New supervisory control strategy designed for wastewater treatment plant (WWTP) with nitrogen removal.
The control strategy combines three complementary control loops to optimise the nitrogen removal inpre-denitrifying activated sludge plants. The first controller is designed to maintain the average concentration of the ammonia in the effluent via the automatic selection of the most appropriate DO set point in the aerobic reactors. The second control loop optimises the use of the denitrification potential and finally, the third control loop maintains the selected amount of biomass in the biological reactors by automatic manipulation of the wastage rate. Mobile-averaged windows have been implemented to incorporate commonly used averaged values in the control objectives. Description including main features/advantages The main objective of the supervisory control strategy designed for Activated Sludge treatment plants with nutrient removal is the optimisation of the existing plant capacity, improving process stability and effluent quality with minimum cost. Therefore, the automatic supervision was conceived as a way to select, at any moment, the most appropriate set-point for the lower-level controllers, leaving the human decision for the top of the hierarchical plant control structure. It is well known that in a conventional or ‘rigid’ operational strategy of a predenitrificatrion activated sludge plant, the process operator has to select (based on their knowledge and experience) several parameters such as the DO concentration in the aerated volumes, the recirculation flow of nitrates to the anoxic zone and the wasting of the excess of sludge. The concentration of DO in the aerated zones should be maintained at a sufficiently high level to support the growth of the adequate organisms, to guarantee the required nitrification rate and to maintain the required level of mixing, but sufficiently low to save energy, avoid the excess mixing and to reduce, as much as possible, the oxygenation in the anoxic volumes induced by the nitrate recirculation. Nitrification is improved by high DO values; however, lowering the DO set-point not only favours the denitrification in the anoxic zone but also some simultaneous nitrification–denitrification in the aerated zone. It is important to point out that for a conventional WWTP, this DO set-point is not able to eliminate completely the fluctuations in the effluent ammonia. Therefore, it is suggested that a variable DO set-point should be selected to only reach the required (not the minimum) averaged value of the ammonia concentration at the end of the aerated volume. The synergetic combination of three supervisory control loops facilitates a stable operation of the biological process, improves the effluent quality and lowers simultaneously the air flow needed. Innovative aspects The control system here presented is based on measurements of real sensors and the full-scale implementation of the control algorithms is based on experimental data combined with simulation programs and requires many additional improvement to the computing code so as to guarantee a safe operation of the process (upper and lower limits for all the manipulated variables, digital filtering and sampling of the signals, detection and elimination of outliers, automatic procedures for the initialisation and commutation of the control loops, etc.). Current and potential industrial users/domains of application WWTP operators and technology suppliers. Current state of development Tested at lab, pilot and full scale urban WWTP (Galindo, Bilbao, Spain) Contact details Organisation CEIT Website www.ceit.es Contact person Eduardo Ayesa Address P Manuel Lardizabal 15 20018 San Sebastian Spain Phone 0034943212800 Fax 0034943213076 Email firstname.lastname@example.org