Organicnitrogen

Figure 9.2 Nitrogen-fixing by algae. A large number of algae, including those that grow in tetrad formation (colony of four) can fix molecular nitrogen. When algae fix molecular nitrogen, the nitrogen is converted to ammonium ions and then incorporated in an organic molecular to form amino acids and proteins.

Municipal, activated sludge processes receive large quantities of organic-nitrogen wastes. Some of the organic-nitrogen wastes release ammonium ions in the sewer system as they are hydrolyzed and dea-minated by organotrophs. Due to organotrophic activity within the sewer system, municipal, activated sludge processes have an influent ammonium ion concentration of usually 15 to 30 mg/l. Approximately 40% of the nitrogen in domestic wastewater is in the form of ammonium ions, and the rest of the nitrogen is primarily in the form of organic nitrogen.

The production of nitrite ions and nitrate ions within the sewer system is rare. Conditions in the sewer system are not favorable for the production or nitrification of these ions. Adverse conditions within the sewer system that prevent nitrification include a lack of adequate oxygen, a small nitrifying bacterial population, and a short retention time. However, relatively large quantities of nitrite ions and nitrate ions may be found in the sewer system, if they are discharged to the sewer system by industrial wastes containing these ions, such as steel mill wastewater.

The amino acids and proteins within plant tissues, roots, and seeds and livestock meats are discharged directly to the sewer system (garbage disposal waste and food processing wastewater) and indirectly to the sewer system (fecal waste). Many bacteria within the sewer system deaminate some amino acids and proteins (Equation 9.2). Deami-nation is achieved with the enzyme deaminase and results in the production of ammonium ions. The production of ammonium ions also is known as ammonification. Deamination of the amino acid phenyl-alanine is shown is Equation (9.3).

Amino acid — Organotrophs ! NH+ + Acid (9.2) Phenylalanine — Proteus ! NH+ + Phenylpyruvic acid (9.3)

Urea is an organic-nitrogen compound that is found in urine, fertilizers, and stockyard wastes. When hydrolyzed by the bacterial enzyme urease, ammonium ions are released (Equation 9.4). Urease is found in many organotrophs associated with fecal waste including Citrobacter. The hydrolysis of urea into ammonia and carbon dioxide by bacterial activity is very rapid. At the pH of sewer system ammonia is quickly converted to ammonium ions.

Amino acids and proteins that are not degraded in the sewer system may be degraded in the aeration tank. Degradation of amino acids or proteins in the aeration tank also results in the production of ammonium ions.

Ammonium ions within the activated sludge process have several fates (Figure 9.4). They may be used as a nutrient source for nitrogen by organotrophs and nitrifying bacteria. They may be air-stripped to the atmosphere as ammonia at high pH, and under appropriate operational conditions, Nitrosomonas may oxidize them to nitrite ions. If ammonium ions are not used as a nutrient source, air-stripped, or oxidized, they are discharged in the aeration tank effluent.

Nitrogen-fixing Bacteria

Nitrogen-fixing Bacteria

Figure 9.3 Wastewater nitrogen cycle. In the wastewater nitrogen cycle, molecular nitrogen is removed from the atmosphere and returned to the atmosphere. The movement of nitrogen through the cycle involves a number of conversions (oxidation and reduction reactions) and a variety of bacteria. Molecular nitrogen (N2) is removed from the atmosphere by nitrogen-fixing bacteria. These bacteria produce ammonium ions (NH¿) from molecular nitrogen. The ammonium ions produced by nitrogen-fixing bacteria are incorporated into amino acids and proteins within green, leafy plants. The plants (fruits, leaves, roots, seeds, and stems) are consumed by humans, and some of the nitrogen within the plants is released in human bodily wastes (urine and fecal material) into the sewer system in the form of urea, amino acids, and proteins. Some of these organic-nitrogen wastes undergo hydrolysis and deamination in the sewer system and release ammonium ions. Some of these organic wastes undergo deamination in the aeration tank and release ammonium ions. There are several fates for the ammonium ions within the aeration tank. If the pH of the tank increases to 9.4 or greater, some of the ammonium ions are converted to ammonia (NH3) and are lost to the atmosphere.

