Sewerage service and treatment system

[GRI - 103-1], [GRI - 103-3], [GRI - 303-2], [GRI - 303-4], [GRI - 103-2], [GRI - 303-1],

Water resources, after uses for the various civil purposes, is collected through the sewer pipes and sent to the treatment plants. There, wastewater is treated enabling removal of pollutants via physical processes (filtering, sedimentation, flocculation) and biological methods (aerobic and/or anaerobic decomposition of the organic substance with bacteria), and the production of sludge.

With 865 treatment plants (of which 484 managed by Acea Ato 2, Acea Ato 5, AdF, GORI and Gesesa), the total volumes of water processed by the Group149 in 2021, were 981 Mm3, of which 779 Mm3 by Acea Ato 2, Acea Ato 5, GORI, AdF and Gesesa150. The total number of Group treatment plants has decreased, from 895 plants in 2019 to 865 in 2021, on the basis of the project for centralisation of treatment of wastewater in order to streamline the service, which involves the main Companies (see info. box for more details on Acea Ato 2). For the Companies Acea Ato 2, Acea Ato 5, GORI, AdF and Gesesa, volumes of wastewater processed and percentage coverage of sewerage and treatment services for the total number of users served by the aqueduct are presented in Tables 55 and 56. The sewerage networks managed in 2021 total 22,381 km, of which 13,712 km relate to the five Companies listed.

Table no. 55 – Volumes of wastewater treated by Water Companies operating in Latium, in Campania and in Tuscany (2019-2021) (Mm3)

company 2019 2020 2021 destination
Acea Ato 2 599,8 596,9 601,5 returned to the environment (river/channel) and sea (in sea 0.3%)
Acea Ato 5 21,3 21,2 25 surface water body (river)
Gori 45,2 70,1 124 surface water body and sea (in sea, in 2021, 23%, equal to approximately 28 million cubic metres151)
AdF 25,8 23,3 25,9 surface water body and sea (0.9% in sea)
Gesesa (*) n.d. 2,2 2,3 surface water body (river)

(*) In 2020, Gesesa began installing flow meters at the entry to treatment plants. Estimated data.

Chart no. 57 – Sewer networks of the main Group Companies in Italy (2021)


Table no. 56 – Percentage coverage of the sewer and purification services for total user accounts of the Water Companies in the NFS (2019-2021)

company 2019 2020 2021
  sewer purification sewer purification sewer purification
Acea Ato 2 91,50% 88,10% 91,70% 88,40% 91,5 88,3
Acea Ato 5 66,50% 55,90% 66,80% 57,30% 67,1 57,7
Gori 82,30% 66,00% 84,00% 70,40% 86,70% 76,10%
Gesesa 80,30% 30,40% 80,60% 33,90% 80,6 34,8
AdF 84,20% 73,50% 84,20% 73,60% 84,10% 74,80%

The water in output from the plants cited, after having undergone the purification treatments described, has chemical and biological properties compatible with the life of the receiving body of water and in accordance with the parameters established (as per Italian Legislative Decree no. 152/2006).

Almost 100% of the wastewater treated, which can be defined en- tirely as “fresh water”, containing less than 1,000 mg/l of total dis- solved solids, flows into bodies of surface water. Only 0.9% of the water treated by AdF is discharged into the sea and 23% of the water treated by GORI, approximately 4% of total water treated152. The portion of water discharged into the sea travels through underwater pipes, following treatment at the coastal treatment plants of the Sorrento Peninsula (Sorrento, Massa Centro and Marina del Cantone), the island of Capri (Gasto, Occhio Marino and La Selva) and Foce Sarno. The main basins affected by discharge are presented in Table no. 57.

Table no. 57 – Hydrographic basins affected by discharges of water companies within the scope of the NFS

company hydro graphic basins affected
Acea Ato 2 basins of rivers Tiber, Aniene, Mignone and Arrone
Acea Ato 5 basins of rivers Gari, Sacco, Cosa and Liri, Fosso della Maddalena, tributary of the River Sacco, Fosso del Diluvio, tributary of Lago di Canterno
Gesesa basins of rivers Calore, Sabato, Isclero and Tammaro
Gori hydrographic basin of the river Sarno and Regi Lagni canals
AdF basins of the rivers Ombrone, Orcia, Fiora, Albegna, Elsa, Pecora

NOTE: prior to discharge, wastewater is treated in the treatment plants managed by the Companies themselves.


