A newly developed biosensor for nitrite having a 90% response time of about 1 min was used to monitor nitrite concentration in activated sludge exposed to oxic/anoxic cycles. The NO2 biosensor contains bacteria that reduce NO2, but not NO3, to N2O that is subsequently monitored by a built-in electrochemical sensor. Nitrite plus nitrate (NOx) was simultaneously monitored by a NOx biosensor. The maximum operational lifetime of the NO2 biosensor was 6 weeks, but much longer lifetimes can be expected as malfunctioning by the 3 sensors used for longer periods was due to either mechanical damage or ineffective internal sterilization during the construction. Insufficiently sterilized sensors became sensitive also to NO3 after some time due to development of NO3-reducing bacterial populations within the sensor. The fraction of NO2 as compared to NO3 in the activated sludge was very dependent on prehistory, actual loading, and aeration. During balanced operation with NH4+ being exhausted during the later parts of the aerobic cycle, NO2 increased in concentration up to about 50 μM during the early part of the aeration cycle until NH4+ became limiting. At that time the NO2 concentration decreased to low levels. Under some operating conditions a peak of NO2 also appeared in the beginning of the anoxic period. NO2 and NO3 were depleted simultaneously during the anoxic period.

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