nh4+

O

NO2-

Figure 9.4 Fate of ammonium ions in the activated sludge process. There are four significant fates for the ammonium ion (NHf) in the activated sludge process. First, at a pH of 9.4 or higher, some of the ammonium ions in the aeration tank are converted to ammonia (NH3). Ammonia escapes from the aeration tank in the form of a gas. Second, some of the ammonium ions are used as a nitrogen nutrient by the bacteria within the aeration tank for growth and reproduction, namely an increase in MLVSS or organic-nitrogen content of the process. Third, under favorable operational conditions, some of the ammonium ions are oxidized by Nitrosomonas to nitrite ions (NO2,)- Fourth, the ammonium ions may leave the aeration tank and enter the secondary clarifier.

Under cold temperature or a limiting process condition, nitrite ions may accumulate in the activated sludge process. Nitrite ions also have several fates in the activated sludge process (Figure 9.5). They may be chemically oxidized to nitrate ions if chlorine is being used to control the undesired growth of filamentous organisms. Nitrite ions may be biological oxidized by Nitrobacter under appropriate operational conditions. If ammonium ions and nitrate ions are not available in the aeration tank, nitrite ions may be used as a nutrient source

Some of the ammonium ions are used as a nitrogen nutrient for the growth of new bacteria or MLVSS (organic nitrogen). If operational conditions are favorable, Nitrosomonas oxidizes some of the ammonium ions to nitrite ions (NO2,), and Nitrobacter oxidizes some of the nitrite ions to nitrate ions (NO3). If an anoxic condition occurs within the treatment process, the nitrate ions are reduced to molecular nitrogen (N2) by facultative anaerobic or denitrifying bacteria. Molecular nitrogen escapes from the treatment plant to the atmosphere through denitri-fication.

g n02"

n02"

o

NO2-

NO3-

Figure 9.5 Fate of nitrite ions in the activated sludge process. There are four significant fates for nitrite ions (NO2,) in the activated sludge process. First, in the absence of ammonium ions (NH¿) and nitrate ions (NOj), some of the nitrite ions are used as a nitrogen nutrient by the bacteria within the aeration tank for growth and reproduction, meaning that an increase occurs in MLVSS or organic-nitrogen content of the process. Second, under favorable operational conditions, some of the nitrite ions are oxidized by Nitrobacter to nitrate ions. Third, if the MLVSS are being chlorinated to control undesired filamentous growth, some of the nitrite ions are chemical oxidized by chlorine to nitrate ions. Fourth, the nitrite ions may leave the aeration tank and enter the secondary clarifier. Here the nitrite ions may undergo denitrification if an anoxic condition develops in the secondary clarifier.

for nitrogen by organotrophs. If nitrite ions are not oxidized or used as a nutrient source for nitrogen, they are discharged in the aeration tank e¿uent. In the secondary clarifier, nitrite ions may be denitrified.

Nitrate ions within the activated sludge process also have several fates (Figure 9.6). In the absence of ammonium ions in the aeration tank, nitrate ions may be used as a nutrient source for nitrogen. If nitrate ions are not used as a nutrient source for nitrogen, they are discharged in the aeration tank e¿uent. In the secondary clarifier, nitrate ions may be denitrified.

Nitrate ions are of central importance in the wastewater nitrogen cycle. They are the product of nitrification, the substrate of denitri-fication, and the source for the nitrogen nutrient when ammonium ions are not available. Nitrate ions are used as a nutrient source for nitrogen through a biological process known as nitrate assimilation. Nitrate ions are the most abundant, inorganic nitrogen source in most waters.

