It has been demonstrated (Anthonisen et aj_, 1976) that the nitrification 

 process can be inhibited in the presence of nitrous acid {HNO2) and un- 

 ionized ammonia (NH3). The total ammonia in a wastewater treatment system 

 is present as ammonium ion (NH4"*') and un-ionized ammonia {NH3). If the pH 

 of the solution increases, either naturally or by addition of a base, the 

 concentration of un-ionized ammonia will increase. Un-ionized ammonia in- 

 hibits nitrobacters at concentrations (0.1-1.0 mg/1 NH3) appreciably lower 

 than those (10-150 mg/1) at which it inhibits nitrosomonads. This impedes 

 the conversion of nitrite to nitrate, causing nitrite to accumulate. When 

 the pH decreases, as ammonium and nitrite are oxidized, an increase in ni- 

 trous acid (HNO2) concentration occurs. Nitrous acid inhibits both nitro- 

 bacters and nitrosomonads at concentrations between 0.22 and 2.8 mg/liter. 

 This inhibition of the process can also result in an increase in nitrite. 



Several organic compounds likely to be found in significant concentra- 

 tions in industrial wastes have been shown to inhibit the nitrification pro- 

 cess (Hockenbury and Brady 1977). Dodecyl amine, aniline, and r[-methyl ani- 

 line at concentrations less than 1 mg/liter caused 50 percent inhibition of 

 ammonia oxidation by Nitrosomonas ; £-nitrobenzaldehyde, £-nitroaniline, and 

 r[-methylaniline at concentrations of 100 mg/liter inhibited nitrite oxida- 

 tion by Nitrobacter . 



The loss of nitrification flora, especially resulting from the use of 

 antibiotics, has also been indicted (Patrick et al_. 1979) as a potential 

 cause of large amounts of nitrite accumulating in natural waters. 



In view of these considerations, nitrite may be present under some cir- 

 cumstances in natural waters at concentrations high enough to be deleterious 

 to freshwater aquatic life. Some field data have been reported documenting 

 this. Klingler (1957) has reported nitrite concentrations of 30 mg/liter 

 nitrite-nitrogen (NO2-N) and higher in waters receiving effluents from 

 metal, dye, and celluloid industries. McCoy (1972) has reported concentra- 

 tions up to 73 mg/liter NO2-N in Wisconsin lakes and streams. We have ob- 

 served levels of 0,1 mg/liter NO2-N in a reasonably clean cold water trout 

 stream in Montana (Russo and Thurston 1974). 



The literature through 1977 on nitrite toxicity to fishes has been sum- 

 marized elsewhere (Russo and Thurston 1977, 1978; U.S. EPA 1977). Most of 

 the data available do not include 96-hour LC50 values, but some comparisons 

 can be made. From this and more recent literature there appear to be some 

 differences, at least on a short term (less than four days) basis, in the 

 relative susceptibilities to nitrite of different fish species. Concentra- 

 tions as low as 0.2 mg/liter NO2-N are acutely lethal to several species, 

 with trout and salmon being the most susceptible. Concentrations in the 

 range of 2 to 15 mg/liter NO2-N have been reported to be lethal to some 

 warmwater species, such as fathead minnows ( Pimephales promelas ) and channel 

 catfish ( Ictalurus punctatus ). Some fish species, such as creek chub 

 ( Semotilus a. atromaculatus ) and carp ( Cyprinus carpio ), succumb only at 

 higher concentrations, up to 100 mg/liter NO2-N. Of the fish species 

 studied, those most tolerant to nitrite were: common white sucker 

 ( Catostomus commersoni ), quillback ( Carpiodes cyprinus ), and mottled sculpin 

 ( Cottus bairdi ). These species incurred no mortalities during short expo- 



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