AERATION


Real-time control delivers plant efficiency


Veolia Eau in France has developed an innovative approach to aeration control. David Thauré, Emmanuelle Vigne and Pierre Girodet explain how both effluent quality and energy consumption can be improved


L


ess energy for a better water quality? Despite the major role played by diffuse pollution, the discharge of nutrients into


water bodies is only regulated through the assessment of the effluent quality of point sources such as wastewater treatment plants (WwTPs). Facing difficulty in controlling diffuse pollution, lawmakers tend to apply more and more stringent regulations to the outlet quality of WwTPs both in Europe and the USA, especially regarding nitrogen compounds. Many responses can be implemented to


address these new regulations. The construction of new plants or the upgrading of existing facilities is common practice but original solutions can also be found such as the nutrient trading. Less known are the development and the implementation of advanced control strategies and decision support tools. In any case, stricter outlet quality means increased energy consumption, which mainly comes from the aeration of biological tanks in case of conventional activated sludge (CAS) process. Control methods offer an efficient means to


target the objectives of WwTPs in terms of treatment performances and reduction of energy consumption. In the CAS process, aeration control is the key parameter to achieve both objectives. An innovative aeration control strategy based on the in situ measurement of ammonium and nitrate concentrations has been developed by Veolia Water to allow the operator to monitor in real time the effluent quality while optimising the energy consumptions of the plant. The implementation of such control tools is still seldom undertaken, but the perspectives they offer for future improvements in the operation and the design of WwTPs will probably favour their wide and rapid spreading at a time when the phrase 'sustainable development' is in the mouth and wallet of many.


Measure and control The recent technical evolution of nutrient sensors has allowed the development of more precise and reliable control strategies regarding nitrogen removal in WwTPs. Among these control tools,


Figure 2. Evolution of the outlet quality management from 2005 to 2008 on WwTP “E”. A steeper S-curve means a better control over the outlet quality.


Figure 1. Typical distribution of the ammonium concentration in the outlet of a WwTP without (a) and with (b) advanced aeration control. Advanced aeration control helps avoid exceeding legal requirements (zone B) and achieving an extremely good, but energy demanding outlet quality (zone A) while offering a great flexibility for outlet quality management. Figure 2. Evolution of the outlet quality management from 2005 to 2008 on WwTP “E”. A steeper S-curve means a better control over the outlet quality.


the STAR system and the AMONIT strategy feature innovative approaches that proved to be very efficient on full-scale applications, both in terms of better effluent quality management (Figure 1 and 2) and energy savings.


Design flexibility Two typical examples (not displayed) of WwTPs show the dependency of the energy consumption with the ratio real load / design load. This dependency is a sign of the lack of flexibility in terms of plant equipment and management – the whole management chain from the sensor to the actuator via the


30 Water & Wastewater Treatment September 2010


control strategy. In addition, the fact that the difference between the real load and the design load can be very significant and can affect a very large number of plants. It seems therefore utopian to base a design on one single hypothetical future load that is more or less known at the time of the construction project. It would certainly be more cost effective to plan enough flexibility in the design and in the equipment (Figure 3) to be able to use control strategies at their best. Indeed these tools – as described before – allow to adjust operational parameters to serve the desired targets in terms of effluent quality and energy consumption depending on current load and plant flexibility.


Control and flexibility The experimental results obtained at different full-scale WwTPs equipped with an advanced control strategy formanaging the aeration of activated sludge tanks have displayed a positive impact on the outlet quality control and the energy management. Together with adequate design of WwTPs, including the implementation of flexible aeration equipment, advanced control tools are thought to open a new way in the global design of treatment facilities thereby providing an appropriate answer to the mitigation between effluent quality and energy


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