  The recent popularity of post-denitrification processes in the greater Paris
area wastewater treatment plants has caused a resurgence of the presence of
nitrite in the Seine river. Controlling the production of nitrite during the
post-denitrification has thus become a major technical issue. Research studies
have been led in the MOCOPEE program (www.mocopee.com) to better understand the
underlying mechanisms behind the production of nitrite during wastewater
denitrification and to develop technical tools (measurement and control
solutions) to assist on-site reductions of nitrite productions. Prior studies
have shown that typical methanol dosage strategies produce a varying
carbon-to-nitrogen ratio in the reactor, which in turn leads to unstable
nitrite concentrations in the effluent. The possibility of adding a model-free
control to the actual classical dosage strategy has thus been tested on the
SimBio model, which simulates the behavior of wastewater biofilters. The
corresponding "intelligent" feedback loop, which is using effluent nitrite
concentrations, compensates the classical strategy only when needed. Simulation
results show a clear improvement in average nitrite concentration level and
level stability in the effluent, without a notable overcost in methanol.
