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Roberto Mazzini, Francesca Pizza, Carlo Candelpergher

EFFICIENZA ENERGETICA E MONITORAGGIO DEI CONSUMI IN UN DEPURATORE DI GRANDI DIMENSIONI

ENERGY EFFICIENCY AND MONITORING IN A LARGE WASTEWATER TREATMENT PLANT

The Nosedo municipal WWTP is the first and the biggest plant serving the city of Milan (Italy), it purifies about 50% of the total urban sewage, corresponding to a treatment capacity 1,250,000 p.e. and an average flow of 5 m3/s. Since the design stage of this plant, a great attention has been paid to some aspects of energy efficiency as, for example, choosing equipment with inverters, installing fine bubble aeration system with high efficiency in oxygen transfer and dO2 probes, to monitor and control the oxygen concentration in each biological treatment line. In 2012 the Nosedo WWTP recorded an average consumption of energy corresponding to 0.266 kWh/m3 of treated wastewater, this value can be placed in the low range of similar data reported in literature (Campanelli M. et al., 2013) also if the values available from different publications indicate a quite variable specific energy consumption for WWTPs, depending on the plant size, the geographical location, the characteristics of the incoming water and the type of treatments. From this side, it’s also worth noticing that the WWTP of Nosedo has biological steps of oxidation - nitrification with high sludge age, tertiary treatments of sand filtration and high level disinfection (bacteriological limits for the discharge are those imposed by the Italian D.M. 185/2003), that are considered notoriously more energy demanding compared to less advanced treatments (Metcalf & Eddy, 2006). In addition to this very good starting condition, in the year 2012 the company in charge of the plant’s operation & maintenance, the Vettabbia S.c.a r.l. (which has among its contractual obligations a rational use of energy) has started a virtuous process for improving energy efficiency.
The path to obtain this improvement started focusing on the principle that “you can only improve what you know” and that’s why the first planned intervention was to increase energy measuring devices present in the WWTP: 17 energy meters are currently installed to allow to measure and accurately monitor more than 80% of total energy consumption. The values provided by the energy meters are connected to the supervision system and are displayed in real time to allow each plant worker to have a quick sight of the power absorbed instant by instant by the WWTP and its subsections.
Another useful and very interesting step has been, since the year 2012, the decision to award employees with a performance bonus linked to the enhancement of energy efficiency indicator (always intended in terms of kWh/m3 of treated wastewater). This allowed to involve every employee and increase alertness by stimulating to avoid even the little waste of energy. In summer 2013, the Vettabbia S.c.a r.l. started also the implementation process of an energy management system (ISO 50001) in parallel with the environmental management system (ISO 14001).
The first phase of this project was a deep Energy Review of the plant, in particular with the definition of an “energy baseline” intended as the quantitative reference providing a basis for comparison of the energy performance. After careful studies on the distribution of the total consumption in each phase of the wastewater treatment and its dependence on flow rate and/or organic load, the energy baseline was defined by an equation that provides the annual consumption of electricity in function of the treated water volumes and BOD (or COD). At the end of this first step, it has been possible to confirm what reported in the literature (Metcalf & Eddy, 2006): more than 50% of the overall energy demand of the Nosedo WWTP comes from the biological process and it’s linked to the daily organic load, about 20% of energy use results from pumping stations, while the rest is basically independent from the flow rate and organic pollution.
In the development of the energy management system other important actions have been: the establishment of a working group on energy management which meets at least every three months, the training and awareness of all working staff, the definition of an energy measurement and monitoring plan, of energy objectives and related action plans. This overall approach to energy management has led the Vettabbia S.c.a r.l. to obtain the certification of the integrated management system according to ISO 14001 and ISO 50001. 
Once set up this system, some further improvement actions have been implemented (or are still being implemented):
• lowering of the oxygen set point used for oxidation-nitrification, reduction of daily hours of operation of mixed liquor recycle pumps;
• optimisation of the operation of mixers installed into denitrification tanks (alternating operation of the two mixers present in each tank);
• optimisation of the operation of coarse screening (activation of waste removal upon level signal);
• realization of an innovative project for the recovery of heat from wastewater for the air conditioning of buildings located at the Nosedo plant;
• maintaining of a certain level of sludge in aerobic stabilization, in order to reduce the load of the compressors;
• optimisation of the power factor correction of electrical loads;
• computerization of preventive maintenance procedures and definition of a more detailed maintenance plan for each component/equipment.

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