Optimization of the energy consumption of the wastewater treatment plant in Castiglione T.se (NW Italy)

The energy consumption represents a significant part of the operative cost of a wastewater treatment plants and this study investigates the energy consumption in the wastewater treatment plant situated in Castiglione T.se (NW Italy), that is the largest biological municipal wastewater treatment plant in Italy. By considering the energy balance of the system, the total amount of energy required is covered for the 43% with electricity bought from the national grid, for the 16% with natural gas purchased externally, for the 39% with energy produced by biogas and in minor part with photovoltaic (2%), both produced in the plant. From the point of view of destination of energy, the study is focused on the energy required in each phase to treat the wastewater flow, in order to estimate which are the most expensive operations; the identification of structural and non-structural potential improvements will be possible. In order to define in detail this type of quite difficult study, first of all it is required to evaluate the electrical consumption; an inventory of the electrical apparatus is required.. In the analyzed wastewater treatment plant, the preparation of it was facilitated by the telecontrol system that allow an immediate overview of the plant. The surveyed devices have three-phase motors, hence it was possible to calculate the power as P = √3•V•I•cosФ in which the term of current was in field measured by using electric meters and the terms of power were collected from the nominal data or from their techniical sheet. To obtain an overview of the consumption, the electrical devices were grouped according to the different phase of the treatment in which they operate; afterwards, by considering the effective operation times, the kWh/d consumed for each part of the treatment were calculated. Looking at the results, the phase that needs the highest amount of electricity is the biological oxidation in account of the electricity that is utilised by the compressors in order to blow the air in the tanks (71 % of the total energy that is used to treat the water). A situation similar to the specific considered system was also found in other Italian plants or in international case studies found in the literature. To allow the comparison with other system of other cities three indicators were calculated: kWh/m3 of treated water, kWh per capita and kWh/ kg COD removed, and the situation in Castiglione seems to be better than in the other considered plants. On account of thei great influence on the consumed energy it was also conducted a more detailed analysis about the compressors efficiency; by using characteristic graphs provided by the plant operator, and by starting from the adsorbed power, the performance of each device for air immission was calculated. Also in this case a quite good condition was observed.