Free Molecular Oxygen

Free molecular oxygen inhibits denitrification by virtue of its competition with nitrite ions and nitrate ions as an electron acceptor for the degradation of cBOD. If free molecular oxygen is in the environment of the bacterial cell and enters the bacterial cell, the cell uses free molecular oxygen (Equation 29.1). The use of free molecular oxygen is preferred over the use of nitrite ions and nitrate ions, because the use of free molecular oxygen yields more cellular energy and cellular growth.

The amount of oxygen that inhibits denitrification is relatively small. Concentrations of dissolved oxygen < 1.0 mg/l inhibit denitrification. However, if a dissolved oxygen gradient exists across a floc particle, denitrification does occur in the core of the floc particle; that is, denitrification occurs in the presence of measurable dissolved oxygen (Figure 29.1). Floc particle > 100 mm in size are large enough to produce a dissolved oxygen gradient.

Under a dissolved oxygen gradient, the bacterial cells within the floc particles respire using oxygen, nitrite ions, and nitrate ions at the same time. The bacterial cells at the perimeter of the floc particle use dissolved oxygen for respiration, while the bacteria in the core of the floc particle use nitrite ions and nitrate ions for respiration.

Figure 29.1 Denitrification in the presence of measurable DO. Although a residual dissolved oxygen level can be measured outside the solids or floc particles in a sludge blanket, denitrification can occur. The occurrence of denitrification in the presence of measurable DO is due to the development of an oxygen gradient from the perimeter to the core of the floc particle. At the perimeter of the floc particle, residual dissolved oxygen is available and is used by the bacteria to degrade cBOD. At the core of the floc particle, residual dissolved oxygen is no longer available, and the bacteria at the core of the floc particle use nitrate ions to degrade cBOD.

Figure 29.1 Denitrification in the presence of measurable DO. Although a residual dissolved oxygen level can be measured outside the solids or floc particles in a sludge blanket, denitrification can occur. The occurrence of denitrification in the presence of measurable DO is due to the development of an oxygen gradient from the perimeter to the core of the floc particle. At the perimeter of the floc particle, residual dissolved oxygen is available and is used by the bacteria to degrade cBOD. At the core of the floc particle, residual dissolved oxygen is no longer available, and the bacteria at the core of the floc particle use nitrate ions to degrade cBOD.

When denitrification occurs, the entire denitrification pathway may not be completed. The denitrification pathway may be shut down at a single oxygen concentration or a significant change in an operational condition. Also some denitrifying bacteria lack key enzymes to complete the entire denitrification pathway.

Denitrification in an activated sludge process can occur whenever appropriate operational conditions exist. These conditions include the presence of an abundant and active population of denitrifying bacteria, an anoxic environment, and the presence of simplistic soluble cBOD. In an anoxic environment a residual amount of free molecular oxygen can be present and measured, but the loss of oxygen across an oxygen gradient results in the use of nitrite ions and nitrate ions at the end of the gradient (Figure 30.1).

Denitrification may be intentional or accidental. Intentional denitrification is the desired use of an anoxic period to achieve a specific operational goal. An anoxic period is established in a denitri-fication tank in order to satisfy a total nitrogen discharge limit. An anoxic period can be used in an aeration tank to improve floc formation, control undesired growth of filamentous organisms, reduce electrical costs to degrade cBOD, or return the alkalinity to the aeration tank that was lost during nitrification (Figure 30.2).

Accidental denitrification is the undesired occurrence of an anoxic period resulting in increased operational costs, operational problems, and permit violations. Although accidental denitrification most often is observed and reported in secondary clarifiers, accidental denitrifi-cation can occur in the sewer system, headworks of the treatment plant, primary clarifiers, chlorine contact tanks, thickeners, and anaerobic digesters (Figure 30.3).

