Temperature

Of all operational factors affecting nitrification, temperature has the most significant influence on the growth of nitrifying bacteria and, consequently, the rate of nitrification (Table 17.1). The rate of nitrification usually is expressed as pounds of ammonium ions oxidized per pound of MLVSS per day. Because nitrifying bacteria are temperature sensitive, nitrification is temperature sensitive. There is a significant reduction in the rate of nitrification with decreasing temperature and, conversely, a significant acceleration in the rate of nitrification with increasing temperature. The rate of growth of nitrifying bacteria increases considerably with temperature over the range of 8° to 30 °C, with Nitrosomonas having nearly a 10% increase in growth rate per 1 °C rise.

Below 10 °C the nitrification rate sharply falls. Above 10 °C the rate of nitrification is almost directly proportional to the temperature. Nitrosomonas isolated from activated sludge processes has an

TABLE 17.1 Temperature and Nitrification

Temperature

Effect upon Nitrification

>45 °C

Nitrification ceases

28° to 32 °C

Optimal temperature range

16 °C

Approximately 50% of nitrification rate at 30 °C

10 °C

Significant reduction in rate, approximately 20% of rate at 30 °C

<5 °C

Nitrification ceases

TABLE 17.2 Temperature and MCRT Required for Nitrification

Temperature

MCRT

10 °C

30 days

15 °C

20 days

20 °C

15 days

25 °C

10 days

30 °C

7 days

optimal growth rate at 30 °C. Therefore, for operational purposes, the optimal temperature for nitrification in the activated sludge process is generally considered to be 30 °C. No growth of Nitrosomonas or Nitrobacter occurs below 4 °C.

Because of the decreased activity and reproduction of nitrifying bacteria during cold temperatures, an increase in the size of nitrifying bacteria (MLVSS) or an increase in MCRT is required in order to maintain effective nitrification (Table 17.2 and Figure 17.1). In gen-

Precent NH4+ Remaining

_ MCRT increasing _

Figure 17.1 MCRT and nitrification. With increasing mean cell residence time (MCRT), more solids including nitrifying bacteria are retained in the aeration tank and more time is provided for nitrifying bacteria to reproduce. Therefore increasing MCRT results in rapid removal or oxidation of ammonium ions in the aeration tank.

_ MCRT increasing _

Figure 17.1 MCRT and nitrification. With increasing mean cell residence time (MCRT), more solids including nitrifying bacteria are retained in the aeration tank and more time is provided for nitrifying bacteria to reproduce. Therefore increasing MCRT results in rapid removal or oxidation of ammonium ions in the aeration tank.

eral, the shift to nitrification is a shift to an older sludge or biomass. A MCRT of 10 days or more is usually required to nitrify effectively. A low temperature may increase the time required to nitrify.

Since nitrifying bacteria are strict aerobes, they are found only in the top one to two inches of the soil. When the ground freezes below two inches, a significant source of ''seeding'' of nitrifying bacteria to the activated sludge process is lost. Due to decreased activity and growth in nitrifying bacteria and loss of a significant ''seed'' source during cold temperatures, many regulatory agencies in temperate regions provide seasonally adjusted nitrification requirements, that is, seasonally adjusted ammonia discharge limits. For example, a higher discharge limit for ammonia is provided during cold temperature months, October through April, and a lower discharge limit for ammonia is provided during warm temperature months, May through September.

The inhibitory effect of cold temperature is greater on Nitrobacter than Nitrosomonas. Therefore it is not uncommon for nitrite ions to accumulate during cold temperatures.

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