Nitrogenous compounds from industrial wastewater (Table 9.3) may release ammonium ions in the sewer system or activated sludge

Mlvss Wastewater

Figure 9.6 Fate of nitrate ions in the activated sludge process. There are two significant fates for nitrate ions (NO3 ) in the activated sludge process. First, in the absence of ammonium ions (NH%), some of the nitrate ions are used as a nitrogen nutrient by the bacteria within the aeration tank for growth and reproduction, meaning that an increase occurs in MLVSS or organic-nitrogen content of the process. Second, the nitrate ions may leave the aeration tank and enter the secondary clarifier. Here the nitrate ions may undergo denitrification if an anoxic condition develops in the secondary clarifier.

Figure 9.6 Fate of nitrate ions in the activated sludge process. There are two significant fates for nitrate ions (NO3 ) in the activated sludge process. First, in the absence of ammonium ions (NH%), some of the nitrate ions are used as a nitrogen nutrient by the bacteria within the aeration tank for growth and reproduction, meaning that an increase occurs in MLVSS or organic-nitrogen content of the process. Second, the nitrate ions may leave the aeration tank and enter the secondary clarifier. Here the nitrate ions may undergo denitrification if an anoxic condition develops in the secondary clarifier.

process depending on their structure and ease of bacterial degradation. Additional sources of nitrogenous compounds include poly-electrolytes and disinfectants, such as chloramines, added to potable water supplies.

Denitrification may occur in the sludge blanket of the secondary clarifier when an anoxic condition develops in the sludge blanket. Here facultative anaerobic bacteria use nitrite ions and nitrate ions to degrade soluble cBOD. This degradation is associated with the release of molecular nitrogen.

An activated sludge process that must satisfy a total nitrogen discharge requirement must nitrify and denitrify. However, denitrifi-cation is controlled. Denitrification occurs in a denitrification tank

TABLE 9.3 Examples of Nitrogenous Compounds Discharged by Industries

Nitrogenous Compounds

Source or Use

Acrylonitrile

Synthetic acrylic fibers

Benzoic acid

Fruit preservative

Dyes

Fabric and paper coloring

EDTA

Soap to remove metallic contaminates

Figure 9.7 Denitrification tank. A denitrification tank is a unit found immediately downstream or following the aeration tank in which nitrification has occurred. The nitrates produced in the aeration tank, as well as bacteria in the aeration tank, are discharged to the denitrification tank. In the denitrification tank, slow subsurface mixing is provided to rapidly remove residual dissolved oxygen and a carbon source or soluble cBOD such as methanol is added. Within the denitrification tank the facultative anaerobic or denitrifying bacteria discharged from the aeration tank use the nitrate ions to degrade the soluble cBOD added to the tank. When this occurs, molecular nitrogen is produced and released to the atmosphere. De-nitrification within the tank takes approximately 30 to 60 minutes. The bacteria or solids within the denitrification tank are discharged to a clarifier where the settled solids are returned to the aeration tank.

Figure 9.7 Denitrification tank. A denitrification tank is a unit found immediately downstream or following the aeration tank in which nitrification has occurred. The nitrates produced in the aeration tank, as well as bacteria in the aeration tank, are discharged to the denitrification tank. In the denitrification tank, slow subsurface mixing is provided to rapidly remove residual dissolved oxygen and a carbon source or soluble cBOD such as methanol is added. Within the denitrification tank the facultative anaerobic or denitrifying bacteria discharged from the aeration tank use the nitrate ions to degrade the soluble cBOD added to the tank. When this occurs, molecular nitrogen is produced and released to the atmosphere. De-nitrification within the tank takes approximately 30 to 60 minutes. The bacteria or solids within the denitrification tank are discharged to a clarifier where the settled solids are returned to the aeration tank.