Denitrification problems associated with the sewer system are due

Figure 30.1 Anoxic environment in the presence of measurable DO. In the presence of an oxygen gradient, an anoxic environment occurs in the core of the floc particle. Operational conditions necessary for the presence of an anoxic environment in the presence of measurable DO include the presence of floc particles that are at least greater than 100 mm in size and the presence of a dissolved oxygen concentration equal to or less than 1.0 mg/l.

Figure 30.1 Anoxic environment in the presence of measurable DO. In the presence of an oxygen gradient, an anoxic environment occurs in the core of the floc particle. Operational conditions necessary for the presence of an anoxic environment in the presence of measurable DO include the presence of floc particles that are at least greater than 100 mm in size and the presence of a dissolved oxygen concentration equal to or less than 1.0 mg/l.

to the discharge of nitrite ions and nitrate ions from specific industries. Denitrification problems associated with the headworks of the treatment plant and the primary clarifiers are due to the discharge of nitrite ions and nitrate ions from specific industries or the recycling of these ions in the RAS to the head of the treatment plant. Denitrification problems associated with the secondary clarifiers, chlorine contact tanks, thickeners, and anaerobic digesters may be due to the discharge of nitrite ions and nitrate ions from specific industries but usually are due to the production of these ions through nitrification in the aeration tanks.

When denitrification occurs in the sewer system, soluble cBOD is rapidly degraded. The degradation of cBOD in the sewer system results in a decrease in the cBOD concentration of the influent wastewater. With a decreased quantity of cBOD in the influent, it becomes more difficult for an activated sludge process to achieve an 85% removal efficiency for cBOD. This difficulty may result in a permit violation for the percentage of cBOD removal.

Denitrification in the sewer system, headworks of the treatment plant, and primary clarifiers reduces the quantity of soluble cBOD or substrate that enters the aeration tank. With reduced substrate in the aeration tank, little bacterial growth occurs. Reduced bacterial growth results in a decrease in MLVSS. Reduced cBOD or substrate may cause many bacteria in the aeration tank to undergo endogenous respiration or die. The occurrence of endogenous respiration

Figure 30.2 Anoxic period in aeration tank. After the production of nitrate ions (NO3) in an aeration tank, aeration of the tank may be terminated for a period of time, such as one to two hours. During this time period, facultative anaerobic bacteria degrade cBOD using nitrate ions. However, a residual concentration of nitrate ions should be maintained in order to prevent septicity within the aeration tank, and the anoxic period should not be extended beyond four hours. If nitrifying bacteria are deprived of dissolved oxygen for more than four hours, damage to the nitrifying bacterial population may result.

Figure 30.2 Anoxic period in aeration tank. After the production of nitrate ions (NO3) in an aeration tank, aeration of the tank may be terminated for a period of time, such as one to two hours. During this time period, facultative anaerobic bacteria degrade cBOD using nitrate ions. However, a residual concentration of nitrate ions should be maintained in order to prevent septicity within the aeration tank, and the anoxic period should not be extended beyond four hours. If nitrifying bacteria are deprived of dissolved oxygen for more than four hours, damage to the nitrifying bacterial population may result.

or death of large numbers of bacteria also results in a decrease in MLVSS.

The death of large numbers of bacteria results in cellular lysis. As bacteria lyze or break open, they release their cellular contents. The contents include ammonium ions. The release of ammonium ions contributes to an elevated level of ammonium ions in the e¿uent.

Figure 30.3 Occurrence of denitrification. Denitrification can occur wherever and whenever an anoxic condition develops. For denitrification to occur, denitrifying bacteria must be present, dissolved oxygen must be absent or a dissolved oxygen gradient must be present, and soluble cBOD must be present. These conditions for the occurrence of denitrification may be found in the conveyance system, headworks of the activated sludge process, primary clarifier, aeration tank during nonaerated periods, secondary clarifier, chlorine contact tank, thickener, and anaerobic digester.

