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Todi and Porchiano WWTP (Italy)

REALIZATION OF A NEW FOUL SEWAGE COLLECTION NETWORK IN THE MUNICIPAL AREA OF TODI AND A NEW WASTEWATER TREATMENT PLANT WITH A TREATMENT CAPACITY OF 25,000 P.E. IN TODI-PORCHIANO (PERUGIA, ITALY).


Project: Sewerage network in the town of Todi and new wastewater treatment plant in Porchiano


Location: Province of Perugia, Italy


Client: Costruzioni Dondi S.p.A.


Cost of total Works: € 5.577.779,33 € 2.801.560,55 (sewage network) € 1.347.389,46 (WWTP)


Services: Final Design and Environmental Mitigation Study


Period: 2010



The town of Todi and the surrounding area were served by numerous small treatment plants discharging their outfall directly into the Tiber river or its tributaries. The whole area is situated inside the Tiber River Nature Reserve and therefore required high efficiency treatment plants in terms of suspended solids and nutrients such as BOD, nitrogen and phosphorous. Many parts of the towns in the depuration district of Todi were without an adequate foul sewage network, and, in some cases, any network at all, so that outfalls were discharged directly into the ground. This situation called for the design of a new and integrated foul sewage network, linking the small treatment plants and directing the urban wastewater towards the new treatment plant.

The entire area of intervention was reduced to two main zones, denoted the “northern part” and the “southern part”. In particular, the solution required the design of a completely new foul sewage network, to connect the existing plants with the new treatment plant, for a total length of 18 km, with 9 pumping stations.
Both the zones were designed to host a single gravity or pressure pipeline system: since Todi and the surroundings towns are situated in a hilly area, the gravity sewerage system (made of PVC pipes) was preferred in order to save energy. Where the ground was almost flat or the slope negative and interferences such as roads, railways and rivers or streams had to be crossed, a pressure sewerage system (made of stainless iron or HDPE pipes) was adopted: the existing small plants, usually made of one circular concrete tank, were converted into pumping stations.
The site assigned to the new plant in Porchiano is located inside the Tiber River Nature Reserve, on the sheltered side of a hill close to the river. This particular conformation of the site needed a reconfiguration, in order to create a flat area and to limit the excavation works. For this reason the new plant, receiving the wastewater directly from the last pumping station of the pressure pipeline system, is set at two different levels, at different elevations: the water treatment line is higher than the sludge line, located below. The soil excavated from the lower area was used to make levees all around the plant, offering the possibility of masking the plant with shielding trees.
The wastewater treatment line was composed of:
  • preliminary treatment: fine screening and horizontal aerated grit removal and equalization tanks
  • secondary or biological treatment: nutrients removal using activated sludge process in mixed and aerated tanks operating in a total oxidation – long aeration system (denitrification, oxidation/nitrification steps) and secondary sedimentation in order to separate the liquid from the sludge phase
  • tertiary treatment: membrane filtration and disinfection with UV light in open concrete channel
  • sludge treatment: gravity thickening tanks and dewatering of the sludge by centrifuging
Since the effluent from the plants was directly discharged into the Tiber River, within the Nature Reserve, and the dried sludge could be re-used for agricultural purposes, the values in concentrations and the removal degree of nutrients were lower than the limits required by the Italian Water Treatment Regulations.