(Figure 9.7). MLVSS and nitrate ions produced in the aeration tank are added to the denitrification tank along with a source of soluble cBOD such as methanol. The denitrification tank is slowly mixed. Facultative anaerobic bacteria within the MLVSS degrade the soluble cBOD using nitrate ions. Nitrogen within the wastes entering the activated sludge process have been nitrified to nitrate ions, and the nitrogen within the nitrate ions is loss to the atmosphere as molecular nitrogen, not the receiving water, through the use of the denitrification tank.

Much of the proteinaceous waste associated with suspended solids in the wastewater is removed by sedimentation in primary clarifiers, and some is solublized to soluble cBOD in the aeration tank (Figure 9.8). Proteinaceous wastes from the primary clarifier and the acti-

Collodial BOD, Protein Containing Sulfur and Phosphorus

Solublization by Exoenzymes 1

Soluble cBOD

Solublization by Exoenzymes 1

Soluble cBOD

Oblique Cylinder Drawing

Figure 9.8 Removal and degradation of proteinaceous waste. Proteins are colloidal in nature and must be solublized in order to enter bacterial cells and undergo degradation. Solublization of proteins is achieved through the use of exo-enzymes. Once solublized, bacterial cells rapidly absorb soluble cBOD. Inside the bacterial cells, endoenzymes oxidize the soluble cBOD into new cells and several inorganic compounds including carbon dioxide (CO2), water (H2O), ammonium ions (NHj~), phosphate ions (PO%~), and sulfate ions (SO%~). Proteins that contain phosphate groups and thiol groups (-SH) serve as the compounds that yield phosphates and sulfates when oxidized.

vated sludge process are degraded in aerobic digesters or anaerobic digesters.

Ammonium ions may be removed by mixing action or air stripping to the atmosphere as ammonia. However, the loss of ammonia through air-stripping is relatively small, that is, less than 10%.

An overview of the changes in quantities of nitrogenous wastes in the activated sludge process is illustrated in Figure 9.9. In a municipal, activated sludge process receiving no nitrite ions and no nitrate ions from an industrial source, the concentration of ammonium ions

_ Time Increasing _^

Figure 9.9 Changes in principle nitrogenous wastes in the aeration tank. The principle nitrogenous wastes in an aeration tank are organic-nitrogen compounds or TKN, ammonium ions (NH¿), nitrite ions (NO2), and nitrate ions (NO3). As TKN and ammonium ions enter the aeration tank, TKN is deaminated to release ammonium ions, and bacteria remove ammonium ions as a nitrogen nutrient. However, TKN is deaminated more rapidly than ammonium ions are removed, and the amount of ammonium ions in the aeration tank initially increasing. Following this increase the amount of ammonium ions within the aeration tank begins to drop. The drop in the amount of ammonium ions is due to two factors. First, little TKN remains to replenish the amount of ammonium ions removed as a nitrogen nutrient. Second, some of the ammonium ions are oxidized to nitrite ions by Nitrosomonas. If no adverse operational condition develops in the aeration tank, nitrite ions do not accumulate and are oxidized to nitrate ions by Nitrobacter. With extended aeration or nitrification, the amount of ammonium ions within the aeration tanks continues to decrease, while the amount of nitrate ions continues to increase.

and organic nitrogen is relatively high in the aeration tank. Although ammonium ions are removed as a nutrient source for nitrogen, its concentration increases as organic-nitrogen wastes are deaminated. As the quantity of organic-nitrogen wastes decreases, the quantity of ammonium ions increases.

When the organic-nitrogen wastes are no longer available to release ammonium ions, the quantity of ammonium ions decreases. The decrease in ammonium ions occurs as they are used as a nutrient source for nitrogen and oxidized to nitrite ions and nitrate ions. If nitrification is proceeding properly, no accumulation of nitrite ions occurs. Some of the nitrate ions may be removed as a nutrient source for nitrogen when ammonium ions are depleted. If denitrification occurs, the quantity of nitrate ions would be greatly reduced, perhaps eliminated.

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