Figure 30.3 Occurrence of denitrification. Denitrification can occur wherever and whenever an anoxic condition develops. For denitrification to occur, denitrifying bacteria must be present, dissolved oxygen must be absent or a dissolved oxygen gradient must be present, and soluble cBOD must be present. These conditions for the occurrence of denitrification may be found in the conveyance system, headworks of the activated sludge process, primary clarifier, aeration tank during nonaerated periods, secondary clarifier, chlorine contact tank, thickener, and anaerobic digester.

With a decrease in the quantity of cBOD entering the aeration tanks and reduced MLVSS in the aeration tanks, the activated sludge process becomes highly vulnerable to upsets from slug discharges and toxicity. Also with reduced MLVSS it becomes difficult for an activated sludge process to successfully nitrify.

Denitrification in the primary clarifiers results in the rising of solids to the surface of the clarifiers. These solids must be collected and transferred to appropriate tanks for thickening, digesting, de-watering, and disposal. These operational tasks represent increased costs.

Denitrification in the secondary clarifiers presents several operational concerns. Molecular nitrogen entrapped in the sludge blanket causes a thinning of the sludge blanket and a decrease in the number of bacteria that are returned to the aeration tank in the RAS. Buoyant sludge rising to the surface of the clarifiers represents a loss of solids and bacteria to the receiving water. This loss of solids may represent a permit violation for total suspended solids (TSS).

Coliform bacteria and pathogenic organisms within the solids lost from the secondary clarifiers and discharged to the chlorine contact tanks are protected from disinfection by chlorination. This protection is provided by the presence of solids that surround the organisms and the presence of nitrite ions that may be in the clarifier effluent. The discharge of elevated levels of coliform bacteria to the receiving water may result in a permit violation for coliform bacteria.

Denitrification in the thickener results in poor compaction of solids and floating solids. Poorly compacted solids may require the use of polymers or metal salts to improve compaction. The use of polymers or metal salts represents an increase in operational costs. Poorly compacted solids transferred to an anaerobic digester result in a ''washout'' of digester alkalinity and a decrease in retention time in the digester. Floating solids due to the entrapment of molecular nitrogen result in an overflow of solids from the thickener to the head of the treatment plant.

Nitrite ions and nitrate ions transferred to an anaerobic digester have two significant and adverse impacts on digester performance. First, the rapid depletion of these ions through denitrification in the digester and the release of molecular nitrogen result in sudden and severe foaming. Second, the presence of nitrite ions and nitrate ions in the digester increases the redox potential of the digester sludge. An increase in redox potential above —300 mv inhibits the activity of methane-forming bacteria that convert volatile acids (cBOD) to methane. Inhibition of methane-forming bacteria permits the accumulation of volatile acids and the production of a ''sour'' digester.

The most common occurrence of accidental denitrification is in the secondary clarifier. The occurrence of denitrification in the secondary clarifier is often termed "clumping" or "rising sludge.'' During denitrification large clumps of dark sludge can be observed rising from the bottom to the top of the clarifier. Numerous bubbles (molecular nitrogen, carbon dioxide, and nitrous oxide) are associated with the sludge. The sludge is dark due to the high MCRT provided for the growth of nitrifying bacteria that produce the nitrite ions and nitrate ions that are used during denitrification.

In the secondary clarifier the bacteria rapidly consume dissolved oxygen as the sludge separates from the clarified supernatant. The loss of dissolved oxygen in the sludge blanket allows for development of an anoxic condition. Under this anoxic condition, nitrite ions and nitrate ions are reduced to molecular nitrogen and nitrous oxide by denitrifying bacteria.

The rate of gas production in the secondary clarifier can vary significantly. Gas production can be extremely high during warm temperatures causing significant turbulence within the clarifier and can inhibit normal settling of solids. Gas production can be very low if an insufficient carbon source (cBOD) is present. Regardless of the rate of gas production, rising sludge is more evident if the floc particles have numerous filamentous organisms. These organisms easily entrap large quantities of gases.

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