TABLE 151 Oxygen Requirement Groups of Bacteria

Group Respiration Examples Significance

Aerobes Aerobic (O2)

Facultative anaerobes

Strict anaerobes

Aerobic (O2) or anaerobic

Anaerobic; killed by O2

Nitrosomonas Nitrobacter

Bacillus Pseudomonas

Methanococcus Methanosarcina

Nitrification cBOD removal Denitrification Floc formation

Methane production in anaerobic digesters biological and operational purposes. These purposes are (1) the oxidation of cBOD to provide carbon and energy for cellular activity, growth, and reproduction (production of MLVSS), (2) oxidation of cBOD to provide energy for endogenous respiration (destruction of MLVSS), and (3) oxidation of nBOD or nitrification.

Of the many operational requirements known to affect nitrifying bacteria or nitrification, dissolved oxygen (DO) concentration is one of the most important requirements. However, an optimal DO concentration to achieve nitrification is relatively low, from 2 to 3 mg/l, and unfortunately, many activated sludge processes are over aerated to achieve nitrification.

The practice of over aeration is not cost-effective and may contribute to shearing of floc particles or enhance foam production. For effective nitrification the amount of DO maintained in the aeration tank should be adjusted to ensure permit compliance and acceptable, mixed liquor effluent concentrations for ammonium ions, nitrite ions, and nitrate ions. In addition to the concentration of DO in the aeration tank, sufficient mixing must be maintained to prevent DO stratification (Figure 15.1), and the DO must penetrate to the core of the floc particles.

Because nitrifying bacteria are strict aerobes, they can only nitrify in the presence of dissolved oxygen (Table 15.2). At DO concentrations <0.5 mg/l, little, if any, nitrification occurs. Factors responsible for this limited amount of nitrification are the lack of oxygen diffusion through the floc particle and competition for oxygen by other organisms. With increasing DO concentration, nitrification accelerates.

Increasing the DO concentration permits better DO penetration

Nitrification Oxygen Curve

Figure 15.1 DO stratification. Dissolved oxygen (DO) stratification can occur in an aeration tank due to short-circuiting of flow or lack of cleaning. DO stratification results in zones of varying dissolved oxygen concentration within the aeration tank. DO stratification may cause increased operational costs in order to achieve nitrification or the occurrence of incomplete nitrification. DO stratification can be determine through periodic monitoring of the dissolved oxygen profile of the aeration tank and can be corrected through the use of baffles or routine cleaning of the aeration tank.

Figure 15.1 DO stratification. Dissolved oxygen (DO) stratification can occur in an aeration tank due to short-circuiting of flow or lack of cleaning. DO stratification results in zones of varying dissolved oxygen concentration within the aeration tank. DO stratification may cause increased operational costs in order to achieve nitrification or the occurrence of incomplete nitrification. DO stratification can be determine through periodic monitoring of the dissolved oxygen profile of the aeration tank and can be corrected through the use of baffles or routine cleaning of the aeration tank.

of the floc particle and encourages more nitrification (Figure 15.2). Within the DO range of 0.5 to 1.9 mg/l, nitrification accelerates, but it does not proceed efficiently. Significant nitrification is achieved at DO concentrations from 2.0 to 2.9, while maximum nitrification occurs near a DO concentration of 3.0 mg/l (Figure 15.3). However, if higher DO concentration is maintained in the aeration tank, and cBOD is removed more rapidly due to the higher DO concentration, increased nitrification time will be provided, and additional nitrification can be achieved.

