TABLE 113 Limiting Factors Responsible for Incomplete Nitrification

Cold temperature

Deficiencies of key nutrients

High influent ammonium ion concentration

Inhibition/toxicity pH swing or extreme pH

Short retention time in the aeration tank

Slug discharge of soluble cBOD

Temporary low dissolved oxygen level

As the bacteria in the mixed liquor suspended solids are removed, the filtrate will not contain ammonium ions used as the nitrogen nutrient that may have been released by the bacteria during testing. Therefore these ammonium ions are not measured. Since the mixed liquor effluent is used, no significant loss of nitrite ions or nitrate ions would have occurred through denitrification. If a sample of clarifier effluent had been used, loss of these two ions through denitrification may have occurred.

The wastewater sample selected for testing should not be taken after chlorination or dechlorination. Chlorine oxidizes nitrite ions to nitrate ions, while dechlorination reduces nitrate ions to nitrite ions. These samples do not provide for a proper measurement of the amounts of nitrite ions and nitrate ions that were produced biologically in the aeration tank. Chemicals commonly used for dechlorination are sulfur dioxide (SO2) and sodium bisulfite (NaHSO3).

Although a rate of nitrification can be determined easily in a laboratory where only nitrifying bacteria are involved in a reaction and all operational conditions are controlled, a rate of nitrification can be difficult to determined in an activated sludge process. In this case there exists a mixed culture of bacteria, organotrophs and nitrifying bacteria, and operational conditions change often. Due to the presence of a mixed culture, changing operational conditions, and the numerous fates of the ammonium ions, nitrite ions, and nitrate ions in the aeration tank, it is preferred to express nitrification in an activated sludge process as a form rather than a rate.

The release of ammonium ions in the aeration tank further complicates a rate determination for nitrification. Ammonium ions are produced in the aeration tank through deamination of organic-nitrogen compounds. If the production of ammonium ions is significant, the ammonium ion concentration leaving the aeration tank may be higher than the ammonium ion concentration entering the aeration tank.

Also cationic (polyacrylamide) polymers that enter an aeration tank are degraded. Cationic polymers are commonly used at activated sludge processes for sludge thickening, solids capturing, and sludge dewatering. If cationic polymers are overdosed or misapplied, the excess polymer may eventually end up in the aeration tank and undergo degradation. The degradation of cationic polymers results in the production of ammonium ions (Figures 11.1 and 11.2).

Nitrification is the oxidation of ammonium ions and nitrite ions. An activated sludge process nitrifies if the production of nitrite ions or nitrate ions occurs in the aeration tank. The production of these

Acrylamide

Acrylamide

Ammonia nh3

Quartemary Amine

Figure 11.1 Chemical components of polyacrylamide polymers. The two basic, chemical compounds used in the production of polyacrylamide polymers are acrylamide and acrylic acid. When these two compounds are joined together to form a very long polymer chain, the resulting polymer is a negatively charged or anionic polymer. To produce a positively charge polymer or cationic polymer, the carboxyl group (-COOH) on the acrylic acid undergoes a quarternarization process using ammonia. This process converts the negative charge of the carboxyl group to a positive charge.

ions can be demonstrated by testing for their presence in the aeration tank effluent. The reduction in ammonium ion concentration across an aeration tank does not demonstrate nitrification. Nitrification is not the reduction of ammonium ions.

An activated sludge process that is required to nitrify usually tests the ammonium ion concentration in the final effluent to determine compliance with its permit requirement. However, the activated sludge process probably does not test for ammonium ion concentration, nitrite ion concentration, and nitrate ion concentration leaving

Figure 11.2 Degradation of polyacrylamide polymers and ammonium ion release. When cationic polyacrylamide polymers are degraded in an aeration tank, the amino groups on the polymer are released. The released amino groups are converted quickly to ammonium ions in the aeration tank.

the aeration tank effluent. If testing for these nitrogenous ions were preformed on a routine basis, the test results could be used to

• demonstrate nitrification,

• determine the form of nitrification,

• determine the limiting factor for incomplete nitrification,

• determine the corrective factor for incomplete nitrification, and

• provide for cost-effective operation.

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