soil and thereby reduces environmental pollution caused by leaching of inorganic fertilizer. It also plays a vital role in increasing crop productivity. Apart from improving soil quality, biochar provides various other benefits such as (i) mitigation of greenhouse gases (such as methane, CH4, nitrous oxide, N2O, and carbon dioxide, CO2), (ii) a decrease in the dissipation rate of herbicide in soil, and (iii) wastewater treatment. Due to large availability of biomass resources, biochar can be a prime product in many countries.
See also: Pyrolysis.
Biochemical Conversion
Biochemical conversion is the use of (i) fermentation or (ii) anaerobic digestion to produce fuels and chemicals from organic sources. In the general sense, fermentation refers to any chemical change of organic material that is accompanied by effervescence, normally without the participation of oxygen. The important differences between fermentation and anaerobic digestion are the nature of the product produced and the character of the biological contribution. Fermentation produces a liquid product in the presence of enzymes, while anaerobic digestion yields a gaseous product as a result of the metabolic activity of bacteria (Table B-3).
Ethanol is the principal product of the fermentation processes appropriate to biomass conversion, although other alcohols, as well as organic acids, ketones, and aldehydes, may be produced either as main products or as by-products. Anaerobic digestion is the decomposition of any organic material by the metabolic action of bacteria without the participation of atmospheric oxygen. Methane and carbon dioxide are the main products of the decomposition. The source of the oxygen in the carbon dioxide is the combined oxygen in the organic molecules and in the water.
Table B-3 Biomass conversion processes.
| Process | Biomass feedstock | Scale* | Product |
|---|---|---|---|
| Combustion | Wood, municipal solid waste, grasses, crop residue | Small, large | Heat, steam, electricity |
| Gasification | Wood, municipal solid waste (grasses, crop residue) | Large | Low-heat content gas, synthesis gas ethanol |
| Pyrolysis | Wood, sewage sludge | Large | Medium-heat content gas tar |
| Fermentation | Grain and sugar crops | Small, large | Ethanol |
| *Small implies domestic or farm application; large is industrial-scale processing of up to 1,000 t/d of biomass. | |||
Bacterial digestion is in effect accomplished by enzymes. Further, certain bacteria produce acids and alcohols as the principal degradation products. In some cases, it is not clear whether the degradation proceeds as a result of bacterial metabolism, or whether it can be achieved by non-growing cells. Nevertheless, the distinction between the two processes is convenient for presentation purposes, and should not cause confusion in classifying the important biochemical processes currently in contention.
Biomass fermentation to produce ethanol is similar to glycolysis (the fermentation that occurs in muscle tissue and converts glucose to lactic acid with the release of energy), but the use of different enzymes results in different end products.
See also: Aerobic Digestion, Anaerobic Digestion, Bioconversion, Fermentation.
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is a chemical procedure for determining the rate of uptake of dissolved oxygen by the rate biological organisms in a body of water use up oxygen. It is a chemical measure of the power of an effluent to deoxygenate water. The test is widely used as an indication of the quality of water. The biochemical oxygen demand can be used as a gauge of the effectiveness of wastewater treatment plants. There are two recognized methods for the measurement of biochemical oxygen demand which are (i) the dilution method and (ii) the manometric method.
In the dilution method, a small amount of microorganism seed is added to each sample being tested. This seed is typically generated by diluting activated sludge with de-ionized water. The test is carried out by diluting the sample with oxygen saturated de-ionized water, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen, and then sealing the sample to prevent further oxygen dissolving in. The sample is kept at 20°C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final dissolved oxygen and initial dissolved oxygen is the biochemical oxygen demand. The apparent biochemical oxygen demand for the control is subtracted from the control result to provide the corrected value.
In the manometric method, the sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide (typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. Oxygen is consumed, and dioxide is released. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From the drop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen.
Biochemicals
Biochemicals, as opposed to petrochemicals, are in the context of this encyclopedia, chemicals produced from biomass.
The production of chemicals from biomass, a renewable feedstock, is highly desirable in replacing petrochemicals to make biorefineries more economical. The best approach to compete with fossil-based refineries is the upgradation of biomass in integrated biorefineries. The integrated biorefineries employed various biomass feedstocks and conversion technologies to produce biofuels and bio-based chemicals. Bio-based chemicals can help to replace a large fraction of industrial chemicals and materials from fossil resources. Biomass-derived chemicals, such as 5-hydroxymethylfurfural (5-HMF), levulinic acid, furfurals, sugar alcohols, lactic acid, succinic acid, and phenols, are considered platform chemicals. These platform chemicals can be further used for the production of a variety of important chemicals on an industrial scale. However, current industrial production relies on relatively old and inefficient strategies and low production yields, which have decreased their competitiveness with fossil-based alternatives.
Biomass feedstocks, such as agricultural residues and wood chips, constitute an inexpensive renewable resource for commercial large-scale biorefineries, as these waste products are widely available and can sequester carbon. The target chemicals include alcohol derivatives, organic acid derivatives such as formic acid and levulinic acid, and furan derivatives such as 5-hydroxymethylfurfural (5-HMF) and furfural derivatives. These chemicals can further be converted to a range of derivatives that have potential applications in biofuels, polymers, and solvent industries. Due to these differences in their chemical composition and structure, cellulose, hemicellulose, and lignin have different chemical reactivities. In addition to the complex nature of bio-resources, the inert chemical structure and compositional ratio of carbon, hydrogen, and oxygen in molecules in biomass present difficulties in the chemo-catalytic conversion of biomass to fuels and chemicals. Therefore, besides using the natural lignocellulosic biomass as a reactant, researchers often use model compounds for conversion process