Wastewater finishing by combined algal and bacterial biomass in a tidal flow channel. Modeling and field experiences in Cesenatico.

Freshwater coming from discharging systems of WWTP, if released in coastal areas, is often responsible of low seawater quality in bathing sea zone. Usually in, urbanized coastal territories,treated and untreated effluents outfall into transition volumes, which provide to discharge into sea according with unsteady tidal flow. The last internal part of the channel system of Cesenatico harbour has been modelled as an unsteady flow reactor tank, where variations in depth and input/output fluxes are provided by tide, but prevalent freshwater conditions are maintained by the high rate input of treated wastewater. Four fixed point, in output, intermediate and input cells, are monitored in order to verify active phenomena and finishing effects on discharged wastewater in terms of C, N biological removal. 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. A few usual formulations, utilized in aerobic lagoons design, have been tested on the studied system and compared with model results and monitoring data, verifying the capability of scheme plug flow reactor+first order kinetic to interpret the finishing effect on BOD and Ammonium residual from WWTP.