TABLE 191 Operational Factors Favoring Nitrification

Operational Factor Range/Value

MCRT >8 days (increasing with decreasing temperature)

MLVSS >2000 mg/l

HRT >10 hours during cold temperatures

F/M 0.5 pounds ammonium ions per pound MLVSS

Ammonium ion <400 mg/l

There are several advantages of maintaining as many nitrifying bacteria as possible. First, having more nitrifying bacteria in the aeration tank can offset the reduction in nitrification due to cold temperature. Second, by increasing the MCRT to provide for more nitrifying bacteria, the increased MCRT also provides for more or-ganotrophs and more rapid removal of soluble cBOD. The rapid removal of soluble cBOD provides for more aeration time for nitrification. Third, a decrease in ''sensitivity'' to inhibiting and toxic wastes is provided for nitrification by maintaining more nitrifying bacteria and organotrophs.

By increasing the MCRT and MLVSS, the inhibitory or toxic mass to biomass ration is lowered; that is, the ratio of the quantity of inhibitory or toxic wastes to the quantity of nitrifying bacteria is lowered. The lower ratio provides for more viable nitrifying bacteria after a discharge of inhibitory or toxic wastes to the aeration tank.

There are several disadvantages of maintaining as many nitrifying bacteria as possible. First, increasing the quantity of solids discharged to the secondary clarifiers may contribute to poor settling of solids in the clarifier and loss of solids from the clarifier. Second, rapid depletion of DO may occur due to the presence of rapid DO consumption by a relatively large population of bacteria. Third, de-nitrification may occur in the secondary clarifier.

TABLE 19.2 Temperature, MLVSS, and Aeration Time Required to Completely Nitrify

Temperature

MLVSS

Aeration Time at 12

°C Aeration Time at 17 °C

2000 mg/l

8 to 16 hours

6 to 12 hours

4000 mg/l

6 to 9 hours

4 to 5 hours

Note: Moderate strength BOD used (200 mg/l).

Note: Moderate strength BOD used (200 mg/l).

In addition to the presence of an adequate number of nitrifying bacteria and organotrophs, an adequate HRT in the aeration tank must be provided. A high HRT is required for organotrophs to efficiently remove cBOD and for nitrifying bacteria to oxidize ammonium ions and nitrite ions. Increased HRT may be achieved in the activated sludge process by (1) placing more aeration tanks in service, (2) thickening solids in the secondary clarifier and reducing the RAS rate, and (3) reducing inflow and infiltration.

Increasing HRT provides more efficient cBOD removal and more time for nitrification. During cold temperatures, an HRT of at least 10 hours may be required to nitrify.

As HRT is increased, more particulate BOD is solublized. The increased presence of soluble cBOD places an increased oxygen demand upon the aeration tank. The increased oxygen demand may hinder nitrification. Therefore improved particulate BOD removal in the primary clarifiers may be required to maintain efficient nitrification at an increased HRT value.

There are three means of improving primary clarifier efficiency. First, more primary clarifiers may be placed in service. Second, settled solids may be removed more quickly, and third, a polymer or metal salt (coagulant), such as lime, may be added to the primary clarifier influent to remove additional solids.

The maintenance of a low F/M of sufficient duration provides a relatively long time for the nitrifying bacteria to increase significantly in number. The effect of F/M on nBOD removal efficiency is shown in Figure 19.1. In terms of ammonium ions (food or "F") to MLVSS (microorganisms or "M") ratio, a threshold of 0.5 pounds of ammonium ions applied per day per pound of MLVSS may be applicable.

AMMONIUM ION CONCENTRATION

Excess ammonium ion concentration as well as excess cBOD adversely affects the growth of nitrifying bacteria. At high pH and low pH, excess ammonium ions may contribute to substrate inhibition or toxicity. Excess cBOD causes a significant oxygen demand. This de-

Precent NH4+ Remaining

Figure 19.1 F/M and nitrification. With decreasing F/M the amount of substrate or food is decreased while the number of bacteria is increased. With decreasing amounts of substrate and increasing numbers of bacteria, the substrate (cBOD or nBOD) is more rapidly removed or oxidized. Therefore decreasing F/M results in more rapid and efficient removal or oxidation of ammonium ions and nitrite ions.

mand may cause a drop in DO that adversely affects nitrifying bacteria.

The rapid loss of DO within an aeration tank ''starves'' the nitrifying bacteria of essential DO. Due to fluctuations in cBOD loading, nitrification may occur intermittently in an activated sludge process.

The impact of cBOD loading upon nitrification can be observed by monitoring aeration tank influent cBOD/TKN. The rate of nitrification increases as the cBOD/TKN decreases. The impact of cBOD loading also can be observed by monitoring the aeration tank influent cBOD and MLVSS. Most municipal activated sludge processes experience a decrease in the rate of nitrification when the cBOD exceeds 0.5 pounds per pound of MLVSS.

The adverse impacts of excess ammonium ion concentration and excess cBOD loading may be prevented by (1) equalizing flow from dischargers of industrial wastes and (2) requiring dischargers of industrial wastes to provide the earliest possible notification of significant loading changes. By providing the earliest possible notice, more time would be made available to implement necessary process control measures.

Figure 19.1 F/M and nitrification. With decreasing F/M the amount of substrate or food is decreased while the number of bacteria is increased. With decreasing amounts of substrate and increasing numbers of bacteria, the substrate (cBOD or nBOD) is more rapidly removed or oxidized. Therefore decreasing F/M results in more rapid and efficient removal or oxidation of ammonium ions and nitrite ions.

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