Micronutrients

Because methane-forming bacteria possess several unique enzyme systems, they have micronutrient requirements that are different from those of other bacteria. The need for several micronutrients, especially cobalt, iron, nickel, and sulfide, is critical. Additional trace elements on which enzymes of methane-forming bacteria are dependent include selenium and tungsten. The incorporation of micronutrients in enzyme systems is essential to ensure not only proper degradation of substrate but also efficient operation of the digester. Cobalt, iron, nickel, and sulfide are obligatory micronutrients, because they are required by methane-forming bacteria to convert acetate to methane. Therefore, attention to macronutrient needs alone is grossly inadequate for methane-forming bacteria.

Molybdenum, tungsten, and selenium may be obligatory micronutrients. Additional micronutrients of concern are barium, calcium, magnesium, and sodium. Deficiencies for micronutrients in anaerobic digesters often have been mistaken for symptoms of toxicity.

Although these micronutrients are usually present in sufficient quantities in municipal wastewater, the digester effluent should be analyzed to ensure that residual soluble quantities of these nutrients exist, especially in industrial waste-water treatment plants. The presence of adequate nutrients, especially micronu-trients, helps to minimize digester upsets caused by the accumulation of volatile fatty acids.

Methane-forming bacteria are able to easily remove or "harvest" micronutrients from the bulk solution. The harvesting of micronutrients is accomplished through the production and excretion of extracellular "slime" that chelates and transports the nutrients into the cell (Figure 15.3). The use of extracellular slime permits "luxury" uptake of micronutrients, that is, the removal and storage of nutrients beyond the quantity that is needed.

If it is necessary to add micronutrients to an anaerobic digester, yeast extract can be used. Yeast extract contains numerous amino acids, minerals, and vitamins, including the B vitamins biotin and folic acid. The addition of 1.5kg/m3 of yeast extract at all loading rates should provide adequate micronutrients.

Slime

Figure 15.3 Relatively large quantities of micronutrients are removed from the bulk solution by methane-forming bacteria through their adsorption to the slime that coats the bacterial cells. Once adsorbed the nutrients are then absorbed by the bacterial cells.

Slime

Cell wall

Figure 15.3 Relatively large quantities of micronutrients are removed from the bulk solution by methane-forming bacteria through their adsorption to the slime that coats the bacterial cells. Once adsorbed the nutrients are then absorbed by the bacterial cells.

Cobalt is required as an activator of enzyme systems in methane-forming bacteria. The incorporation of cobalt into enzyme systems provides for more efficient conversion of acetate to methane.

Although methane-forming bacteria have a relatively high iron requirement, and iron usually exists in high concentrations in the environment, it is difficult for methane-forming bacteria as well as anaerobic bacteria in general to assimilate iron. For iron to be assimilated, it must be in solution. Unfortunately, this requirement is usually not satisfied in the environment of methane-forming bacteria and other anaerobic bacteria.

Nickel is a unique micronutrient requirement for methane-forming bacteria, because nickel is generally not essential for the growth of most bacteria. For example, the F430 enzyme in methane-forming bacteria contains nickel. The addition of nickel can increase acetate utilization rate of methane-forming bacteria.

The requirement for nickel has long been overlooked because of the high background level or presence of nickel in bacterial growth media. However, the lack of adequate usable nickel in the bulk solution of an anaerobic digester results in a

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