TABLE 183 Inhibitory Threshold Concentrations of Some Organic Wastes

Organic Waste Concentration, mg/l

Allyl alcohol 20.0

Aniline 8.0

Chloroform 18.0

Mercaptobenzothiazole 3.0

Phenol 6.0

Skatol 7.0

Thioacetamide 0.5

Thiourea 0.1

Free ammonia inhibits Nitrosomonas and Nitrobacter. Free ammonia can inhibit Nitrosomonas at concentrations as low as 10 mg/l. Free ammonia can inhibit Nitrobacter at concentrations as low as 0.1 mg/l.

The conversion of nitrite ions to free nitrous acid and its accumulation are a function of nitrite ion concentration and aeration tank pH. With decreasing pH, nitrite ions are more easily converted to free nitrous acid (Equation 18.2).

Free nitrous acid inhibits Nitrosomonas and Nitrobacter at very low concentrations. Both genera of bacteria may be inhibited by free nitrous acid at concentrations as low as 1.0 mg/l.

Substrate inhibition in an activated sludge process usually occurs at a concentration of 400 to 500 mg/l ammonium ions or when ammonium ions are converted to nitrite ions at a faster rate than nitrite ions are converted to nitrate ions. Therefore excessive ammonium ion discharge or deamination of organic-nitrogen compounds may inhibit nitrification.

Inhibition due to high ammonium ion concentration or high organic-nitrogen levels can be prevented. Reducing or equalizing nitrogenous waste discharges to the activated sludge process helps to prevent substrate inhibition or toxicity, and maintaining proper pH and alkalinity in the aeration tank also helps to prevent substrate inhibition or toxicity.

Except for the photosynthetic bacteria, ultraviolet radiation or light harms most bacteria, including nitrifying bacteria. The lethal

Photosynthetic Bacteria Tank

Angstroms

Figure 18.1 Ultraviolet radiation. Ultraviolet radiation from approximately 2000 to 4000 Angstroms in length is highly lethal to nitrifying bacteria. The lethal effect of ultraviolet radiation upon nitrifying bacteria prevents their growth on the surface of soil and the surface of biofilm that coats aeration tanks and secondary clarifiers.

Angstroms

Figure 18.1 Ultraviolet radiation. Ultraviolet radiation from approximately 2000 to 4000 Angstroms in length is highly lethal to nitrifying bacteria. The lethal effect of ultraviolet radiation upon nitrifying bacteria prevents their growth on the surface of soil and the surface of biofilm that coats aeration tanks and secondary clarifiers.

effects of ultraviolet radiation are limited to only short, invisible wavelengths of light (Figure 18.1). The most harmful wavelength is 2650 Angstroms (265 nm or 0.265 mm). It is suspected that ultraviolet radiation causes inactivation of enzyme systems, especially in young or rapidly growing cells.

The mode of operation of an activated sludge process addresses those critical factors that have significant influence on the population size of nitrifying bacteria and the ability of the process to nitrify. Those operational factors that influence nitrification most in an activated sludge process are MCRT, MLVSS, HRT, F/M, and ammonium ion concentration (Table 19.1)

MCRT AND MLVSS

In order to establish a large population of nitrifying bacteria, most activated sludge processes operate at a relatively high MCRT. The minimum MCRT required to nitrify is affected by several operational factors, especially temperature. However, the MCRT needed to achieve significant nitrification is usually two to three times the generation time of nitrifying bacteria. The generation time of nitrifying bacteria in an activated sludge process is considered to be two to three days.

Because temperature affects biological activity and generation time of nitrifying bacteria, the MCRT, MLVSS, and HRT required to achieve complete nitrification is inversely related to temperature (Table 19.2). With increasing temperature, increased biological activity and shorter generation times occur for nitrifying bacteria. Therefore a lower MCRT, lower MLVSS and smaller HRT are required. The reverse is true for colder temperature.

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