Because nitrifying bacteria must reduce oxidized carbon (CO2) for cellular growth and reproduction and obtain little energy from the

TABLE 15.2 DO Concentration and Nitrification Achieved

DO Concentration

Nitrification Achieved

<0.5 mg/l

Little, if any, nitrification occurs

0.5 to 1.9 mg/l

Nitrification occurs, but inefficiently

2.0 to 2.9 mg/l

Significant nitrification occurs

3.0 mg/l

Maximum nitrification

O Bacteria in the presence of dissolved oxyen; nitrifying bacteria oxidize NH4+ and N02"

Bacteria in the absence of dissolved oxyen; nitrifying bacteria do not oxidize NH4+ and N02"

Figure 15.2 DO penetration of the floc particle and nitrification. Nitrification occurs only in the presence of free molecular oxygen. As long as dissolved oxygen is present around the perimeter of a floc particle, the bacteria around the perimeter of the floc particle may use the dissolved oxygen to degrade cBOD or oxidize ammonium ions and nitrite ions. As dissolved oxygen penetrates the floc particle, bacteria within the floc particle use the dissolved oxygen and may exhaust any residual dissolved oxygen. In the absence of dissolved oxygen, the bacteria in the core of the floc particle experience an anoxic condition. Also, in the absence of dissolved oxygen, nitrification cannot occur.

oxidation of ammonium ions and nitrite ions, they compete poorly with organotrophs for DO in the aeration tank. Therefore the DO level within the aeration tank should be carefully monitored and not allowed to drop below 1.5 mg/l. Below this value a diminution of nitrifying activity occurs.

Nitrifying bacteria can survive in the absence of DO for only a relatively short period of time. An absence of DO for less than 4 hours does not adversely affect the activity of nitrifying bacteria when DO is restored. An absence of DO for more than 4 hours adversely affects the activity of nitrifying bacteria when DO is restored. An absence of DO for 24 hours or more can destroy the nitrifying bacterial population.

The oxygen demand for complete nitrification is large. For most municipal activated sludge processes the demand will increase the required oxygen supply and power requirement significantly. This

Nitrification Oxygen Curve
DO (mg/1)

* pound of ammonium ion oxidized per pound MLVSS per day Figure 15.3 DO concentration and nitrification. With increasing dissolved oxygen (DO) concentration, the rate of nitrification increases. In laboratory studies, the rate of nitrification eventually levels off at a dissolved oxygen concentration of 30 mg/l. However, in an aeration tank, increasing dissolved oxygen concentration above 30 mg/l may improve nitrification if the increased dissolved oxygen concentration helps to more rapidly remove cBOD from the aeration tank. With more rapid removal of cBOD in the aeration tank, more time is provided for nitrification. It is the increase in time for nitrification, not the dissolved oxygen concentration, that is responsible for improvement in nitrification.

increase is due to the consumption of approximately 4.6 pounds of oxygen for each pound of ammonium ions oxidized to nitrate ions (Table 15.3).

The consumption of 4.6 pounds of oxygen for the oxidation of one pound of ammonium ions to one pound of nitrate ions is the theoretical value. The actual or observed amount of oxygen consumed is 4.2 pounds or slightly less than the theoretical value, attributable to the fact that some ammonium ions are not oxidized but are assimilated into cellular material (C5H7NO2).

TABLE 15.3 Oxygen Consumption (Theoretical) during Nitrification

Biochemical Reaction

Pounds O2 Consumed

1 pound NHJ to 1 pound NOg

3.43

1 pound NOg to 1 pound NO3

1.14

1 pound NH4+ to 1 pound NO3

4.57

The influent wastewater of municipal, activated sludge processes usually contains 15 to 30 mg/l of ammonium ions. Because these processes are required to achieve stable operation, namely 85% removal of BOD, nitrification easily occurs in these processes regardless of permit requirements for nitrification.

The additional oxygen demand to nitrify may be substantial, namely 30% to 40% higher, in comparison for cBOD degradation. Although the oxygen requirement for the treatment of BOD within an activated sludge process depends on the HRT, MCRT, and temperature of the aeration tank, several general comments apply. First, increasing HRT, MCRT, and temperature increase oxygen requirements. Second, with increasing HRT, MCRT, and temperature, nitrification occurs more easily in the aeration tank. Nitrification increases the oxygen requirement. Third, approximately 1 pound of oxygen is required to oxidize 1 pound of cBOD, and 4.2 pounds of oxygen are required to oxidize 1 pound of nBOD.