To improve the quality of treated water, Acea Ato 2 has defined a Centralisation Plan for treatment plants aimed at streamlining the service, centralising treatment, where sustainable, at a limited number of sites identified through analysis of the land from a geomorphological and urban-planning perspective.
In fact, with a high number of small and medium-sized treatment plants managed (117 treatment plants with capacity below 10,000 P.E.), service coverage is guaranteed primarily by large and medium- large treatment plants (43 treatment plants with capacity above 10,000 P.E.). From the date of acquisition of the Integrated Water Service (2003), and subsequent steps, 14% of treatment plants with low and medium capacity have already been eliminated. The reduced fragmentation in favour of medium-large plants, combined with integration of sewerage collector systems, has allowed greater control of treatment efficacy and optimisation of management and energy costs.
Acea Ato 2 has therefore prepared a rationalisation plan, which it keeps up to date, choosing between centralisation and upgrading of small plants on a case-by-case basis. The optimal solution depends on many factors that must be carefully evaluated for the specific case. In 2021, the Centralisation Plan reached the goal of eliminating a further 5 minor treatment plants (Guado Tufo Sacrofano, Grotte Portella Frascati, Valle Focicchia and Valle Vergine Rocca di Papa and La Botte Guidonia) as well as one medium-sized plant (Lucrezia Romana – Ciampino).

The Company manages treatment processes in line with the provi- sions of the authorisations required for each plant and on the basis of the regulatory context in which they operate. The discharge limits are established for each plant, through an authorisation issued by the competent administrative body, which, on the basis of technical and environmental assessments during evaluation, may set stricter parameters compared to those applicable nationally. In this regard, for example, the regulatory framework in which Acea Ato 2 operates is characterised by prescriptive standards for discharge which are slightly higher compared to the national regulatory reference level, and similarly, for Acea Ato 5, in the Province of Frosinone, authorisations regarding the quality of water discharged are stricter than those established by sector-wide regulations. This is a precautionary approach.

The Company that performs analyses to verify the proper treatment of waters indicated the percentages for non-compliance with discharge limits, which are nevertheless very low, relative to the total quantities analysed: 3.3% for Acea Ato 2, approximately 3.4% for Acea Ato 5 and Gesesa, 0.8% for GORI and 4.3% for AdF. In 2021, no hazardous substances were identified in analyses of Group wastewater.

Chart no. 58 – Analytical checks on wastewater, total and by company (2021)

Reti fognarie delle principali Società del Gruppo in Italia (2021)

Specifically, for Acea Ato 2, 127,417 analyses performed confirm the high purification performance achieved by the treatment process. In the “historic” area managed by Acea Ato 2, which includes Rome and Fiumicino, the main treatment plants treated in 2021 approx- imately 516 million of cubic metres of wastewater, a figure that is slightly higher than the previous year (512 million cubic metres in 2020). Considering also the smaller treatment systems and the plants of the municipalities acquired in OTA 2 (a total of 164) a total volume of approximately 602 million cubic metres of wastewater treated, in line with 2020 (597).

Table no. 58 shows the details of the most important parameters from the main treatment plants of Acea Ato 2, Acea Ato 5, GORI, AdF and Gesesa. Other indicators of the efficiency of purification are described in the section Key environmental performance indicators – Water Segment of the Environmental Accounts.

Table no. 58 – Output parameters of the main treatment plants managed by Acea Ato 2, Acea Ato 5, GORI, AdF And Gesesa (2021)

  Acea Ato 2 Acea Ato 5 Gori AdF Gesesa (Benevento)

concentration limits in surface water
(Legislative Decree no.152/06)

parameter average of values (mg/l)
BOD5 5 4 8 9 8 ≤ 25
COD 20 21 15 32 19 ≤ 125
SST 8 3 19 10 5 ≤ 35
(ammoniacal, nitrate
and nitrous)
6 6 7 20 3 -
phosphorous 1 3 1 4 1 -
  quantity output (t)
COD 16.041 925 1.892 585 25 -
SST 7.443 313 2.402 196 6 -

The sludge produced during the purification process is mostly sent for recovery of material (see the section Intermediation and trans- port of waste in Environment segment).

In 2021, actions continued to reduce the quantity of sludge produced by treatment plants managed by the Group Companies. Specifically, Acea Ato 2 began use of its thermal dryer at the Ostia plant. As a result of actions taken in recent years, in 2021 the reduction in the quantity of dehydrated/dried sludge was over 5% (compared to 2019), in line with the targets defined in the 2020- 2024 Sustainability Plan. In 2024, it is expected that the sludge produced will be dried at the largest plants (Roma Est, Roma Nord, Roma Sud, Ostia and COBIS).

