Wastewater finishing in Cesenatico’s Channels open to tidal flow

Aim of the present investigation is to verify the effect of transitory water volumes in completing water treatment before discharging in coastal sea and to give information about water depth regulation and flowrate management in order to influence natural phenomena. Freshwater coming from discharging systems of Wastewater Treatment Plants (WWTPs), and discharged 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. When ground and basin morphology implicate an extended system of channels and storage capabilities permit Hydraulic Retention Time (HRT) of the same order of HRT in WWTPs, significant finishing effects are foreseeable on water quality discharged to sea, because of combined aerobic and anoxic natural phenomena. This work, following the first experimental campaigns (ML Mancini in MEDCOAST 07 proceedings) extends the investigations to the role of algal/bacterial balance in conditioning the main finishing effects. The research is carried out taking samples for laboratory analysis and monitoring profiles, by an YSI multiparameter probe, into an internal channel of Cesenatico harbour basin at five Km from the sea outlet. Along his length (almost 2000 m) this tract receives, in dry water conditions, only residual organic loading from Cesena municipal wastewater treatment plant (200.000 AE). It presents low average depth (max 0,9-1,5 m) and almost constant width. Four fixed point, in output, middle and input cells, are monitored in order to verify active phenomena and finishing effects on discharged wastewater such in terms of C, N, P biological removal as in terms of physical removal during sedimentation phases related to low velocity cycles. In different tidal conditions are measured water depth, velocity, temperature, salinity, pH, redox, oxygen and are taken samples to analyse concentrations of ammonium, nitrate nitrogen, Chl a, Escherichia Coli (E. coli), Total Coliforms, suspended solids. The focused internal part of the channel system has been modelled as an unsteady flow reactor tank where variations in depth and input/output discharge 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. Modelling 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. Monitoring data present as largely prevalent, along water column, aerobic conditions supporting oxidizing and nitrifying activities. The natural disinfection efficiency has been evaluated enumerating the E. coli in the Cesena WWTP outfall section and four times during the day in the last section (Visdomina). The results show that the disinfection capacity of the channel depend on the tidal variation because they influence the HRT. The disinfection efficiency was 65.3 % in the afternoon when the tidal level was at its maximum as also showed by the modelling study of the trajectories.