Groundwater Management at the Variante di ValicoDuring tunnelling, the groundwater seeping from the tunnel face must be treated before being discharged in superficial waters or sewers because, as a result of the excavation process, these waters are polluted and do not comply with some parameters imposed by the regulation.For this reason, Fraccaroli & Balzan collaborates with the main tunnelling companies to make water decantation and sludge dewatering plants, operated concurrently with the tunnel construction, whether it be driven by classical means or TBM. In addition to the Martignano tunnel, the Naples metro or the Gotthard tunnel in Switzerland, the company collaborates at the moment with Toto Costruzioni and Baldassini-Tognozzi Pontello at the 'Variante di Valico' highway bypass. Visit www.fraccarolibalzan.itObjectivesThe design of water settling and sludge dewatering plants is based on the objective of preserving the environment. The objective set by these plants is, in particular, the limit values for discharge to surface waters or sewers with respect to pH and the quantities of suspended solids (mg/litre) and hydrocarbons (mg/litre). Such parameters comply with the values imposed by the Italian regulation (legislative decree #152 of 11th May, 1999). Operating cycleSeepage water from the tunnel is transported to the decanter. The type of decanter is static or radial. In the decanters, the suspended solid particles deposit, while clean water appears at the surface. The range of tanks is very wide, with a capacity of treatment ranging from 200 litres/min up to 25,000 litres/min and more, according to requirements.During the travel of cloudy water to the decanter, alluminium polichloride is added together with a water/flocculant solution, prepared in a purposely-built automatic mixing station. The flocculant is composed of a non toxic biodegradable powder used to aggregate the suspended solid particles, thereby accelerating sedimentation. The solid particles then deposit at the bottom of the decanter while clean water appears at the top. The decanter is equipped with an oil separator consisting of a sheet metal ring, concentric with the skimmer channel of the decanter. The function of this separator is to hold the oil standing at the surface of the settled water and make possible its removal manually using adecuate piping.The quantity of suspended solids contained in the water at the exit of the decanter equals or is less than 80 mg/litre, a value imposed by the legislation for discharge to surface waters. In case it is necessary to reduce later this value, it is possible to filter the water a second time downstream the final treatment to remove smaller suspended solids. For this purpose, sand filters are used. These filters function discontinuously, since after a certain period of operation, they are regenerated by countercurrent flushing in order to remove the solids retained. Lastly, the utilization of active carbon filters may be necessary to reduce hydrocarbons.Sludge cycleThe sludge is unloaded from the plant at regular intervals and is collected in a basin equipped with level sensors and agitator to prevent sedimentation. The sludge is subsequently sent to a filter press with filter plates of the recessed chamber plate type, where it is later pressed to extract the residual water, if any is left.The sludge is conveyed from the basin to the filter press chambers with a high pressure pump, while the contrary pressure is given by a piston activated from a hydraulic station. The residual water contained in the sludge and extracted by the decanter is resent to the back-flow water collecting shaft. The dewatered sludge deposits at the surface of the filter plates, that are like blades removed from under the filter press. The filter press may be equipped with an automatic flushing system of the filter cloth using a high pressure water gush.pH meterThere is a system to control and possibly correct the pH value of cloudy waters. The system allows to execute a first pH measurement of the back-flow water at the entrance of the decanter. The pH measurement is carried out by an instrument connected to a sensor. According to the measured value, an acid solution is injected upstream the measuring point by means of an inverter-controlled dosing pump. The second control measurement is achieved by a second sensor in the dewatering basin. If the pH value observed by this sensor exceeds the tolerance, it needs to be corrected by a second dosing pump.AnalizerFor a better management, the plants are equipped with an automatic control and dosing device for the flocculant, named analizer, which controls the presence of sludge in water by analyzing a water sample under treatment. The system changes therefore automatically the working parameters of the flocculant pump in accordance with the variations of sedimentation speeds of the suspended solid particles contained in the analysed sample, in order to keep the sedimentation speed within the set parameters. The analysis is carried out on water samples taken at the entrance of the decanter and the pump power is adjusted accordingly.Automation and innovationThrough remote supervision, it is also possible to control the operation of the full plant. The system integrates a software installed on a remote control station (typically a PC) which, through a series of transducers (level, power, pressure, etc.), checks the operating status of the components (pumps, valves, motors, etc.). This way, it is possible to identify possible abnormalities with a list of alarms. 16/06.