and store waste-activated sludge generated from the liquid treatment process and/or primary sludge from primary sedimentation tanks. The waste-activated sludge and primary sludge in an aerobic digestion system can be processed separately or can be combined into one product. Air is introduced to the tank(s) from an aeration system typically coarse or fine bubble diffuser equipment with the air being supplied by a positive displacement or centrifugal blower.
The bacteria continue metabolism as they do in the liquid process, but without new food, they use their own biomass (endogenous respiration). This stabilizes the sludge so that it is safer for human contact, does not attract vermin (vectors), and odors are reduced. This method of digestion is capable of handling waste-activated, trickling filter, or primary sludges as well as mixtures of the same.
Biological Conversion – Anaerobic Digestion
Anaerobic digestion is the decomposition of biological wastes by microorganisms, usually under wet conditions, in the absence of air (oxygen), to produce a gas comprising mostly methane and carbon dioxide. A digester system (the anaerobic digester) is a device that promotes the decomposition of manure or digestion of the organics in manure to simple organics and gaseous biogas products.
During anaerobic digestion of an organic material such as biomass, a varied mixture of complex compounds is converted to a very narrow range of simple compounds, mainly methane and carbon dioxide. The anaerobic bacteria are responsible for the biochemical transformation of the biodegradable organic fraction (BOF). The AD of organic material basically consists of hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These transformations are involved in the breakdown of complex polymers, such as cellulose, fats, and proteins to long and short chain fatty acids, and finally to methane, carbon dioxide, and water.
Any organic substance can become subject to anaerobic digestion so long as there are warm, wet, and airless conditions. For example, marsh gas is a product of the anaerobic digestion of vegetation at the bottom of ponds; this gas rises to the surface and bubbles, and the gas is also combustible. With the aid of human intervention, there are two products of this process, biogas and landfill gas. The chemical processes behind the production of these gases are complex.
The anaerobic digestion process focuses on hastening the natural process of biomass conversion to a gaseous fuel (biogas). Research has been conducted to ascertain optimal conditions for anaerobic digestion. These include (i) the feedstock, (ii) the nutrients, (iii) the temperature, (iv) the moisture content of the feedstock, (v) the pH of the system, and (vi) the atmospheric conditions. Most of the biomass waste feedstocks (municipal solid waste, agricultural waste, farm waste, crop waste, and forestry waste) studied have produced a biogas rich in methane. This medium to high Btu gas can, in some instances, be upgraded to a substitute natural gas (SNG). Depending on the feedstock, sulfur may also be produced.
Anaerobic digestion is a multi-stage biological waste treatment process whereby bacteria, in the absence