In 2021, AdF completed works at the Grosseto San Giovanni plant for centralisation and thermochemical hydrolysis of sludge produced by treatment plants under its management, launching activity in the second half of the year. The works enabled a 30.5% reduction in volumes of sludge produced compared to 2019. With the planned integration of anaerobic digestion and cogeneration processes, it will be possible to contribute to the energy independence of the plant through the production of biogas.

For Acea Ato 5, increasing focus on identifying innovative technological solutions aimed at recovering material from treatment sludge, in line with circular-economy principles, guided the choice towards use of the Fiuggi Colle delle Mele treatment plant, which is particularly efficient and has an appropriate residual capacity, for treatment of liquid waste produced. In 2021, the Company launched works for activation of the same sludge treatment methods for the Ceccano treatment plant.

At Gesesa, in 2021 the centrifuge was installed at the Ponte delle Tavole treatment plant, which was already operational, and assessment is underway for further specific actions. At the end of 2020, GORI began using a sludge-drying system at the Angri treatment plant. Authorisation and technical activity is underway for the upgrading of existing dryers at the treatment plants of Nocera Superiore and Foce Sarno, which came under management in 2019 and 2020, respectively, and for which activation is expected within two years. Construction of a drying system is also planned at the Nola plant.
In 2021, Acea Ato 2 also completed preparatory activities for the production of biomethane, with a project target of 1 million Sm3 per year in 2024, as an opportunity for water operators in the circular-economy context. Once fully operational, the “biomethane” project aims to produce over 2 million Sm3 of biomethane per year through upgrading of biogas available at the two large treatment plants for civil wastewater of Roma Est and Roma Nord. This is a renewable energy source, the development of which was included

by the Italian Government in the National Recovery and Resilience Plan (NRRP) with the provision of capital funding to incentivise production. In the year, the Company completed design of the two plants, including the points of input of biofuel to the gas network managed by Italgas Reti and verification of compliance with fire-prevention regulations with the collaboration of the Rome Fire Department. According to plans, the two plants should be operational by the first quarter of 2023.


Together with SIMAM and another 24 European partners, in November 2021 Acea Elabori saw the kick-off of the Promisces (Preventing Recalcitrant Organic Mobile Industrial chemicalS for Circular Economy in the Soil-Sediment-Water System) in Paris, with the aim of identifying the current barriers that prevent circularity in the soil-sediment-water (SSW) system and defining strategies to overcome them. More specifically, the project is intended to remove highly persistent, mobile and potentially toxic substances (identified within the European REACH regulations153), to help contribute to the goal of zero pollution and to improve protection of human health.
The project, financed by the European programme Horizon 2020, calls for seven case studies in Europe. Acea Elabori will work to remove PFAS (per- and polyfluoroalkyl substances) from dredged up materials to allow for reuse. PFAS are a highly persistent type of pollutants that damage ecosystems, creating high water and soil reclamation costs and with significant impacts on human health. The project will take place over multiple years, during which techniques to monitor and model the substances and sources of pollution will be analysed, in order to enhance relevant scientific knowledge and support the promotion of solutions to prevent, mitigate and remedy this issue in the SSW system. In cooperation with interested parties, tools and strategies will be developed (as well as techniques and regulations) for sustainable management in the context of the zero pollution action plan.
By defining advanced treatment techniques for water purification and leachates from landfills, the project will contribute to circularity and the protection of ecosystems, as well as to the activities of the various companies in the Acea Group working in the integrated water system and protection of water.

Acea also works with ENEA on projects aimed at sustainable management of the waste and water cycle, with the objective of apply- ing innovative technologies and solutions to industrial projects. In the context of the Framework Agreement signed with the Agency, in 2021 Acea began two studies. The first focuses on the creation of a self-tracking tool for continuous improvement of the quality and reliability of analysis services offered by Acea Elabori. The second is aimed at preliminary investigation for the definition and implementation of sampling and MP (microplastics) analysis methodologies on water lines of treatment plants and in recipient bodies of water.

149Again in this case, the data relating to the number of treatment plants, the volumes treated, the size of the networks and the controls refer to the main Group companies operating.
in the water sector, including those not included in the full scope of consolidation.
150Gesesa started installing the first flow meters on certain plants in 2020 and estimating the quantities of wastewater treated.
151Plants that discharge into the sea for the Company GORI are those on the islands of Capri, the Sorrento Peninsula and that of Foce Sarno.
152The discharge of water, as for intake, occurs in areas at potential risk of water stress, as defined by the cited Aqueduct Water Risk Atlas.​​​​​​​
153Regulation (EC) 1907/2006, the integrated European regulation concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH).