Sewerage service and treatment system

[GRI - 103-3], [GRI - 302-1], [GRI - 303-1], [GRI - 303-2], [GRI - 303-4], [GRI - 306-1],
Sewerage service

Once it has been used for civil purposes, wastewater is collected through the sewer system and sent to the treatment plants. The treatment process enables the removal of solids and 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 864 treatment plants (of which 490 managed by Acea Ato 2, Acea Ato 5, AdF, Gori and Gesesa), the total volumes of water processed by the Group main companies224 in 2023, were 978 Mm3, of which 798 Mm3 by Acea Ato 2, Acea Ato 5, Gori, AdF and Gesesa225. The total number of treatment plants has come down in recent years, from 895 between 2019 and 2023, to 864 plants, on the basis of the progressive centralisation of the treatment of wastewater, where possible, with the improvement of certain purification plants and decommissioning of others, from a perspective of rationalising and making the service more efficient (see also the information box on Acea Ato 2).

The volumes of wastewater treated and the percentage coverage of sewerage and treatment services, out of the total number of users served by the aqueduct, for Acea Ato 2, Acea Ato 5, Gori, AdF and Gesesa are shown in Tables 60 and 61. The sewerage networks managed in 2023 total 23,534 km, of which 14,565 km relate to the five Companies listed.


Table no. 60 – Volumes of wastewater treated by Water Companies operating in Lazio, in Campania and in Tuscany (2021-2023) (mm3)

company 2021 2022 2023 destination
Acea Ato 2




99.8% returned to the environment (river/channel), sea (0.2%) and soil (0.01%).

Acea Ato 5




surface water body (river)





surface water body and sea (in sea, in 2023, 22%, equal to approximately 30.6 million cubic metres226)





surface water body and sea (0.5% in sea)

Gesesa (*) 2.3 1.8


surface water body (river)

(*) Since 2020, Gesesa began installing flow meters at the entry to treatment plants. Installations continued in 2023. The data is estimated.

Chart no. 56 – Sewer networks of the main Group Companies in Italy (2023)

Chart no. 56

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 entirely as “fresh water”, containing less than 1,000 mg/l of total dissolved solids, flows into bodies of surface water. In 2023, only 0.2% of the water treated by Acea Ato 2, 0.5% of the water treated by AdF and 22% of the water treated by Gori is discharged into the sea, representing approximately 4% of total water treated227. The portion of water discharged into the sea from Gori travels through underwater pipes, following treatment at the coastal purification plants on 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 the discharge from the 490 plants228 are presented in Table No. 62.

Table no. 61 – Percentage coverage of the sewer and purification services for total user accounts of the Water Companies in the NFS

company 2021 2022 2023
  sewer purification sewer purification sewer purification
Acea Ato 2 (*)







Acea Ato 5




























(*) The 2023 data include an estimated percentage of users in newly acquired municipalities not yet migrated to Acea Ato 2's commercial systems.

Table no. 62 – Hydrographic basins aected by discharges of COMPANIES within the scope of 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


basins of rivers Calore, Sabato, Isclero and Tammaro


basins of the river Sarno and Regni Lagni canals


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.

The companies operating in the water sector are committed to increasing the capacity and efficiency of their treatment plants by upgrading certain facilities and decommissioning others (see the box on Works by Acea Ato 2).


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 geo- morphological and urban-planning perspective. In fact, with a high number of small and medium-sized treatment facilities and sew- age systems managed (127 treatment plants with capacity below 10,000 P.E.), service coverage is guaranteed primarily by large and medium-large treatment plants (44 treatment plants with capacity above 10,000 P.E.). In the last three years (2021-2023), 13 small and medium-capacity treatment facilities were decommissioned. 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 care- fully evaluated for the specific case. In 2023, the Centralisation Plan reached the goal of eliminating a further 3 minor treatment plants (Giustinianella, Trigoria e Case and Campi in the Municipality of Rome), in addition to one medium-large plant “Palmarola” again in the Municipality of Rome.

Water companies manage treatment processes in line with the authorisations required for each plant and on the basis of the regulatory context in which they operate. The discharge limits each plant are established through an authorisation issued by the competent administrative body, which, on the basis of technical and environmental assessments during the evaluation stage, 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 generally higher than the national regulatory reference level, while Acea Ato 5 is subjected to stricter authorisations regarding the quality of water discharged than those established by sector-wide regulations. This is a precautionary approach.

The companies that perform analyses to verify the proper treatment of water report the percentages of non-compliance with discharge limits, which are very low relative to the total quantities analysed: 2.2% for Acea Ato 2, approximately 0.6% for Acea Ato 5, 0% for Gori, 3.6% for AdF and 0.2% for Gesesa229.

Chart no. 57 – Analytical checks on wastewater, total and by company (2023)

Chart no. 57

The 145,889 controls conducted by Acea Ato 2 on wastewater confirmed the high purification standards achieved by the treatment process.
In the “historic” area managed by Acea Ato 2, which includes Rome and Fiumicino, the main treatment plants in 2023, treated approximately 515 million of cubic metres of wastewater, a figure that is slightly lower than the previous year (510 million cubic metres). Considering the 171 smaller treatment systems and the plants of the municipalities acquired in OTA 2, this makes a total volume of 604 million cubic metres of wastewater treated (589 Mm3 in 2022).

Table no. 63 shows the figures of the most important parameters from the main treatment plants of Acea Ato 2, Acea Ato 5, Gori, AdF and Gesesa. The section Sustainability performance: Water Business in the Environmental Report details other treatment efficiency indicators.

Table no. 63 – Output parameters of the main treatment plants managed by Acea Ato 2, Acea Ato 5, Gori, AdF And Gesesa (2023)

  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 4 3 9 7 10 ≤ 25
COD 22 9 17 36 10 ≤ 125
SST 6 5 20 14 10 ≤ 35

nitrogen (ammoniacal, nitrate,

6 3 7 12 5 -


2 1 1 3 1 -

quantity output (t)






30 -





9 -


The sludge produced during the treatment process is mostly sent for recovery of material (see Intermediation and Transport of Waste in the Environment Business, and the chapter on Waste).
In 2023, work continued to reduce the sludges produced by treatment plants managed by Group companies230 and make treatment processes more efficient (see Management and minimisation of waste produced in the chapter Waste).
Acea Ato 2, completed the upgrading of two plants producing biomethane in 2023, which is fed into the gas distribution network managed by Italgas Reti, from the biogas produced by the anaerobic digestion processes at the Roma Nord and Roma Est treatment plants.

224 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 three subsidiaries not consolidated on a line-by-line basis. Acque, Publiacqua and Umbra Acque.
225 Gesesa started installing the first flow meters on certain plants in 2020 and estimating the quantities of wastewater treated; this activity continued in 2023 as well.
226 Plants that discharge into the sea for the Company Gori are those on the islands of Capri, the Sorrento Peninsula and that of Foce Sarno.
227 The discharge of water, as for intake, occurs in areas at potential risk of water stress, as defined by the cited Aqueduct Water Risk Atlas.
228 The wastewater treatment plants for the NFS companies, Acea Ato 2, Acea Ato 5, AdF, Gori and Gesesa total 490. By also including Acque, Publiacqua and Umbra Acque, the number goes up to 864.
229 The percentages correspond to the M6 value, as defined by ARERA, but still in the consolidation stage, except for Gesesa, which reports non-compliances according to an alternative calculation method and where the M6 data was not available at the time of publication.
230 It is noted with regard to AdF that the production of sludges increased for a specific reason: the urban wastewater treatment plant IDL S. Giovanni - at Pianetto in the Munici- pality of Grosseto - stopped hydrolysis treatment in January 2023 and restored aerobic treatment, as had been done in the past. This was in the context of criminal proceedings, where the Preliminary Investigations Judge ordered the preventative seizure of only the areas intended for the surplus transfer of sludge. Investigations are still ongoing. ADF complied with all the prescriptions issued by the Judicial Authorities and Administrative Authorities.