A 10 years old CW multistage system (5 stages: Imhoff+HF+VF+HF+FWS) treating the municipal wastewater for a town of 3500 inhabitants.

The use of CWs for small-medium size settlements is set to increase sharply in the Mediterranean countries. The hybrid designs incorporating multiple stages of CWs is now becoming more common (Gomez et al., 2001; Peng et al., 2005; Brix et al., 2011; Ghrabi et al., 2011; Rivas et al., 2011; Vymazal et al, 2011; Jóźwiakowski K., 2009), due to the higher tolerance and efficiency of this kind of systems from one side and the often lower footprint in comparison to single stage ones when facing flow and loads variations, or even for giving chances of different effluents quality to be chosen on seasonal basis and depending on the relative final disposal. Furthermore, single typology CW systems have intrinsically specific limits in terms of processes which occur inside the reactors, like as the commonly scarce nitrification for the HF systems or the low denitrification rate for the VF unsaturated systems. The main concept of the multistage systems is the assignment of a specific role and process to each stage, in order to reach a final effluent with the highest and more appropriate quality.

The obtained results in Dicomano. by a 5 years monitoring programme, demonstrate that multistage CWs provide an excellent secondary treatment for wastewaters with variable operative conditions, reaching also an appropriate effluent quality for reuse. Dicomano CWs has shown good performances, in average 86% of removal for the Organic Load, 60% for Total Nitrogen (TN), 43% for Total Phosphorus (TP), 89% for Total Suspended Solids (TSS), 76% for Ammonium (NH4+). Even the disinfection process has performed in a very satisfactory way, reaching up to 4-5 logs of reduction of the inlet pathogens concentration, with an E. coli average concentration in the outlet often below 200 ufc/100ml.

Additional Info

  • In operation since: 01/01/2003
  • Type of wastewater treated: Domestic, office/industry
  • Hydraulic load: 525 m³/day
  • Organic load (PE): 3500 PE
  • Organic design load (kg BOD/day): 210
  • Location: Dicomano (Florence) - Italy
  • Client: Municipality of Dicomano
  • Stage 1 type: Horizontal sub-surface flow
  • Stage 1 surface area (m²): 1000
  • Stage 2 type: Vertical flow, unsaturated
  • Stage 2 surface area (m²): 1680
  • Stage 3 type: Horizontal sub-surface flow
  • Stage 3 surface area (m²): 1800
  • stage 4 type: Free water surface flow
  • stage 4 surface area (m²): 1600
  • Cost: 550.000 €
  • Needs:

    This project started by a Feasibility Study by ARPAT on commission of “Comunità Montana del Mugello, Alto Mugello e Val di Sieve” in 1997. Wastewaters produced by the whole Dicomano settlement (3.500 p.e.) were discharged in the Sieve River untreated. This multi-stage plant is working since september 2003. At the moment, it is the biggest secondary treatment Constructed Wetland system in Italy and on the biggest in the world.

  • Solution:

    The wastewater, after a primary treatment, flows into an horizontal subsurface flow system as secondary treatment (1st stage), then into a vertical subsurface flow system (2nd stage) and into an horizontal subsurface flow system again (3rd stage). At least, wastewater is received by a free water system as a tertiary treatment (4th stage). The free water system is used as polishing stage. It is conceived in order to obtain a highbiodiversity area (16 Tuscany's autoctone species of vegetation have been planted). The hydraulic loading (considering the sewage restitution coefficient) has been fixed at 150 lt/p.e. per day: the plant will treat 525 m3 of wastewater per day, in average. The organic load considered in the design has been based on the results obtained in a former analyses campaign of wastewater produced by urban settlements in the project area and has been fixed at 140 mg/l (BOD 5); inlet Ammonia concentration has been assumed as 35 mg/l. Minimal winter wastewater temperature has been conservatively fixed as 6°C.

  • Benefits:

    This system configuration is able to perform a good nitrogen removal, especially during summer, when the receiving water body has the lowest flow, and achieves the purification targets required by the italian law (D.L. 152/2006).

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