Wastewater finishing by facultative lagoons open to tidal flow: field experiences in the internal system of channels at Cesenatico FC

Aim of the present investigation is to verify the effect of transitory water volumes in completing urban water treatment before discharging in coastal sea. An internal channel of the Cesenatico harbour canal basin have been modeled as a sequence of unsteady flow reactor tanks, where variations in depth and in input/output flowrates are provided by tide, but prevalent freshwater conditions are maintained by the high rate input of treated wastewater. Unsteady flow conditions, regulated by downstream level, according with tidal oscillations, are schematized, for water quality calculations, as four sequencing daily cycles of storage and discharge. Wastewater finishing effects, provided by channel, are described by classic facultative lagoon biological processes developed in three plug flow reactor tanks. In two tidal conditions are measured water depth, velocity, temperature, salinity, pH, redox, oxygen, BOD, ammonium and nitrate nitrogen. Data present as largely prevalent, along water column, photosynthetic and aerobic oxidizing activities but reveal also an under placed active layer varying between 10 to 20 cm where anoxic denitrification conditions are maintained by oxygen consumption of bottom activities. Modeling of oxygen and biomass balances permit to quantify the different role of productive and oxidizing activities supported by mixed microalgal and bacterial populations during daily and nightly cycles.