Nitrite Ion Accumulation

The production and accumulation of nitrite ions in an activated sludge process can occur during some forms of incomplete nitrification. The accumulation of nitrite ions is due to partial inhibition of enzymatic activity within nitrifying bacteria. This inhibition prevents the rapid oxidation of nitrite ions to nitrate ions. Operational factors responsible for this inhibition include cold temperature, deficiencies in key nutrients, high influent ammonium ion concentration, inhibitory and toxic wastes, pH changes, short retention time in the aeration tank, slug discharge of soluble cBOD, and temporary low dissolved oxygen level.

Nitrite ion accumulation in an activated sludge process may be seasonal in occurrence due to a change in temperature and MLVSS concentration. Nitrite ion accumulation usually occurs during late winter or early spring in an activated sludge process with an ammonia discharge limit. Nitrite ion accumulation usually occurs during late spring or early summer and late winter in an activated sludge process without an ammonia discharge limit.

Nitrite ions in relatively low concentrations may be toxic to not only many aquatic organism but also many wastewater organisms, including floc-forming bacteria. The critical concern with the accumulation of nitrite ions is the increased chlorine demand that occurs as a result of the nitrite ion accumulation. This increase in chlorine demand is known as the ''chlorine sponge'' or ''nitrite kick.'' Nitrite ions interfere with the chlorination of the final effluent and undesired filamentous organisms. The interference with chlorination of the final effluent may result in a violation of the discharge permit for coliform bacteria.

Chlorine is a strong oxidizer. If Nitrobacter does not oxidize nitrite ions to nitrate ions, chlorine will oxidize the ions. Therefore an activated sludge process that produces and accumulates nitrite ions, meaning it experiences incomplete nitrification, will have variations in chlorine residuals and possible problems with the use of chlorine to achieve coliform kills and filamentous organism control.

When chlorine is added to the effluent for disinfection, hypo-chlorous acid (HOCl) is formed (Equation 13.1). Hypochlorous acid is the ''free available chlorine'' and the effective bactericide or killing agent of coliform bacteria and disease-causing organisms. However, the hypochlorous acid is a relatively weak acid and dissociates under the dilute aqueous solution that exists in the final effluent. When hy-pochlorous acid dissociates, the acid releases a hydrogen ion and a hypochlorite ion (Equation 13.2).

Below pH 7.5 the hypochlorous acid predominates over the hypochlorite ion (OCl~) (Figure 13.1). However, before the hypochlorous acid can effectively kill coliform bacteria, the hypochlorite ion reacts quickly with the nitrite ion. In this reaction the nitrite ion is oxidized to the nitrate ion, and the hypochlorite ion is reduced to the chloride ion (Cl~) (Equation 13.3). As more and more hypochlorite ions react with the nitrite ions, more hypochlorous acid dissociates. With more and more dissociation of hypochlorous acid, less destruction of coli-form bacteria occurs. The hypochlorite ion and the chloride ion are poor bactericides.

The production and accumulation of nitrite ions not only adversely affect permit compliance for a coliform requirement but also interferes with process control. The ''chlorine sponge'' often is responsible for the inability of chlorine to effectively control undesired growth of filamentous organisms.

If the accumulation of nitrite ions is not excessive, then the

Percent H0C1 Percent OC1"

Percent H0C1 Percent OC1"

Figure 13.1 Relative quantities of HOCl and OCl~ as affected by pH. The relative amount of HOCl and OCl~ within the chlorine contact tank, or the mixed liquor, is influenced strongly by the pH of the wastewater. At pH values less than 7.5, HOCl dominates, while atpH values greater than 7.5, OCl~ dominates.

Figure 13.1 Relative quantities of HOCl and OCl~ as affected by pH. The relative amount of HOCl and OCl~ within the chlorine contact tank, or the mixed liquor, is influenced strongly by the pH of the wastewater. At pH values less than 7.5, HOCl dominates, while atpH values greater than 7.5, OCl~ dominates.

amount of chlorine needed to satisfy the "chlorine sponge'' can be calculated. The amount of chlorine needed is approximately 13 pounds per milligram per liter (mg/l) of nitrite ion produce per million gallons per day of flow.

Chlorine Sponge

Calculate the amount of chlorine consumed in the chlorine contact tank of an activated sludge process by the presence of 7.5 mg/l of NOj in the secondary effluent? The activated sludge process has a flow of 4.0 MGD.

The amount of chlorine consumed by the ''chlorine sponge'' is 13 pounds per milligram per liter (mg/l) of NOj per MGD. Therefore the amount of chlorine consumed by the ''sponge'' is

13 pounds of chlorine per mg/l NO2 per MGD

x 7.5 mg/l NO22 x 4.0 MGD = 390 pounds of chlorine

Adjusting chlorine feed for disinfection of the effluent in the presence of the ''chlorine sponge'' should only be attempted if nitrite ion concentration and flow are monitored periodically, and the chlorine demand is calculated and adjusted accordingly. However, identifying and correcting the operational factor responsible for incomplete nitrification of the production and accumulation of nitrite ions is the better process control measure.

Nitrite ion production and accumulation can occur during nitrification and denitrification. Coliform bacteria can reduce nitrate ions to nitrite ions, and some species of Nitrobacter also are capable of reducing nitrate ions to nitrite ions.

There are several, bacterial substrates that enter an activated sludge process. These substrates provide carbon and energy for bacterial activity, growth, and reproduction. The strength of each substrate is measured as milligrams per liter of biochemical oxygen demand (Table 14.1). The types of biochemical oxygen demand or BOD that are found in an activated sludge process include total (tBOD), par-ticulate (pBOD), soluble (sBOD), colloidal (coBOD), carbonaceous (cBOD), and nitrogenous (nBOD) (Figure 14.1).

BOD not only provides energy for organotrophic bacteria and nitrifying bacteria but also provides energy for the higher life forms in the activated sludge process including ciliated protozoa, rotifers, and free-living nematodes (Figure 14.2). The higher life forms obtain carbon and energy when the ciliated protozoa consume bacteria, and the

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment