The effect of incubation time on most-probable-number estimates of autotrophic nitrifying bacteria was investigated by using waters, rooted aquatic plants, sediments, and slimes as inoculum sources. Maximum most probable numbers of the NH,+-oxidizing group were attained in 20 to 55 days (median, 25). Estimates of NO2-oxidizers were highest at termination (103 to 113) days.

This paper advances the hypothesis that nitrification exists in shallow streams as a result of surface activity and in estuaries due to growth in the water phase. Between these zones no significant levels of nitrification occur. Field measurements of ammonia, nitrite and nitrate nitrogen, enumerations of Nitrosomonas and Nitrobacter, and respirometry on two small streams and two moderately large rivers are presented. The appropriate mathematical models to describe nitrification must be based on the mechanisms involved ‐ bacterial growth kinetics for estuaries and zero kinetics for the surface activity in shallow streams.

The abundance of nitrifying bacteria, determined by most-probable-number procedures, within habitats of the Passaic River was as follows: rooted aquatic plants > algae _ rocks > sediments >> water. On the-average, NH4' oxidizers were M40-fold more abundant in the topmost 1 cm of sediment than in the water, and N02oxidizers were 250-fold more abundant. The population densities in this surface sediment at two nearby stations, one with a predominantly mineral stream bed and the other an organic ooze, did not differ significantly. Large numbers of nitrifiers were present to a depth of about 5 cm in a mineral sediment core.

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Estimates of NH4+and NO.-oxidizers in samples from four activated sludge plants treating mainly domestic sewage were obtained using a most-probablenumber (MPN) technique. Ranges of concentrations per milliliter of each, respectively, were 1,010 to 3,880 and 79 to 145 in settled sewages, 32 to 7,420 and 2 to 1,010 in secondary effluents, and <0.1 to 622 and 0.1 to 70 in chlorinated secondary effluents. The results of this field study indicated that nitrifiers were more resistant to chlorination than fecal streptococci, which were also enumerated. In laboratory studies the survivals of these bacterial groups in secondary effluents were determined after exposure to chlorine residuals of up to 2 mg/liter for 0 to 60 min. The nitrifiers proved considerably more resistant than fecal streptococci, with NO.-oxidizers showing greater resistance than NH4+-oxidizers. Below the outfall of one of the plants that discharges heavily chlorinated unnitrified effluent, NH4+-oxidizers amounted to approximately 200 x 10W per g of slime scraped from stream-bed rocks. Upstream of the outfall this was approximately 3 x 105/g.