On-line determination of nitrite in wastewater treatment by use of a biosensor

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 microM 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.