Anaerobic Digester Significant Abiotic And Biotic Factors

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Significant abiotic factors in the anaerobic digester include ionized ammonia (NH4+), alkalinity, carbon dioxide, hydrogen, nitrate (NO3-), nutrients, pH, quantity and types of substrates, sulfate (SO42-), temperature, toxic wastes, and volatile acids. Significant biotic factors include acetogenic bacteria, fermentative (acid-forming) bacteria, hydrolytic bacteria, methane-forming bacteria, solids retention time (SRT), sulfur-reducing bacteria, and volatile suspended solids (VSS).

Within each biological treatment unit, different groups of organisms transfer carbon and energy from one trophic (food) level to the next trophic level (Figures 2.1 and 2.2). In the activated sludge process, carbon and energy enter the process as nonliving substrates or BOD. In the soluble form, BOD is absorbed by a variety of organisms, mostly bacteria. Some of the absorbed BOD is transformed into new bacterial cells (sludge) or living BOD. Each organism in the food chain or food web represents BOD, because living organisms are consumed (predator-prey relationships); for example, bacteria are consumed by protozoa and metazoa, and dead organisms are decomposed by living organisms.

As carbon and energy move up the food chain or food web, the quantity (weight) or biomass of each group of organism in the higher trophic level decreases (Figure 2.4). With each move to a higher trophic level, more carbon and energy are lost in waste products and heat, thus leaving less carbon and energy for the synthesis of cellular material (biomass). However, the transfer of carbon and energy from one group of organisms to another is not as simple as a food chain, because several groups of organisms often feed off the same substrates or lower trophic level. The transfer or movement of carbon and energy here is referred to as a food web. The food web better illustrates the activated sludge process than the food chain, because organisms here work mostly side-by-side (Figure 2.2). For example, nitrifying bacteria oxidize nBOD, while organotrophic bacteria oxidize cBOD, and biological phosphorus removal occurs, while organotrophic bacteria oxidize cBOD.

The food chain better illustrates the anaerobic digester than the food web, because bacteria in the digester work in step-by-step fashion to produce methane

Anaerobic Sludge Digester

FIGURE 2.4 Activated sludge food pyramid. As carbon and energy move up the food pryramid from one tropic level to the next, a smaller quantity of organisms (biomass) is produced in the higher trophic level, due to the loss of some carbon and energy as carbon dioxide and waste products. The loss occurs as a result of the biochemical reactions involved in the degradation of the substrate (biomass) from the lower trophic level.

FIGURE 2.4 Activated sludge food pyramid. As carbon and energy move up the food pryramid from one tropic level to the next, a smaller quantity of organisms (biomass) is produced in the higher trophic level, due to the loss of some carbon and energy as carbon dioxide and waste products. The loss occurs as a result of the biochemical reactions involved in the degradation of the substrate (biomass) from the lower trophic level.

(Figure 2.1). Hydrolytic bacteria solubilize complex organic wastes to soluble wastes. The soluble wastes then are degraded to simple wastes by fermentative bacteria. These simple wastes finally are converted to methane by methane-forming bacteria.

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