Wastewater is normally alkaline. It receives its alkalinity from the potable water supply, infiltration of groundwater, and chemical compounds discharged to the sewer system.

Alkalinity is lost in an activated sludge process during nitrification. This loss occurs through the use of alkalinity as a carbon source by nitrifying bacteria and the destruction of alkalinity by the production of hydrogen ions (H+) and nitrite ions during nitrification. Hydrogen ions are produced when ammonium ions are oxidized to nitrite ions (Equation 16.1). Significantly more alkalinity is lost through the oxidation of ammonium ions than through the use of alkalinity as a carbon source.

NH+ + 1.5O2 — Nitrosomonas ! 2H+ + NO22 + 2H2O (16.1)

When hydrogen ions are produced during the oxidation of ammonium ions, nitrous acid (HNO2) also is produced (Equation 16.2). Nitrous acid destroys alkalinity. The amount of nitrous acid and nitrite ions produced is dependent on the pH of the aeration tank (Figure 16.1).

As alkalinity is lost in the activated sludge process and the pH of the aeration tank drops below 6.7, a significant decrease occurs in

1000

Free Nitrous Acid as HN02 (mg/1) Figure 16.1 Free nitrous acid and pH. The concentrations of free nitrous acid and the concentration of nitrite ions within an aeration tank are influenced by the pH of the aeration tank. Increasing pH results in a decrease in the concentration of nitrous acid and an increase in the concentration of nitrite ions. Decreasing pH results in an increase in the concentration of nitrous acid and a decrease in the concentration of nitrite ions.

nitrification (Figure 16.2). Therefore it is important to maintain an adequate amount or residual buffer of alkalinity in the aeration tank to provide pH stability and ensure the presence of inorganic carbon for nitrifying bacteria. The residual amount of alkalinity desired in the aeration tank after complete nitrification is at least 50 mg/l.

Alkalinity refers to those chemicals or alkalis in wastewater that are capable of neutralizing acids. There is a large variety of chemicals in wastewater that provide alkalinity. These chemicals include bicarbonates (HCOj), carbonates (CO2~), and hydroxides (OH~) of calcium, magnesium, and sodium (Table 16.1).

These alkalis provide inorganic carbon (CO2) for nitrifying bacteria. Nitrifying bacteria prefers bicarbonate alkalinity. When carbon dioxide dissolves in wastewater, it reacts with water to form carbonic acid (H2CO3) (Equation 16.3). Carbonic acid dissociates in wastewater to form a hydrogen ion and a bicarbonate ion (Equation 16.4). The bicarbonate ion provides alkalinity.

Figure 16.2 pH and nitrification. With increasing pH, the rate of nitrification increases. The improvement in the rate of nitrification with increasing pH is due to the presence of increased alkalinity and more efficiently operating enzyme systems within the nitrifying bacteria.

Figure 16.2 pH and nitrification. With increasing pH, the rate of nitrification increases. The improvement in the rate of nitrification with increasing pH is due to the presence of increased alkalinity and more efficiently operating enzyme systems within the nitrifying bacteria.

Although alkalinity in wastewater is provided by a variety of chemicals, all chemicals are grouped together, and alkalinity is computed as though the alkalinity is all calcium carbonate (Table 16.2). Approximately 7.14 mg (theoretical) of alkalinity as CaCO3 are destroyed per milligram of ammonium ions oxidized. The actual amount of alkalinity destroyed is 7.07 mg. Some ammonium ions are

TABLE 16.1 Alkalis in Wastewater

Chemical Name

Chemical Formula

Calcium bicarbonate

Ca(HCO3)2

Calcium carbonate

CaCO3

Calcium hydroxide

Ca(OH)2

Magnesium bicarbonate

Mg(HCO3)2

Magnesium carbonate

MgCO3

Magnesium hydroxide

Mg(OH)2

Sodium bicarbonate

NaHCO3

Sodium carbonate

Na2CO3

Sodium hydroxide

NaOH

TABLE 16.2 Alkalinity as CaCO3

Chemical Name

CaCO3 Equivalent

Calcium bicarbonate

0.62

Calcium carbonate

1.00

Calcium hydroxide

1.35

Magnesium bicarbonate

0.68

Magnesium carbonate

1.19

Magnesium hydroxide

1.13

Sodium bicarbonate

0.60

Sodium carbonate

0.94

Sodium hydroxide

1.25

not oxidized but are assimilated as a nutrient for nitrogen. If the ammonium ions are assimilated, alkalinity is not destroyed.

