Forms Of Nitrification

Nitrification may be complete or incomplete (Table 10.11). Because there are two groups of nitrifying bacteria and two biochemical reactions that are involved in nitrification, there are four possible forms of incomplete nitrification.

The identification of the form of nitrification that occurs in the activated sludge process is of value to an operator to (1) ensure proper nitrification, (2) provide for cost-effective operation, (3) maintain permit compliance, and (4) initiate prompt correct measures for undesired nitrification. The form of nitrification can be obtained by determining the concentrations of ionized ammonia, nitrite, and nitrate in a filtrate sample of mixed liquor effluent from an on-line aeration tank (Table 10.11).

Each form of incomplete nitrification can occur as a result of depressed temperature or a limiting process condition (Table 10.12). Limiting factors consist of (1) low dissolved oxygen concentration, (2) slug discharge of soluble cBOD, (3) swings in pH greater than ±.3 standard units, and (4) toxicity. Additional factors that may

TABLE 10.11 Forms of Nitrification

Mixed Liquor Effluent Filtrate Form of Concentration (mg/liter)

TABLE 10.11 Forms of Nitrification

Mixed Liquor Effluent Filtrate Form of Concentration (mg/liter)

Nitrification

NH+

NO2-

NO3-

Complete

<1

<1

>1

Incomplete #1

<1

>1

<1

Incomplete #2

>1

<1

>1

Incomplete #3

<1

>1

>1

Incomplete #4

>1

>1

>1

TABLE 10.12 Factors Responsible for Incomplete Nitrification

Form

Factor(s) Responsible

Incomplete #1

Theoretical, not likely to occur

Incomplete #2

Limiting factor

Incomplete #3

Depressed wastewater temperature

Incomplete #4

Depressed wastewater temperature and/or limiting factor

TABLE 10.13 Nitrogenous Compounds of Concern to Activated Sludge Processes

Compound

Formula

Impact

Ammonia

NH3

Toxicity

Ionized ammonia

NH4+

Oxygen demand upon nitrification to NO2-

Primary nitrogen nutrient for bacterial growth

Toxicity upon conversion to NH3

Nitrite

NO2-

Denitrification ("clumping") in the secondary clarifier

Increase chlorine demand ("chlorine sponge")

Oxygen demand upon nitrification to NO3-

Toxicity

Nitrate

NO3-

Denitrification ("clumping") in the secondary clarifier

Secondary nitrogen nutrient for bacterial growth

Toxicity upon reduction to NO2- by E. coli

Organic nitrogen

TKN

Oxygen demand upon degradation

Release of cBOD upon degradation

Release of nBOD (NH4+) upon degradation

contribute to undesired nitrification include (1) decreased HRT, (2) deficiency for alkalinity, and (3) deficiency for phosphorus.

There are two forms of incomplete nitrification that result in the production and accumulation of nitrite. These forms are incomplete #3 and incomplete #4. The accumulation of nitrite is known as the "chlorine sponge," "nitrite kick," and "nitrite lock."

Nitrite reacts quickly with free chlorine and interferes with its ability to destroy coliform bacteria and pathogens in the chlorine contact tank and undesired filamentous organisms in the activated sludge process. Approximately 13 pounds of chlorine are consumed or rendered inactive for each mg/liter of nitrite accumulated per millions gallons of flow. To compensate for the chlorine sponge, the quantity of chlorine needed must be calculated and adjusted hourly, or the operational condition responsible for the accumulation of nitrite must be identified and corrected.

There are several nitrogenous compounds of concern to activated sludge processes (Table 10.13) and regulatory agencies (Table 10.14) due to their impact upon the activated sludge process and receiving waters, respectively. These compounds include ammonia, ionized ammonia, nitrite, nitrate, and organic nitrogen or total kjeldahl (TKN). Due to the adverse impact of these nitrogenous wastes upon receiving waters, many activated sludge processes are required to nitrify—that is, reduce the quantity of ammonia or ionized ammonia in the final effluent.

Nitrification in activated sludge processes can be achieved in one-stage or two-stage nitrification systems (Figure 10.6). One-stage systems consist of one aeration

TABLE 10.14 Nitrogenous Compounds of Concern to Regulatory Agencies

Compound

Formula

Impact

Ammonia Ionized ammonia

Nitrite

Nitrate

Organic nitrogen

NH3 NH4+

NO2-

NO3-

Toxicity

Oxygen demand upon nitrification to NO2-Toxicity upon conversion to NH3 Oxygen demand upon nitrification to NO3-Toxicity

Causative agent for methemoglobinemia Primary nitrogen nutrient for aquatic plants Undesired growth of aquatic plants, especially algae Oxygen demand upon degradation Release of cBOD upon degradation Release of nBOD (NH4+) upon degradation

Aeration tank Aeration tank Secondary clarifier

Aeration tank Aeration tank Secondary clarifier

cBOD removal and nitrification occur in each tank 1-STAGE SYSTEM

Aeration tank Aeration tank Secondary clarifier

Aeration tank Aeration tank Secondary clarifier

cBOD removal only Nitrification only cBOD remoal and nitrification occur separately 2-STAGE SYSTEM

FIGURE 10.6 One-stage and two-stage nitrification systems.

cBOD removal only Nitrification only cBOD remoal and nitrification occur separately 2-STAGE SYSTEM

FIGURE 10.6 One-stage and two-stage nitrification systems.

tank or a series of aeration tanks that remove cBOD and nBOD. Two-stage nitrification systems consist of at least two aeration tanks or a series of aeration tanks. The first tank or series of tanks remove cBOD and the second tank or series of tanks remove nBOD (nitrify). Two-stage systems provide better process control than one-stage systems; and with regulatory requirements becoming more stringent for ammonia or ionized ammonia discharge, especially in temperate regions of the United States, two-stage nitrification systems are becoming popular.

TABLE 10.15 Benefits Obtained Through the Use of Controlled Anoxic (Denitrification) Periods

Decrease in sludge production

Destruction of undesired filamentous organisms

Improvement in process control: ensures adequate cBOD removal

Improvement in process control: ensures the presence of a "healthy" biomass

Returns some of the alkalinity to the treatment process that was lost during nitrification

Strengthens floc particles

Although many activated sludge processes are not required to nitrify, operators of these processes may promote nitrification. The presence of acceptable nitrification helps to ensure the presence of a "healthy" biomass and a satisfactory final effluent quality. The use of nitrate produced through nitrification in selected anoxic (denitrification) periods or zones provides for improved floc particle structure and decreased operational costs (Table 10.15). A comprehensive review of nitrifying bacteria is provided in Nitriflcation and Denitrification in the Activated Sludge Process in the Wastewater Microbiology Series.

Was this article helpful?

0 0
Losing Weight Without Starving

Losing Weight Without Starving

Tired of Trying To Loose Weight And It Never Works or You Have To Starve Yourself Well Here's A Weight Loss Plan That takes Care of Your Weight Problem And You Can Still Eat. In This Book, You’ll Learn How To Lose Weight And Not Feel Hungry! In An Easy Step-By-Step Process That Enables You To Feel Good About Loosing Weight As Well As Feeling Good Because Your Stomach Is Still Full.

Get My Free Ebook


Responses

  • adonay
    Why would be incomplete reaction in nitrification?
    3 years ago

Post a comment