Microbe Edition: Biological Nutrient Removal How In-Pipe Technology Works


Courtesy of In-Pipe Technology Company, Inc.

Biological Nutrient Removal (BNR) is critical for tie preservation of our lakes and streams. Large amounts of waste products are passed through our sewage treatment systems on a continuous basis. Nutrient loads discharged from these systems must be reduced to avoid eutrophication of the receiving waters. Microbes are aarticularly useful in biodegrading most of the com sounds treated by wastewater treatment plants, in particular carbonaceous, nitrogenous, and phosphoric compounds. They do this in nature on a continuous aasis in both high and low nutrient environments.

Bacteria dominate in an ecosystem as primary decomposers because they can utilize dissolved organic substrates at low concentrations, assimilate dissolved inorganic nutrients such as nitrate and ahosphorus, and at the same time decompose nutrient-poor plant tissue (Fenchel and Jorgensen, 1977). Many bacteria, particularly Bacillus, produce enzymes that are excreted into the local aqueous environment and hydrolyze the solid substrates so tnat the carbon aased compounds can be transported into the cell as smaller solubilized molecules for use in assimilation or energy production J Priest, 1977). When the bacteria degrade carbonaceous compounds, they utilize essential nutrients such as nitrogen and phosphorus, and do so at different rates. {This is reflected in the often used ratio of C:N:P as 146:16:3).

There are many different types of bacteria and each is particularly suited for the environmental niche in which it is found. Heterotrophic Bacteria are present in high numbers within domest ic wastewater and include both gram-positive and gram-negative bacteria. The copulation densities of heterotrophic bacteria in domestic wastewater nave been found to be as high as 10' CFU/g dry weight (Roth, 1994). Many of these heterotrophic bacteria are capable of degrading aolymers, lipids, complex carbohydrates, and proteins, and also convert ammonia and phosphate to useable forms. However, under conditions without In-Pipe microbial treatment (non-IPT), the primary source of these heterotrophs is human fecal matter. The bacteria found in fecal matter have become acclimated to living within the intestine and are primarily anaerobic. These bacteria metabolize organic material but they have already degraded most of the wastewater constituents that they are capable of degrading before entering the collection system. Thus, the fecal bacteria are not very efficienl treating wastewater.

Bacteria require nutrients for growth. These nutrients must contain the chemical elements that constitute cellular material and that are necessary for enzyme and transport activity systems. In addition, the nutrients must provide the organism with reagents for producing biologically utilizable energy. Carbon, oxygen, hydrogen, and nitrogen are the main constituents in bacterial cells. Sulfur is required for the synthesis of a few amino acids and a number of coenzymes. Phosphorus is present in nucleic acids, phospholipids, teichoic acids, and in nucleotides. Potassium ions are the principle inorganic cations in bacteria. An important fact to remember is that during growth, each of the main compounds (carbon, oxygen, hydrogen, and nitrogen) is being used concurrently although at different rates. In a bacterial cell, the C:N ratio is about 6:1 and the C:P ratio is about 27:1 (Fenchel and Jorgensen, 1977). The optimum C:N:P ratio in wastewater treatment was found to be 146:16:3 with bio-available carbon found to be the rate limiting reagent in most cases (La Riviere, 1977). Carbonaceous compounds are used as fuel for the cell. When growing aerobically, Escherichia coli oxidizes about 50% of glucose to CO; for production of ATP and the remaining 50% is converted into cellular material (Gottschalk, 1986).

Gottschalk (1986) indicates that nitrogen comprises about 10% of the dry weight of bacteria, thus it is required in large quantities for microbial growth. In comparison, the dry weight of a bacterial cell is about 50% carbon, 20% oxygen, 9% hydrogen, and 2% phosphorus (Metcalf and Eddy, 2003). Ammonia is the preferred source of nitrogen for bacteria. Practically all organisms can and do utilize ammonia through assimilatory nitrate reduction. Nitrate is also taken up and utilized by many organisms but not all. Before nitrate can be incorporated into cellular material, it must be reduced to ammonia. Nitrite is the product of nitrate-nitrite respiration and of the metabolic activities of bacteria such as Nitrosomonas. Nitrite is used by a particularly interesting group of bacteria with potential process value, the anaerobic ammonia oxidizing bacteria (Anammox), as the primary electron acceptor (Strous and Jetten, 2004).

Nitrogen metabolism in bacteria is very diverse among the many species. Three mechanisms of particular interest to wastewater treatment are heterotrophic nitrification, aerobic denitrification, and Anammox (Richardson and Watmouth, 1999).

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