Sources of Nitrite Ions and Nitrate Ions

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Unless nitrite ions and nitrate ions are discharged to an activated sludge process in specific industrial waste streams, nitrite ions and nitrate ions that are found in an activated sludge process must be produced in the aeration tank. The production of nitrite ions and nitrate ions is achieved through the biologically mediated reactions of nitrification. The reactions involve the oxidation of ammonium ions and nitrite ions.

Industrial waste streams that contain relatively high concentrations of nitrite ions or nitrate ions are listed in Table 26.1. Nitrification or the production of nitrite ions and nitrate ions in the sewer system does not occur due to unfavorable conditions. These conditions include the presence of large quantities of soluble cBOD, absence of dissolved oxygen or presence of only a low level of dissolved oxygen, short retention time, and relatively small population size of nitrifying bacteria.

Nitrite ions and nitrate ions are very mobile compounds due to their high solubility in water. Because of their mobility, settled solids do not need to be mixed to enhance denitrification.

TABLE 26.1 Industrial Waste Streams Containing Ammonium Ions, Nitrite Ions, and Nitrate Ions

Industrial Waste Steam

Nitrite Ions or Nitrate Ions

Ammonium Ions

Automotive

x

Chemical

x

Coal

x

Corrosion inhibitor

no2

Fertilizer

x

Food

x

Leachate

x

Leachate (pretreated)

NOg, NO3

Livestock

x

Meat

x

Meat (flavoring)

no3

Meat (pretreated)

NOg, NOg

Petrochemical

x

Ordnance

x

Pharmaceutical

x

Primary metal

x

Refinery

x

Steel

NOg, NO3

x

Tannery

x

There are several operational factors that strongly influence denitrification. These factors include the presence of substrate (cBOD), the absence of free molecular oxygen, the presence of an adequate and active population of denitrifying bacteria, pH, temperature, nutrients, and redox potential. The most critical factors are the presence of substrate or readily available carbon and the absence of free molecular oxygen. These two critical factors are reviewed in separate chapters.

Approximately 80% of the bacteria within the activated sludge process are facultative anaerobes and are capable of denitrification. Bacteria in the activated sludge process are present in very large numbers in the bulk solution and MLVSS. Unless the treatment process is experiencing a start-up condition, wash out, toxicity, or recovery from toxicity, an adequate and active population of denitrifying bacteria should be present to ensure denitrification under favorable operational conditions. Denitrifying bacteria can be added to the treatment process by augmenting with commercially prepared bioaugmentation products or seeding with the mixed liquor from another treatment process.

Denitrification can occur over a wide range of pH values. Denitri-fication is relatively insensitive to acidity but may be slowed at low pH. The range of pH values acceptable for proper floc formation by facultative anaerobes, 6.5 to 8.5, also is acceptable for denitrification. To ensure acceptable enzymatic activity of facultative anaerobe and nitrifying bacteria, the pH in the aeration tank should be maintained

at a pH value greater than 7.0. The optimal pH range for denitrifi-cation is 7.0 to 7.5.

Because denitrification is biologically mediated, denitrification occurs more rapidly with increasing temperature, and conversely, denitrification occurs more slowly with decreasing temperature. Denitrification is inhibited at wastewater temperature below 5 °C. To compensate for decreased denitrification at cold temperature, increasing the MLVSS can increase the number of denitrifying bacteria.

Because denitrification is linked to nitrification, and nitrification also is biologically mediated, warmer temperature favors rapid proliferation of nitrate ions. Warmer wastewater also has less affinity for dissolved oxygen than colder wastewater. Therefore dissolved oxygen is exhausted more easily during warm wastewater conditions, and denitrification occurs more easily during warm wastewater conditions.

Major nutrient needs for facultative anaerobes are nitrogen and phosphorus. Because of the greater energy yield and greater cell production of facultative anaerobes during aerobic respiration of cBOD as compared to anoxic respiration of cBOD, nutrient guidelines for facultative anaerobes during aerobic respiration can be used for these bacteria during anoxic respiration. These guidelines for nitrogen and phosphorus during aerobic respiration are 1.0 mg/l for ammonium ions or 3.0 mg/l for nitrate ions and 0.5 mg/l for orthophosphate ions (HPO2") in the mixed liquor effluent filtrate at all times.

Nitrite ions and nitrate ions are present and used for bacterial degradation of cBOD in an operational condition having a redox potential of +50 to —50 millivolts (mv). Redox is the measurement of the amount of oxidized compounds, such as NO— and NO—, and reduced compounds, such as NH+ in a wastewater sample. Within the range of +50 to -50 millivolts, oxygen is either absence or present in a relatively small quantity, while nitrite ions and nitrate ions are present in relatively large quantities (Table 27.1).

TABLE 27.1 Redox Potential and Respiration

Redox Potential (mv)

Respiration

Electron Acceptor

Condition

>+50 mv

cBOD removal

O2

Aerobic or oxic

nitrification

+50 mv to -50 mv

cBOD removal

NO-, NO-

Anoxic or

denitrification

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