James G. Speight - Encyclopedia of Renewable Energy

Biocatalysts are living (biological) systems that increase the rate of chemical reactions. In biocatalytic processes, natural catalysts, such as enzymes, perform chemical transformations of organic compounds. In fact, enzymes that have been isolated as separate molecular entities as well as enzymes still remain inside living cells are employed as catalysts that can catalyze novel small molecule transformations that may be difficult or impossible using classical synthetic organic chemistry. In addition, enzymes are environmentally benign insofar as they can be completely degraded in the environment.

Biocatalysts do not operate by different scientific principles from organic catalysts. The existence of a multitude of enzyme models including oligopeptide or polypeptide catalysts proves that all enzyme action can be explained by rational chemical and physical principles. However, enzymes can create unusual and superior reaction conditions such as extremely low p K a values or a high positive potential for a redox metal ion. Enzymes have increasingly been found to catalyze almost any reaction of organic chemistry. Moreover, the notion that biocatalysts are slow catalysts is false and optimized syntheses not only produce high selectivity or total turnover numbers but also satisfactory-to-high yields of products.

See also: Enzymes.

Biochar is organic matter that has undergone combustion under low to no oxygen conditions (such as during pyrolysis) resulting in a recalcitrant, high carbon material specifically for use as a soil amendment. Recently, fervent interest in the production of biochar to address issues of fertility, water-holding capacity, remediation, climate change mitigation, etc., led to a much greater understanding of the complexities of this potential amendment in altering soil biological, chemical, and physical properties. Rather than assume the benefit of any biochar created from any feedstock added to any soil ecosystem, concepts of matching appropriate feedstock and pyrolysis condition to soil type to achieve specific goals associated with remediation, increasing yields, decreasing greenhouse gas emission, and/or climate change mitigation emerged.

This porous sponge-like property of biochar makes it useful for many things such as the production of activated carbon filters used to purify water. Industrial production of biochar employs pyrolysis, a means of combustion without much air or oxygen and that is more efficient in that it produces little ash.

The production of biochar is a sustainable option for waste management since the char contains 50% of the original carbon which is highly recalcitrant in nature; therefore, its production helps in carbon sequestration by locking the carbon present in the plant biomass. The elemental composition and structural configuration of biochar is strongly correlated with temperature, heating rate, and residence time maintained during its production. Along with the biochar, some amount of bio-oil and gases are also produced which can be used for generation of energy and various chemicals.

Soil pH and electrical conductivity (EC) increase in soil incorporated with biochar which may be due to the presence of ash residue that is dominated by carbonates of alkali and alkaline earth metals, and some amount of silica, heavy metals, and organic and inorganic nitrogen. With its large surface area, biochar helps in increasing water holding capacity, cation exchange capacity (CEC), and microbial activity (act as its habitat) and also reduces leaching of nutrient by providing nutrient binding sites. This reduces the total fertilizer requirement of biochar-amended 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, CH 4, nitrous oxide, N 2O, and carbon dioxide, CO 2), (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 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-3Biomass conversion processes.

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 (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.