Alkalinity is produced in an activated sludge process when organic-nitrogen compounds are deaminated and nitrate ions are destroyed during denitrification. The amount of alkalinity produced or returned to an activated sludge process during denitrification is 3.57 mg as CaCO3 per milligram of nitrate ions that are reduced to molecular nitrogen. This amount of alkalinity that is returned during denitri-fication is approximately one-half the amount of alkalinity that is lost during nitrification. Therefore the net alkalinity change in an activated sludge process through bacterial activity is a function of

• organic-nitrogen compounds deaminated,

• ammonium ions converted to nitrite ions,

• ammonium ions assimilated into new cells or MLVSS, and

• nitrate ions destroyed during denitrification.

It is essential for successful nitrification that an activated sludge process be adequately buffered with alkalinity to counteract its tendency to become more acidic over time through nitrification. To ensure that an adequate amount of alkalinity is maintained in the aeration tank during nitrification, a residual concentration or target value of at least 50 mg/l of alkalinity is recommended after complete nitrification. If this value for alkalinity is not present, then alkalinity should be added to the aeration tank.

Although numerous chemicals may be used to add alkalinity, commonly used chemicals for alkalinity addition are listed in Table 16.3.

TABLE 16.3 Alkalis Suitable for Alkalinity Addition

Chemical Name

Formula

Common Name

Sodium bicarbonate

NaHCO3

Baking soda

Calcium carbonate

CaCO3

Calcite

Limestone

Whiting chalk

Sodium carbonate

Na2CO3

Soda ash

Calcium hydroxide

Ca(OH)2

Lime

Sodium hydroxide, 50%

NaOH

Caustic soda

The addition of alkalinity to the aeration tank through the use of calcium carbonate is shown in Equation 16.5.

H2O + CO2 + CaCOs ! Ca(HCO3)2 $ Ca2+ + 2HCO3 (16.5)

Aside from temperature, the hydrogen ion concentration or pH of an organism's environment exerts the greatest influence upon the organism. Nitrification proceeds much more slowly at low pH, and it is likely that in most environments, nitrification below pH 5.0 is not due to nitrifying bacteria but to organotrophs, including fungi. At neutral pH values nitrifying bacteria are dominant, and at alkaline pH values nitrification is due mostly, if not entirely, to nitrifying bacteria.

Low pH in wastewater has a primary effect on nitrifying bacteria by inhibiting enzymatic activity and a secondary effect on the availability of alkalinity. Nitrification in an activated sludge process begins to accelerate above pH 6.7 (Table 16.4), and the optimal pH range for nitrification is 7.2 to 8.0. At the pH range of 7.2 to 8.0 the rate of nitrification is assumed to be constant, and many activated

TABLE 16.4 pH and Nitrification

pH

Impact upon Nitrification

4.0 to 4.9

Nitrifying bacteria present; organotrophic nitrification occurs

5.0 to 6.7

Nitrification by nitrifying bacteria; rate of nitrification sluggish

6.7 to 7.2

Nitrification by nitrifying bacteria; rate of nitrification increases

7.2 to 8.0

Nitrification by nitrifying bacteria; rate of nitrification assumed

constant

7.5 to 8.5

Nitrification by nitrifying bacteria

sludge processes nitrify at a pH close to neutral. Although a higher pH would appear to be more desirable for nitrification, the higher pH would adversely affect many organotrophs that are required to degrade cBOD. Fortunately nitrifying bacteria are able to slowly acclimate to a pH less than optimal. However, this acclimation may require a gradual increase or decrease of pH. The pH at which nitrifying bacteria acclimate must be maintained at a steady-state condition.

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